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AN ANALYSIS OF BOND STRESS
BETWEEN HI-BOND BARS AND CONCRETE

Thesis for tho Degm of B. S.
MICHIGAN STATE COLLEGE
W. J. Cofiron - C. E. BM

1949

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An Analysis of Bond Stress Between.

Hi-Bond Bars and Concrete

A Thesis Submitted to
The Faculty of
MICHIGAN STATE COLLEGE
of
AGRICULTURE AHD APPLIED SCIEHCE

b“!

W. J. ggffron Co E. Bauer

Candidates for the Dagree of

Bachelor of Science

Juno 1 9 49

 

DIED IS A“? 1’. ON
we wish to dedicate this thesis to the

Inland Steel Company. Their interest and couperetion

in furnishing the material made this project possible.

216949

ZE-I'I‘HODUC‘I'IOI'I

When a reinforcing rod is embedded in concrete, the letter
adheres to its surface. resisting any force that tends to
pull or push out the red. This is called the "bond" between
the concrete and the steel. The intensity of this adhesive
force is called the "bond stress." In rceli y. this bond
stress is a resistance to shearing between the surfaces of
the steel and the concrete. The action is that of resistance
to forces that try to break the concrete away from the sur-
face of the steel in a direction parallel to that surface.

The function of bond in a reinforced concrete member
is the same as that of rivets in structural steeleork. It
is the force that holds the two materials together so as to
deveIOp their simultaneous and mutually helpful action. If
the rods have no chanse of stress ~ end therefore no change
of length - as a result of the application of a load on the
member. then there will be no bond stress set up by the load.
However. as soon as flexural action causes the steel to st-
retch or compress, the bond stresses must come into action
in order to cause these changes.

When a rod as in plate I figure I is stretched, the elon-
gation is greatest at the point where the steel enters the
concrete. It then decreases to zero at somewhere below the
top end of the red. It follows that the intensities of the
bond stresses alone the rod must vary somewhat in proportion
to the stretching of the rod inside the cancrete block.
Probably the bond stresses are very high near the point at

which the rod enters the concrete. The distribution of the

stresses is uncertain. For design purposes, it is assumed

to be uniform. It should be noted, however, that the bond

will develop the rod as quickly as possible so that a part

of the rod which is a long way from the point of entry of

the rod may have no stress at all. In other words, anchorage

far from the p int where the rod is needed may be ineffective.
An eXpression for the magnitude of the bond stress can

be derived as follows. Referring to plate I figure I, let

i.‘ the length of embedment of the bar;.let "u" t the aver-

ane unit bond stress; let d = the diameter of the bar; and

let fs = the stress in the steel. fhen according to statics

“miss/4 :; 1rd"u"L or L: era /4"u"

The above formula applys also for square bars. It
shows that the length of imbedment needed to develop the
strength of the bar in tension or compression increases
with the strength of the steel and decreases with an in-
crease in the magnitude of the permissible bond stress.

The bond stress alone the reinforcement can be increas-
ed by making the surface of a rod rough or irregular. Such
rods are called “deformed rods". The lugs, or corrugations,
on the bars produce a mechanical bond which helps to lock
the concrete and steel together. These lugs are generally
some form of a raised rib which extends directly or diag-
onally around the rod. }

In many cases, it is not possible to extend straight

rods for enough to develop their strength sufficiently by

5.

bond alone. A common way to remedy this trouble is to hook
the rods so as to obtain additional length for their devel-
opment through bond. when a hooked rod is pulled, it tries
to slip or slide around the curve. The hook provides a
certain amount of mechanical locking of the steel into the
concrete, but this is too indefinite to be relied on.

The tests described in this thesis were made to attempt
to substantiate the claims of the Inland Steel Company that
the newly designed Ui-Bond Ber could develoo the same bond
stress that could be obtained with a standard deformed bar
with a hooked end. To determine this,pullout tests from
concrete were make. Hi-Bond bars were tested using as con-
trols both straight standard deformed hers and standard
deformed bare with hooked ends. The variables in the in-
vestigstion were the kind and size of bar. the length of
inbedment and the strength of the concrete mix. The max-
imum narce required to cause the bars to pull out was observ-

ed;
3.! ATEI-i I ALE;

Reinforcing Bars
4 - é“ die. Hi-Bond bars. straight

5 . 3/4" dis. Hi-Bond bars, straight
. é" din. straight deformed bars

- 3/!" die. straight deformed bars

k” die. deformed bars with standard hook

chibtb
I

- 5/3" dia. deformed bar with standard hook.

4.

Gene 11 t
The cement used was standard portland cement with spec-

ific gravity of 3.15.

Aggregates

Coarse gravel - maximum size 1%" in diameter. Specific
gravity of 2.55.

Sand - River sand of medium fineness and grading was used.

The specific gravity was 2.65 (saturated, surface dry).

water

East Lansing City water.

Concrete

TIC different strength concretes were used. One ml:
was designed for 2800 p.s.i. in compression and the other
mix was designed for 4000 p.s.i. in compression. All of
the concrete was mixed by hand in the mixing trough. A
slump of 2 to 4 inches was allowed. Standard cylinders
6” in diameter by 12" in height. each placed and rodded in
3 layers. were cast for each batch or concrete. These cyl-
inders were cured in the damp room along with the specimens
and were tested in compression after 28 days. The average
28 day compressive strength of the 2300 p.s.i. concrete was
1860 p.s.i.. The average 28 day compressive strength of the
4000 p.s.i. concrete was 3515 p.s.i.. The weight of the
fresh concrete was calculated from the weight and absolute
volumes of the ingredients. The weights obtained in this

manner were 151 pounds per'cubic foot for the 2300 p.s.1.

5.

concrete and 149 pounds per cubic foot for the 4000 p.s.i.
concrete. The percentage of sand for both concrete mixes
was 57 %. by absolute volume. or the tatal aggregate. The

water cement ratio was .67 for the 2300 p.s.i. concrete and
.53 for the 4000 p.s.i. concrete. The following table shows

the particulars of both concrete designs.

Tflflel

 

height of materials per Cubic Yard of Concrete

 

 

 

 

Enterials 2300 p.s.i. 4000 p.s.i.
*_poncrete _goncrete#‘

water 280 lbs. 230 lbs.

cement 419 lbs. 528 lbs.

land 1245 lbs. 1180 lbs.

coarse agiregste 2140 lbs. 2050 lbs.
spscxssss

Straight Bars
Both i” and 3/4” round bars were used. The fi' round
bars were imbedded 6%" in concrete cylinders 6" in diameter
by 12" in height. The 3/4" diameter bars were imbedded in
the same size cylinders to a depth of 8%" with the exception
of one Hi-Bond bar which was imbedded 11%”.
Hooked Bars

Stlndard books were used as shown in plate I figure 2.
The é” bars were imbedded a total of 12" in 9"XL11"X.11'
concrete specimens. The 5/!" bar was imbedded a total of

15%" in a 11”x.12"x ll” concrete specimen.

6.

ASSEMBLY AND CONSTRUCTICN OF SPHCIEiWS

The specimens were constructed in the College Concrete
Laboratory. All concrete was mixed by hand in the mixing
troughs. The fresh concrete was placed in the containers
in 5 increments. Each increment was roddsd 20 times. The
bars were then imbedded in the concrete to the desired depth.
This was accomplished by cutting holes in cardboard. wrapping
rubber bands around the here just above the desired level
of imbedment. inserting the here through the hole in the
cardboard. and setti a the rod and cardboard on the container
in the specified position. Each container was then tapped
lightly several times to insure consolidation of the concrete
around the bars. The specimens were allowed to set for one
day in the laboratory. They were then taken to the damp
room, where they were cured for 26 days.

TESTIHG THE 3 3013338

On the 27th. day the specimens were removed from the
damp room and allowed to dry. After they had dried, they
were capped with plaster-of~paris to secure an even bearing
surface. This was accomplished by boring a 7/0" hole in
a smooth board, placing the plaster-of-paris on the surface
of the specimen, and applying pressure on the surface with
the board. This method produced quite even bearing surfaces.

0n the 28th. day the specimens were tested. The machine
used for testing was the Universal testing machine as shown
in plate 2. This machine consists of a mavable head whi h
Operates up and down inside a steel frame that sets on a

platform. This platform is in turn connected to a system

7.

of levers which are arranged so that the amount of force
exerted by the machine during testing can be measured on
the beam balance.

is is shown in plate 2. each concrete specimen was pla-
ced in an inverted position on top of the steel bearing plate
with the steel bar extending down through a drilled hole in
the bearing plate. The steel bar was then connected to the
movable head by s set of grips. Tension was then applied by
running the movable head down at a speed of .06" per min..
keeping the bean balanced at all times. The maximum force
required to cause failure of the specimen was in this way

determined and recorded.

TEST DATA
The data for the tests is recorded in table 2. This
table indicates the actual compressive strength (p.s.i.) of
the concrete, the type of bar imbedded in the concrete. the
length of imbedment of the bar, the actual bond stress at the
time of failure of the sample, and the type of failure of the

i

specimen.
DISCUSSION of the TEST DATA

The compression results obtained from testing the com-
pression specimens showed that the actual compressive stren-
gths were smaller than what we had designed forl The ever.
one of the 2300p.s.i. specimens tested was 1860 p.s.i. at

3, While the average at 28 days for the 4000 p.s.i.
28 days

8.

specimens was 5515 p.s.i.. This failure of the concrete to
come up to design specifications did not matter since we
were interested in the compressive strength only as an indi-
cation of the strength of the concrete.

It should be noted teat not all succimens failed in
bond. Twenty- two specimens were tested. Sixteen of these
failed 5n bond. Three failed by the concrete block splitting
and three failed bv the steel pulling in two. Of the five
specimens containing s+andard hooks, none failed in bond.

In three, the concrete sclit along the plane the hook was
imbedded in, and in two, the steel bar snapped.

The formula used in Table 2. column 6, in computing
the actual bond stress at the time of failure of the spec-
imen is the one derived in the introduction ---

"u" : P/(fid)L-. where "u" g the bond stress (Pos.io).

P 3 the maximum load sustained by the sample, (we) 3 the
circumference of the bar, and L: the length of imbedment
of the bar.

In Table 2. column 7. we have included the recommended
working stresses in bond for deformed bars as computed from
the formulas given in the 1940 Report of the Joint Committee
on Standard Specifications for Concrete and Heinforced Con-
crete. These recommendations are: .05f; (f; a compressive
strength of concrete) for deformed bars, but not to exceed
200 poeoio. and .075f; flar bars with hooked ends. but not to
exceed 250 p.s.i. . Those recommended working stresses. when

compared with the ultimate bond stresses obtained by our

experiments. seem quite conservative. Safety factors of 5

to 6 are indicated. However. those working stresses are in-
tended for construction conditions which are not as favorable
as the ones we worked under. such as individual consolidation
of the specimens, curing in the damp room, etc.. Allowance
apparently is made for such factors as failure of the con-
crete to fill in around the bars, exposure of reinforcing
bars, faulty placing of bars. etc..

In Table 2, column 8, the tyne of failure of each spec-
imen has been listed. he trouble was experienced in pulling
the é" straight bars. Examination of the concrete cylinders
showed no traces of radial cracks outward from the center.
However, examination of cylinders which contained 3/4" round
straignt bars showed that in failing cracks had proceeded
from the center to the outside of the cylinders. This in-
dicates that the 3/4" round bars should have been imbedded
in larger blocks. Doing this would probably eliminate the
radial cracks from the center to the outside of the block.

As has been stated before. 5 of the 5 hooked specimens
failed by the concrete block splitting along the plans of the
hook. Ezemination of these blocks showed that the hooks
had not started to pull. This splitting of the blocks could
probably be eliminated by imbedding the hooked bars in larger
blocks than those used in our experiments.

A comparison of tee actual bond stresses (col. 6. Table 2)

at the time of failure of the specimens shows that the

Hi-Bond bars have a definite edge in band. This is to be
expected since the Hi-Bond bars are designed to produce
higher bond values. This hi her bond design consists of
larger lugs on the bar thereby increasing the bearing area
per lineal inch of the bar. Comparisons of %" round Hi-Bond
and %" round deformed bars imbedded in 1375 p.s.i. concrete
show that the Hi-Bond bars developed an average of 955 p.s.i.
of bond and that the deformed bars developed an average of
795 p.s.i. of bond. The_3/4" round Hi-Bond bars imbeddod in
1375 p.I.io concrete developed an average of $05 p.s.io of
bond while 3/4" round deformed bars,devoloped an average of
522 p.s.i. of bond. No comparisons could be made with the

hooked bars since they did not fail in bond.

SEEZMARY and COHC LUS I AIS

Pullout tests were made on anchorage specimens consist-
ing of straight and hooked deformed bars. and of straight
HinBond bars. of s" and 3/4" iameter embedded in 1875 p.s.i.
ant 3515 p.s.i. concrete. These specimens were assembled
and allowed to cure in the damp room for 28 days. After this.
they were tested on the Universal Testing Machine to deter-
mine the maximum bond between the concrete and the bars.
Twenty two specimens in all were tested.

or the five hooked specimens, two failed by the bar
snapping, and three failed by the concrete blocks splitting
along the plane of the hook. With the exception of one rod

that snapped, all of the straight bar specimens failed in bond.

11.

It was noticed on pulling out the Hi-Bond bars that the con-

crete between the lugs at the pull out ends had failed in

sheer. All results of the tests are contained in Table 2.
The following conclusions were reached:

1. The HicBond bars deveIOped more bond per square inch
than the ordinary deformed bars. This is a result of
the greater bearing area per square inch of the lugs
on the bar.

2. The hooked specimens should be imbedded in larger con-
crete blocks than we used to prevent splitting of the
blocks along the plane of the hook.

3. The 3/4" round straight bars should also be imbedded in
larger blocks than the ones we used - 6" round cylinders.

12" high - to prevent radial cracks in the concrete.

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