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This is to certify that the

dissertation entitled

ENHANCEMENT OF THE CRACKING RESISTANCE AND
TOUGHNESS CHARACTERISTICS OF CONCRETE WITH
RECYCLED SYNTHETIC INCLUSIONS

presented by

Aly I. Eldarwish

has been accepted towards fulfillment
of the requirements for

Doctor of Philosophy degree in Civil Engineering

 

 

' Major professor

Date March 11, 1994

 

MSU is an Affirmative Action/Equal Opportunity Institution 0-12771

 

 

 

 

 

LIBRARY
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University

 

 

PLACE IN RETURN BOX to remove this checkout from your record.
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(:me

WIS-9.1

ENHANCEMENT OF THE CRACKING RESISTANCE AND TOUGHNESS
CHARACTERISTICS OF CONCRETE WITH RECYCLED SYNTHETIC
INCLUSION

Aly 1. Eldarwish

A DISSERTATION

Submitted to
Michigan State University
in partial fulfilhnent of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Civil and Environmental Engineering

1994

ABSTRACT
ENHANCEMENT OF THE CRACKING RESISTANCE AND
TOUGHNESS CHARACTERISTICS OF CONCRETE WITH RECYCLED
SYNTHETIC INCLUSIONS
By

Aly 1. Eldarwish

The main intent of this research was to determine the feasibility of utilizing
recycled plastics and tires, through mechanical size reduction, as soft reinforcing
inclusions in concrete.

Plastic flakes obtained through size reduction of high-density polyethylene
and mixed plastic wastes were used at relatively high volume fractions as
replacement for fine aggregate in light-weight concrete, and as an additive at
relatively low volume fractions in normal-weight concrete. Plastic inclusions were
effective in enhancing the flexural toughness, post-cracking impact strength and
resistance to shrinkage cracking of light-weight concrete. The flexural strength of
light-weight concrete could be maintained at an optimum plastic volume fraction;
the compressive strength, however, was reduced in the presence of plastics. The
positive effects of plastics could be attributed to their micro crack arrest and

deflection, and particularly crack bridging effects. In compression, the adverse

efl‘ects of stress redistribution and concentration resulting from the presence of
low-modulus inclusions seem to overshadow any positive effects resulting from
the interaction of plastics with cracks. In normal-weight concrete, relatively low
volume fraction of plastic flakes had significant effects on reducing the restrained
shrinkage crack widths; slight improvements in flexural toughness were also
observed in the presence of plastics. Various aspects of the long-term
performance of both the light-weight and normal-weight plastic-concrete
composites under severe exposures were also investigated. While long-term
performance was generally good, plastic flakes at relatively high volume fractions
were observed to have adverse effects on de-icer salt scaling resistance; the
normal-weight concrete with optimum plastic flake content exhibited satisfactory
scaling resistance.

Tire inclusions were observed to be dimensionally unstable in concrete
under moisture effects; coated tire particles at relatively low volume fractions,
however, performed satisfactorily in reducing restrained shrinkage crack widths in

nonnal-weight concrete.

DEDICATED T 0: MY FA T HER PROFESSOR IBRAHIA/I ELDAR WISH,
MY MOT HER NAGIA ELDAR WISH, AND MY SISTER INGY
ELDAR WISH, FOR THEIR GREAT LOVE AND ENDLESS

ENC OURAGEMENT.

iv

ACKNOWLEDGMENTS

I would like to thank Dr. Parviz Soroushian for his intellectual inspiration,
encouragement, and patience. The completion of this work is deeply indebted to
his support. My thanks are also to members of my guidance committee: Dr.
William Saul, Dr. Ronald Harichandran, Dr. Mark Snyder, and Dr. Gerald
Ludden.

The computing resources provided by the College of Engineering A.H.
Case center for Computer-Aided Engineering and the Division of Engineering
Research at Michigan State University are also acknowledged.

Financial support of this research was provided by the US. EPA, Coalition
Technologies Ltd., and the Research Excellence Fund of the State of Michigan.

Last but not least, I would like to express my appreciation and gratitude to
my wonderful family, my father professor Ibrahim A. Eldarwish, my mother
Nagia Eldarwish, and my sister Ingy Eldarwish, who stood by me during my ups
and downs throughout my graduate studies in the USA. Their emotional support,
understanding, love and constant encouragement made my life much happier and

my work much easier.

TABLE OF CONTENTS

Chapter 1: INTRODUCTION ............................................................. 1
1.1 Concrete Materials ........................................................ l

1.2 Hypothesis of Using Plastics in Light-Weight
Concrete ........................................................................ 2
1.3 Objectives ...................................................................... 5

PART I: PLASTIC LIGHT-WEIGHT CONCRETE

Chapter 2: BACKGROUND ................................................................ 7
2.1 Recycled Plastics ........................................................... 7
2.1.1 Plastic Types ................................................... 7
2.1.2 Processing Procedures of Recycled
Plastics ............................................................ 9
2.2 Sulfonation .................................................................. 12
2.2.1 Rationale for the Use of Surface
Treatment ...................................................... 12
2.2.2 Nature of Banier Layer ................................ 14
2.2.3 Sulfonation ................................................... 15
2.3 Light-Weight Concrete ..... 17
2.3.1 Classification .............................................. 18
2.3.2 Light-Weight Aggregates ............................. 19
2.3.3 Peculiarities of Light-Weight
Concrete Mix Proportioning .......................... 20

2.3.4 Peculiarities in Manufacturing

Light-Weight Concrete ............................................ 21
2.3.5 Properties of Light-Weight Aggregate

Concrete ................................................................. 23
2.3.5.1 Compressive Strength ................................. 23
2.3.5.2 Modulus of Elasticin and Poison’s

Ratio .......................................................... 24
2.3.5.3 Ductility ...................................................... 25
2.3.5.4 Shrinkage ................................................... 25

Chapter 3: DETERMINATION OF INFLUENTIAL VARIABLES
IN THE PRODUCTION OF PLASTIC LIGHT-

WEIGHT CONCRETE ............................................................ 26
3.1 Introduction ................................................................. 26

3.2 Experimental Program ................................................. 27

3.3 Materials and Mix Proportions ..................................... 28

3.4 Test Procedures ........................................................... 31

3.5 Test Results, Analysis and Discussion ......................... 35

3.5.1 Hardened Unit Weight .................................. 35

3.5.2 Flexural Performance .................................... 36

3.5.3 Compressive Strength ................................... 41

3.5.4 Impact Resistance .......................................... 44

3.5.5 Restrained Drying Shrinkage ......................... 46

3.5.6 Microstructural Observations ........................ 47

3.6 Assessment of the Sulfonation Effects ......................... 49

3.7 Summary and Conclusions ......................................... 52
Chapter 4: OPTIMIZATION OF INFLUENTIAL VARIABLES .. 54

4.1 Introduction ................................................................ 54
4.2 Optimization Experimental Program .......................... 54
4.3 Test Results and Analysis .......................................... 55
4.4 Evaluation of the Optimized Composite ..................... 61
4.4.1 Flexural Performance .................................... 61

4.4.2 Compressive Strength .................................... 63

4.4.3 Impact Resistance .......................................... 65

4.5 Summary and Conclusions .......................................... 66

vii

Chapter 5: THEORETICAL MODELING ...................................... 67

5.1 Introduction ................................................................. 67
5.2 Modeling Under Compression ..................................... 68
5.3 Results and Discussion ................................................ 72
5.4 Summary and Conclusion ............................................ 85

Chapter 6: LONG-TERM DURABILITY AND
ENVIRONMENTAL MACT OF PLASTIC

LIGHT-WEIGHT CONCRETE ............................................... 87
6.1 Introduction ................................................................. 87
6.2 Background .................................................................. 88

6.2.1 Freeze-Thaw Durability ................................ 88

6.2.2. De-Icer Salt Scaling ...................................... 90

6.3 Experimental Design .................................................... 91
6.4 Test Procedures ........................................................... 92
6.4.1 Freeze-Thaw Durability ................................ 92

6.4.2. De-Icer Salt Scaling ..................................... 94

6.4.3 Exposure to Temperature Cycles .................. 96

6.5 Experimental Results and Discussion ......................... 96

6.5.1 Freeze-Thaw Durability ............................... 96

6.5.2 De-Icer Salt Scaling ...................................... 98

6.5.3 Exposure to Temperature Cycles .................. 99

6.6 Environmental Impact .............................................. 101

6.7 Assessment of the Sulfonation Efi‘ects ...................... 101
6.8 Summary and Conclusions ........................................ 102

PART II: PLASTIC N ORMAL-WEIGHT CONCRETE

Chapter 7: SHRINKAGE CHARACTERISTICS OF

NORMAL-WEIGHT CONCRETE ........................................ 104
7.1 Introduction ............................................................ 104
7.2 Background .............................................................. 104
7.3 Experimental Program .............................................. 111

viii

 

Chapter 8:

Chapter 9:

7.4 Materials, Mix Proportions and Test

Procedures ................................................................ l 12
7.5 Test Results and Analysis ......................................... 112
7.6 Summary and Conclusions ........................................ 124
EVALUATION OF THE OPTIMIZED PLASTIC
NORMAL-WEIGHT CONCRETE MIXTURE .......... 125
8.1 Introduction .............................................................. 125
8.2 Materials, Mix Proportions and Test
Procedures ................................................................. 125
8.3 Test Results and Discussion ...................................... 127
8.3.1 Mechanical Properties ................................. 127
8.3.1.1 Flexural Strength and Toughness .............. 127
8.3.1.2 Compressive Strength ............................... 128
8.3.1.3 Impact Resistance ..................................... 129
8.3.2 Long-Term Durability Characteristics ......... 130
8.3.2.1 Freeze-Thaw Durability ............................ 130
8.3.2.2 Scaling Resistance .................................... 131
8.4 Summary and Conclusions ....................................... 132
PART III: TIRE CONCRETE
RECYCLING OF TIRES IN CONCRETE

MATERIALS .................................................................. 134
9.1 Introduction ............................................................... 134
9.2 Background .............................................................. 135
9.3 Production and Disposal ........................................... 136

9.3.1 Volume of Production ............................... 136
9.3.2 Disposal Problems ..................................... 138
9.4 Properties of Tires .................................................... 140
9.4.1 Chemical Composition ............................... 140
9.4.2 Physical Composition ................................ 141
9.5 Resolving of Pop-Out of Tire Particles ...................... 144
9.5.1 Objectives .................................................. 144
9.5.2 Approach ................................................... 144

ix

9.6 Tires as Reinforcing Inclusions in Concrete .............. 148

9.6.1 Objective ................................................... 148

9.6.2 Materials and Mix Proportions ................... 148

9.6.3 Test Procedures .......................................... 149

9.6.4 Test Results and Discussion ....................... 150

9.6.4.1 Flexural Strength and Toughness ............ 150

9.6.4.2 Compressive Strength .............................. 152

9.6.4.3 Impact Resistance ................................... 153

9.6.4.4 Restrained Drying Shrinkage ................... 154

9.6.4.5 Permeability ............................................ 155

9.7 Summary and Conclusions ........................................ 156
Chapter 10: SUMMARY AND CONCLUSIONS ............................. 158
10.1 Introduction ............................................................. 158
10.2 Plastic Light-Weight Concrete ................................. 158
10.3 Plastic Normal-Weight Concrete ............................. 162
10.4 Tire Concrete .......................................................... 163

APPENDIX A: EFFECT OF AGGREGATE PROPERTIES

ON CONCRETE PROPERTIES .............................. 165
Al Introduction .............................................................. 165
A2 Effect of Aggregate Grading ..................................... 165
A3 Effect of Particle Shape and Surface Texture ........... 168
A4 Effect of Strength and Modulus of Elasticity

of Aggregate ............................................................. 170

A5 Effect of Porosity and Absorption
of Aggregates ............................................................ 171
A6 Effect of Bond of Aggregate ..................................... 173
A7 Effect of Thermal Properties of Aggregates .............. 173
A8 Effect of Aggregate Properties on ............................. 174
A81 Mixture Proportions ................................ 174
A82 Fresh Mix Properties ............................... 175
A83 Shrinkage ................................................ 176
A84 Creep ....................................................... 178
A85 Durability ................................................ 178
BIBLIOGRAPHY ............................................................................... 184

Table 3.1

Table 3.2

Table 3.3

Table 3.4

Table 3.5

Table 3.6

Table 3.7

Table 3.8

Table 3.9

Table 4.1

Table 4.2

Table 4.3

 

LIST OF TABLES

Experimental Program For Light-Weight

Concrete Incorporating Recycled Plastics .................................. 27
Chemical Composition of Type I Cement Used

in This Investigation .................................................................. 28
Types and Percentages of Different Plastics,

by Weight .................................................................................. 30
Average Thickness of the Different Types

of Plastics .................................................................................. 30
Mix Proportions, lb/yd3 ............................................................. 31

F lexural Strength Test Results at 28 Days:

Means, ksi ................................................................................. 38

Flexural Toughness Test Results at 28 Days:

Means, kin ................................................................................ 39
Compressive Strength Test Results at 28 Days:

Means, ksi ................................................................................. 42
Impact Resistance Test Results at 28 Days:

Means (# of Blows) ................................................................... 44
Optimization Experimental Program .......................................... 55
Flexural Performance ................................................................. 57

Compressive Strength and Impact Resistance
Test Results ............................................................................... 58

 

Table 4.4

Table 4.5

Table 5.1

Table 6.1

Table 6.2

Table 6.3

Table 7 .1

Table 7.2

Table 8.1

Table 8.2

Table 9.1

Table 9.2

Table 9.3

Table A. 1

Results of the Analysis of Variance (F lexural

Strength and Toughness) ............................................................ 62
Results of the Analysis of Variance

(Compressive Strength) .............................................................. 64
Material Properties .................................................................... 69
Minimum Moist-Curing Times to Develop

Salt Scaling Resistance .............................................................. 91
Experimental Program For Light-Weight

Concrete Incorporating Recycled Plastics .................................. 92
Scaling Resistance Test Results ................................................. 98

Summary of Restrained Shrinkage Test
Methods and Conditions Reported By

Different Investigators .............................................................. 109
Optimization Experimental Program ........................................ 111

Mix Proportions for Normal-Weight
Concrete (lb/yd3) ..................................................................... 126

Scaling Resistance Test Results ............................................... 132

Typical Chemical Composition of
Tire Rubber ............................................................................. 141

Physical Properties of Reinforcement
Structures in Tires .................................................................... 143

Typical Properties of Cross-Linked Rubber
Compounds Used in Tires ........................................................ 143

Relationship Between Drying Shrinkage
and Elastic Modulus ................................................................ 176

xii

Figure 1.1

Figure 1.2

Figure 2.1

Figure 2.2

Figure 2.3
Figure 2.4
Figure 3.1

Figure 3.2
Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

LIST OF FIGURES

Concrete Microcracks .................................................................. 3

Mechanism of Action of Plastic
Inclusions in Concrete ................................................................. 5

A Schematic Presentation of the Granulator

Cutting Chamber ......................................................................... 10
Chemical Bonding of Sulfonated Plastic Surface to
the Cement-Based Matrix ............................................................ 12
Schematic Picture of the Sulfonation Apparatus ........................... l7
Classification of Light-Weight Concretes ..................................... l8
Light-Weight Aggregate Gradation ............................................ 29
Flexural Testing ......................................................................... 33
Impact Resistance Test Set-Up ................................................... 34
Hardened Unit-Weight Test Results (Means
and 95% Confidence Level) ....................................................... 36
Typical 28-day Flexural Load-Deflection
Curves ....................................................................................... 37
Flexural Strength Test Results at 28 Days
(Means and 95% Confidence
Interval) ..................................................................................... 39
F lexural Toughness Test Result at 28 Days
(Means and 95% Confidence
Interval) .................................................................................... 4O

Figure 3.8

Figure 3.9

Figure 3.10
Figure 3.11

Figure 3.12

Figure 3.13
Figure 4.1

Figure 4.2

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7
Figure 5.1
Figure 5.2
Figure 5.3
Figure 5.4

Figure 5.5

Compressive Strength Test Results at 28 Days

(Means and 95% Confidence Interval) ....................................... 43
Impact Resistance Test Results at 28 Days

(Means) ..................................................................................... 45
Crack Width Vs. Drying Time ................................................... 46
Shrinkage Cracking Conditions .................................................. 47
SEM Micrographs of Plastic-Concrete

Composites ................................................................................ 48
Assessment of The Sulfonation Effects ...................................... 51
Typical F lexural Load-Deflection Curves .................................. 56

Regression Curves for Different Light-Weight
Concrete Mixtures ..................................................................... 60

F lexural Strength Test Results (Means and

95% Confidence Intervals) ......................................................... 62
F lexural Toughness Test Results (Means and

95% Confidence Intervals) ......................................................... 63
Compressive Strength Test Results (Means and

95% Confidence Intervals) ......................................................... 64
Impact Resistance Test Results .................................................. 65
Geometric Configurations .......................................................... 72
Deformations .............................................................................. 75
Longitudinal Stresses ................................................................. 78
Transverse Stresses .................................................................... 81

Effects of Soft Inclusions on Light-Weight
Concrete Properties .................................................................... 84

xiv

Figure 5.6

Figure 6.1

Figure 6.2

Figure 6.3
Figure 6.4
Figure 6.5

Figure 6.6

Figure 7.1

Figure 7.2

Figure 7.3
Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 7.8

Figure 7.9

Experimental Results Versus Theoretical Model
Predictions ................................................................................. 85

Creation of Hydraulic Pressure in Frozen Paste ......................... 88

Test Set-Up for Measurement of Fundamental

Transverse Frequency ................................................................ 94
Specimen Used in Scaling Resistance Test ................................. 95
Freeze-Thaw Durability Test Results ......................................... 97
Effect of Exposure To Temperature Cycles .............................. 100

Assessment of Sulfonation Effects on
Freeze-Thaw Durability Characteristics

of Light-Weight Concrete ........................................................ 102
Effect of Water-Cement Ratio on Vertical and

Horizontal Shrinkage ....................... f ........................................ 106
Principle of Stress and Tensile Strength

Development of Concrete at Early Stage .................................. 106
Restrained Shrinkage Test Specimen Types ............................. 110
Test Results ............................................................................. 117

Restrained Drying Shrinkage Versus Drying
Time ........................................................................................ 1 18

Effect of Fiber Width at Similar Volume
Fractions on Restrained Drying Shrinkage

Cracking .................................................................................. 119
Optimization Test Results ........................................................ 121
Crack Width Versus Time in Days ........................................... 123

Comparison Between Plastic Flakes and
Plastic Fibers ........................................................................... 123

Figure 8.1

Figure 8.2

Figure 8.3

Figure 8.4
Figure 8.5

Figure 9.1

Figure 9.2
Figure 9.4
Figure 9.5

Figure 9.6

Figure 9.7

Figure 9.8

Figure 9.9

Figure 9.10

Figure 9.11

Figure A1

Flexural Strength Test Results (Means and 95%

Confidence Intervals) ............................................................... 127
Flexural Toughness Test Results

(Means and 95% Confidence Intervals) .................................... 128
Compressive Strength Test Results

(Means and 95% Confidence Intervals) ..................................... 129
Impact Resistance Test Results (Means) .................................. 130
Freeze-Thaw Durability Test Results ....................................... 131

Flow Diagram Showing Estimated Destination
of Scrap Tires in 1990 (In Millions of

Tires and Percent ...................................................................... 137
Destination of Waste Tires in 1990 .......................................... 138
Volume Change of Tire Particles ............................................. 146
Epoxy Coating Eflects on Pop-Out in Hot

Water ....................................................................................... 147
Flexural Strength Test Results at 28-Days

(Means and 95% Confidence Level) ........................................ 150
Flexural Toughness Test Results at 28-Days ............................ 151
Compressive Strength Test Results at 28-Days

(Means and 95% Confidence Level) ........................................ 152
Impact Resistance Test Results at 28-Days .............................. 153
Restrained Drying Shrinkage Vs. Time .................................... 154
Chloride Permeability Test Results .......................................... 155

Effect of Increasing Maximum Size On the
Void Content of a Well-Graded Aggregate .............................. 166

Figure A2 Influence of Maximum Aggregate Size and
Water-Cement Ratio on Concrete
Permeability ............................................................................. 180

CHAPTER 1

INTRODUCTION

1.1 CONCRETE MATERIALS

The most widely used construction material is concrete, commonly made by
mixing Portland cement with sand, aggregate, water and some admixtures. The
consumption of concrete in the US. is close to two tons per year for every US.
resident. No other material except water is consumed in such tremendous
quantities. Concrete has emerged as the most widely used engineering material
because: (1) it possesses excellent resistance to water; (2) structural concrete
elements can be easily formed into a variety of shapes and sizes; (3) concrete is
usually the cheapest and most readily available material on the job site; (4)
compared to most other engineering materials, the production of concrete requires
considerably less energy input; and (5) large amounts of many industrial wastes
can be recycled as a substitute for various virgin materials in concrete.

There are some key advantages associated with recycling in concrete
construction, including: (1) the potential to develop of large-volume markets for
waste products; (2) potentially reduced need for purification of waste; and (3)
long-term removal of recycled materials from the waste stream, noting that
concrete products typically have a service life exceeding 40 years.

Improvements in some key aspects of concrete performance can make important

contributions toward developing a more reliable infrastructure. Recycling of

 

 

 

plastics in concrete can help overcome problems with the brittleness, low cracking
resistance and the relatively high unit weight of concrete. It can also help control
shrinkage cracking of concrete. The study presented herein is concerned with the
recycling of plastics as reinforcing and light-weight inclusions in light-weight

concrete, and as reinforcing inclusions in normal-weight concrete.

1.2 HYPOTHESIS: Reinforcing Action of Plastic Inclusions

It is now known that even before the application of external loads,
microcracks already exist in the transition zone between the mortar matrix and
coarse aggregates in concrete as shown in Figure 1a (Hsu et al, 1963). The
number and width of these cracks in concrete would depend, among other factors,
on the bleeding characteristics and strength of the transition zone, and the curing
history of concrete. Under ordinary curing conditions (when a concrete element is
subjected to drying shrinkage or thermal strains), due to the differences of
dimensional movements and elastic moduli, diflerential strains will be set up
between the matrix and the coarse aggregates, generating the cracks in the
transition zone. Under load and enviromnental effects, the transition zone
microcracks begin to increase in length, width and number, initially within the
transition zone and later into the matrix and (in the case of light-weight
aggregates) through the aggregates (Figure 1b). The relatively low fi'acture
energies required for the propagation of cracks in brittle concrete matrices result in

relatively low toughness, impact resistance and tensile strength of concrete.

 

 

 

 

 

 

 

 

 

(b) Under Load/Environment Effects
FIGURE 1.1 Concrete Microcracks (Hsu et al, 1963)

3

In plastic-concrete composites, the encounter of propagating microcracks
with the tough and well-bonded plastics leads to relaxed intensity of stresses at the
crack tips, a phenomenon which increases the fi'acture energy and thus the
toughness and impact resistance of the composite as well as its resistance to
shrinkage cracking. Delayed propagation of microcracks encountering the plastic
inclusions would take place within the plastic-cement paste transition zone rather
than through the plastic inclusions (Figure 2a). Eventually, when increased load
levels lead to interconnection and rapid growth of microcracks, the bridging of
plastic inclusions across the resulting cracks helps control the crack widths and

maintain the integrity of the composite after cracking (Figure 2b).

Plastic Inclusion

 

(a) Arrest and Deflection of Microcracks

 

Coarse Aggregate

  

Plastic
Inclusion

 

 

Matrix

(b) Bridging of Cracks
FIGURE 1.2 Mechanism of Action of Plastic Inclusions in Concrete

1.3 OBJECTIVES

The main thrust of this research is to assess the feasibility of using recycled
plastics as reinforcing inclusions in concrete.

The work reported herein is divided into 3 parts. Part I deals with recycled
plastics as soft inclusions in light-weight concrete; part H considers recycled
plastics as additives, to control shrinkage cracking, in nonnal-weight concrete.
Part III deals with recycled tire as reinforcing inclusion in normal-weight concrete.

Chapter 2 gives a background on recycled plastics, surface treatment
through sulfonation, and light-weight concrete. Chapter 3 is concemred with
determining the influential variables in the production of light-weight concrete

incorporating recycled plastics. Chapter 4 deals with optimizing the influential

 

 

variables identified in Chapter 3. Theoretical modeling of light-weight concrete
incorporating plastics is presented in Chapter 5 which emphasizes compressive
behavior. Long-term durability characteristics and environmental impact of light-
weight concrete materials incorporating recycled plastics are assessed in Chapter
6.

Part H, which deals with recycled plastics as reinforcing additives in
normal-weight concrete, is divided into two chapters. The first chapter (Chapter
7) deals with optimizing the potentially influential variables in application of
recycled plastics to normal weight concrete. The composites were optimized
considering their resistance to cracking under restrained drying shrinkage. Both
short- and long-term properties of the optimized composite were evaluated in
Chapter 8 in order to ensure the added values of using recycled plastics in normal-
weight concrete.

In the final part of this research (Chapter 9), potential chemical and
physical causes of dimensional instability of tires in concrete, causing pop-out
were investigated. Following the elimination of pop-out problem through coating
of tire particles, the effectiveness of recycled tires as reinforcing inclusions in
nonnal-weight concrete was demonstrated.

The summary and conclusions of research as well as the recommendations

for further research are presented in Chapter 10.

PART 1: PLASTIC LIGHT-WEIGHT CONCRETE

CHAPTER 2
BACKGROUND

2.1 RECYCLED PLASTICS

2.1.1 PLASTIC TYPES

Plastics can be classified into four groups: commodity thermoplastics,
engineering thermoplastics, thermosets and multicomponent plastics. Commodity
thermoplastics are produced at low costs in high volumes. They include the five
resins that account for about two-thirds of all plastic sales: low-density
polyethylene (LDPE), high-density polyethylene (HDPE) polyvinyl chloride
(PVC), polypropylene (PP) and polystyrene (PS). They also include polyethylene
terephthalate (PET), which has only recently been used in sizable quantities for
packaging. Other commodity thermoplastics include acrylonitrile butadine styrene
(ABS) and nylon. In general, commodity thermoplastics can be reheated and
reshaped many times and are therefore eminently recyclable.

Engineering thermoplastics are produced at high costs and in low volumes.

Examples include polycarbonate and polytetrafluoroethylene

(PTFE). Engineering thermOplastics are used in the construction,
electric/electronic and transportation markets. These plastics are not considered a
major component of the municipal solid waste, MSW (Facing America's Trash,
1990). Engineering thermoplastics can be used in structural panels, building
insulation and other long-life construction markets.

Thermosets, compared to thermoplastics are low-volume materials but they
still compromise about 20% of the US plastics market. Two resins, phenolic and
polyurethane, are sold in sizable quantities. They can be used in building and
construction, transportation, and furniture. Unlike thermoplastics, thermosets
generally are not considered recycable because they do not soften when heated and
thus can not be remolded.

Multicomponent plastics and larninations are combinations of different
plastics or of plastics and other material such as paper or metal foil. These
materials are primarily used for packaging. Multicomponent plastics provide an
economical way of combining the needed properties of different materials.
Recently, available plastic ketchup bottles are made of several plastics including
an exterior plastic for appearance and strength and an interior plastic to resist fats
and acids. This combination of materials makes these plastics technically difficult
to recycle except into mixed plastic products. Medical devices also are fabricated
of or contain a variety of polymeric resins such as polyolefins. Most of the
recycling handlers, reclairners and distributors refer to the thermoplastics that are

not pure (i.e. colored or mixed thermoplastics) as commingled plastics.

2.1.2 PROCESSING PROCEDURES FOR RECYCLED PLASTICS

To produce virgin substitute resins suitable for use by manufacturers, post-
consurner plastics must go through 3 stages: separation, washing and granulation.

Granulators are designed and engineered to meet the specific requirements
on the size, shape, amount, and type of plastic scrap being produced. It is usually
a mechanical procedure. The number and design of the knife blades and their
rotational speed, rotor design and screen size are all essential to the reliable
production of high quality scrap recycling. Granulators all work on a similar
principle; they consist of knives mounted around the perimeter of a cutting
chamber. There can be 1 to 6 stationary knives and as many as 32 rotating knives
depending on the rotor design. Although granulate is easy to use, there has been
problems with contamination.

Recycling collectors are broadly using Granulators to process scrap plastics.
A brief review of the granulation process and how it has been applied to
processing is presented below.

The purpose of a granulator is to reduce materials to a uniform and
consistent size so it can be reclaimed for use in any of the various plastic
manufacturing processes. A granulator is a very simple piece of equipment which

has 3 major components:

(a) the hopper, designed to accept the subject feed stock;
(b) the cutting chamber, where size reduction takes place; and
(c) the base, where granulated material is collected .
Several factors must be considered to determine the throughput capability of a

granulator. These factors are:

(i) hopper opening and configuration;
(ii) rotor design and knife configuration;
(iii) available screen area and hole size;
(iv) horsepower; and

(v) method of evacuation.

The proper rotor design is essential. An open rotor will increase and help
prevent heat build-up in the cutting chamber because of better air flow. It has
been reported that HDPE has been known to plasticize in cutting chambers
(Changon, 1989).

A slant knife design rotor with counter angled rotor to bed knives provide

cleaner and more uniform granulate with less dust, see Figure 2.1.

   

Cutting circle _.__>.

Primary
bed knife

 

 

 

 

Rotor kniio

FIGURE 2.1 A Schematic Presentation of the Granulator Cutting
Chamber (Changon, 1989)

10

It has been reported by several people that an absolute necessity is to feed
the blown bottles to the cutting chamber on a tangent. A tangential feed cutting
chamber presents the feedstock to the down stroke of the rotor and eliminates
"rotor bounce".

The open screen area and the diameter of the holes in the screen will
determine how quickly the granulated product can exit the cutting chamber.

Following the granulation process, the separation process takes place. The
separation process is the backbone of any plastics recycling process. Automated
separation technologies have been developed. Such technologies separate out
component resins by combination of floatation, electrostatic processes and air
classification.

The first separation procedure used is gravity separation. In this method,
separation is by blowing air through a porous deck shaped like a trapezoid. The
lighter material or the material with the lowest terminal velocity will escape from
the deck and will be collected while the remainder will be conveyed onto the next
stage of processing. The final stage of separation is the is the electrostatic
separation.

The last step for preparing the recycled plastics is the washing process.
Water hoses are used to wash the recycled plastics.

Sulfonation of plastics is used in this investigation to make the plastic
surfaces wettable and provide for chemical bonding of the cement-based matrix to

plastic surfaces (see Figure 2.2).

11

 

Plastic Cement-Based Matrix

    

Sulfonation Bridge

FIGURE 2.2 Chemical Bonding of Sulfonated Plastic Surfaces to the Cement-
Based Matrix

2.2 SULFONATION

Chemical modification of the surface of engineering polymers is an
attractive approach to the problem of economically producing a barrier or a

permelective structure with tailored properties.

2.2.1 RATIONALE FOR THE USE OF SURFACE SULFONATION

In a study by Walles, 1990, involving the use of fluorine treatments
resulted in containers with a good barrier to gasoline, however, upon repeated

flexing, a loss of barrier properties can results. The barrier layers in these cases

12

were up to 0.1m thick. Additional studies using 25m (82 ft) films of polystyrene,
polyethylene and polypropylene exposed to fluorine for a longer time led to total
pulverization of the films. The loss in mechanical properties appears to be
associated with the localization of energy associated with the C-F bond formation,
which in turn can break neighboring C-C-bonds of the polymer backbone. Under
certain rigorously controlled treatments, the use of fluorine has proven workable,
but it was felt that a less aggressive agent allowed added process flexibility to
sulfonation.

Surface chlorination can produce good barrier layers with less tendency to
provide flexure failures. Unfortunately, chlorine reacts too slowly to be practical
as a treatment agent in the absence of promotion by ultra-violet light.

Chemical activation of the chlorination reaction was shown to be possible
using a mixture of 10% 803 and 90% C12 as such or further diluted with air or
nitrogen at 25 0C (77 0F). This procedure produced good barriers to gasoline,
particularly to the ethanel-and methanel- containing types.

Treatments of the inside of high density polyethylene (HDPE) automotive
gas tanks with about 20% 803 in air followed by air purging with subsequent
neutralization with N113 gas resulted in an excellent gasoline barrier.

It was reported by Walles (1990) that concentrations of about 75 and 200
micrograms 803 per cm2 (0.484 to 1.29 psi) of surface reduces permeation losses

by 90 and 99% in ambient temperature permeation tests.

13

2.2.2 NATURE OF THE BARRIER LAYER

Much work has been done on solution sulfonation, but solution sulfonation
causes problems such as polymer swelling and solvent contamination (Tradiff,
1993). A gaseous sulfonation method is preferred, using sulfur trioxide gas. The
small gas molecule can diffuse farther into a polymer sample than the bulky liquid
group, resulting is a more thorough and efficient surface modification. Sulfur
trioxide gas is difficult to use due to its high affinity for water. Much of the work
that has been done to characterize gaseous sulfonation has been conducted by
Walles (Walles, 1992).

The sulfur trioxide molecule is a resonance hybrid, which means that the
oxygen atoms are equivalent. The oxygen atoms are stingily bound, with a large
degree of double bond character. Due to the binding nature of the oxygen atoms,
the sulfur atom is strongly electron-deficient and the oxygen atoms are electron-

rich (W alles, 1992). A typical reaction with a polymer is;

- [ CHZCHQ]n - CHZCHz- + $03 --------> -[CH2CH2]3-CH2CHSO3H-

where the sulfonic acid group (-SO3H) is added to the polymer chain. In reactions
with polymers, the sulfur atom attacks the electron-rich sites, abstracts a proton,
and the oxygen atoms accept the acidic protons. Since sulfur trioxide often exists
as a complex, it has been found that the modification surface group may initially
exist as SO3SO3H, with one S03 group being more strongly bound than the other.
With a water wash after sulfonation, one 803 group is removed as H2304, thus
producing the SO3H group on the polymer chain.

The sulfonation reaction is a reversible reaction. Due to its strong affinity

for water, the sulfonic acid group may be removed with water, producing sulfuric
l4

acid. To prevent the loss of the sulfonic acid group, the sulfonated material is

neutralized with ammonia or ammonium hydroxide;

- [ CHZCHZIn -CH2CHSO3H- + NH3 -----> - [ CH2CH2]n CH2CHSO3'NH3+-

The neutralized sulfonic acid group (CHSO3‘NH3+) is a stable complex
which resists the attack of solvents such as water due to the strong attraction
between the sulfonic acid ion and the ammonium ion.

The neutralized sulfonic group, as well as the unneutralized sulfonic acid
group, is polar thus making the polymer water-wettable. Water-wettability is
important for the use of recycled plastics in concrete materials.

All commercially available films containing either a CH or an NH bond
have been found to be treatable via the sulfonation process.

The time required to produce a desirable banier improvement varies with
the type of polymer, its degree of crystallinity and particular penetrant being
considered.

Polyethylene generally requires a degree of sulfonation equivalent to about
0.1 mg 803 per cm2 (142* 10‘5 lb/inz). For polypropylene a degree of sulfonation
as low as 0.015 mg 803 per cm2 can be effective (213"‘10‘6 lb/inz).

2.2.3 SULFONATION

The process of sulfonation starts with delivering a stream of sulfur
trioxide reagent and maintaining the reagent stream within narrow, but adjustable

concentration limits. The sulfur trioxide reagent is a reagent of sulfur trioxide in a

15

canier. The carrier may be either a liquid halocarbon or an inert gas such as dry
air.

In either way, the process for generating the reagent includes the step of
introducing a source of sulfur trioxide into a suitable vessel such as a reagent
generator. The source of sulfur trioxide may be gaseous sulfur trioxide or sulfur
trioxide in a liquid vehicle, such as oleum counting 10-90% sulfur trioxide. That
step may actually include the introduction of a source of sulfur trioxide into a
separate contact chamber and, from there, to the reagent of 803 in canier. The
process also includes the step of introducing the carrier to the reagent generator,
either as a liquid halocarbon or an inert gas. Finally, the process includes the step
of introducing oleum into the reagent generator.

In this approach, a process for generating and recycling a solution of sulfur
trioxide in a liquid halocarbon as the carrier is provided. The process includes the
steps of introducing a source of $03, a halocarbon, and oleum into a reagent
generator. The $03, halocarbon, and oleum are contacted and mixed in the
reagent generator to provide a reagent solution of 803 in the halocarbon.
Additionally, the oleum phase which forms is removed from the reagent generator
for recycling. The sulfur trioxide in halocarbon solution phase is sent from the
reagent generator to a parts treatment chamber located in the parts treatment loop.
This separation of the two phases is readily accomplished due to the immiscibility
of the halocarbon and oleum as well as the differences in density between the two
and the high solubility of sulfur trioxide in the halocarbon.

The process for generating SO3 reagent solution also includes the surface
treatment of polymer resins in a treatment chamber in the parts treatment loop.
Figure 2.3 shows a schematic picture of the sulfonation apparatus.

16

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2.3 LIGHT-WEIGHT CONCRETE

Light-weight concrete may weigh from 320 kg/m3 (20 pct) to 1844 kg/m3
(115 pct), depending on the type of light-weight aggregate used and the method of
production. The strength of light-weight aggregate is roughly proportional to its
density. The advantages of light-weight concrete generally result fi'om the
decrease in member size because of reduced weight, increase in fire resistance,
and insulation against heat and sound when light-weight concrete is used. Light-
weight concrete, however, is typically 30-5 0% more expensive than normal-weight
concrete (N ilson and Winter, 1986), and has a greater porosity and more drying

shrinkage than ordinary concrete.
17

 

2.3.1 CLASSIFICATION

Light-weight concrete can be classified in accordance to its density, or
based on the purpose for which it is to be used. Distinctions can be made between
structural and insulating light-weight concrertes. Structural light-weight concretes
has a unit weight between 1443-1844 kg/m3 (90-115 pet), with a 28-day
compressive strength not less than 17 MPa (2500 psi). Insulating light-weight
concrete has a density lower than 802 kg/m3 (50 pct) and a 28-day compressive
strength between 0.69-6.9 MPa (100-1000 psi). Light-weight concrete can be
classified as moderate strength light-weight concretes with properties in between
those of structural and insulating concretes (Neville, 1982).

A variety of light-weight aggregates have been used for the production of
light-weight aggregate concrete. Figure 2.4 shows the classification of light-weight
concretes produced with different light-weight aggregates.

 

ATE —— UNIT ‘.
AGGREG 30 was" 7‘s
20

   

 

      

      
  

5 60 7O 80

0
no TE UNIT WE
ORE conc
co“ Stilt co~2”°

   

 
 

e °
6‘ I
< > IOpcf=I6.02 liq/m3

IODSI = 00069 MPO

 

 

 

FIGURE 2.4 Classification of Light-Weight Concretes (Mehta, 1993)

18

2.3.2 LIGHT-WEIGHT AGGREGATES

There are many types of aggregates available which are classified as being
light-weight. Their individual characteristics are so variable that it is impossible to
_ consider a typical light-weight aggregate to produce a similar (standard) concrete
mix for which any type of light-weight aggregate can be used with equal success.

The process by which light-weight aggregates are obtained has a
considerable bearing on their resulting characteristics and hence, the uses to which
they can be put. Their chemical composition is variable, some are inert while
others may be corrosive to steel.

Light-weight aggregates can be classified into two groups, natural and
manufactured, including recycled waste products and industrial by-products.
Pumice, which are common rocks of volcanic origin which occur in many parts of
the world and are light enough and generally strong enough to be used as light-
weight aggregates. Their lightness is due to their being spongy laves, the cells
having being formed by gases expanding with release of pressure when the
material was still plastic. Pumice is usually light colored or nearly white and has a
fairly even texture of small interconnected cells. Pumice is the oldest known light-
weight aggregate and from about 100BC onwards was commonly used as an
aggregate in the concrete roofs and walls of Roman buildings, notably baths and
temples, the best known surviving example being the Pantheon in Rome, in the
dome of which pumice concrete was used. In the mid-nineteenth century pumice
as an aggregate was revived in Germany, where there were large deposits of the
rock, and its use subsequently spread to other parts of Europe. Considering its
mode of formation, it is not surprising that pumice varies in quality in different
localities, and that it is not all equally suitable as aggregate. While the strength of

pumice aggregate is an important factor in the achievements of strength in concrete
19

 

 

produced from it, the particle grading and the degree of compaction attainable
under different manufacturing conditions influence the density and strength of
concrete.

Another natural light-weight aggregate is Diatonrite, which is essentially a
hydrated amorphous silica derived from the skeleton remains of microsc0pic
aquatic plants called diatoms. When pure, diatomite has an average density of 448
kg/m3 (28 pct), but due to impurities such as sand clay, it may be much heavier
than this. It is mined in many parts of the world. Diatonrite has many uses, as a
filtering aid in sugar refining, and as a workability aid in concrete. In the USA, it
is used for concrete for the insulation of high temperature firmaces. Low-grade
diatomite and diatomaceaous earth are sintered in rotary kilns at about 1100°C
(20120F) to produce light-weight aggregates, such as Diacrete, Raycrete and
Arrox.

2.3.3 PECULIARITIES OF LIGHT-WEIGHT CONCRETE MIX
PROPORTIONING

Light-weight aggregates tend to segregate and float on the surface of
concrete in high-consistency mixtures. This problem can be controlled by limiting
the maximum slump and by entraining air in fresh concrete mixtures made with
light-weight aggregates.

A major factor necessitating adjustments in proportioning and control
procedures of normal-weight concrete when applied to light-weight concrete is the
greater water absorption and higher absorption rate of light-weight aggregates.
Light-weight aggregate mixtures are usually proportioned by trial on a cement and

air content basis at the required consistency rather than on a water-to-cement ratio

20

basis, because water-to-cement ratio can not be established with accuracy due to
absorption of water by light-weight aggregates.

In a recent study, it was observed that during mixing of light-weight
aggregate concrete the cement paste will penetrate most of the open pores in a
surface layer of the aggregate. The amount of paste penetration depends on the
nricro structure of the surface layer of the light-weight aggregate, the particle size
distribution of the cement, and the viscosity of the paste (Zhang and Gjorv, 1992).

The particular properties of light-weight aggregates pose special problems
in calculating rrrix proportions for lightweight concrete. The absolute volume
method, which is the basis of the American Concrete Institute method for
proportioning normal-weight concrete, can not be used with confidence for
lightweight concrete. This is due to :

1. Variations in bulk specific gravity of lightweight aggregates;

2. Changes in lightweight aggregate moisture content;

3. Difficulty to quantify the absorption, penetration of cement paste.

2.3.4 PECULIARITIES IN MANUFACTURING LIGHTWEIGHT
CONCRETE

Production of concrete with light-weight aggregate involves all the
procedures and precautions that are necessary for ordinary concrete. After mix
design, the ingredients should be mixed according to ASTM C-94 as in the case of
normal weight concrete. The problem is more difficult when light-weight
aggregates are used because of greater variations in absorption, specific gravity,

moisture content and gradation of aggregates. Uniform results can be obtained if

21

the unit weight and slump tests are performed frequently, and the water content of
the mix is adjusted as necessary to compensate for variations in properties.

Dry light-weight aggregates should not be used at the mixing stage because
continuous water absorption by dry light-weight aggregates will cause concrete to
segregate and stiffen before placement is completed.

In order to ensure unifomrity, light-weight aggregates should be
presaturated. Presaturation of aggregates will help achieve uniformity but may
reduce freeze-thaw durability (particularly if concrete is not allowed to dry before
exposure to freezing temperature).

Workability of freshly made light-weight aggregate concrete requires
Special attention because, with high consistency mixtures, the aggregate tends to
segregate and float on the surface. To combat this tendency, it is necessary to
limit the maximum slump and to entrain air. Approximately, 5-7% air
entertainment is generally required to lower the mixing water requirement while
maintaining the desired slump and reduce the tendency for segregation and
bleeding. A slmnp of 51-76 mm (2-3 inches) represents a relatively high
workability. A slump in excess of 51-102 mm (2-4 inches) may cause segregation
with light-weight aggregate particles floating to the top. The tendency towards
floating of larger particles of light-weight aggregates may be improved by
adjusting the grading of aggregates. Concretes made with light-weight aggregates
may be difficult to place and finish because of porosity and angularity of the
aggregates. The placeablity of concrete can be improved by adding air-entraining

agents.

22

2.3.5 MECHANICAL PROPERTIES OF LIGHT-WEIGHT
AGGREGATE CONCRETE

The following properties of light-weight aggregate concrete will be
discussed in this section: Compressive, tensile and flexural strengths, modulus of
elasticity and Poison's ratio, impact resistance, ductility, shrinkage, abrasion

resistance, permeability and creep characteristics.

2.3.5.1 Compressive Strength

Design compressive strengths of 21-28 MPa (3000-4000 psi) at 28 days are
common for structural light-weight concrete. Light-weight aggregates with
controlled microporosity have been developed to produce 69-76 MPa (10000-
11000 psi) light-weight concretes which generally weigh 1435 to 1735 kg/m3
(Zhang and Gjorv, 1992).

The following factors associated with light-weight aggregates may
contribute towards the differences in compressive strength of light-weight
concretes:

i) Aggregate strength;

ii) Aggregate stiffness;

iii) Aggregate surface texture;

iv) Differences in surface area of the aggregate particles;

v) Aggregate shape

Concretes made with round aggregates have 28-day compressive strengths
of about 6-8 MPa (870 to 1160 psi) higher than those obtained with elongated

aggregates (length/thickness=4.0). The differences in surface area associated with
23

the differences in shape contribute to strength variations ( Soroushian et al, 1989).
This could be due to the fact that the shape of light-weight aggregates affects
stresses concentrations in concrete under load causing differences in the
compressive strength of light-weight aggregate concretes made with different

aggregate shapes (quryszkiewiez, 1973).

2.3.5.2 Modulus of Elasticity and Poison's Ratio:

The modulus of elasticity of light-weight concrete EC tends to increase with
increasing strength of concrete. Modulus of elasticity of light-weight concrete is
typically lower than that of normal-weight concrete. The following equation is
recommended for calculating EC with compressive strengths ranging from 21-48

MPa, 3000 to 7000 psi (Mehta, 1993).

Ec : (We) 15 V33* f'c

It has been reported that the elastic properties of the light-weight aggregates
are characteristic of the material property and are therefore not affected by particle
shape (quryszkiewiez, 1973). Thus, it can be assumed that Young's modulus of
all shapes is the same.

Typical Poison's ratio for light-weight concretes range from 0.17 to 0.21.

24

2.3.5.3 Ductility

Ductility is the ability to sustain inelastic deformations after peak load
without significant drop in load-resisting capacity prior to collapse. The ductility
of reinforced concrete members depends on the stress-strain behavior and
ductility of concrete materials.

Very little information is available on complete compressive stress-strain

behavior of light-weight concretes.

2.3.5.4 Shrinkage

The shrinkage of light-weight concrete is generally greater than that for
normal weight concrete with similar mix proportions and consistency. Some light-
weight aggregates produce concretes having relatively low shrinkage. light-weight
aggregates usually give higher shrinkage because they have lower modulus of
elasticity and thus produce smaller restraint against the shrinkage movements of
the cement paste. Aggregates with a larger fractions of fines smaller than #200
sieve (75 microns) show higher shrinkage movement as they lead to larger void
content.

Light-weight aggregate concrete exhibits higher moisture movements and a
somewhat higher ultimate shrinkage when compared with normal—weight concrete.
It has been reported that the shrinkage of light-weight concrete might be 6 to 38%

higher than that of normal-weight concrete.

25

CHAPTER 3

DETERMINATION OF INFLUENTIAL VARIABLES IN THE
PRODUCTION OF PLASTIC-LIGHTWEIGHT CONCRETE

3.1 INTRODUCTION AND OBJECTIVES

The main thrust in this phase of research was to check the validity of the
hypotheses presented in Chapter 1 regarding microcrack arrest and deflection, and
bridging of cracks by plastic inclusions in concrete. Two different types of
recycled plastics were used, and the plastic content was optimized. An effort was
made to modify the matrix through partial substitution of cement with fly ash in
order to achieve better interface characteristics. Mechanical and physical
properties of light-weight concrete incorporating recycled plastics were assessed
with emphasis on flexural performance. The results obtained in this study were
analyzed statistically using the analysis of variance and multiple comparison

techniques in order to derive statistically reliable conclusions.

26

 

3.2 EXPERIMENTAL PROGRAM

An experimental program was conducted at this stage of research on light-
weight plastic-concrete composites. This experimental program was concerned
with identifying the influential variables in the production of light-weight plastic
concrete materials.

The experimental program (Table 3.1) was designed based on the statistical
concepts of factorial analysis of variance. The purpose of this experimental
program was to investigate the following three variables: plastic type, plastic
content, and composition of the cementitious paste. The composites were
optimized considering their flexural performance (strength, toughness, and initial
stiffness), compressive strength, and impact resistance. In Table 3.1, each "*"
reflects one batch of concrete for which the standard number of specimens were

tested; two batches (i.e. two replications) were considered for each mix.

TABLE 3.1 Experimental Program For Light-Weight Concrete
Incorporating Recycled Plastics

 

 

 

 

 

 

 

 

 

Plastic Type Replacement Level of Sand With Plastics, by volume
7.5% 15%
Paste Composition Paste Composition
W/O Fly Ash With F 1y Ash W/O Fly Ash With Fly Ash

HDPE * * t *

it * t #

MIXED * * t t

It: II: t a:
CONTROL * *
* *

 

 

 

 

27

 

3.3 MATERIALS AND MIX PROPORTIONS

The basic mix ingredients were Type I Portland cement, light-weight coarse
aggregate, light-weight fine aggregate, recycled HDPE and MIXED plastics, Class
F fly ash, water, and air-entraining agent. The chemical composition of the
cement are shown in Tables 3.2. Sulfonated plastics were used in this
investigation, and results are compared with some results obtained with sulfonated

plastics in section 3.6.

TABLE 3.2 Chemical Compostion of Type I Cement Used in This

 

 

 

 

 

 

 

Investigation
Chemical Composition Percent
Tricalcium Silicate (C; S) 43.3
Dicalcium Silicate (C28) 26.3
Tricacium Aluminate (C3A) 11.0
Tetracalcium Aluminoferrite (C4AF) 8.6
Insoluble Residue 0.12

 

 

 

The light-weight aggregate used in this investigation (Tufflite) was volcanic
rock-based with a maximum aggregate size of 12mm (0.5 in). The specific
gravity of light-weight coarse and fine aggregates were 1.2 and 1.5, respectively.

The gradation of the light-weight fine aggregates, see Figure 3.1,
incorporating the two replacement levels of recycled plastics satisfied ASTM C-

330 requirements for structural light-weight concrete.

28

 

 

 

 

100

80—

60-

% Passing

40»

20-...

 

 

 

 

O r r r I
No. 100 No. 50 No. 16 No. 4 3/8 in.
Seive Designation

 

[* Lower Limit "' 7.5% Plastic 9 15% Piaatic * Upper Limit)

 

FIGURE 3.1 Light-Weight Aggregate Gradation

Recycled "MIXED" plastic, as will be referred to in this research, is a
combination of high density polyethylene (HDPE), polystyrene (PS),
polypropylyne (PP), polyvinyl chloride (PVC), polyethylene terephlthalate (PET),
and acrylonitrile butadiene styrene (ABS), all obtained fiom the solid waste

stream.
Table 3.3 shows the percentages, by weight, of these plastics in municipal

solid waste and those used in this investigation.

29

 

 

 

 

TABLE 3.3 Types and Percentages of Different Plastics, by Weight

Plastic T MSW“ Used
HDPE 21 3 1

PP 16 24
PS 16 24

PVC 7 10

PET 4 6

ABS 3 5
OTHER 33 -

 

‘* MSW: Muncipal Solid Waste

Both recycled HDPE and "MIXED" plastic particles are irregular (relatively
flat) in shape. The specific gravity of different types of plastics ranged from 0.9 to
1.1. Both HDPE and "MIXED" plastics have a nominal planar dimension of 10
m (3/8 in). The average and the 95% confidence interval for the thicknesses of
the recycled HDPE and "MIXED" plastic are shown in Table 3.4.

Table 3.4 Average Thickness of the Different Types of Plastics

 

 

 

 

 

 

 

 

Plastic Type Thickness, mm
(in)

HDPE 1.90 (0.075 )
PP 1.60 (0.063)

PS 2.40 (0.095)
PVC 1.70 (0.067)
PET 1.04 (0.041)
ABS 2.31 (0.091)

 

 

 

Different trial mixtures were done to optimize the cement content and the
fine aggregate/coarse aggregate ratio, in order to achieve the maximum
replacement level of fine aggregates with HDPE or "MIXED" plastics. The

cement content and fine aggregate/coarse aggregate ratio were 450 kg/m3 (750

30

 

 

 

 

lb/yd3) and 4 (by volume), respectively. It should be noted that a relatively high
fine/coarse aggregate ratio was necessary in order to achieve desirable workability,
compactability and fmishability when part of the light-weight fine aggregate was
replaced with HDPE or "MIXED" plastic. Both the cement content and fine
aggregate/coarse aggregate ratio were kept constant in all mixtures. The water
content was adjusted to give comparable slumps of 38-51 mm (1.5-2.0 in). An
air entraining agent (water based) was used at 0.06% by weight of cement to

produce resistance against frost attack. Table 3.5 presents the optimized mix

 

 

 

 

 

proportions.
Table 3.5 Mix Proportions, lb/yd3*
Matrix Cement Coarse Fine Agg. Recycled Water AEA %
Comp. Agg. Plastic
Control 750 170 850 - 735 0.06
7.5% 750 180 719 120 698 0.06
Plastic
15% 750 193 579 258 638 0.06
Plastic

 

 

 

 

 

 

 

 

*1 lb/yd3 = 0.594 kg/m3; ABA = Air Entraining Agent, by weight of cement

3.4 TEST PROCEDURES

The fresh mix workability was assessed by the slump test (ASTM C-143)
and the hardened unit weight was measured following the ASTM C-567
procedures.

For the hardened materials, the flexural strength and toughness,

compressive strength, impact resistance and restrained shrinkage cracking

31

 

 

characteristics were investigated experimentally in order to develop an overall
understanding of the various aspects of material behavior.

The flexural tests were conducted by four-point loading on a span of 12 in
(305 mm). Deflections were measured at the center of the specimen with respect
the loading point (Figure 3.2). This method of displacement measurement
eliminates any errors associated with the rigid body movements of the specimen or
penetration at support and loading points into the specimen.

F lexural loading was displacement-controlled at a quasi-static deflection
rate of 1/ 1000 times the span length per minute. These flexural tests produced
load-deflection curves which were characterized by flexural strength (modulus of
rupture) and toughness. The Japanese Concrete Institute approach was followed
for calculating flexural toughness, defined as the area underneath the flexural load-
deflection curve up to a deflection equal to the span length divided by 150 (J CI-
Sp,1979) A

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(a) Japanese Standard Flexural Test Set-Up

32

 

(b) Photograph of Flexural Test Apparatus
FIGURE 3.2 F lexural Testing

The compressive tests were performed according to the Japanese code (J CI-
SP, 1978); in this approach compressive toughness is defined as the area
underneath the compressive load-deflection crnve up to a strain of 0.0075.

In the flexural and compression tests, both load and deflection were
monitored through about the test in order to obtain complete load-deflection
relationship.

The impact test was conducted following the procedures recommended by
ACI Committee 544. This test measures the amount of impact energy (represented
by the number of blows) necessary to start a visible crack in the concrete
incorporating recycled plastics and then to continue opening that crack until
failure. The equipment for impact test (Figure 3.3) consists of a standard 4.54-kg

(IO-lb) compaction hammer repeatedly and recording the number of blows

33

required to cause the first visible crack on the specimen top surface and then

failure.

2.5 in. dia hardened steel ball
Note: free (it between ball and pipe

  
 
   
 

2.5 in. dia. steel pipe

2 X 0.25 in. steel bar

+

    
     
  

2 X 3.375 X 0.25 in. bar

 

1.25 in.

    

 
 

Specimen Zj-g ;
. "' "' ‘ ~.-‘.'2.5 in...
dra.

2 :< 2 x 0.5 in. bar

   

'_ f ’6' in.

 
 

/

,1

0.1875 in. 3 in.

    
 

    

12 in. dirt

FIGURE 3.3 Impact Resistance Test Set-Up

Ring type specimens were used for restrained drying shrinkage tests on
mortar. The specimen is cast in two equal layers, leveled by trowel, and then
covered with plastic sheets for 6 hours. The specimen is then exposed to air at
approximately 23°C (73°F) and 40% RH. Restraint of shrinkage movements by

the steel ring inside the specimen creates internal tangential tensile stresses which

cause cracking.

34

3.5 TEST RESULTS AND DISCUSSION
3.5.1 Hardened Unit Weight

The hardened unit weight test results are presented in Figure 3.4. The
addition of recycled plastics tends to reduce the hardened unit weight which adds
value to concrete properties. The reduction in hardened unit weight can be
attributed to the fact that the light-weight sand used in this investigation had a
higher specific gravity than that of the recycled plastics used.

HARDENED UNIT WEIGHT (lb/ft3)

 

120

I 95% Confidence Interval
100 _........ .. _ ___ .__..._. .._.-_.-i.-.... _ _. ..

 

 

 

80 V ~  . , -------

 

 

 

60-:

40»-

20—

 

 

 

 

 

 

 

 

 

 

 

 

 

CONTROL 7.5% HDPE 15% HDPE 7.5% MIXED 15% MD(ED

(a) Without Fly Ash

35

HARDENED UNIT WEIGHT (lb/ft3)

 

120

195% Confidence Interval

 

100 --

 

 

 

80 ~~~ ~~~~~~

 

 

60 _ ..._ __ ": ;':'

4O _.. -. __ :Eji‘g'. .: j .j pppppp

 

 

20 - --------

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CONTROL 7.5% HDPE 15% HDPE 7.5% MIXED 15% MIXED

(b) With Fly Ash

FIGURE 3.4 Hardened Unit-Weight Test Results (Means and 95%
Confidence Level)

3.5.2 Flexural Performance

Typical 28-Day flexural load-deflection curves for light-weight concrete
and plastic concretes incorporating 7.5%, 15% (without fly ash), and 7.5% and
15% (with fly ash) recycled plastics, respectively, are shown in Figure 3.5. Table
3.6 and Figure 3.6 present the flexural strength test results, and Table 3.7 and

Figure 3.7 presents the flexural toughnesses test results.

36

 

 

 

 

 

 

 

3
j [— CONTROL - 7.5% PLASTIC “159: PLASTIC]
2;
”a? I
.9 «
é ..
'0 ,
(U 'l
O .
_J
1 ‘ ............
o 1 ......... , ......... , ......... , ........ WAN-.7551-T.’..‘,.........
0 0.0 0.02 0.03 0.04 0.05 0.06 0.07
Displacement (in)
(a) Without Fly Ash
3

 

l *CONTROL "7.5% PLASTIC — 15% PLASTIC)

 

11111111

 

 

 

 

O YYYTYYYVYYIYVYY
O 0.01

TYY Y'VYY'VY'YYIVY'ITerTIVI IYTVYYT'YYY'TY'TTY'V‘VTYTY'Y

0.02 0.03 0.04 0.05 0.0 0.07
Displacement (in)

(b) With Fly Ash
FIGURE 3.5 Typical 28-day Flexural Load-Deflection Curves

37

 

LLLLLLLLLLLLLLLLLLLLLL

 

 

PPPPPPPPPPP

\\ \

ooooooooooooooooo

 

 

 

 

 

 

 

 

 

7 FLEXURAL STRENGTH (ksi)

 

0.6 n

0.5 r

0.4 -

 

0.3 r

0.2 ‘

 

 

 

I 95% Confidence Interval

 

 

 

l: HDPE Plastic
MIXED Plastic

 

 

 

Control

7.5% Plastic

(b) With Fly Ash

 

 

 

15% Plastic

FIGURE 3.6 Flexural Strength Test Results at 28 Days (Means and 95%

Confidence Interval)

TABLE 3.7 Flexural Toughness Test Results at 28 Days: Means, kin

 

 

 

 

 

 

 

 

 

 

Plastic Type Replacement Level of Sand With Plastics, by volume
7.5% 15%
Paste Composition Paste Composition
W/O Fly Ash With Flv Ash W/O Fly Ash With Fly Ash
HDPE 0.027 0.029 0.046 0.057
0.038 0.034 0.035 0.058
MIXED 0.028 0.034 0.049 0.04
0.025 0.036 0.043 0.041
CONTROL 0.005 0.004
0.006 0.003

 

 

 

 

39

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Statistical analysis of the results through comparison of means indicated
that the addition of plastics, whether HDPE of "MIXED" plastic, produce flexural
strengths that are statistically comparable to those of the control mixtures without
plastics. This was confirmed statistically at 95% level of confidence.

Analysis of variance showed that the plastic content, and the interaction
between plastic content and composition of cementitious matrix (i. e. use of fly
ash) had significant effects on flexural toughness. This was confirmed statistically
at 95% level of confidence. The flexural toughness increased 4.5 and 8 times that
of the control light-weight concrete at 7 .5% and 15% plastic contents,
respectively. The addition of fly ash further pronounced the positive effects of
plastics on flexural toughness. The positive effects of fly ash can be attributed to
their capability to enhance the structure and properties (bonding) at the transition
zone between plastics and cement-based matrix.

In general, the positive effects of plastics on flexural toughness reflect their
capability to bridge cracks and mitigate brittle modes of failure in concrete

materials by their pull-out resistance across cracks.

3.5.3 Compressive Behavior

The compressive strength test results (means and 95% confidence intervals)
for different plastic light-weight concretes at 28 days are shown in Table 3.8 and
Figure 3.8. It was confirmed statistically, at 95% level of confidence, that plastics
had adverse effects on compressive strength when fly ash was not used. However,
comparison of means of the compressive strength test results showed that 7.5%
plastics (HDPE or "MIXED") in the presence of fly ash produced compressive
strength comparable, at 95% level of confidence, with that of the control fly ash

concrete. At 15% plastic contents, HDPE with fly ash also produced a

41

compressive strength comparable to that of the control fly ash concrete. The
general drop in compressive strength with the addition of plastics may be
attributed to the relatively low modulus of elasticity of plastics which leads to a
redistribution of stresses into the more rigid inorganic matrix. It should,-however,
be noted that limits on load-canying capacity and service life of concrete
structures are generally provided by the resistance of concrete to tensile stresses
and impact loads. Concrete is fairly strong in compression and concrete structures
rarely fail due to material failure in compression. In the presence of fly ash, one
may expect that plastic-concrete composites may reach compressive strengths

approaching that of control concrete.

TABLE 3.8 Compressive Strength Test Results at 28 Days: Means,

 

 

 

 

 

 

 

 

 

 

kin
Plastic Type Replacement Level of Sand With Plastics, by volume
7 .5% 15%
Paste Composition Paste Composition
W/O Fly Ash With Fly Ash W/O Fly Ash With Fly Ash
I-IDPE 2.25 2.22 2.13 1.89
2.36 2.18 1.57 1.78
MIXED 2.16 2.05 1.97 1.62
2.14 1.96 1.87 1.67
CONTROL 2.87 2.08
2.90 2.12

 

 

 

 

42

4 COMPRESSIVE STRENGTH (Ksi)

 

 

I 95% Confidence Interval [:1 HDPE plastic
3 _ , W MIXED Plastic

 

 

 

 

    

 

 

 

 

 

 

h

Control 7.5% Plastic 15% Plastic

 

(a) Without Fly Ash

4 COMPRESSIVE STRENGTH (Ksi)

 

 

I HDPE Plastic
f' .
3 q 95% Con Idence Interval m MIXED Plastic

 

 

 

 

 

 

 

 

 

 

 

 

Control 7.5% Plastic 15% Plastic
(b) With Fly Ash

FIGURE 3.8 Compressive Strength Test Results at 28 Days (Means and
95% Confidence Interval)

43

3.5.4 Impact Resistance

Table 3.9 and Figure 3.9 give the mean values of the 28-day impact
resistance test results for light-weight concrete, presented as the number of blows
to first crack and failure. Statistical analysis (comparison of means) showed, at
95% level of confidence, that recycled plastics have a significant positive effect on
the impact resistance of concrete beyond the initial crack up to failure.

The improvements in ultimate impact resistance in the presence of plastics
further validate the hypothesis that tough plastic inclusions help enhance the
fracture energy and toughness characteristics of concrete materials through

bridging across cracks.

TABLE 3.9 Impact Resistance Test Results at 28 Days: Means (# of

 

 

 

Blows)
Plastic Type Replacement Level of Sand With Plastics, by volume
7.5% 15%
Paste Composition Paste Composition

 

W/O Fly Ash With Fly Ash W/O Fly Ash With Fly Ash
Initial Final Initial Final Initial Final Initial Final

 

 

 

 

 

 

 

 

 

 

HDPE 5.8 8.0 5.8 8.0 6.6 9.4 8.8 12.2
MIXED 7.4 10.0 3.4 5.4 5.4 9.0 5.4 7.2
CONTROL 5.8 6.4 4.2 4.4

 

44

3.5.5 Restrained Drying Shrinkage

Figure 3.10 shows the crack width versus time in restrained
shrinkage test on light-weight concrete. Figure 3.11 compares the shrinkage
cracking conditions of control and plastic concretes. The addition of recycled
plastics to light-weight concrete helps control the drying shrinkage cracks. This
can be attributed to the fact that recycled plastics (HDPE or "MIXED") act as
reinforcing inclusions that arrest microcracks and bridge across cracks to restrain

their widening.

 

p—A
co

h—c
O\

]
n.'.°.'.Q9NTBQI-. T? 72596 PLASTIC. . I

   
 

h-b
&
l

p—s
N
l

.o
oo i—l
l ' T

O
O\
l

   
 

Crack 1

Crack Width (mm)

.°
A
1

Crack 2

~L.

1

53
N
l

 

 

O
l
l
l
l
>—
i
}_
l

 

o 5 - 10 15 20 25 3o
Drying Time (Days)

FIGURE 3.10 Crack Width Vs. Drying Time

46

Concrete Concrete

(a) Control Concrete (b) Plastic Concrete

Rigid Ring Rigid Ring

FIGURE 3.11 Shrinkage Cracking Conditions

3.5.6 Microstructural Observations

In order to confirm the crack arrest and bridging mechanisms of plastic
inclusions in concrete, fracture surfaces of flexural specimens were observed
under a Scanning Electron Microscope. A typical arrest of microcracks by plastics
is shown in Figure 3.12a (at 200x magnification), and Figure 3.12b (at 50x
magnification) presents a typical condition of plastic inclusions bridging cracks.

These observations further validate the hypotheses of this investigation.

47

 

(a) Arrest of Microcrack

 

FIGURE 3.12 SEM Micrographs of Plastic-Concrete Composites

48

3.6 ASSESSMENT OF THE SULFONATION EFFECTS

In order to further investigate the effects of sulfonation, some critical tests
were performed on specific concrete mixtures incorporating unsulfonated recycled
plastics. These results are compared in this section with those of corresponding
concrete mixtures incorporating sulfonated recycled plastics.

For the mix with 7.5% HDPE without fly ash, as shown in figures 3.13a
and 3.13b, sulfonation of the recycled plastics helped increase both flexural
strength and toughness by 2 and 65%, respectively. This increase was due to the
improved bond between the cementitious matrix and recycled plastic inclusion.
However, the addition of sulfonated recycled plastics in light-weight concrete
produced comparable compressive strength and impact resistance to those with
unsulfonated recycled plastics (see Figure 3130 and d). It is important to note that
increased level of sulfonation of the recycled plastics may show some

improvements in some other mechanical properties.

49

FLEXURAL STRENGTH (ksi)

 

0.7

 

El UNSULFONATED [Z] SULFONATED

 

0.5—--

 

 

 

 

04~

 

:     .

 

 

 

 

 

 

 

 

HDPE PLASTIC MIXED PLASTIC

(a) F lexural Strength, ksi

FLEXURAL TOUGHNESS (k.in)

 

0.05
UNSULFONATED lZl SULFONATED

 

 

0.04- *

0.03 J // » /
/ _ W

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.01 — . /
w /
HDPE PLASTIC MIXED PLASTIC

(b) F lexural Toughness, k.in

50

COMPRESSIVE STRENGTH (ksi)

 

UNSULFONATED [Z] SULFONATED

 

 

 

 

 

 

7

 

 

 

 

 

 

 

 

 

HDPE PLASTIC MIXED PLASTIC
(c) Compressive Strength, ksi

IMPACT RESISTANCE (# of Blows)

 

l

 

14 -. First Crack E] Failure

If; ‘ /   //
m, / y /

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7.5% HDPE 7.5% SHDPE 7.5% MIXED 7.5% SMIXED

 

 

 

(d) Impact Resistance (# of Blows)
FIGURE 3. 13 Assessment of The Sulfonation Effects

51

 

3.7 SUMMARY AND CONCLUSIONS

The effects of partial substitution of light-weight aggregates with recycled
plastics on concrete pr0perties were investigated. Two plastic types (HDPE and
"MIXED"), two levels of replacement of fine aggregate (7.5% and 15% plastics
by total volume of concrete) were used, and two paste compositions (without and
with fly ash) were considered.

The hardened material mechanical properties were assessed through
flexure, impact, compression and restrained drying shrinkage tests. The following

conclusions were derived through analyses of the generated data:

1. The addition of recycled plastics to light-weight concrete helps reduce the
drying shrinkage crack widths.

2. Recycled plastics at 7.5 and 15% volume fractions produce flexural
strengths comparable to that of control concrete mix. However, flexural
toughness increases by 4.5 and 8 times, respectively. This was confirmed

statistically at 99% level of confidence.

3. Compressive strength test results were indicative of the adverse effects of
recycled plastics on compressive strength; however, the addition of fly ash
to the cementitious matrix produced compressive strengths comparable to
control mixtures. It should be noted that concrete is fairly strong in
compression, and brittle failure mode and low crack resistance are the main
problems which are targeted to be overcome through the use of reinforcing

inclusions. Furthermore, since the reduction in compressive strength is

52

accompanied with the reduction in unit weight, the situation would be

improved if one looks at the compressive strength-to-weight ratio.

Recycled plastics have a significant positive effect on the impact resistance

of concrete beyond the initial crack up to failure.

Although some of the flexural performance of the plastic light-weight
concrete incorporating sulfonated recycled plastics improved in some cases,
the overall mechanical properties (i.e. compressive strength and impact
resistance) produced test results that are comparable to these that were
unsulfonated. It is important to note that by increasing the level of

sulfonation one may see some improvement in these mechanical properties.

CHAPTER 4
OPTIMIZATION OF INFLUENTIAL VARIABLES

4.1 INTRODUCTION

The only influential variable identified in the previous phase of the study
(plastic content) was selected to be optimized for the production of light-weight
concrete incorporating sulfonated recycled plastic flakes. The composites were
optimized considering their flexural performance (strength and toughness),
compressive strength, and post-cracking impact resistance i.e. the difference
between the number of blows to initial cracking and final break).

The optimized plastic light-weight concrete composites identified in this
phase of research were produced with sulfonated recycled plastic flakes, and their
mechanical properties (flexural performance, compressive strength, and impact

resistance) where compared with control mixtures (i.e. without plastics).

4.2 OPTIMIZATION EXPERIMENTAL PROGRAM

The experimental program for optimization is presented in Table 4.1.

Various levels of the influential variable (plastic content) are considered in this

experimental program for the production of plastic light-weight concrete.

54

TABLE 4.] Optimization Experimental Program

 

 

 

 

 

 

Experiment # Plastic Content, % by total
volume
1 7.5
2 11.25
3 15 .0
4 0

 

 

 

For each of the mixtures presented in table 4.1, two replications (i.e. two
batches) in two blocks (i.e. with two replications with HDPE and two with
"MIXED" plastics) were considered. It should be noted that HDPE and "MIXED"
plastics have been observed to perform similarly in concrete. For each batch of
concrete the standard number of specimens specified by ASTM were tested. The
binder in those experiments was Type I Portland cement.

Materials, production and curing procedures were the same as these

presented in Chapter 3.

4.3 TEST RESULTS AND ANALYSIS

Typical flexural load-deflection curves produced for the light-weight
concrete incorporating recycled plastics (at different plastic contents) are shown
in Figure 4.1. Flexural strength and toughness test results are shown in Table 4.2,
and Table 4.3 shows the compressive strength and post-cracking impact resistance.
Figure 4.2 presents the test results and regression curves indicating the effects of

plastic content on various material properties.

55

 

 

 

, \
‘ l| [_ CONTROL ‘ '11% PLASTIC
_ l
2 ._ l .............................................................
l
‘ l
- l
- l
_ l
.9— - .
x l
v
v - ‘.
(U ‘ l
3 1 _l - 4 u .\ .........................................................
- l
- l
_ l
l
q l
‘ l
- \~
0

 

 

 

......... ,-,,,..,,
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07
Displacement (in)

FIGURE 4.1 Typical Flexural Load-Deflection Curves

56

 

 

 

TABLE 4.2 F lexural Performance

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flexural Mean Flexural Mean
Experiment # Strength (ksi) Flexural Toughness Flexural
Strength (kin) Toughness

HDPE (1) 0.467 0.025
0.422 0.027
0.430 0.026

1 HDPE (2) 0.443 0.460 0.031 0.028
0.474 0.033
0.425 0.026
MIXED (1) 0.503 0.026
0.480 0.028
0.439 0.023
MIXED (2) 0.511 0.022
0.488 0.027
0.446 0.024
HDPE (1) 0.438 0.031
0.437 0.035
0.402 0.030

2 HDPE (2) 0.398 0.432 0.023 0.035
0.440 0.034
0.408 0.043
MIXED (1) 0.461 0.046
0.428 0.042
0.447 0.040
MIXED (2) 0.441 0.038
0.442 0.033
0.440 0.029
HDPE (1) 0.406 0.046
0.385 0.048
0.369 0.047

3 HDPE (2) 0.360 0.371 0.043 0.048
' 0.363 0.051
0.435 0.049
MIXED (1) 0.431 0.049
0.401 0.046
0.454 0.051
MIXED (2) 0.429 0.048
0.433 0.049
0.412 0.053

CONTROL 0.467 0.442 0.003 0.005
0.456 0.006
0.0.41 0.006

 

 

 

 

 

 

TABLE 4.3 Compressive Strength and Impact Resistance Test Results

 

Experiment #

Compressive

Strength (ksi)

Mean
Compressive
Strength

Diff. between
Initial and
Final Crack
(# of blows)

Mean Diff.
bet. Initial
and Final

Crack

 

HDPE (1)

2.27
2.22
2.20

 

HDPE (2)

2.28
2.10
2.15

2.26

 

1 MIXED (l)

2.22
2.15
2.13

 

MIXED (2)

2.33
2.27
2.34

2,3,2,1,3

 

3,2,2,2,2

 

4,3,2,2,2

 

2,2,2,3,l

2.4

 

HDPE (1)

2.15
1.96
2.06

 

2 HDPE (2)

2.03
2.00
2.11

2.19

 

MIXED (1)

2.25
2.26
2.24

 

MIXED (2)

2.75
2.25
2.315

4,3,2,3,5

 

2,694,393,

 

3,3,2,2,l

 

3,3,3,4,4

 

HDPE(1)

2.15
2.09
2.10

 

3 HDPE (2)

1.52
1.60
1.58

1.78

 

MIXED(1)

1.84
1.94
2.14

 

MIXED (2)

 

1.56
1.36
1.56

1,3,2,6,2

 

6,6,5,5,5

 

6,3,3,4,2

 

6,6,5,5,5

4.3

 

 

CONTROL

 

3.08
2.74
2.80

2.87

 

 

0,0,0,1, l

 

0.6

 

58

 

Flexural Strength, ksi

 

 

 

 

 

 

 

 

 

 

0.6
+ HDPE
3" MIXED
0,5 .....................................................................
)K
I// +
+ i
o 4 .....................................................................
0.3
0 5 10 15
Plastic Content, %
(a) F lexural Strength, ksi
Flexural Toughness, k.in
0.07
+ HDPE 9" MIXED
0.06 ....................................................................
0.05 ................................................................... it

0.04

0.03

0.01

 

 

 

 

0 ‘7 .5 11.25 15
Plastic Content, %

(b) Flexural Toughness, km

59

 

 

Compressive Strength, ksi

 

+ HDPE
3K MIXED

 

 

 

 

 

 

0 5 10 15
Plastic Content, %

(c) Compressive Strength, ksi
Difference Between First And Final Crack (# of Blows)

 

+ HDPE 3" MIXED

 

 

 

 

 

 

0 5 10 15
Plastic Content, %

((1) Difference Between First and Final Crack in Impact Resistance

FIGURE 4.2 Regression Curves for Different Light-weight Concrete
Mixtures

60

The results presented in Figure 4.2 suggest that increasing plastic content
leads to improved flexural toughness and post-cracking impact resistance but
damages compressive strength. These trends were confirmed statistically, through
analysis of variance, at 95% level of confidence (in the case of compressive
strength, the increase in plastic content beyond 7.5% up to 15% did not cause any
statistically significant drop at 95% level of confidence). Plastic contents
exceeding 11% started to also damage flexural strength.

The optimum plastic content would depend on the importance of difi‘erent
properties for various applications. If one can tolerate a drop of roughly 25% in
compressive strength, it seems that approximately 11% plastic content can provide
major improvements in flexural toughness and post-cracking impact resistance

without any adverse effects on flexural strength.
4.4 EVALUATION OF THE OPTIMIZED COMPOSITE

F lexural performance, compressive strength and impact resistance tests
were carried out on the optimized composite with 11% of HDPE or "MIXED"
plastics. The optimized plastic light-weight concrete mixtures were evaluated

versus control light-weight concrete mixtures (no plastics).

4.4.1 FLEXURAL PERFORMANCE

The flexural load-deflection curves for the plastic light-weight concretes are
presented in Figure 4.3. Analysis of variance, Table 4.4, shows that light-weight
concrete composites with 11% plastic content produced flexural strengths

statistically comparable to those of connol mixtures (without plastic); however,

61

 

 

 

 

 

mmmmmmmm

Flexural Toughness, K.in

 

0.035
95 "
093 _.... -, . [Mt .99"I',9°'19£.'_"I9[Y.al .

0.025 .__._._._ ... __L, 22,, ..___, . _ In»... . . m,

 
    

0.02 r

0.015 -'

    

0.005

 

 

t\\

I \ t
Control 11% HDPE 11']. MIXED

 

FIGURE 4.5 Flexural Toughness Test Results (Means and 95%
Confidence Intervals)

The higher flexural toughness of plastic light-weight concretes
incorporating sulfonated recycled plastic flakes at 11% (optimized content) shows

the reinforcement efficiency of the plastic flakes.

4.4.2 COMPRESSIVE STRENGTH

Optimized composites with sulfonated recycled plastic flakes are observed
in Figure 4.6 to show reduced compressive strength when compared with control
composites (i.e. no plastics). This was confirmed statistically at 95% level of
confidence. The reduction of compressive strength was about 25%. Table 4.5

shows the results of the analysis of variance. It is important to note that concrete

63

 

 

 

 

4.4.3 INIPACT RESISTANCE

Figure 4.7 shows the post-cracking impact resistance test results (presented
as the number of blows between initial cracking and failure). The addition of
sulfonated recycled plastic flakes at 11% (optimum content) causes major
improvements in the post-cracking impact resistance of the resulting composite.
This can be attributed to the reinforcing action of the tough plastic flakes when

incorporated in a brittle cementitious matrix.

Post-Cracking Impact Resistance (# of Blows)

 

 

 

 

 

 

 

 

 

 

 

 

 

4
95% Confidence Interval
3 I /Cl7//
,l
//////
/ 4 //
2 // . ’ ////
// / V/l
/_///. / //
/ ' ’//’ t
if” .
, /;"/,
/’i/ / x, //// /
0 AM. //////.
Control MIXED

 

FIGURE 4.7 Impact Resistance Test Results

65

4.5 SUMMARY AND CONCLUSIONS

The influential variable (plastic content) in the production of light-weight
concrete incorporating recycled plastics was optimized. Optimization was based
on flexural strength and toughness, compressive strength, an impact resistance
with sulfonated plastics. The optimized mixture was then compared to the control
concrete mixture (i.e. without plastics). The following conclusions could be

derived:

(1) Increasing the plastic content leads to improved flexural toughness and
post-cracking impact resistance; but adversely influences the compressive

strength; excess plastic contents may also damage flexural strength; and

(2) The optimum plastic content would depend on the importance of different
properties for various applications; if one can tolerate a drop of 25% in
compressive strength, when compared to control mixtures, 11% plastic
content (by total volume) will improve flexural toughness (about 6 times)
and post-cracking impact resistance (about 3 times) without adversely
influencing flexural strength, this was confirmed statistically at 95% level

of confidence.

66

CHAPTER 5
THEORETICAL MODELING

5.1 INTRODUCTION

The drop in compressive strength and elastic modulus are the key adverse
effects of plastics in light-weight concrete. Due to their low elastic modulus,
plastic inclusions tend to redistribute compressive stresses to the stiffer concrete
matrix; they also act as stress risers producing sharp local increases in stress
values. The nature of plastic-concrete interface could also be influential. While
the crack-arrest action of plastics is also expected to play a role in compression,
the work reported herein intends to partially understand the role plastic inclusions
play under compression through modeling of plastic-concrete composites. Linear
elastic finite element analysis techniques were used to investigate stress
distributions and deformations in plastic-concrete composites subjected to
compressive stresses. The effects of plastic content and interface conditions on
compressive performance were investigated.

“ANSYS” which is a computer program for finite element analysis and

design was used in numerical analysis of plastic—concrete composites

67

under compression. The maximum compressive stresses, maximum tensile
stresses and average displacements of different concretes were obtained through
“AN SYS” while the modulus of elasticity of the composites were computed

numerically.

5.2 MODELING UNDER COMPRESSION

Two different inclusions (plastics and empty void), two different volume
fraction of inclusions (7.5% and 15%), and two different interface conditions
(perfect and imperfect bonds), were investigated. A two-dimensional elastic finite
element analysis on a plate 152 mm (6.0 in) high and 305 mm (12 in) wide
subjected to a compressive stress of 7 MP3 (1000 psi) was performed; the
inclusions were assumed circular with a diameter of 10 mm (0.375 in), and the
imperfect coating was represented by a 1.00 mm (0.04 in) thick interface zone
with lower modulus and higher Poisson’s ratio than concrete matrix. The key
properties used for the plastic inclusions, the bulk concrete matrix and the
interface zone (in case of imperfect bond) are presented in Table 5.1. A control
plate (without any soft inclusions) was also considered. At each volume fraction,

the inclusions (voids or plastics) were distributed randomly in the plane (see

Figure 5.1)

68

TABLE 5.1 Material Properties

 

 

 

 

 

 

 

Material Modulus of Elasticity Poisson’s ratio
(107). Psi
Light-Weight Concrete 2.6 0.3
Plastic Particles 0.02 0.45
Coating Material 1.31 0.35

 

 

 

(i) 7.5% Volume Fraction

69

 

ANSYS 5.0
DEC 2! 1993
10:15:30
PLOT 90
m
m: NUH
W I].
DIST-1.3
XI‘ IL!

I! I3
CENTROID HIDDEN

 

 

 

ANSYS 5.0
m 5 1!"
10:10:01
PM HO. 1
ms
m m
on

2V '1
DIST-3 . 3
X! I] . S

Y? I3
curmxo HIDDEN

 

 

 

(ii) 15% Volume Fraction
(a) Empty Voids

 

ANSYS 5.0

2V -1
DIST-3 . 3
If -1 . 5

Y! .3
minnow HIDDEN

 

 

 

(i) 7.5% Volume Fraction

70

 

 

M313 5.0
m l 1934
17:00:21
PLO! N. 1
ms
m: m
7.

2" II.
DIST-3.3
at ‘1.

I? .3
MID HIDDEN

 

(ii) 15% Volume Fraction
(b) Plastics With Perfect Bond

 

AISYS 5.0
DEC 29 1993
14:01:10
pm l0.
ms
nu WI!
nu

I

W .1
DIST-3 . 3
X? I1 . 5

Yr -!
CIHIIOID IXDDBN

 

 

 

(i) 7.5% Volume Fraction

71

 

 

 

 

(ii) 15% Volume Fraction
0) Plastics With Imperfect Bond (i.e. coated)

FIGURE 5.1 Geometric Configurations

5.3 RESULTS AND DISCUSSION

Figures 5.2, 5.3 and 5.6 present the deformations, longitudinal stresses (in
the direction of compressive loading) and transverse stresses, respectively, for

different modeling conditions.

72

 

ANSYS 5.0
09: 2! 1533
19:15:55

I .
DISILICIIINT
STEP-1

SUI I1

TIHIDI

ISYS-0

W 20.2295—03
awe-25.192
DECK-13"
DIST-3.102

XI -1.4!7

KY I2.I¢9
mm 3100:!

 

 

 

 

Y! -2.l76
cannon: HIDDEN

 

 

 

(ii) 15% Volume Fraction

(a) Empty Voids

ANSYS 5.0
DEC 2! 1!!)
14: :1

EXIT-3.101
X7 Il.l!l
"-

2.060
CENTROID HIDDEN

 

 

 

ARSYS 5.0

_;_ | | I ll ll:
WIII:IIIIIII=’.I=_II ‘ n 0141:,“
I x :

' l . 4
nrsrumm
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DHX I0.266E-03
SB'C-90.744
PIS.

DSCA-IIDI

1V I1
DIST-3.169

XF I1.493

YE -2.075
CENTROID HIDDEN

 

 

(ii) 15% Volume Fraction
(b) Plastics With Perfect Bond

74

 

 

 

 

 

(i) 7 .5% Volume Fraction

ISIS-0

DHX I0.243E-03
SEC-49.197
I”

INCH-1302

 

 

 

(ii) 15% Volume Fraction
(c) Plastics With Imperfect Bond

FIGURE 5 .2 Deformations

75

nan-o
no: .0.300I-03
SEC-55.001
"as
uswlou

-
DIST-3.101
X? IIJDC
¥l' I2.Bl'l
CENTHDID HIDDEN

 

 

ANSYS 5.0
DEC 2! 1933
19:16:40
Pm IO. 5
HOD“ SOLUTION
STEP-1

SUB =1

 

 

 

 

 

M513 5.0

W 5 1594
16:19:53

rm ID.
:0me

noon
“'1

 

 

 

(b) 15% Volume Fraction
(a) Empty Voids

76

 

(AWE
ISIS-0

-iZ.l I
214.744

 

 

 

(i) 7.5% Volume Fraction

 

ANSYS 5.0

JR! I 1934
17r01|54

P T . 3
HODAL SOLUTION
STEP-1

-42.
409.224

 

 

(ii) 15% Volume Fraction
(b) Plastics With Perfect Bond

77

 

ANS‘IS 5.0
DEC 29 1993
14:02:16

PLOT NO. 4
NODAL SOLUTION

IIDIIIIII E

103.644
457.531

 

 

 

(i) 7.5% ’Volume Fraction

 

ANS?! 5.0
JAN 10 1934
11:37:30

I . 4
IODAL SOLUTION
5! -1

 

 

 

(ii) 15% Volume Fraction
(c) Plastics With Imperfect Bond
FIGURE 5.3 Longitudinal Stresses

78

 

ANSYS 5.0
DEC 20 199]
19x16:19
NOD“ SOLUTION
STD-1

I1

TIMI-1
5X

 

 

 

(i) 7.5% Volume Fraction

 

"49!: 5.0
5 1994

10:19:23
”DAL SOLUTION
STU-1
SDI '1
TIMI-1
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(b) 15% Volume Fraction
(a) Empty Voids

 

”4515 5.0

i use as 1993

' 14:20:16
nor no. 4
noon. SOLUTION
arse-r

SU'H ‘

TIME-1

5X (AW)
REVS-0

mot -0.226l-03
SHN I-34ll

   

 

(i) 7.5% Volume Fraction

 

 

ANSYS 5.0

JAN 4 1994
17:01:25
PLOT I90. 1
mu. SOLUTION
STEP-1

SUI-1

 

 

 

(ii) 15% Volume Fraction
(b) Plastics With Perfect Bond

80

 

 

 

 

 

 

 

 

(ii) 15% Volume Fraction
(c) Plastics With Imperfect Bond

FIGURE 5.4 Transverse Stresses

81

MSYS 5.0
DEC 29 1.993
14:01:43
PM No

. 5
NODAL SOLUTION
am I 1

 

ANSYS 5.0
10 1994

11 t 37 : 10
PLOT NO. 3
”GOAL SOLUTION
STEP-1

 

The effects of inclusions (plastics or empty voids) on some key aspects of
light-weight concrete performance are summarized in Figure 5.5. Increasing levels
of maximum compressive and particularly tensile stresses are generated by the
inclusion of bonded plastics, imperfectly bonded plastics and empty voids (see
Figures 5.5 a and b); the increase in the inclusion volume fraction leads to
increased maximum compressive and particularly tensile stresses. These elevated
stresses are consequences of the relatively low elastic modulus (and also the
relatively high Poisson’s ratio) of the inclusions. Apparently, any positive effects
of soft plastic inclusions on the arrest, deflection and bridging of cracks (not
accounted for in this modeling) are overshadowed by the adverse effects of
plastics on the maximum stress levels under compression. The presence of soft
inclusions is also observed in Figures 5.5 c and 5.5 d to increase displacements

and reduce the elastic modulus of light-weight concrete.

Maximum Compressive Stress, psi (Thousands)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5 ..... '7.5% .......... w”
I15% §

4 .............................................. ~§ ..

, .................... §

1 _. ....... §

2::i\

0 SEN 533%
Control Plastics w/ Plastics w/ Empty

(no inclusions) Per. Bond Imp. Bond Voids

(a) Maximum Compressive Stress

82

Maximum Tensile Stress, psi

 

3,500

 

3,000 """ ° 0 .......................... . ______________

 

 

 

2,500 A ..........................................................
2,000 ............. - ........................................
1,500 ............... , .......
1,000 .............. . ......

500 ..... .. ..............

 

 

 

 

 

 

 

 

 

Control Plastics w/ Plastics w/ Empty
(no inclusions) Per. Bond Imp. Bond Voids

(b) Maximum Tensile Stresses

Average Displacement, in/in

 

0.0003

 

I7.5% 15%
0.00025 ........................

 

 

 

...............................

0.0002 ------------ ....

0.00015

0.0001 ~-

..........

J

0.00005 ----

.........

 

///////WWW

 

 

 

 

 

 

 

 

 

 

 

0

 

 

Control Plastics w/ Plastics w/ Empty
(no inclusions) Per. Bond Imp. Bond Volds

(c) Average Displacement

83

 

 

Modulus of Elasticity, ksi (Thousands)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

40
__1 [317.5%
'15%
so
0 33::
Control Plastics w/ Plastics w/ Empty
(no inclusrons) Per. Bond Imp. Bond Voids

(d) Modulus of Elasticity

FIGURE 5.5 Effects of Soft Inclusions on Light-Weight Concrete
Properties

Figure 5.6 shows elastic moduli of plastic-lightweight concrete composites
as predicted by the theoretical model and measured in laboratory experiments.
The laboratory specimens were prepared and tested as presented earlier in Chapter
3; the test procedure followed ASTM C-469. The predictions of elastic modulus
compare reasonably well (considering phenomenological nature of the model) with
experimental results. The model, however, does not consider the arrest and
bridging of cracks by the soft inclusions and thus is only capable of illustrating
some aspects of the soft inclusion effects on certain properties of light-weight

concrete .

84

2.5

1.5

0.5

FIGURE 5.6 Experimental Results Versus Theoretical Model Predictions

MODULUS OF ELASTICITY, ksi

 

 

\f

 

.........

.......

 

 

.........

//

 

 

 

E3 EXPERIMENT MODEL

 

..............................................

.....................

 

 

..........
..........
..........
..........

........

..........

.........

..........
..........
..........

........

..........

 

..........

 

 

..........

 

7.5% PLASTIC

5.4 SUMMARY AND CONCLUSIONS

A phenomenological model based on linear elastic using finite element
analysis techniques was used to investigate some effects of plastic inclusions on
some aspects of light-weight concrete behavior under compressive stresses. The
inclusions were either empty voids or plastics which were perfectly or imperfectly
bonded to the concrete matrix. Two diflerent volume fractions of inclusions were

considered. The analytical results, which did not account for the arrest and

bridging of cracks by plastics, indicated that:

(1) The redistribution and concentration of stresses induced in concrete by

soft inclusions (with relatively low elastic modulus and relatively high

85

15% PLASTIC

 

(2)

(3)

Poisson’s ratio) leads to increased maximum compressive and particularly
tensile stresses, increased displacements and reduced elastic modulus of

light-weight concrete;

Increasing levels of maximum stress and deformation are observed with
perfectly bonded plastics, imperfectly bonded plastics, and empty voids;
increased volume fraction of the soft inclusions leads to increased maximum

stress and deformation;

The experimentally observed negative effects of plastics on compressive
performance suggest that any positive effects of plastic inclusions (through
the arrest and bridging of cracks, which are not accounted for in this
analytical study) would be overshadowed by the adverse effects of plastics on

maximum compressive and particularly tensile stresses.

86

CHAPTER 6

LONG-TERM DURABILITY CHARACTERISTICS AND
ENVIRONMENTAL IMPACT OF PLASTIC LIGHT-WEIGHT
CONCRETE

6.1 INTRODUCTION

Concrete materials incorporating recycled plastic flakes as light-weight
aggregates and reinforcing fibers for enhancing toughness characteristics, impact
resistance and shrinkage cracking characteristics at reduced unit weight present
new developments in concrete technology. In order to fully develop this class of
materials it is important to ensure their satisfactory long-term performance under
severe environmental effects; this was the main thrust of the work reported in this
chapter.

The long-term durability tests performed on light-weight plastic-concrete
composites include: freeze-thaw durability (ASTM C-666), scaling resistance
(ASTM C-672), and exposure to temperature cycles.

The results obtained in this study were analyzed statistically using analysis
of variance and multiple comparison techniques in order to derive statistically

reliable conclusions.

87

 

 

6.2 BACKGROUND

6.2.1 F REEZE-THAW DURABILITY

One of the major problems in cold climates requiring heavy expenditures
for repair and replacement of concrete pavements, retaining walls and bridge decks
is the damage caused by frost-action (freeze-thaw cycles). The frost damage in
concrete can take several forms. The most common is cracking and sapling of
concrete that is caused by aggressive expansion of cement paste or aggregate under
repeated freeze-thaw cycles.

When water turns to ice, the corresponding volume increase (about 9%)
causes the residual water in capillary pores to be compressed. This pressure can
be relieved if the water can escape fi'om the capillary to a free space by diffusing
through unfrozen pores, but if the water has too far to move to an escape
boundary, the capillary will tend to dilate and the surrounding material will come

under stress (see Figure 6.1).

 

Microcrack Water migrates to

 

airv ida d
X + X... \ f 2:, freezoes thgre
/ i i x / X i t x /
cap
\ f/ / \ ® \1‘ / \
5 W
* l i l Q X 1‘ x t
l i ‘ y i \ i
(a) Non Air-Entrained Paste (b)Air-Entrained Paste

FIGURE 6.1 Creation of Hydraulic Pressure in Frozen Paste (Mindess et a1
1981)

88

 

 

The superposition of pressure from adjacent capillaries will eventually
cause the tensile strength of paste to be exceeded and rupture will occur. As the
temperature is progressively lowered, more capillary water is involved in freezing,
increasing the hydraulic pressures and thereby increasing microcracking and
dilation. In a saturated non-air-entrained paste, the only free space is the exterior
of the specimen, and the diffusion of water to the outside is very slow to relieve
the hydraulic pressure. Thus, the inclusion of entrained air provides empty space
within the paste to which the excess water can move and freeze without damage.
The bubbles act as "safety valves" and the spacing factor determines the average
distance the water must travel to reach the free space. This distance must not be
too great if hydraulic pressure is to be relieved.

Air entertainment in concrete provides escape boundaries in the cement
paste matrices. It is not the total air, but the void spacing of the order of 0.1 to 0.2
mm (0.004 to 0.008 in) within every point in the hardened cement, that is
necessary for protection of concrete against frost damage.

The water-to-cement ratio also has an important effect on the freeze-thaw
durability of concrete. In general, the higher water-cement ratio for a given degree
of hydration, the higher will be the volume of large capillary pores in the hydrated
cement paste, and therefore the greater the amount of freezable water. It was
found that for a water-cement ratio of more than 0.4, at a cement content less than
415 kg/m3 (700 lb/yd3), a maximum aggregate size of 25 mm (1 in), and an air
content of 6 to 8% should be used to avoid deterioration under freeze-thaw cycling
(Mindess et al, 1981). But since in most field conditions the water-cement ratio is
greater than 0.4, if the structure is going to be exposed freezing and thawing,
sufficient entrained air should be specified.

The aggregate grading also affects the volume of entrained air, which tends
to be decreased by an excess of very fine sand particles. Addition of admixtures

89

such as fly ash has a similar effect. In general, a more cohesive concrete mixture
is able to hold more air than either a very wet or a very stiff concrete.

A number of different tests have been developed to assess the long-term
frost resistance of concrete. These involve subjecting concrete to difierent freeze-
thaw cycles and measuring the progressive internal damage by monitoring weight
loss, length change (dilation), and decrease in strength or dynamic modulus of
elasticity. Tests differ in the nature of freeze-thaw cycles used and the condition

of the specimen during the test.

6.2.2 DE-ICER SALT SCALING

Concrete that is adequately air-entrained for frost resistance may
nevertheless be damaged by repeated application of de-icer salts. Concrete that
has suffered salt scaling becomes roughened due to the sapling of small pieces of
mortar.

The exact causes underlying salt scaling are not known, but they probably
involve more than one process. It has been reported that the consumption of heat
required to melt ice when the de-icer is applied causes a rapid dr0p in the
temperature of the concrete just below the surface, which may cause damage from
either the effects of rapid freezing or the stress caused by differential thermal
strains. The additional free moisture now present at the surface of the concrete
may encourage the growth of microscopic or macroscopic ice lenses near the
surface, where ice formation can still occur. Osmosis has also been suggested as a
mechanism of salt scaling. De-icing chemicals can accumulate in the concrete just
below the surface to form relatively concentrated solutions. When rainwater

accumulates on the surface, the phenomenon of osmosis occurs, where water flows

90

to equalize concentration differences. Considerable hydraulic pressures can be
created by this effect, causing rupture of the paste. The most damage usually
occurs at places where rainwater can accumulate and remain for some time.
Scaling is most likely to occur on the surfaces that have been over vibrated,
trawled too early or too long, subjected to plastic shrinkage, or where excessive
bleeding occurs. Such surfaces tend to have a weak layer of paste or mortar either
at the surface or just below, and may have microcracks or bleeding channels that
can transport surface solutions to lower levels. Careful attention to mix design,
placing and finishing should eliminate many potential problems. Table 6.1
indicates the recommended curing times for air entrained pavements. If adequate
moist curing is followed by a period of drying before de-icing chemicals are

applied, scaling should not be a problem.

TABLE 6.1 Minimum Moist-Curing Times to Develop Salt Scaling
Resistance (Woods, 1968)

Cement Type Minimum C ' Period
23°C 4°C 3

I 7 15
II 7 12
7 7

 

6.3 EXPERIMENTAL PROGRAM

An experimental program has been designed based on the statistical
concepts of factorial analysis of variance (22 factorial design). This experimental
program (Table 6.2) investigates the following two variables: plastic type (HDPE
and MIXED), plastic content (two different levels, 20 and 40% replacement of

fine light-weight aggregate by volume corresponding to 7.5% and 15% by total

91

volume of concrete). The sulfonation of plastics was considered in the case of
light-weight concrete (for both HDPE and MIXED plastics) for freeze-thaw

ductility, scaling resistance and exposure to temperature cycles.

TABLE 6.2 Experimental Program For Light-Weight Concrete Incorporating

 

 

 

 

 

 

Recycled Plastics.
Replacement Level of Sand with Plastics, by volume“ Plastic Type
HDPE MIXED
20% - -
40% - -
Control -

 

 

 

 

*20% and 40% replacements of fine aggregate correspond to 7.5% and 15% of total
volume.

The mix proportions used in the phase of research are similar to these

presented earlier in Chapter 3.

6.4 TEST PROCEDURES

6.4.1 FREEZE-THAW DURABILITY

The purpose of this test is to study the effects of repeated freezing and
thawing cycles on the durability of concrete. The test method used to perform this
task is ASTM C-666 (procedure A, Rapid Freezing and Thawing in Water). Two
specimens were prepared for each of the mixes in Table 6.2. The specimens were
rectangular prisms of 76x102x406 mm (3x4x16 in) in dimensions. They were
cured for one day inside their molds underneath a wet burlap and a polyethylene

film, and then moist cured at 23°C (74°F) and 100% RH. until the age of 28 days;

92

thereafter, they were immersed in a cold water bath at 7°C (45°F) for two hours,
and then put inside the freeze-thaw machine to start the testing cycles. The test
procedure consists of subjecting the specimens alternatively to fieezing and
thawing periods. This is accomplished by lowering the temperature of the
specimens (that are surrounded by not less than 1 mm (1/32 in) nor more than 3
m (1/8 in) of water) from (4.4 to -l7.8°C (40 to 0°F) and then raising it from -
17.8 to 44°C (0 to 40°F) with not less than 2 nor more than 5 hours spent on the
complete cycle. At least 25% of this time is used for thawing. The specimens are
removed from apparatus, in a thawed condition, at intervals not exceeding 36
cycles of exposure to the freezing-and-thawing. The freeze-thaw damage was
assessed through measurement of the fundamental transverse frequency of
specimens when simply supported, see Figure 6.2, from which the dynamic

modulus of elasticity (PC), was derived using the following equations:

Pk = (n1)7- / (n) x 100

Where n = flmdamental transverse frequency at 0 freeze-thaw
cycles;

111 = fimdarnental transverse frequency at k freeze-thaw
cycles.

The length change (LC) of the specimens was calculated using the following

equation:

LC = (12 -11)/lg x 100
Where Lc = length change of test specimen after C cycles of
freezing and thawing, %;

11 = length compactor reading at 0 cycles;

93

12 = length compactor reading at c cycles;
lg = the eflecfive gage length between the innermost ends

of the gage studs.

 

 

 

FIGURE 6.2 Test Set-Up for Measurement of Fundamental Transverse
Frequency

6.4.2 SCALING RESISTANCE

The test Procedure outlined in ASTM C-672 was used for this purpose.
This test method determines the resistance to scaling of a horizontal concrete
surface exposed to repeated freezing-and-thawing cycles in the presence of deicing

chemicals. Two specimens were prepared for each of the nine mixes presented in

94

Table 6.2. The prismatic specimens were 254x203x76 mm (10x8x3 in) in
dimensions (see Figure 6.3); they were moist-cured inside their molds for the first
24 hours, then demolded and cured in a curing room at 23°C (74°F) and 100%
RH. until the age of 28 days. The specimens were left to dry in room temperature
at 23°C (74°F ) and 60% RH. for one day. At the age of 29 days, the flat finished
surface of each specimen was covered with approximately 6 mm (0.25 in) of a
solution of calcium chloride and water, having a concentration such that each 100
ml (6.1 m3) of solution contains 4 grns of anhydrous calcium chloride. The
specimens were then placed in a freezing environment for 16 to 18 hours. At the
end of this time, they were removed from the freezer and placed in laboratory air
at 233:1.70C (73i30F) for 6 to 8 hours. This cycle was repeated daily with the
surface being flushed off thoroughly at the end of each 5 cycles. A visual
examination was then made and the solution was replaced. This test was carried

out for 50 cycles.

 

 

  

 

Concrete
Specimen
Mortar
Dike
/ f __ 0.5 in
/ 3.0 in.

 

 

 

 

 

.1.

10.0 in.

FIGURE 6.3 Specimen Used in Scaling Resistance Test.

95

6.4.3 EXPOSURE TO TEMPERATURE CYCLES

In this test, after 28 days of moist curing, the specimens were subjected to
cycles of temperature change. This was accomplished by subjecting the specimens
to a temperature of 1.7°C (35°F) for a period of 12 hours followed by exposing the
specimens to a temperature of 30°C (86°F) for another 12 hours. This cycle was
repeated 30 times before testing. Specimens for flexure strength and toughness,
100x100x350 mm (4x4x14 in), were the tested in flexure.

6.5 EXPERIMENTAL RESULTS AND DISCUSSION

6.5.1 FREEZE-THAW DURABILITY

The relative dynamic modulus test results for light-weight concrete
incorporating recycled plastics are presented in Figure 6.4(a). HDPE and MIXED
plastics produced essentially similar performance. Each value plotted is an
average of two specimens. Statistical analysis (using split plot analysis of variance
with repeated measurement and separation of means) of results between 12 and 48
cycles confirmed the improvements in freeze-thaw characteristics obtained through

incorporating plastics into light—weight concrete.

96

Relative Dynamic Modulus, %

 

200
*CONTROL +75% HDPE *7.5% MIXED

*1573 HDPE +15% MIXED

L
150-

100 v

 

50—

 

 

 

0 I U l I I
0 20 40 60 80 100

Number of Cycles

(a) Relative Dynamic Modulus
Length Change, ‘23

120

 

+CONTROL +7.52; HDPE 9167.57. MIXED

5S *1573 HDPE +15% MIXED
\

 

 

 

 

60 80 100

Number of Cycles

(b) Length Change in Percent
FIGURE 6.4 Freeze-Thaw Durability Test Results

97

120

As seen in figures 6.4a and b, the control mixture showed poor freeze-thaw
durability characteristics. This poor behavior was attributed to the specific light-
weight aggregate, Tufflite, used in this investigation. The fine pore system in
Tufflite provides it with an extremely high capacity for absorption, about 50
percent, and apparently does not allow quick escape of water to allow release of
the pressure associated with ice formations. The addition of recycled plastics to
light-weight concrete improved the freeze-thaw durability characteristics of the
resulting composite. This could be due to the fact that plastic particles act as
reinforcing inclusions, bridging across cracks and mitigating the widening of

cracks and disintegration of concrete.

6.5.2 SCALING RESISTANCE

Table 6.3 shows the scaling resistance test results. Recycled plastics, at
relatively high volume fractions of 7.5 and 15% by total volume of concrete, had
an adverse effect on scaling resistance. This effect, however, is not reflected in
Table 6.3 because the specific light-weight concrete used in this investigation itself
produced the lowest scaling resistance specified by ASTM. In the presence of
plastics, the surface damage under de-icer salt scaling effects occurred at a faster

rate.

TABLE 6.3 Scaling Resistance Test Results

 

 

 

 

# of Cycles Rating
Control 50 5
7.5% PLASTIC“ 50 5
15% PLASTIC* 50 5

 

 

 

 

 

* HDPE and "MIXED" produced comparable results

98

 

 

 

6.5.3 EXPOSURE TO TEMPERATURE CYCLES

The flexural strength and toughness test results associated with exposure to
temperature cycles of light-weight concretes incorporating recycled plastics
(HDPE) are shown in Figure 6.5. Exposure to temperature cycles led generally
consistent damage to control specimens and those incorporating plastics at 7 .5%
by total volume; the average strength drops of control specimens and those with
7 .5% by total volume of plastics were 60% and 52%, respectively. The major
drop in strength of control concrete, which was also reflected in plastic concretes;
is due to the weakness of the specific light-weight aggregates used in this
investigation. Analysis of variance and comparison of means of the flexural
strength test results indicated that all drops in strength after 30-day exposure to
temperature cycles are statistically comparable at 95% level of confidence.

After 30 day exposure to temperature cycles, the flexural toughness
increased about 6 times that of control at 7 .5% plastic content. This was
confirmed statistically at 99% level of confidence. Hence, if the light-weight
aggregate type is selected to provide resistance to temperature cycles, the
pronounced toughening mechanism of plastics could highly be advantageous under

temperature cycles.

99

FLEXURAL STRENGTH (Ital)

 

0.3

0.25 ... ....................................................................

 

 

0-2"‘""""iiiiiiiiéiiiiiiééé """""

0.154 """""""""" """""

°~1T """""""""" iiizszéixzfiiiéai """""

005... ............................................................................
. ....................................
....................................
....................................
...................................
....................................
....................................
..................

 

 

 

 

 

 

 

Control 7.5% HDPE

(a) F lexural Strength After Exposure to Temperature Cycles

FLEXURAL TOUGHNESS (KJn)

 

 

0.02- ........ .........

0.01... ......................................... ........

 

....................................

 

 

 

 

 

 

....................................

Control 7.5% HDPE

 

(b) F lexural Toughness After Exposure to Temperature Cycles
FIGURE 6.5 Effect of Exposure To Temperature Cycles

100

 

 

6.6 ENVIRONMENTAL [IMPACT

Table 6.4 present the leaching test results for plastic light-weight concrete
incorporating 7 .5% HDPE. The results show that the addition of recycled plastics

to light-weight concrete produced enviromnently acceptable composites.

TABLE 6.4 Toxicity Characteristics Test Results For Plastic Light-Weight

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Concrete, ug/L
Analyte Result Limit Allowed

Arsenic <200 200
Barium <100 100
Cadmium <100 100
Chromium <100 100
Cgper <100 100
Lead <100 100
Selenium <100 100
Silver <100 100
Zinc <100 100

Mercury <10 10
Aluminium <2000 2000
Beryllium <2000 2000
Boron <4000 4000
Calcium 680000 6000

Iron 490 50

Manganese <50 50
Molybdenum <2000 2000
Nickel <200 200

 

 

 

 

6.7 ASSESSMENT OF SULFONATION EFFECTS

The effects of sulfonation of recycled plastics on chloride permeability,
freeze-thaw durability characteristics, and scaling resistance of light-weight
concrete mixtures were investigated through comparing the results obtained with

sulfonated and unsulfonated plastics. Sulfonated recycled plastics produced

101

freeze-thaw durability characteristics and scaling resistance which were
comparable to those obtained with usulfonated plastics (see Figure 6.6 for freeze-
thaw durability test results).

Relative Dynamic Modulus, %

 

200
+75% HDPE *7.5% SHDPE
*15% HDPE “#15% SHDPE

 

k
150-4

100

 

50-

 

 

 

 

120

 

Number of Cycles

FIGURE 6.6 Assessment of Sulfonation Effects on Freeze-Thaw Durability
Characteristics of Light-Weight Concrete.

6.8 SUNIMARY AND CONCLUSIONS

The long-term durability of light-weight plastic-concrete campsites
(incorporating recycled sulfonated HDPE and "MIXED" plastics as partial
substitutes for light-weight aggregate in light-weight concrete) were assessed
through investigating the freeze-thaw durability characteristics (ASTM C-666),
scaling resistance (ASTM C-672), and resistance to temperature cycles of the

102

materials. Two different plastic contents, 20 and 40% replacement of fine light-
weight aggregate by volume corresponding to 7.5% and 15% by total volume of
plastics in the case of light-weight concrete were considered. The effects of
sulfonation on fieeze-thaw durability and scaling resistance were investigated. It
was concluded that:

In light-weight concrete: (1) The addition of recycled plastics to light-
weight concrete improved the freeze-thaw durability characteristics of the resulting
composite noting that the specific light-weight aggregate used in this investigation
made concrete highly susceptible to frost attack; (2) Recycled plastics had a
negative effect on scaling resistance of concrete, again noting the very low scaling
resistance of the control light-weight concrete associated wit the frost
susceptibility of the specific light-weight aggregate used in this study; (3) the
addition of recycled plastic flakes to light-weight concrete produced
environmentally acceptable composites; and (4) Sulfonated plastics produced
freeze-thaw durability characteristics and scaling resistance which were

comparable to those obtained with unsulfonated plastics.

103

CHAPTER 7

SHRINKAGE CHARACTERISTICS OF NORMAL-WEIGHT
CONCRETE

7.1 INTRODUCTION

The potentially influential variables (recycled plastic content and recycled
plastic width) were selected to be optimized in application to normal-weight
concrete. The optimization experimental design was formulated based on response
surface analysis techniques. The composites were optimized considering their

resistance to cracking under restrained dying shrinkage.

7 .2 BACKGROUND

Volumetric changes in hardened and plastic concrete are significant in
magnitude and have a serious influence on the performance and durability of
concrete structures. Inadequate allowance for the effects of shrinkage in concrete
can lead to cracking or warping of concrete slabs (20110, 1984). Shrinkage, the

reduction in the bulk volume of concrete, is only a fraction of the volume of the

104

water loss, with larger part of released water coming from the pores in the
concrete. Furthermore, shrinkage dose not depend on water loss alone, but also on
the actual deformability of concrete.

The duration of shrinkage in concrete could be divided into three phases, as
shown in Figure 7.1 (Kral and Geuaver, 1979). During the first phase, concrete
adopts itself to the mold (low resistance to deformation); it does not shrink at all in
this phase (except in the vertical direction, see Figure 7 . 1) and does not suffer
serious cracking. In the second phase, the evaporation of water causes rapid
shrinkage. Thus, the possibility of cracking arises as the tensile strength of
concrete increases more slowly than the induced stresses. Then, in the third phase,
the rapidly increasing resistance against deformation, as shown in Figure 7.1, will
be accompanied by a retardation in shrinkage and faster increase of the tensile
strength. Due to the combination of both efi'ects, the danger of cracking at early
ages ceases in the third phase.

At the beginning and during the period of initial hardening (i.e., in green
concrete), the fracture stress of concrete is going through a minimum while the
tensile strength is low, see Figure 7.2 (Chatterji, 1982). As result, concrete cracks

easily.

105

 

 

 

 

 

 

 

 

 

 

 

 

Cum Content : Joo wat’
.; 0.4 '- W/C
3 aa— 054
O
”‘3 oz .. f 0.58
O ‘ I
I o 0.52 -
8 .I - \
u; 0 l r 4 L I
o
<
§ lat 2nd y 3rd .1
E Phase Phase m“ ‘1
55, 0.5—
0 ‘_ 0.54
3 ' 0.53
5 0.3 -
> 02— as:
0.1 -
I l 1. I l l l
0 I 2 3 4 5 6 7 0

TIME AFTER WATER ADDITION, hr

FIGURE 7.1 Effect of Water-Cement Ratio on Vertical and Horizontal
Shrinkage (Karl and Geuaver, 1979)

TEnsile strength]
fracture stress

 

TEnsile

 

strength

 

 

 

Fracture
stress

 

 

FIGURE 7.2 Principle of Stress and Tensile Strength Development of
Concrete at Early Stage (Chatterji, 1982)

106

Here are some of the many factors that influence the shrinkage of concrete,
which will consequently affect cracking:

1. Material composition;

2 Mix design;

3. Construction Methods;

4 Temperature and relative humidity of the construction

environment;

5 Steel Reinforcement;

6 Curing practices and conditions;

7. Rate of evaporation;

8 Size of the structure;

9 Age of the concrete;

10. Structural loads;

11. Capillary stresses;

12. Desiccating pressure; and

13. Changes in surface pressure.

Un-restrained (free) shrinkage is rarely found in typical concrete structures.
Restraints are always present, either internal or external, resulting from support
conditions, reinforcement, or due to non-uniform drying. These restraints induce
tensile stresses which approach the tensile strength of concrete and cause cracking.

Recent investigation of concrete structures have revealed that cracking can
occur inside the bulk of concrete structures with or without any visible sign on the
outer surfaces. It is also indicated that under certain conditions both micro- and
macro-cracks can form shortly after or even before the formwork is completely
removed. It is more probable that the micro-cracks will widen further due to both

mechanical and environmental stresses. To lengthen the service life of a concrete

107

structure, it will be necessary to avoid the formation of the early cracks as much as
possible.

Shrinkage is a time-dependent phenomenon; if it takes place during the
early age, it is called "plastic shrinkage". Ifit occurs after the concrete has
hardened it is called "Drying shrinkage". Other identifiable types of shrinkage,
such as carbonation shrinkage and autogenous shrinkage, while not caused by the
same mechanisms, are similar in effects to drying shrinkage.

In plastic concrete the stress distribution properties of plastics and their
ability to transfer tensile stresses across cracks are very important. When concrete
reinforced with steel bars is subjected to tension, it has been shown that the
predominant parameter determining the crack spacing and the crack width is the
specific surface of the reinforcement (the surface area of reinforcement per unit
volume of concrete). It can be shown that concrete incorporating discrete plastic
inclusions of small particle size has higher specified surface area of reinforcement
than conventionally reinforced concrete (Krenchel and Shah, 1987). Hence, when
plastic concrete is subjected to restrained shrinkage, the resulting strains can be
distributed over several cracks of limited width instead of a few cracks with
unacceptably large widths as is the case in unreinforced concrete.

Many other factors govern the performance of plastic-concrete subjected to
restrained shrinkage. These include the potential extent of shrinkage, degree of
restraint, time-dependent constitutive properties of concrete, and the plastic to
matrix interfacial bond characteristics (Grzybowski and Shah, 1990; Dahl, 1989;
and Dahl and Holand, 1985).

There are currently no standard tests to assess cracking due to restrained
shrinkage. Table 7.1 presents a summary of the restrained shrinkage test methods
and conditions reported by different investigators. Some researchers have tried to

use long specimens with flared ends (Figure 8.3 a) that were restrained, and used

108

small cross-sectional dimensions at 2.75 x 4 in. (71 x 102 mm) to produce
shrinkage cracking. Two other key types of specimen have been used by other
investigators for restrained shrinkage tests, namely plate-type specimens
(rectangular) and ring-type specimens, see Figures 7.4 b, 7.4c and 7 .4d (Krenchel
and Shah, 1987; and Grzybowski and Shah, 1990). In the case of plate specimens,
when the restraint against shrinkage movement is provided in two directions, a
biaxial state of stress is produced. The results obtained from this type of test may
depend on the specimen geometry in addition to the material properties. The
restrained shrinkage tests using steel rings were conducted as early as 1939 to
1942 (Grzybowski and Shah, 1990). The ring type restrained shrinkage test
apparatus has been found to provide a high and nearly constant degree of restraint,
producing consistent results with cement paste and mortar. In this test a concrete
ring is cast directly around a heavy steel ring. As concrete dries, the concrete ring
tends to shrink while the steel ring provides the restraint and prevents the
shrinkage movements. The steel ring also serves as a sensitive dynamometer to
compute the induced tensile stresses in concrete by measuring the steel strains

using strain gages.

TABLE 7.1 Summary of Restrained Shrinkage Test Methods and
Conditions Reported By Different Investigators

 

 

 

 

 

 

 

 

 

 

Ref # Specimen Curing Condition
Type Dimension
6 Ring 6.7/9.8 in. 1.6 Moist cured for 1-2 weeks then ambient atmosphere
in. thick
7 Ring 6.07/7.36 in. 68°F, 100% RH. for 2.5 hrs. then dry at 40% RH.
5.5 in. thick
8 Ring 20/23 in. 4.0 61°F, 50% RH. (chamber)
in thick

 

109

/ Cir-jock

 

 
   

 

 

 

 

 

 

 

   

 

 

mobile cnchorlng necc
concrete specimen
E (cross section 7cm x 10cm) A 3
8 axis
- trome
fixed head
(a) Flared Ends (b)Plate (Panel)
12' ..i
A.
\ /4‘/2. steel ring
pluigles mold
(c) Plate With Steel Ring (d) Ring Type

FIGURE 7.3 Restrained Shrinkage Test Specimen Types (Mirza, 1992)

110

7.3 EXPERINIENTAL PROGRAM

The main thrust of this phase of research was to determine the optimum
geometry and volume fiaction of plastics for the control of restrained drying
shrinkage cracking. The experimental program for optimization through response
surface analysis (using the "Design Expert" software) is presented in Table 7 .2.
Various combinations of the two influential variables (recycled plastic content and
plastic width) are considered in this experimental program. Recycled plastic
flakes with maximum size of 10 m (3/8 in) were also used at different volume
fractions (in addition to the experimental program of Table 7.2). the materials
considered in this phase of the study were subjected to the restrained shrinkage

test.

TABLE 7.2 Optimization Experimental Program

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Variables
Experiment # Plastic Width, mm Plastic Content (by total
volume), %
l 1.0 0.250
2 2.0 0.250
3 1.0 5.000
4 5.5 1.500
5 5.5 2.625
6 5.5 2.625
7 2.0 2.625
8 5.5 5.000
9 5.5 2.625
10 5.5 2.625
11 5.5 2.625
12 5.5 2.625
13 5.5 2.625

 

 

 

 

 

 

111

7.4 MATERIALS, MIX PROPORTION S AND TEST PROCEDURES

The basic mix ingredients of the mortar mixtures used in this phase of
research were Type I portland cement, normal-weight fine aggregate, water, and
recycled HDPE plastics with variable widths ranging from 1 mm (0.04 in) to 5.5
mm (0.22 in) were used. The length of plastic particles was kept constant at 25.4
mm (1.0 in) for all mixtures. The plastic inclusions had a thickness of 1.90 mm
(0.075 in); they were basically milk jugs cut to the specified dimensions.

The mortar mixtures used in this phase had cement-to-fine aggregate-to-
water ratios of l : 2.2 : 0.5

Ring type specimens (see Chapter 3) were used for restrained drying
shrinkage tests on mortar specimens. The specimen was cast in two equal layers,
leveled by trowel, and then covered with plastic sheets for 6 hours. The specimens
were then exposed to air at approximately 230C (73 0F) and 40% RH. Restraint of
shrinkage movements by the steel ring inside the specimen creates internal

tangential tensile stresses which cause cracking.

7.5 TEST RESULTS AND ANALYSIS

The test results obtained in this phase of study are presented in Figure 7.4
in the form of crack width versus air-drying period. Typical effects of plastic
particle width on maximum crack width in restrained drying shrinkage are
presented in Figure 7.5. Figure 7.6 shows the effect of fiber widths on restrained
drying shrinkage cracking at same volume fractions.

112

“1.2“

i

ck

03

Cr

A

idth (mm

Crack Width (mm
.0 .0
07 CI) —-‘
l l l

1.6—

1.4“

0.6 ~

02‘

1—

ng

04*

 

Rigid Ring

 

Concrete

 

 

O

l

lilllllTlTlllllllllTlllTlllllllllllllTTTTllllllllTllllll

17

41 49 57
Time (Days)

(a) Width = 1 mm @ Volume Fraction = 0.25%

 

1.6“

1.4“

.4

12*

Q
A
l 4

 

.0
O [\D
I J r

E

Rigid Ring

Concrete

 

 

 

....L

ITTTTTTlTlTllllllllllllilTTlTlTlTlTlTlllllllllllllllllll

17

41 49 57
Time (Days)

(b) Width = 2 mm @ Volume Fraction = 0.25%

113

16‘ Concrete
E1::
E _ RigidRing
E.
.9
:08—
6 -
930.6%
0 .
0.4—
0.2—
O lllllTl—TlllllllllllllllllllllTllllllllllllflllllllllllll
1 17 41 49 57
T2ime(Days)
(c) Width = 5.5 mm @ Volume Fraction = 1.5%
1.6“ Concrete
1.4-4 w
”€1.21 ‘ ’
E
v 1.9 Rigid Ring
E.
.9
$0.8“
x
$0.6 CrackZ
0 Crack]
04*
0.2“
1 ”1--” 4.5—K
O llllillllTlTlllllfifl-TTl'llllllilllllllllllTTllllllllTlT
1 9 17 25 83 41 49 57
Time (Days)

 

 

 

 

 

 

 

 

 

 

((1) Width = 2 mm @ Volume Fraction = 2.625%

114

1.64

1

.3.

i\>
I

.0
A
I

.0
|\)
I

A

Crack Width (mm

 

.4~

Crack Width (mm
0
CD
1

.0 .
CD
1 I I

O
I

_L
l

_ Rigid Ring

 

Concrete

.3

Crackl CrackZ

\ \ /

x -4
---
__-'
"--—

 

 

Till]Il[TilllllllTTllltllllllllllllillllTTllllIlllllllll

9 17 25 33 41 49 57

Time (Days)

(e) Width = 5.5 mm @ Volume Fraction = 2.625%

 

 

 

 

 

16F Concrete
1.4~ fl
. I.
1'27 \I
1; RigidRing
1
0.84
0.6-i CrackZ
1 Crack]
0.4“
\
,i ——————————— 1
O lllllllllllll—l—TlllllllllllllellllllllllllllillTTTllllll
1 9 17 25 33 41 49 57
Time (Days)

(t) Width = 1 mm @ Volume Fraction = 5%

115

 

 

 

 

 

 

 

 

 

1-6‘ Concrete
1.4— w
512? q
E, 1‘ Rigid Ring
E
."Q -
g08~
x .
$0.5- CrackZ
g3 .
0.4- Crack]
02- \
. /--"'"3~—"‘
O ITTfllTlTllllllllilillllTlAlllllfijllTllllllllllllllllll
1 9 17 25 33 41 49 57
Time(Days)
(g) Width = 5.5 mm @ Volume Fraction = 5%
1.5““ Concrete
1'4- ‘.
”€1.2j
g 1.4 Rigid Ring
:9
.9
5'08“
6 .
g06~
O .
04*
02'
r
O TTllllllTerTTlllllllllllllllllllllllTTlllllllllllllllll
1 9 17 25 33 41 49 57

Time (Days)

(h) Flakes @ Volume Fraction = 0.25%

116

 

 

_..L
03
r 1

Concrete

2. _.
.
(9

Rigid Ring

_L
l

L

Crack2

nL

Crack WIdth (mm
.0 .0
CD (I)
l l

 

 

 

 

 

04-1 Crackl
1
0.2-
O— WTTIITIIIlITTIllllllilljllllllTTTllTTTllllllllllililli
I 9 17 25 33 41 49 57
Time (Days)

(i) Flakes @ Volume Fraction = 2.625%

 

 

 

 

 

 

1.6‘ Concrete
1.4— a.“
”6‘2“ V
g 1 RigidRing
E 1—
I). .
g 0.8 ~
(x) -I
g O.6~
Q a
0.4 Crackl Crack2
0.2 a
O IIIIIIIIIIIIIIIIIiiriiiii'iiiiIIIIIIIHIHTFTTWWTWT
1 9 17 25 33 41 49 57
Time (Days)
(j) Flakes @ Volume Fraction =5%
FIGURE 7.4 Test Results

117

 

/
1.6“ +Control “‘0.25 HDPE

"2.625 HDPE —5 HDPE
\

 

 

 

 

 

O lllllllTllllllllllllTTTTTTllTlllllTllTTllTllllllTTTTTTll

1 9 17 25 33 41 49 57
Time (Days)

FIGURE 7.5 Restrained Drying Shrinkage Versus Drying Time

 

1-67 *Control “Flakes "'1 mm "2 mm)

 

 

 

 

 

O lIlrlflTlllllllllT—TllllIlllllllllTlTllllllTTTlTTllllllTi

1 9 1 7 25 33 41 49 57
Time (Days)

(a) Volume Fraction of 0.25%

118

 

 

 

 

 

 

 

 

 

1-6" *Control —Flakes “'2 mm "'5.5 mm]
'\
51.4—
E
g1.2~
E
g 1—
x
o
90.85
goe~
.§
360.4—
0.2“
O lllllllTTlIllll7T14I-j’dllTllllllllllllllTlTTIlllllllTllTT
1 9 17 25 33 41 49 57
Time (Days)
(b) Volume Fraction of 2.625%
1-67 *Control —Flakes "1 mm "'5.5 mm]
"\
E14-

Maximum Crack Width (m
.0 o _.
o» oo _. iv
1 I I l

 

 

 

 

 

0.4—
0.2
O lllIIlTTIIllIITTIIII'IillllilTlIIilTlIllilllTllllI—l_17Tl-—l-
1 9 17 25 33 41 49 57
Time (Days)

(0) Volume Fraction of 5%

FIGURE 7.6 Effect of Fiber Width at Similar Volume Fractions on
Restrained Drying Shrinkage Cracking

119

It can be seen in Figure 7 .4 that fiber volume fraction has an important role
on restrained shrinkage cracking. This is due to the reinforcing action of the
plastic particles in brittle concrete matrices. Concretes at same volume fractions
produced restrained shrinkage cracking characteristics that were statistically
comparable at different plastic widths (Figure 7.5). Plastic flakes also produced
results comparable with those obtained with plastic "fibers" having difi‘erent
widths.

Figure 7.7(a) shows the response surface of the results. Analysis of test
results, at the age of 6 weeks showed that the maximum crack width dropped by
about 40% at 0.25% volume fractions when compared to control mixtures without
plastics. The reduction in crack width was about 85% for both 2.625% and 5%
plastic contents, as shown in Figure 7.7(b). The drop in restrained shrinkage crack
width with increasing plastic volume fractions was confirmed statistically at 95%

confidence level.

120

0,6

0.5 T

 

A i .49” 5: '0‘ I
0' 14,3565" ' ’I
.41". ' 7' [’11,

i

03 WW]

1'! {754? 1",,
02 u“ \\\“.‘"/u,

W_fi

Maximum CI ack Width. mm

 

 

 

1 x . . ,. , ”‘25.?
O. “““"“’““’"““§€tttttzzfie§:&zf

 

 

 

(a) Response Surface

Maximum Crack width. mm (6 weeks)

 

1.2 ~ 95% Confidence Interval 1:11 mm ..

" “ ‘ en's's'n‘n
0.8.. .
0.6’ . ,.
0.2- ..

o 1 [’71 Nfl [7711 I33

5

Control 0.25 2.625

 

 

 

 

Volume traction, 96

(b) Effect of Fiber Volume Fraction on Restrained Drying shrinkage Cracking at 6
Weeks of Age

FIGURE 7 .7 Optimization Test Results

121

The optimum levels of the influential variables derived from the response
surface analysis of results were:

Recycled Plastic Content: 2%

Recycled Plastic Width: Due to the insignificant effects of plastic width, a

range of widths would be acceptable.

The above optimum conditions correspond to the minimum plastic content
which effectively reduces restrained shrinkage crack widths. The optimized
mixture with a plastic width of 5.5 mm (0.22 in) at volume fraction of 2% was
then compared to control mixtures (i.e. without plastics) with respect to restrained
drying shrinkage. Figure 7.8 shows the comparison of restrained drying shrinkage
test results (crack width versus time). The addition of recycled plastic distributes
the cracks and thus reduces the crack widths. This can be attributed to the fact that
recycled plastics act as reinforcing inclusions that arrest microcracks and bridge
across cracks to restrain their widening. Statistical analysis showed that at 2%
(optimum) plastic content, plastic flakes produced shrinkage cracking
characteristics comparable to those obtained with plastic "fiber" at a width of 5.5
mm (0.22 in), as shown in Figure 7.9; this was confnmed statistically at 95%
confidence levels. Hence, plastic flakes obtained through a simple mechanical
grinding process would produce satisfactory results as far as the control of

restrained shrinkage cracking is concerned.

122

 

/
1-6— "*Control —2% Plastic
”\

Crack2

O4 _ Crackl

0.2“ \

O llTlTllTllllillllllllllllllllldjllllllllill[Till—ITWTITT

1 9 17 25 33 41 49 57
Time (Days)

 

 

 

FIGURE 7.8 Crack Width Versus Time in Days

 

_L
i\)

_/
. *Control —Flake ""Fiber")

 

   

.0
(I) _.|
I L l 1 l l i

4

Maximum Crack Width (mm)
0
03
l

 

 

 

 

0.4-1
O.2-
O liillllTTlTIl iTill/ITTTITIITIIITIGIIITITIIllliITIIIITTIT
1 9 17 25 33 41 49 57
Time (Days)

FIGURE 7.9 Comparison Between Plastic Flakes and Plastic Fibers.

123

7.6 SUMIVIARY AND CONCLUSIONS

Normal-weight concretes incorporating recycled HDPE plastics, as
secondary reinforcing inclusions (additives), were considered in this phase of
research. Plastic volume fraction and plastic width were optimized in order to
achieve a condition where minimum plastic content would efiecfively reduce the
restrained shrinkage crack width The optimized composite was then compared to
control mixtures ( i.e. without plastic) and also with mixtures incorporating ground
plastic flakes at the optimum plastic content. The conclusions derived are

summarized below.

1) Recycled plastics can effectively reduce the restrained shrinkage crack widths
in concrete; this can be attributed to the capability of plastics to bridge across
cracks, restrain their opening, and provide post-cracking tensile resistance

across cracks.

2) While plastic volume fraction significantly influences restrained shrinkage
crack widths, the width of plastic particles did not have statistically significant

effects.

3) A plastic volume fraction of 2% represent the minimum (optimum) plastic
content where restrained shrinkage crack widths were effectively controlled;
ground plastic flakes and plastic "fibers" where both effective at this

volume fraction.

124

CHAPTER 8

EVALUATION OF THE OPTIMIZED PLASTIC NORMAL-
WEIGHT CONCRETE MIXTURES

8.1 INTRODUCTION

As mentioned in the previous chapter, the addition of recycled plastics to
concrete helps control the drying shrinkage cracks. This valuable contribution of
recycled plastics to concrete shrinkage cracking could be most beneficial if other
qualities of concrete (both short-and long-term properties) are not damaged (or are
improved) in the presence of plastics. The aim of this chapter is to evaluate the
efi‘ect of the optimum plastic content on some mechanical properties (flexural
strength and toughness (ASTM C-78), compressive strength (ASTM C-39), and
impact resistance (ACI Committee 544), and long-term durability characteristics
(freeze-thaw resistance (ASTM C-666) and scaling resistance (ASTM C-672) of

concrete.

8.2 MATERIALS, MIX PROPORTIONS AND TEST PROCEDURES

The basic mix ingredients of the concrete materials used in this phase of

research were: type I Portland cement, normal-weight aggregate (natural sand and

125

 

 

 

 

crushed limestone with maximum size of 12 mm, 0.5 in), water, recycled (ground)
HDPE plastics with maximum particle size of 10 m (3/8 in), and air entraining
agent (water-based). The gradation of both coarse and fine aggregates met the
ASTM C-33 requirements). The concrete mixtures used in this phase of research
had water/cement, fine aggregate/ cement and coarse aggregate/ cement ratios of
0.42, 2 and 2.5, respectively. The optimum plastic content of 2% by total volume
of concrete was used in the production of the plastic normal-weight concrete
mixtures. Air entraining agent was used at 0.06% by weight of cement to produce
resistance against frost attack. Table 8.1 presents the mix proportions.

TABLE 8.1 Mix Proportions for Normal-Weight Concrete (lb/yd3)'

 

 

 

 

 

 

 

 

 

 

 

Mix Cement Water Coarse Fine Plastic AE%
Control 670 281 1675 1340 -- 0.06
Plastic 655 275 1637.5 1310 31 0.06

 

* lb/yd3 = 0.594 kg/m’; AB = Air 13nt Agent by weight of cement

Conventional mixing and curing procedures (ASTM 0192) were used to
prepare the control and plastic concrete materials. External vibration was found to
be suitable for consolidating concrete specimens incorporating recycled plastics.
The optimum vibration time was found to be 25:5 seconds at a frequency of 80
Hz.

Details of test specimens and the test procedures for determining the
mechanical properties and long-term durability characteristics have been

introduced in Sections 3.4 and 6.5, respectively.

126

8.3 TEST RESULTS AND DISCUSSION

8.3.1 Mechanical Properties

8.3.1.1 Flexural Strength and Toughness

Figure 8.1 shows the mean values of flexural strength. Analysis of variance
and comparison of means showed that plastic-concrete with 2% plastics produced
a flexural strength comparable to that of control mixtures. The addition of
recycled plastics produced a ductile mode of failure rather than the conventional
brittle mode of failure, as shown in Figure 8.2. Flexural toughness of plastic
normal-weight concrete improved by about 1.5 times when compared to control
mixtures. This was confirmed statistically at 99% level of confidence.

Flexural Strength, ksi

 

 

 

 

 

 

 

 

 

 

1.5
195% Confidence Interval
1.25 .....................................................................
1 ' """"""""""""""""""""""
I
0°75 """""""""" """
0.5 ----------
0.25 ----------
0 CZIIIIIIIIIIIIIIIT? .IZICIIIIZIIIiiiiii
Control 2% Plastics
FIGURE 8.1 Flexural Strength Test Results (Means and 95%
Confidence Intervals)

127

Flexural Toughness, k.in

 

0.025

I 95% Confidence Interval
0,02 ..................................................................

 

..................

0.015 ---------------------------- ------------~-32353323232333255 ---------

 

0-01 ---------

0.005 ---------

..................

 

 

 

 

 

 

 

Control 2% Plastics

FIGURE 8.2 F lexural Toughness Test Results (Means and 95%
Confidence Intervals)

8.3.1.2 Compressive Strength

The 28-day compressive strength test results for different mixtures are
shown in Figure 8.3. Statistical analysis of variance and comparison of means
indicated that the addition of 2% plastics (optimized plastic content) produced
compressive strengths comparable to those of control normal-weight concrete

mixtures. This was confirmed statistically at 95% level of confidence.

128

 

 

Compressive Strength, ksi

 

I 95% Confidence Interval

 

 

.......................................

.......................................
.......................................
.......................................
.......................................
.......................................
.......................................

.......................................

.......................................
....................
.......................................
.......................................
.......................................
.......................................
.......................................
.......................................

.......................................

.......................................

 

 

 

 

 

 

....................

Control 2% Plastics

 

FIGURE 8.3 Compressive Strength Test Results (Means and 95%
Confidence Intervals)

8.3.1.3 Impact Resistance

Figure 8.4 gives the mean values of the 28-day impact resistance test
results, presented as number of blows to first crack and failure. Statistical analysis
of variance (comparison of means) showed, at 95% level of confidence, that
recycled plastics have a positive effect on the impact resistance beyond the initial

crack up to failure.

129

Impact Resistance (# of Blows)

 

50
First Crack

[Z] Failure r
40 .. ............................... // ......

................
................
............
30 ................
‘:::::::'.:::/: ::::::::::::::::
................................
::::::::::::::.1 ::::::::::::::.
............... ,...........

20_ ::::::::::::::'. __._.___._._,::::::::;::;:: .......
...............................
..............................
..............................
:::::;::::::;:1 ::.:;.:::..:.:.

..............................

 

 

 

 

 

 

...............................

...............................

10—----- 32222332532252; ------

................

.............................

...............................
.......
..............................
...............................
..............................

Control 2% Plastic

 

 

 

 

 

 

 

 

 

FIGURE 8.4 Impact Resistance Test Results (Means)

8.3.2 Long-Term Durability Characteristics
8.3.2.1 Freeze-thaw Durability

The relative dynamic modulus test results for normal-weight concrete
incorporating recycled plastics are presented in Figure 8.5. Optimized mixtures

incorporating 2% recycled plastics produced essentially similar performance when

compared to control mixtures (i.e. no plastics).

130

Relative Dynamic Modulus (%)
120 \
— CONTROL
115 ................................................ ’ ' 2% PLASTIC

 

110 .....................................................................

105

100

 

95

 

 

 

90
O 50 100 150 200

Number of Freeze-Thaw Cycles

FIGURE 8.5 Freeze-Thaw Durability Test Results

8.3.2.2 Scaling Resistance

Table 8.1 shows the de-icer salt scaling resistance test results. Recycled
plastics, at relatively low volmne fractions, produced concretes with scaling
resistance comparable to that of control mixtures. However, at higher volume
fractions, recycled plastics had a negative effect on scaling resistance of resulting
concrete (as observed in some preliminary tests at 5% or more plastic volume
fractions). This can be attributed to the fact that plastic particles, which are
impermeable, tend to block the bleeding water fiom escaping to the surface,
producing cavities underneath plastics where later ice can form to pressure plastics

out of concrete surfaces. The action of impermeable flat particles in preventing

131

water (e. g. osmosis) movements may also contribute to scaling damage at high

plastic contents.

TABLE 8.2 Scaling Resistance Test Results

 

 

 

 

# of Cycles Rating
Control 50
Plastic Concrete (2% by 50
total Volume)

 

 

 

 

8.4 SUMMARY AND CONCLUSIONS

The effects of recycled plastics as secondary reinforcing inclusions
(additives) in normal weight concrete were investigated through evaluating
concrete mixtures, with optimum plastic (ground HDPE) content of 2% versus
control mixtures (without plastics). The results indicated that:

(1) Recycled plastics enhance the restrained shrinkage cracking characteristics of
normal-weight concrete without adversely influencing any of the short-term

mechanical properties of the material;

(2) The optimized mixture gave comparable flexural and compressive

strengths when compared to control mixtures;
(3) The addition of recycled plastics, at 2% by total volume (optimum

content), improved both flexural toughness and post cracking impact

resistance of the concretes;

132

 

 

(4) Recycled plastics had no adverse effects on freeze-thaw durability of

concrete; and

(5) Recycled plastics at the optimum volume fraction of 2% did not have a
negative effect on the scaling resistance of concretes exposed to de-icer salts;
however, at higher volume fractions one may see some adverse efl‘ects of

plastics on the scaling resistance of normal-weight concrete.

133

PART III: TIRE CONCRETE

CHAPTER 9

RECYCLING OF TIRES IN CONCRETE MATERIALS

9.1 INTRODUCTION

Approximately 240 million used vehicle tires are generated in the United
States each year. About 25% of these tires are reused or processed. The
remaining 170 million are discarded in landfills or illegal disposal sites, or
stockpiled above ground.

Tires usually contain a variety of rubber compositions as well as other
constituents such as carbon black, synthetic fibers, steel wires, sulfur and zinc
oxide, each contributing certain properties to the overall performance of tire.
Rubber compounds designed for a specific function will usually be similar but not
identical in composition and properties, although in some cases there can be
significant differences between compounds in tires of various types. When
coupled with cement and moisture, tire constituents tend to swell inside concrete
matrices; this eventually leads to pop-out at outer surfaces of concrete.

Recycled tire inclusions can potentially relieve the stress intensity at sharp
crack tips in brittle concrete matrices, and thus produce a composite material with

enhanced resilience, toughness, energy absorption capacity, impact resistance and
134

cracking strength. The relatively low density of recycled tire inclusions would
also improve the thermal insulation properties of concrete. Many applications of
recycled tires as reinforcing inclusions such as slabs on grade can take advantage
of the cracking resistance and toughness characteristics of tire-concrete

composites.

9.2 BACKGROUND

About 240 million automotive, truck, and ofl-road tires are discarded in the
United States each year. This is approximately equal to one waste tire per person
per year. Additionally, there are 33.5 million tires that are retreated and an
estimated 10 million that are reused each year as second-hand tires. It is estimated
that 7% of the discarded tires are currently being recycled into new products and
11% are converted to energy. Nearly 78% are being landfilled, stockpiled, or
illegally dumped, with the remainder being exported.

Tires are difficult to landfill. Whole tires do not compact well, and they
tend to work their way up through the soil to the top. As a result, tire stockpiles,
which cost less than landfills, have sprung up all over the country. It is estimated
that between 2 and 3 billion tires are stockpiled in the US. at present, with at least
one pile containing over 30 million tires. Tire stockpiles are unsightly and are a
threat to public health and safety. Not only are tire piles excellent breeding
grounds for mosquitoes, but they are also fire hazards.

Tire recycling activities include the use of whole tires or processed tires for
useful purposes. Whole tire applications include reefs and breakwaters,

playground equipment, erosion control, and highway crash barriers. Processed tire

135

products include mats and other rubber products, rubberized asphalt, playground
gravel substitute, and bulking agent for sludge composting.

Scrap tire combustion is practiced in power plants, tire manufacturing
plants, cement kilns, pulp and paper plants, and small package steam plants (EPA,
1991)

9.3 PRODUCTION AND DISPOSAL

9.3.1 Volume of Production

It is commonly accepted in the tire industry that about one tire per person
per year is discarded. Since there is no industry group or governmental agency that
monitors tire disposal in the United States, the best estimates that can be made are
based on tire production. The Rubber Manufacturers Association (RMA) records
the number of original equipment, replacement, and export tires that are shipped
each year in the United States. In 1990, a total of 264,262,000 tires were shipped.
The RMA data include new tire imports, but not imported used tires.

Figure 9.1 shows the estimated disposition of the 240 million scrap tires
generated in 1990. About 16.3 million were recycled, 26 million were recovered
for energy, and about 12 million were exported, leaving 188 million for landfilling,
stockpiling, or illegal dumping. Figure 9.1 shows that in 1990, 17.4% of the tires
scrapped were recycled or burned for energy (EPA, 1991).

136

 

LJ

 

 

 

 

 

 

Reuse 10 7 Vehicles I 33-5 Retread 1
T 33.5 7

 

 

 

 

10 .

 

242 100 %
v

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

‘23 Waste Tire Inventory ‘ ‘2
17.4% 5.0 °/.
187.8 77.6%
1 i i 1’
. Used
Illegal Dumping Landitii Stockpile 1 Export
1.Whole
2. Shredded ’
1
25.9 10.7 '7. 0.3 ‘ 0.1% 16.0 5.5 'A
Combustion Processed Tire Products
1. power plants Whole Tire Applications 1. Processed rubber products
2. Tire plants 1. 99915 30d breakwaters 2. Crumb rubber ior pavements

3. Cement plants

4. Pulp and paper mills
5. Small package boilers

2. Playground equipment
3. Erosion control
4. Highway crash barriers

 

 

 

 

8. Playground gravel
substitute .

4. Sludge composting

5. Split tire products

 

 

 

 

FIGURE 9.1

Flow Diagram Showing Estimated Destination of Scrap
Tires in 1990 (In Millions of Tires and Percent; EPA,

1991)

137

 

 

 

  
    

Illegal Dumping

5%
Used
Expon

6.7 %

Other

Recovery

10.7..)

0

Energy/Burning

FIGURE 9.2. Destination of Waste Tires in 1990 (Fletcher, 1980).

9.3.2 Disposal Problems

A wide range of scrape tire disposal technique are available or being
investigated. Numerous diverse schemes, such as artificial tire reefs, dock fenders,
synthetic turf, motor way crash barriers, fillers for building and road materials, and
filling in river beds, railway cutting and marshlands have been contemplated, but
inevitably have had only minor impact on the total disposal problem.

Methods such as tire splitting, and reclaim and crumb recovery (most of
which is recycled to the tire industry) are carried out, but do not handle large truck
tires and produce by-product wastes with similar disposal problems.

An increasing majority of landfills refuse to accept any tires because they

do not biochemically degrade when buried and can "float" to the top, breaking

138

through the cover causing damage which requires costly repairs. Tire piles
provide excellent breeding places for vermin and insets.

The recycling of rubber through size reduction, either by shredding and
grinding or by cryogenic fi'agmentation, would certainly attack a large market but
is expensive.

In the United States, an oil and ground-tire mixture has been burned to
produce carbon black for re-use in new tires, while a stoker-fired boiler has been
operated using a mixture of 10% shredded tire with coal. Biochemical degradation,
by mixing steel-free tire particles with yeast/frmgi and certain chemicals, has also
been reported for producing a soil conditioner, although, without a market, the
process is uneconomic.

Dumping is generally an expensive operation, is a waste of a value
resource, and is normally discouraged by local authorities because such sites are
fire risks as well as being potentially unsightly. Where sites are not scarce,
inaccessibility can be a problem even when the normally low bulk-density of tires
is more than doubled by shredding. Several furnaces, operating worldwide, have
been designed specifically for burning tires. However, incineration of tires
without heat recovery is disadvantageously placed in an energy-conscious
environment. The incorporation of a waste-heat recovery system is attractive,
provided economical operation is possible. The high calorific value of a tire is
troublesome, the air-fuel ratio must be maintained within close limits to ensure the
required fuel-vaporization rate and maintain the ignition temperature; insufficient
quantities of air result in the production of explosive mixtures. Even so, the
combustion is violent and flame extinction is always a possibility. The relatively
high sulfur content (up to 2%) results in sulfur dioxide formation and subsequent
corrosion problems. To comply with the requirements of the Clean Air Act, it is

necessary to install sophisticated gas clean-up plant. The high temperature of

139

operation, 8710C (16000F) in some cases, necessitates temperature resistant
construction materials with associated additional costs. The high steel content of
tires can cause major problems with moving grate incinerator. For efficient and
acceptable treatment, specially designed incinerators are therefore required,
although tire-shredding and/or mixing with general waste materials can reduce this
problem and allow the use of conventional units ( Fletcher, 1980).

According to the Maine DEP (1989), fire is a particularly important
problem, since tire fires emit heavy black smoke into the air, and release petroleum 1
and other chemical contaminants into surface and ground water. Once ignited,
tire fires are extremely difficult to contain and extinguish, requiring large amounts
of manpower and equipment.

Anyone who has ever handled an old tire realizes the impossibility of
dumping all the water out of one. Such water creates breeding habitat for
mosquitoes, known to carry yellow fever and many forms of encephalitis.
According to the Maine DEP (1989), "In 1986, the US. Environmental Protection
Agency estimated that health care costs for scrap tire related encephalitis totaled
$5. 4 million. This cost includes medical care as well as lost earning due to

illness".
9.4 PROPERTIES OF TIRE
9.4.1 Chemical Composition
Today's tire is made to last under extremely severe physical, thermal, and

chemical conditions and is practically indestructible. Over years tires have

developed to be chemically complex, precision engineered and designed not to

140

 

come apart. A tire only becomes a scrap tire because the tread is worn off or it has
been physically damaged. The material out of which a tire is made the fabric,
fiber glass, wire and rubber remain essentially as good as when they were
introduced to make a new tire. The most common tire rubber is styrene butadiene
copolymer (SBR), containing about 25% by weight styrene. In combination with
SBR, other elastromers such as natural rubber (cis-polyisoprene), synthetic cis-
ployisoprene, and cis-polybutadience are also used in tires in varied amounts. A

typical recipe for tire rubber is given in Table 9.1 ( Fader, 1990)-

Table 9.1 Typical Chemical Composition of Tire Rubber (F ader, 1990)

 

 

 

 

 

 

 

 

 

Component Weight (%)
SBR 62.1
Carbon Black 31.0
Extender Oil 1.9
Zinc Oxide 1.9
Stearic Acid 1.2
Sulfur l.l
Accelerator 0.7

 

 

 

A typical analysis of a de-beades scrap tire suggests that tire is a mixture of
rubber, rayon, nylon, polyester, carbon-black, and natural and synthetic rubbers.
Steel (up to 17%), non-ferrous metal (mainly as the vulcanizing agent, zinc oxide)

and sulfur are also present in significant quantities in the whole tires.

9.4.2 Physical Properties

A pneumatic tire generally fulfills the following functions: (1) Allows a
comparatively free and fiictionless motion of the vehicle through rolling; (2)

141

 

 

Distributes vehicle weight over a substantial area of ground surface, thus avoiding
excessive stress on the latter and on the wheel; (3) Cushion the vehicle against
road shocks; (4) Transmits engine torque to the road surface with a low power
consumption; (5) Permits, through tire adhesion, the generation of substantial
braking, driving and steering loads; and (6) Ensures lateral and directional
stability. No other device exists today which can fulfill these varied function in an
efficient manner as the pneumatic tire. To fulfill these purpose, passenger-car
radial ply tires are usually built of one to four radial plies of rayon, nylon or
polyester. The belt or breaker consists of either two layers of steel cords, or four to
six layers of textile cords. These are generally thicker than the cords used in the
radial portion of the casing and therefore have a considerably increased stiffness.
For truck and heavy-duty tires, it is useful to have a single layer of radial cords and
two or three layers in the belt, all of steel construction. Ofien, additional
reinforcing strips of cross-biased fabric extend a short way up the sidewalls of
radial-ply tires. The introduction of fibrous glass as a substitute reinforcement for
textiles in carcass design deserves mention. Having received special chemical
treatment, glass fibers permit greater adhesion to rubber and give increased
stability in withstanding dynamic stress when embedded in elastomeric material.
Moisture and low elongation no longer appear to be problems in using fibers for
rubber reinforcement. Table 9.2 (Moore, 1975) compares some of the physical
properties of glass, rayon, nylon and polyester reinforcement in tires, while Table
9.3 (Rodrigues, 1970) shows the typical properties of cross-linked rubber

compounds of the most commonly used rubber tires.

142

TABLE 9.2 Physical Properties of Reinforcement Structures in Tires
(Moore, 1975)

 

 

 

 

 

 

 

 

Glass Rayon Nylon Polyester
Ultimate tensile strength
psi‘lOOO 407 94 122 104
Ultimate Elongation, % 4.83 9.8 19.3 18.5
Toughness, psi 9,900 5,800 10,200 9,900
Modulus, 1000 psi 8,450 960 630 570
Breaking strength, lbf 79.0 39.1 33.2 32.1
Filament Dia. (mls) 17 26 21 24

 

 

 

 

 

 

TABLE 9.3 Typical Properties of Cross-Linked rubber Compounds Used
in Tires (Rodrigues, 1970)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Polyisoprene (natural . Styrene-butadine
rubber, also made Polybutadrne random copolymer 25
synthetically) wt% styrene SBR
Gum Stock (cross-
linked, unfilled):
Density, gm/em3 0.93 0.93 0.94
Tensile Str., psi 300-2500 200-1000 ZOO-400
Reinforced Stock:
Tensile Str., psi 3000-4000 2000-3500 2000-3500
Elong. at Break, % 300-700 300-700 300-700
Resistance To:
Acid Good Good Good
Alkali Good Good Good
Oxidation Good Good Good

 

 

 

 

143

 

 

9.5 RESOLVING OF POP-OUT OF TIRE PARTICLES

9.5.1 Objectives

Preliminary work on tire-concrete composites indicated that dimensional
instability of tire in the presence of moisture, particularly at elevated temperatures,
is a problem; surface pop-out is a result of the dimensional movements of tire
particle.

Potential chemical and physical causes of dimensional instability of tires
causing pop-out were investigated in order to resolve the problem. For this
purpose, concrete specimens incorporating tire particles, subjected to different
treatments (aimed at controlling the chemical or physical causes of pop-out) were

exposed to different environments where pop-out could occur.

9.5.2 Approach

Based on alternative hypotheses regarding the pop-out mechanisms,
different treatments were considered for mitigating the pop-out problem. These
treatments included: immersion in lime solution, immersion in cold and hot water
and other chemicals, and coating with epoxy and acrylic latex. Immersion of tire
particles in lime solution for one week prior to mixing in concrete helped reduce
the pop-out problem in moist conditions at ambient temperature 23°C (73°F ), but
when immersed in water at high temperatures of approximately 600C (1400F ), it
actually pronounced pop-out.

To further investigate the pop-out problem of tires in concrete materials at

high temperatures, difi‘erent chemical pretreatrnents were investigated with limited

144

 

success. These treatments included immersion in solutions of coal fly ash, coal fly
ash with lime, cold and hot water, sodium hydroxide, aluminum oxide, Kaolin,
calcium chloride, sodium silicate, potassium hydroxide, aluminum sulfate, and
silica fume slurry. These treatments were supposed to remove different chemical
causes of pop-out or physical inhibit it. The lack of any considerable success led
to the hypothesis that pop-out is a physical problem caused by expansion of tire in
the presence of moisture (and particularly at high temperatures) rather than a
chemical one.

Tires were subsequently immersed in cold/hot water at different time
intervals in order to measure the volume change and water absorption. Figures
9.3a and 9.3b shows the volume changes of the tire particles in cold and hot water,

respectively.

2 VOLUME CHANGE (%)

 

 

 

 

 

 

 

 

 

 

 

AGE (WEEKS)

(a) Ambient Water Immersion

145

 

VOLUME CHANGE (7e)

 

 

 

 

 

 

 

 

 

 

 

AGE (WEEKS)

(b) Hot Water Immersion
FIGURE 9.3 Volume Change of Tire Particles

It is observed in Figure 9.3 that tire particles, when immersed in ambient
temperature water, tend to shrink and at longer periods of time start to expand, but
when immersed in hot water they expand from the beginning. The volume change
of tires at different water temperatures is partly due to thermal expansion. It
should be noted that the expansion of tires in moist/hot condition is more severe
than that in moist/cold condition. This further explains why tire particles at
moist/hot condition tend to cause more severe pop-out than in moist/cold
environments.

In order to prevent tire expansion upon exposure to moisture, tire particles
were coated with HydroEpoxy 1041 (water based), which is commercially
available. This resulted in eliminating the tire pop-out( problem (see Figure 9.4).

 

1 HydroEpoxy 104 is a commercially available Epoxy produced by ACME Chemicals Ltd, New York.

146

The combination of reducing the amount of water reaching the tire particles and
providing an elastic layer around the tire particles prevented the tire particles from
causing pop-out in concrete materials. This was partly due to the fact that as tires
particles tend to expand, the thin elastic layer around the particles was able to
withstand the preSsure caused by the tires and act as a "bumper" to prevent the
tires from applying pressure on the thin concrete surface which leads to pop-out
However, this method proved to be relatively costly due to the high cost of
HydroEpoxy 104.

Acrylic latex, Synthemul 40401-002 which is water based and
commercially available, was then used to coat the tire particles. This method

produced same results as the HydroEpoxy coating at reduced cost.

 

(a) Uncoated Tire Particles (b) Coated Tire Particles
FIGURE 9.4 Epoxy Coating Effects on Pop-Out in Hot Water

 

2 Synthemul 40401-00 is a commercially available Acrylic latex produced by Reichhold Chemimls, Ltd.

147

 

 

9.6 TIRES AS REINFORCING INCLUSION IN CONCRETE

9.6.1 Objective

Following the elimination of pop-out problem through coating of tire
particles with polymeric materials, the effectiveness of coated and uncoated tires
as reinforcing inclusions in concrete was demonstrated. The fact that coating
increases the cost of recycled tire particles encourages higher-value use of the
coated particles (e. g. as reinforcing inclusions rather than light-weight aggregates).
Coated tire particles were incorporated into concrete at 2.5 and 5% volume
fractions and uncoated tires at 5% volume fraction. Control mixtures without tire
were also considered. The effects of tire inclusions on the following properties of
concrete were investigated: restrained drying shrinkage, flexural strength and

toughness, compressive strength, impact resistance and chloride permeability.

9.6.2 Materials and Mix Proportions

The basic ingredients of the concrete materials used in this phase of the
research were type I Portland cement, normal-weight aggregate (fine and coarse
with a maximum size of 12 mm, 0.5 in), water, recycled tires with maximum size
of 12mm (0.5 in), and air entraining agent (water based). The gradation of coarse
and fine aggregates met the ASTM C-33 requirements.

The concrete mixtures used in this phase had fine aggregate/cement and
coarse aggregate/cement ratios of 2 and 2.5, respectively. Air entraining agent was

used at 0.06% by weight of cement to produce resistance against fi'ost attack. The

148

 

water content was adjusted for all mixtures to give similar slumps (44.5 to 57.2
mm, 1.75 " to 2.25"). Water-cement ratio ranged fiom 0.39 to 0.43.
For the normal-weight mortar mixtures, the cementzsandzwater ratio was

1:2.4:0.5. The flow (ASTM C-230) ranged from 80 to 95%.

9.6.3 Test Procedures

For the hardened materials, the flexural strength and toughness,
compressive strength, impact resistance, restrained shrinkage cracking
characteristics and chloride permeability properties were investigated
experimentally in order to develop an overall understanding of various aspects of
material behavior.

The flexural test specimens were 100x100x350 mm (4"x4"x14”) prisms,
and the compressive strength test specimens were 3"x6" (75x15 mm) cylinders.
Flexural and compression tests were conducted following ASTM C-78 (four point
loading) and C-39 procedures. Midspan deflection as well as loads were
monitored in flexure tests. The impact test was conducted following the
procedures recommended by ACI Committee 544. This test measures the amount
of impact energy (represented by the number of blows) necessary to start a visible
crack in concrete and then continue the opening of crack until failure.

Ring type specimens are used for restrained drying shrinkage test on mortar.
The specimen is cast in two equal layers, leveled by trowel, and then covered with
plastic sheets for 6 hours. The specimens are then exposed to air at approximately
23°C (73°F ) and 40% RH. Restraint of shrinkage movements by the steel ring
inside the specimen creates internal tangential tensile stresses which cause

cracking.

149

Permeability tests were conducted using AASHTO T-277 (Rapid
Determination of the Chloride Permeability of Concrete). This test measures the
amount of charge passed through a concrete specimen subjected to permeation of
chloride ions at 60 VDC for 6 hours. The total charge passed (in Coulombs) is
related to chloride ion permeability. The more permeable the concrete, the higher
would be the Coulombs. A cylindrical specimen. 102 mm (4 in) in diameter by 51

mm (2 in.) in thickness is used for this test.

9.6.4 Test Results and Discussion

9.6.4.1 Flexural Strength and Toughness

Figures 9.5 and 9.6 show the flexural strength and toughness test results of

normal-weight concrete incorporating recycled tires.

FLEXURAL STRENGTH (ksi)

 

1.4

1.2 — A
195% Confidence Interval

 

 

 

0.8 “

 

0.2 ‘

 

 

 

 

 

 

 

 

 

 

O ; :
CONTROL(0%) 2.5% COATED 5% COATED 5% UNCOATED
TIRE VOLUME FRACTION

 

FIGURE 9.5 Flexural Strength Test Results at 28-Days (Means and 95%
Confidence Level)

150

FLEXURAL TOUGHNESS (k.in.)

 

0.03

 

 

0.02..-.

 

 

 

 

0.01 .7. .... .. ,, “m_“,_,_,,_Vfl_w .-. ______g '4:>::

 

 

0,00 5 _

 

 

 

 

 

 

 

 

 

 

 

 

 

TIRE VOLUME FRACTION

FIGURE 9.6 Flexural Touglmess Test Results at 28-Days

Although the flexural strength of concrete incorporating recycled tires
dropped by 6, 35 and 9% at 2.5 and 5% coated, and 5% uncoated, respectively,
when compared to control mixtures (without tires), they were statistically
comparable at 95% level of confidence, except for 5% treated tire. The dr0p in
flexural strength, particularly at 5% addition of coated tire can be attributed to the
weak bond between coated tire inclusions and cementitious matrices.

Figure 9.6 is indicative of the positive effects of recycled tires on the post-
peak ductility and toughness of concrete materials in flexure. The flexural
toughness of concrete incorporating 2.5 and 5% coated tire, and 5% uncoated (as
additives), improved by about 3, 4 and 6.5 times, respectively, when compared to
control mixture. The significant increase in flexural toughness in the presence of
recycled tires can be attributed to the reinforcing action of the tough tire inclusions

in the brittle cementitious matrices. In spite of the potentially weak bonding of the

151

tire particles to concrete matrix, the fact that cracks can not go through tire

inclusions but have to follow longer paths around their interfaces seem to enhance

the fracture properties of concrete.

9.6.4.2 Compressive Strength

The compressive strength test results are shown in Figure 9.7. The addition

of tire inclusions as additives in normal-weight concrete had an adverse eflect on

compressive strength. .

7 COMPRESSIVE STRENGTH (ksi)

 

 

I 95% Confidence interval

 

 

 

 

 

 

 

 

 

 

 

 

 

o . * 21:2:
CONTROL (0%) 2.5% COATED 5% COATED 5% UNCOATED
TIRE VOLUME FRACTION

 

FIGURE 9.7 Compressive Strength Test Results at 28-Days (Means and
95% Confidence Level)

The compressive strength dropped by 18, 33 and 32% when 2.5 and 5%

coated, and 5% uncoated tires were added to normal-weight concrete, respectively.

152

The reduction in compressive strength was confirmed statistically at 95% level of
confidence. The drop in compressive strength was expected because of lower
modulus of elasticity of tire when compared to that of concrete (about 10 times
less) leading to a redistribution of stresses into concrete which is a more rigid
matrix. It should, however be noted that limits on load-carrying capacity and
service life of concrete structures are generally provided by the resistance of
concrete to tensile stresses and impact loads. Concrete is fairly strong in

compression and concrete structures rarely fail in compression.

9.6.4.3 Impact Resistance

Figure 9.8 shows the impact resistance test results measured as the number
of blows up to the initial crack and failure. The difference between the initial
crack and failure represent the post-cracking energy absorption capacity of the

material.

IMPACT RESISTANCE (# of Blows)

 

200

 

First Crack
150‘ Failure

 

 

 

 

100*'“

50‘

       

 

 

 

 

 

 

 

0 .,; ' ; 3:15.. Te: 7:29.: I}: .a;
CONTROL(0%) 2.5% COATED 5% COATED 5% UNCOATED
TIRE VOLUME VRACTION

 

 

FIGURE 9.8 Impact Resistance Test Results at 28-Days

153

 

One-way analysis of variance and comparison of means showed that, at
99% level of confidence, the recycled tires have a significant positive effect on the
impact resistance of concrete beyond the initial crack up to failure. The
improvements in impact resistance between initial crack and failure in the presence
of recycled tires in concrete materials further validates the hypothesis that tough
tire inclusions help enhance the fi'acture energy and toughness characteristics of

concrete materials through bridging of cracks by the tire inclusions.

9.6.4.4 Restrained Drying Shrinkage

Figure 9.9 shows the maximum crack width versus time in restrained
shrinkage test on normal-weight concrete. The addition of recycled tires to
normal-weight concrete helps delay and connol the drying shrinkage cracks. This
can be attributed to the fact that recycled tire inclusions act as reinforcing

inclusions that arrest microcracks and bridge across cracks to restrain their

 

       
 

 

 

 

 

 

widening.
1.2
+
+
1
11 Control
1 5% Crack2
’E‘ 4
£08 4
c 1
,9 0.6 ‘1 2.5% Crack3
g ‘ 2.5% Crack2
f) .
(204-1
0 I
0.2“
O Herrrwrera's—rrrfisfififihrfirttfifiéefifi'rmI
1 9 17 25 33 41 49

Time (Days)

FIGURE 9.9 Restrained Drying Shrinkage Vs. Time

154

9.6.4.5 Permeability

The chloride permeability test results (Figure 9.10) indicates that
incorporating coated tire particles at 2.5 and 5% volume fractions and uncoated
tires at 5% volume fraction produced permeability levels comparable to that of
control. Although the addition of 5% coated tires as additive in normal-weight
concrete increased permeability by an average of 6%, one way analysis of variance
and separation of means showed that this increase was not statistically significant
at 95% level of confidence. In actual feild conditions, control of restrained
shrinkage cracking bytire inclusions could lead to reduced permeability of tire-

concrete composites.

0 PERMEABILITY (Coulomb8)

 

 

 

 

 

2500*

2000-

1500— ~ _-_. ~ — _

1000~ . __

500-

 

 

 

 

 

 

 

 

 

 

 

0
CONTROL (0%) 2.5% COATED 5% COATED 5% UNCOATED
TIRE VOLUME FRACTION

FIGURE 9.10 Permeabilty Test Results.

155

9.7 SUMMARY AND CONCLUSIONS

The dimensional instability of tire particles in the presence of moisture,
which is pronounced at elevated temperatures and eventually leads to surface pop-
out was investigated. F ollowing the elimination of pop-out through coating of tire
particles with polymeric materials, the efi‘ects of tire inclusions on: restrained
drying shrinkage, flexural strength and toughness, compressive strength, impact
resistance were investigated. The environmnetal impact of tire concrete was alo

checked. The following conclusions were derived:

1. Coating of tire particles with polymeric materials proved to eliminate surface
pop-out of concrete particles even in extreme conditions of immersion in hot

water.

2. The addition of recycled tires to normal-weight concrete helps delay restrained
drying shrinkage cracking and reduce the crack width.

3. Coated recycled tires at 2.5% and uncoated recycled tire at 5% volume fraction
produced flexural strengths comparable to that of control concrete mixtures.
However, statistical analysis of variance showed that coated tire at 5% volume
fraction reduced flexural strength. This was confirmed statistically at 95%
level of confidence.

4. The flexural toughness of concrete incorporating 2.5 and 5% coated tire, and
5% unc0ated tire, increased by about 3, 4 and 6.5 times when compared to
control concrete mixtures. this was cnfinned statistically at 99% level of

confidence.

156

 

5. Compressive strength test results were indicative of the adverse effects of
recycled tires on compressive strength. It should be noted that concrete is
fairly strong in compression, and brittle failure mode and low crack resistance
are the main problems which are targeted to be overcome through the use of

soft reinforcing inclusions.

6. Recycled tires have a significant possitive effect on impact resistance of

concrete beyond the initial crack upto failure.
7. Chloride permeability test results showed that coated tire and uncoated tire

produced results which were statistically comparable to those obtained with

control concrete.

157

 

CHAPTER 10

SUMMARY AND CONCLUSIONS

10.1 INTRODUCTION :
Recycling in construction presents the potentials for high-volume use of

waste materials in products with long service life, while avoiding costly separation

and purification steps. This research focused on the use of recycled plastic flakes

as reinforcing and light-weight inclusions in light-weight concrete, and as

secondary reinforcing inclusions in normal-weight concrete. The effectiveness of

recycled tires as secondary reinforcement in normal-weight concrete was also

demonstrated.

10.2 PLASTIC LIGHT-WEIGHT CONCRETE

The effects of partial substitution of light-weight aggregates with recycled
plastic flakes on concrete properties were investigated. The short-and-long-term
properties of the materials were assessed. The following conclusions were

derived through analysis of the generated data:

158

 

(1)

(2)

(3)

(4)

The addition of recycled plastics to light-weight concrete helps reduce

the drying shrinkage crack widths;

Recycled plastics at 7.5 and 15% volume fractions produce flexural strengths
comparable to that of control concrete mix. However, flexural toughness
increases by 4.5 and 8 times, respectively. This was confirmed statistically at

99% level of confidence;

Compressive strength test results were indicative of the adverse effects of
recycled plastics on compressive sfiength; however, the addition of fly ash to
the cementitious matrix produced compressive strengths comparable to
control mixtures. It should be noted that concrete is fairly strong in
compression, while brittle failure mode and low crack resistance are the
main problems which are targeted to be overcome through the use of
reinforcing inclusions. Furthermore, since the reduction in compressive
strength is accompanied with the reduction in unit weight, the situation

would be improved if one looks at the compressive strength-to-weight ratio;

Recycled plastics have a significant positive effect on the post- cracking

impact resistance of concrete;

159

 

(5)

(6)

(7)

(8)

Some aspects of the flexural performance of light-weight concrete

materials incorporating plastics are improved through sulfonation of plastics;
the overall mechanical properties (i.e. compressive strength and impact
resistance), however, were not substantially improved by the sulfonation of

plastics;

Increasing the plastic content leads to improved flexural toughness and post-
cracking impact resistance; but adversely influences the compressive

strength; excess plastic contents may also damage flexural strength;

The optimum plastic content would depend on the importance of different
properties for various applications; if one can tolerate a drop of 25% in
compressive strength, when compared to control mixtures, 11% plastic
content (by total volume) will improve flexural toughness (about 6 times) and
post-cracking impact resistance (about 3 times) without adversely influencing
flexural strength; this was confirmed statistically at 95% level of confidence;

Theoretical studies of plastic-lightweight concrete composites under
compression indicated that the redistribution and concentration of
stresses induced in concrete by soft inclusions (with relatively low elastic
modulus and relatively high Poisson’s ratio) leads to increased maximum
compressive and particularly tensile stresses, increased displacements and
reduced elastic modulus of light-weight concrete; increasing levels of

maximum stress and deformation are observed with perfectly bonded plastics,

160

 

 

imperfectly bonded plastics, and empty voids; increased volume fraction of

the soft inclusions leads to increased maximum stress and deformation;

(9) The experimentally observed negative effects of plastics on compressive
performance suggest that any positive eflects of plastic inclusions (through
the arrest and bridging of cracks) would be overshadowed by the adverse

effects of plastics on maximum compressive and particularly tensile stresses;

(10) The addition of recycled plastics to light-weight concrete improved the
freeze-thaw durability characteristics of the resulting composite, noting that
the specific light-weight aggregate used in this investigation made concrete

highly susceptible to frost attack;

(11) Recycled plastics had a negative effect on scaling resistance of concrete,
again noting the very low scaling resistance of the control light-weight

concrete associated with its frost susceptibility; and
(12) Sulfonated plastics produced levels of fieeze-thaw durability characteristics

and scaling resistance which were comparable to those obtained with

unsulfonated plastics.

161

10.3 PLASTIC NORMAL-WEIGHT CONCRETE

Normal-weight concretes incorporating recycled HDPE plastics, as
secondary reinforcing inclusions (additives) were considered in this research.
Plastic volume fraction and plastic width were optimized in order to achieve a
condition where minimum plastic content would effectively reduce the restrained
shrinkage crack width. The optimized composite was then compared to control

mixtures (i.e. without plastic). The conclusions derived are summarized below.

(1) Recycled plastics can effectively reduce the restrained shrinkage crack widths
in concrete; this can be attributed to the capability of plastics to bridge across
cracks, restrain their opening, and provide post-cracking tensile resistance

across cracks;

(2) While plastic volume fraction significantly influenced restrained shrinkage
crack widths, the width of plastic particles did not have statistically

significant effects;

(3) A plastic volume fraction of 2% represented the minimum (optimum) plastic
content where restrained shrinkage crack widths were effectively controlled;
ground plastic flakes and plastic "fibers" used in this investigation where both

effective at this volmne fraction;

(4) The optimized mixture gave comparable flexural and compressive

strengths when compared to control mixtures;

162

 

 

(5) The addition of recycled plastics, at 2% by total volume (optimum
content), improved both flexural toughness and post cracking impact

resistance of concrete;

(6) Recycled plastics had no adverse effects on freeze-thaw durability of

concrete; and

(7) Recycled plastics at the optimum volume fraction of 2% did not have a
negative eflect on the scaling resistance of concretes exposed to de-icer salts;
however, at higher volume fractions one may see some adverse effects of

plastics on the scaling resistance of normal-weight concrete.

10.3 TIRE CONCRETE

The dimensional instability of tire particles in the presence of moisture,
which is pronounced at elevated temperatures and eventually leads to surface pop-
out, was investigated. Following the elimination of pop-out through coating of tire
particles with polymeric materials, the effects of tire inclusions on some

mechanical properties were determined. The following could be concluded:

(I) Coating of tire particles with polymeric materials proved to eliminate
surface pop-out of concrete particles even in extreme conditions of

immersion in hot water;

163

(2)

(3)

(4)

(5)

(6)

(7)

The addition of recycled tires to normal-weight concrete helps delay
restrained drying shrinkage cracking and reduce the crack width;

Coated recycled tires at 2.5% and uncoated recycled tire at 5%

volume fraction produced flexural strengths comparable to that of

control concrete mixtures. However, statistical analysis of variance showed
that coated tire at 5% volume fraction reduced flexural strength. This was

confirmed statistically at 95% level of confidence;

The flexural toughness of concrete materials incorporating 2.5 and 5%
coated tire, and 5% uncoated tire, increased by about 3, 4 and 6.5 times,

respectively, when compared to control concrete mixtures;

Compressive strength test results were indicative of the adverse effects of
recycled tires on compressive strength. It should be noted that concrete is
fairly strong in compression, and brittle failure mode and low crack
resistance are the main problems which are targeted to be overcome through

the use of soft reinforcing inclusions (e.g. plastics and tire);

Recycled tires have a significant positive effect on impact resistance of

concrete beyond the initial crack up to failure; and
Chloride permeability test results showed that coated tire and uncoated tire

produced results which were statistically comparable to those obtained with

control concrete.

164

APPENDIX A

 

APPENDIX A

EFFECT OF AGGREGATE PROPERTIES ON CONCRETE
PROPERTIES

A.1 INTRODUCTION

Aggregates are the major constituents of concrete, they usually occupy
about 75% of the concrete's volume. They are critical to the performance of fresh
mixed and hardened concrete. In addition to serving as an inexpensive filler, they
have certain positive benefits to concrete. This report will give a brief review on

the effect of aggregate properties on some Portland cement concrete properties.

A.2 EFFECT OF AGGREGATE GRADING

It has been reported by the American Concrete Institute (AC1, 1978) that
there are several reasons for specifying both grading limits and maximum
aggregate size in concrete applications. Aggregates having a smooth grading curve
and neither a deficiency nor excess of any one particle size will generally produce

mixtures with fewer voids between particles. Since cement costs more than

165

 

aggregate and the cement paste requirement for concrete increases with increasing
void content of the combined aggregates, it is desirable to keep the void content
aslow as possible. Ifthere is not enough sand to fill the voids between coarse
aggregate particles, the space must be filled with cement paste. These mixtures
also tend to be harsh and difficult to finish. On the other hand, aggregate
combinations produce uneconomical concrete because of the larger surface area of
finer particles. When the surface area increases, concrete will be too stiff and hard
to place and compact. If we make the paste more fluid by adding water, the
concrete strength and durability will suffer.

The maximum size of coarse aggregate used in concrete also has an effect
upon surface area and economy. Usually, as the maximum size of well-graded
coarse aggregate increases, the amount of paste required to produce concrete of a
given slump or consistency decreases. Figure A.1 shows the effect of increasing

maximum size upon the void content of a well graded aggregate.

 

 

 

 

 

' L
Well-graded aggregate Well-graded aggregate
7: -ln. maximum size i - In. maximum size

FIGURE A.1 Efl‘ect of Increasing Maximum Size On the Void Content of
a Well-Graded Aggregate (ACI, 1978)

166

One of the most important characteristics of the fine aggregate grading is
the amount of material passing numbers 50 and 100 sieves. Inadequate amounts of
materials in these size ranges can cause excessive bleeding, difficulties in pumping
concrete, and difficulties in obtaining smooth trawled surfaces.

In a study by Czuryszkiewicz (1973), it has been reported that the size of
aggregate has less influence on the strength of concrete having a higher water-
cement ratio; however, for lower water-cement ratios a difference of
approximately 15 Mpa (2200 psi ) was observed between a concrete having a
maximum aggregate size of 76 mm (3 in.) and one having a maximum aggregate
size of 9.5 mm (3/ 16 in. ); the one with larger aggregates gave less strength. They
concluded that the smaller size aggregates produce higher strength concrete at a
given water-cement ratio, as discussed previously; however, the water and cement
requirements of the small maximum size aggregate mixes are much greater than
for the large aggregate mixes. A similar trend for the effect of aggregate size on
flexural strength Walker, 1960), although the difference in strengths are not so
great. Walker also reported that shrinkage and loss of water during drying
increased with size. In the range of aggregates sizes from 19 to 63.5 m (3/4 in.
to 2.5 in.), differences in shrinkage amounted to only about 0.5 percentage points,
which would seem to be of little or no significance.

It has been reported that small size aggregates would increase the volume
changes. The drying shrinkage is closely related to the quantity of water and
hence reflect the greater amounts required by the small-size materials

(Czuryskiewicz, 1973).

167

A.3 EFFECT OF PARTICLE SHAPE AND SURFACE TEXTURE

The shape and surface texture of the individual aggregate particles has an
important influence on the workability of freshly mixed concrete and the strength
of the hardened material. Fine aggregate particle shape and texture affect concrete
in one major way (ACI, 197 8). Angular rough sands will require more mixing
water in concrete than rounded smooth fine aggregates to obtain the same level of
slump or workability (thus decreasing both the strength and eventually durability).
This, in turn, will affect the water-cement ratio if the cement content is to be held
constant; or it will require an adjustment in the cement content if a certain water-
cement ratio is needed. The influence of fine aggregate shape and texture on the
strength of hardened concrete is ahnost entirely related to its influence on the
resulting water-cement ratio of concrete if the fine aggregate has a grading within
the normally accepted limits.

Coarse aggregate shape and texture also affects mixing water requirements
and water-cement ratio in a similar manner to fine aggregates. But coarse
aggregate particles, due to their much smaller ratio of surface area to volume,
affect strength through a more complex relationship involving the bond
characteristics to cement paste and concrete water-cement ratio (AC1, 1978). It
has been demonstrated by different researchers that the microcracks between the
paste or mortar and the surfaces of the largest coarse aggregate particles play
critical roles in the response of concrete to bond. Angular rough-textured
aggregates, for example, have an increased surface area for bond to the cement
paste when compared to similar size round particles, and this provides favorable
conditions in concrete.

Considering all of the factors relating to aggregates which have an effect on
concrete strength, the following appear to be most important:

168

i) The water-cement ratio of the concrete as affected by fine and coarse

aggregate shape and texture.

ii) The surface area available for bond to the cement paste; here the shape

and texture of the largest particle is most important.

iii)The surface texture of the largest pieces affect interfacial bond strength.

The mineralogy and crystal structure of these pieces will also affect bond
strength.

iv)As the size of the larger particles is increased then there is more
likelihood of a paste aggregate bond failure since stresses at the interface
will be higher than those for smaller particles.

Factors which give higher intrinsic bond strength are relatively unimportant
in fine aggregates because of the large total surface area available for bond and the
lower stresses around small particles. Likewise, the larger surfaces of angular
sands compared to rounded sands are of no particular benefit to bond strength.
This leads to the conclusion that fine aggregate shape and texture affect the
amount of mixing water required for a given slump level and that the effects of
different fine aggregates on concrete strength can be predicted from a knowledge
of their effects on mixing water and water-cement ratio.

For a coarse aggregate the situation is quite different, and the final effects
on strength are more difficult to predict due to the importance of bond strength
characteristics in the larger particles. This is the fimdamental reason why different
maximum sizes of coarse aggregate, different gradings, and different sources of

coarse aggregate will produce different water-cement ratio vs. strength curves.

169

A.4 EFFECT OF STRENGTH AND MODULUS OF ELASTICITY OF
AGGREGATES

Clearly, the compressive strength of concrete cannot significantly exceed
that of the major part of the aggregate contained, although it is not easy to state
what is the strength of the individual particles. It has been reported that
inadequate strength of aggregate represents a limiting case as the properties of
aggregate have some influence on the strength of concrete even when the
aggregate by itself is strong enough not to fracture prematurely (Neville, 1986). It
has also been reported by different researchers that if we compare concrete made
with different aggregates we can observe that the influence of aggregate on the
strength of concrete is qualitatively the same whatever the mix proportions, and is
the same regardless of whether the concrete is tested in compression or in tension.
It is possible that the influence of aggregate on the strength of concrete is due not
only to the mechanical strength of the aggregate but also, to a considerable degree,
to its absorption and bond characteristics.

In general, the strength and elasticity of aggregate depend on its
composition, texture and structure, as discussed previously. Thus a low strength
may be due to the weakness of constituent grains or the grains may be strong but
not well knit or commented together.

The modulus of elasticity of aggregate is rarely determined; this property
however, can not be neglected as the modulus of elasticity of concrete is generally
increased with increasing modulus of elasticity of the constituent aggregate; the
concrete modulus depends on other factors as well. The modulus of elasticity of
aggregate also affects the magnitude of creep and shrinkage that can be realized by
the concrete. It should be noted that the required strength of aggregate is

considerably higher than the normal range of concrete strengths because the actual

170

stresses at the points of contact of individual particles within the concrete may be
far in excess of the nominal compressive stresses applied. On the other hand,
aggregate with moderate or low strength and modulus of elasticity can be valuable
in preserving the durability of concrete. It has also been reported that the volume
changes of concrete, arising from thermal reasons, lead to a lower stress in the
cement paste when the aggregate is compressible. Thus compressibility of
aggregate would reduce distress in concrete while a strong and rigid aggregate

might lead to cracking of the surrounding paste (Neville, 1986).

A5 EFFECT OF POROSITY AND ABSORPTION OF
AGGREGATES

The porosity of aggregate, its permeability, and absorption, influence such
properties of aggregate as the bond between it and the cement paste, the resistance
of concrete to freezing and thawing, as well as its chemical stability and resistance
to abrasion. The specific gravity of aggregate also depends on its porosity and, as
a consequence, the yield of concrete for a given weight of aggregate is affected.
The pores in aggregate vary in size over a wide range, the largest being large
enough to be seen under a microscope or even with the naked eye, but even the
smallest aggregate pores are generally larger than the gel pores in the cement
paste. It was reported that pores smaller than 4pm are of special interest as they
are generally believed to affect the durability of aggregate under alternating
freezing and thawing (Mehta, 1993).

Some of the aggregate pores are wholly within the solid. Other pores are
open to the surface of the particle. The cement paste, because of its viscosity,

cannot penetrate to a great depth into any but the largest of the aggregate pores, so

171

that it is the gross volume of the particle that is considered solid for the purpose of
calculating the aggregate content in concrete.

Although there is no clear-cut relation between the strength of concrete and
the water absorption of aggregate used, the pores at the surface of the particle
affect the bond between the aggregate and the cement paste and thus exert some
influence on the strength of concrete (Neville, 1986).

It has also been reported that the porosity of the aggregates not only affect
the workability of concrete but also the mixing procedures, pumbability and
finishing. Recycled plastics do not absorb water, thus problems associated with
workability are reduced.

Normally, it is assumed that at the time of setting of concrete the aggregate
is in a saturated and surface-dry condition. If the aggregate is batched in a dry
condition, it is assumed that sufiicient water will be absorbed from the mix to
bring the aggregate to a saturated condition, and this absorbed water is not
included in the net or effective mixing water. It was reported that when dry
aggregate is used, the particles become quickly coated with cement paste, which
prevents further inpress of water necessary for saturation. This is particularly so
with coarse aggregate, where water has further to travel from the surface of the
particle. As a result, the effective water-cement ratio is higher than would be the
case if full absorption of water by the aggregate had been possible. This effect is
significant mainly in rich mixes where rapid coating of aggregate can take place,
while in lean, wet mixes, the saturation of aggregate proceeds undisturbed.

The absorption of water by aggregates also results in some loss of
workability with time, but it has been reported (AC1, 1989) that beyond 15

minutes the loss becomes small.

172

A.6 EFFECT OF BOND OF AGGREGATE

Bond between aggregate and cement paste is an important factor in the
strength of concrete, especially the flexural strength. The full role of bond in
cement is only being realized now. Bond is due, in part, to the interlocking of the
aggregate and the paste owing to the roughness of the surface of the former. A
rougher surface results in a better bond. A better bond is also usually obtained
with softer, porous and mineralogically heterogeneous particles (Nichols et al,
1970)

Generally, texture characteristics which permit no penetration of the surface
of the particles are not conductive to good bond. In addition, bond is affected by
other physical and chemical properties of aggregate, related to its mineralogical
and chemical compositions and to the electrostatic condition of the particle
surface. Due to the hydrophilic nature of recycled plastics, the bond between the
recycled plastics and the cementitious matrix is weak resulting in strength and
durability problems.

A.7 EFFECT OF THERMAL PROPERTIES OF AGGREGATE

There are three thermal properties of aggregate that may be significant in
the performance of concrete: coefficient to thermal expansion, specific heat, and
conductivity. The last two are of importance in mass concrete or where insulation
is required, but not in ordinary structured work.

The coefficient of thermal expansion of aggregate influences the value of
this coefficient for concrete containing the given aggregate: the higher the

coefficient of the aggregate, the higher the coefficient of the concrete, but the latter

173

depends also on the aggregate content in the mix and on the mix proportions in
general.

It was reported that there is another aspect of the problem. It has been
suggested that if the coefficient of thermal expansion of the coarse aggregate and
of the cement differ too much, a large change in temperature may introduce
differential movement and a break in the bond between the aggregate particles and
the surrounding paste. However, possibly because the differential movement is
affected also by other forces, such as those due to shrinkage, a large difference
between the coefficients is not necessarily detrimental when the temperature does
not vary outside the range of 4 to 60 0C (39 to 1400F ), for example. Nevertheless,
when the two coefficients differ by more than 5.5x10'6 / 0C (3x10‘6/ 0F) the
durability of concrete subjected to freezing and thawing may be affected.
Recycled plastics, due to their higher thermal coefficient will cause problems with

durability, specially freezing and thawing and scaling.

A.8 EFFECT OF AGGREGATE PROPERTIES ON

A.8.1. MIXTURE PROPORTIONS

The grading and particle shape of aggregates influence the proportions
needed to obtain workable freshly mixed concrete and at the same time provide
needed hardened concrete properties with reasonable economy. The amount of
water needed to obtain a desired slump or workability depends on the maximum
size of the coarse aggregate, particle shape and texture of both the fine and coarse

aggregates, and particle size range of coarse aggregate.

174

 

Increased angularity and rouglmess of the coarse aggregate can also
increase the mixing water requirement (and needed mortar content) of concrete for
a given level of workability; however, its effect is generally as great as the shape
and texture properties of fine aggregate. Large amounts of flat and elongated
pieces of aggregate in concrete can make it too harsh for some placement methods

resulting in voids, honeycombing, or pump blockage.

A.8.2. FRESH MIX PROPERTIES
Slump and Workability

The strength, appearance, permeability, and general serviceability of
concrete is dependent on the effective placement and consolidation of freshly
mixed concrete without undesirable voids and honeycombing. It must be workable
enough for a given formwork, reinforcement spacing, placement procedure, and
consolidation technique to completely fill spaces around the reinforcement and
flow into corners. Aggregate grading, and in particular the maximum aggregate
size, as mentioned earlier, controls the amount of paste in a properly designed mix
and, therefore, has an important influence.

The effect of aggregate on the cohesive properties of a concrete
mixture depends on factors such as the maximum size of the coarse aggregate, if
larger than 10 m (3/8 in), the overall combined grading fine and coarse (and
percentage of sand on the basis of total aggregate), and the amount of clay size
fines present.

Another source of slump loss, as mentioned earlier, is the absorption of

mixing water by the aggregates.

175

 

A.8.3. SHRINKAGE

The shrinkage of concrete is less than that of neat cement owing to the
restraining influence of the aggregate and may be one-fifth to one-tenth, or even
less, of that of neat cement. The aggregate is surrounded by cement paste which,
in shrinking, places the aggregate under compression and itself becomes subjected
to tensile forces. These tensile forces may be greater than the strength of the
paste, in which case cracking will occur and shrinkage measurement will be
unreliable. From this reasoning it would be expected that aggregate with high
modulus of elasticity would give a concrete with less shrinkage than aggregate
with a low modulus of elasticity. Table A] shows the relationship between drying
shrinkage and elastic modulus.

TABLE A.l Relationship Between Drying Shrinkage and Elastic Modulus
(Orchard, 1973)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Aggregate Type Elastic Modulus, Absorption, % Drying
x106 psi by Volume Shrinkage at 1 yr

x 10"6
Basalt 13.63 3.3 300
Rounded Quartz . 12.27 4.7 180
Crushed Quartz 3.37 6.6 330
Marble 6.61 8.0 250
Granite , 6.18 5.5 290
River Gravel Mixed 5.60 3 .2 280
Calcareous Sandstone 2.80 9.7 1020
Ferruginous Sandstone 1.37 13.6 630

 

It has been reported by that the differences in shrinkage cannot be explained
entirely by differences in the mineralogical composition of the aggregate and the
anomalies may be due to cracking of the mortar paste; thus the shrinkage of the

176

 

concrete using the calcareous sandstone is much greater than that of the concrete
using the ferrugunous sandstone in spite of the fact that the ferruginous sandstone
has a lower modulus of elasticity (Orchard, 1973) .

The shrinkage of concrete is largely governed by the compressibility of the
aggregate and its own shrinkage properties on drying. Carlson (193 8) made some
concrete using rubber as an aggregate. This concrete contracted almost as much as
the corresponding neat cement paste and about 8 times as much as ordinary
concrete.

It has been reported by several researchers that aggregate shape has little
effect on shrinkage, except in so far as it affects the amount of mixing water
required to maintain workability.

It has been shown by Carlson (193 8) that below a No. 4 sieve the aggregate
size had little effect on shrinkage; the abrupt reduction in shrinkage with aggregate
above No. 4 sieve size indicates, however, that cracking of the cement paste
probably occurred in these regions and the cement and aggregate were both of a
type likely to cause cracking . The aggregate grading, and in particular the
maximum aggregate size, as mentioned earlier, controls the amount of paste in a
properly designed mix and, therefore, as the paste has an important influence it
would be expected that the minimum drying shrinkage would be obtained with the
concrete using an aggregate grading promoting the minimum amount of paste. In
general, the shrinkage reduced considerably as the aggregate size is increased. It is
also known that the reduction in shrinkage is obtained by using a large maximum
size of aggregate on account of the reduction in the water-cement ratio which can

be effected and due to the increased likelihood of cracking of the cement paste.

177

A.8.4 CREEP

It is reasonable to suppose that if creep occurs in the mortar paste the
progressive effect will be to transfer more and more applied load on to the
aggregate and further deformation will then be governed by creep of the aggregate
particles.

Orchard (1973) reported that the effect of modulus of elasticity of the
aggregate does not appear to follow a regular law and variability in the test results
may be due to the effect of breakdown of the bond between the mortar paste and
the aggregate with consequent internal cracking. There is little difference in the
creep of concrete made with sharp or with rounded aggregate, but concrete made
with porous aggregate generally has a grater creep.

It has been reported that there appears to be considerable conflict on the
effect of the water absorption of the aggregate and its modulus of elasticity on the

amount of creep.

A.8.5. DURABILITY

3) Resistance to Freezing and Thawing:

Concrete containing frost-resistance paste may not be totally resistant to
freezing and thawing if it contains aggregate particles which became critically
saturated. An aggregate particle is considered to be critically saturated when there
is insufficient unfilled pore space to accommodate the expansion of water which
accompanies freezing. It has been reported (ACI, 1989) that there is a critical
particle size above which the particle will fail under repeated fi'eezing-thawing

178

cycles of critically saturated aggregate. This size is dependent on pore structure,
permeability, and tensile strength of the particle.

It was also reported that experience has yet to show that fine aggregates are
directly associated with freezing-thawing deterioration of concrete. Some porous
coarse aggregates can, on the other hand, cause deterioration of concrete due to
freezing (Spitzner, 1989).

Various properties related to the pore structure within the aggregate
particles, such as absorption, porosity, pore size, and pore distribution, or
permeability, may be indicators of potential durability problems when used in
concrete which will become saturated and freeze in service. Generally, it is
absorption values, caused principally by medium-sized pore spaces in the range of
0.1 to 0.5m, that are most easily saturated and contribute to deterioration of

concrete.

b) Permeability

Theoretically, the introduction of aggregate particles of low permeability
into a cement paste is expected to reduce the permeability of the system because
the aggregate particles should intercept the channels of flow within the cement
paste matrix. Test data, however, show that this is not the case. Figure A.2 shows
that the addition of aggregate to a cement paste or a mortar increases the
permeability considerably; in fact the larger aggregate size, the greater the

coefficient of permeability.

179

 

 

50.000

3
o
T

        
 

 

 

 

 

 

 

22'

8
c § 20.000
9 MaxAggregate (g E, i0.000 I00
@120 —- 4-I/2In.— -; .
x 3m _ 33 7, cf 5.000 50
.: I00 ""2” 4 so e 0? B 30
c t— \ .. x 2.000 20 m
'2 -1 27 E 5.0 \
*- 80 - — 24 = o o _o
O; '0 U - 500 5 T
O — 2| .. . s x o
U x 8 Q 3 __
>. 60 *- * I8 0 ...- x0 200 ><
.- .K 0- U
: MOTIOI -‘ 15 8 '00 I x
3 40 No.4 I2 0
° 7 3: 50
E r 9 E 2
3 20 a e ‘5 'E 20 —
a o :3

‘1 3 E ..s I0 9
0.4 0.5 0.6 0.7 0.8 0.9 0.4 0.5 0.6 0.7 0.80.9 I.0
Water /Cement Ratio Water/Cement Ratio

FIGURE A.2 Influence of Maximum Aggregate Size and Water-Cement Ratio on
Concrete Permeability (Mehta, 1993)

The explansion as to why the permeability of mortar or concrete is higher
than the permeability of the corresponding cement paste lies in microcracks that
are present in the transition zone between the aggregate and the cement paste. It is
known that the aggregate size and grading affect the bleeding characteristics of a
concrete mixture which, in turn, influences the strength of the transition zone.

Since strength and permeability are related to each other through the
capillary pores, thus the factors influencing the strength will influence the
permeability. As far as aggregates are concerned, the aggregate size and grading,
thermal and drying shrinkage strains, and aggregate permeability will influence the

permeability characteristics of concrete.

180

c) Fire Resistance

The fire resistance of concrete depends largely on the type of aggregate;
aggregates which have been subjected to heat during their formation or
manufacture being the best and siliceous aggregates the poorest.

For this purpose, the Building Regulations deviled the aggregates into two
classes:

Class I: Foamed slag, pumice, blast-fumace slag, pelleted fly
ash, crushed brick and burnt clay products (excluding
expanded clay), well burnt clinker and crushed lime

stone;

Class H: Flint-gravel, granite, and all crushed natural stones

other than limestone.

Expanded clay aggregates and other lightweight aggregates prepared by
sintering would presumably come with Class I.

The failure of concrete under the action of fire is due to differential
expansion between the hot surface layers and the cooler concrete behind, and to
the opposing actions of the cement which shrinks owing to the loss of moisture to
a greater extend than it expands due to the rise in temperature and of the aggregate

which expands continuously with the rise in temperature. These phenomena lead

 

to cracking and sapling and in the case of reinforced concrete to the exposure of
the reinforcement to the fire. Once the reinforcement is exposed, it conducts the
heat rapidly and accelerates the effects of unequal expansion. Flints, siliceous

gravels, and granites are perhaps the worst aggregates and concrete having these

aggregates does not offer high resistance to fire.

181

In reinforced concrete an important factor is the thermal transmittance of
the concrete and in this respect the lightweight aggregate concretes made with
foamed slag, pumice, expanded clays, and sintered materials offer the best

protection.

c) Resistance to Wetting and Drying

The influence of aggregate or durability of concrete subjected to wetting
and drying is also controlled by the pore structure of the aggregate. Difl‘erential
swelling accompanying moisture gain of an aggregate particle with a fine-textured
pore system may be sufficient to cause failure of the surrounding paste and results
in the development of a popout. The amount of stress developed is proportional to
the modulus of elasticity of the aggregate. In some cases this may be a
contributing factor to popouts.

(1) Resistance to Heating and Cooling:

Heating and cooling induce stresses in any non homogeneous material. If
the temperature range is great, damage may result. For aggregates commonly
used, and for temperature changes ordinarily encountered, this is not usually a
critical factor in concrete. However, it has been reported that there has been
speculation that large differences in the coefficient of expansion or thermal
diffusing between the paste and the aggregate can result in damaging stresses in

concrete subject to normal temperature change.

182

e) Abrasion Resistance:

Abrasion resistance and localized impact resistance of concrete is a
property which is highly dependent on the quality of both the cement paste and the
aggregate at and near the surface receiving localized impact and abrasive stresses.
In those cases where the depth of wear is not great, there will be little exposure of
coarse aggregate, and only the presence of hard and strong fine aggregate in good

quality cement paste may be necessary to provide needed surface toughness.

183

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188

 

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