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A STUDY’OF COIHERCIAL CENTRAL CONCRETE IIXIHG PLANTS
with which ie combined
A] ORIGINALIRESEARCH OF SHRINKAGE 0F CONCRETE DUE TO
DELAYED PLA (3133 “D BETEMPERIIIG

A theeie Submitted to
The Faculty of

nihhigea BteCe College
of

Agriculture end Applied Science

By

Glenn J. Egprevy

Candidete for the Degree of Bachelor

of Science

June 1, 1939

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Part I

In line with the tendency of the present age toward
centralization of industries is the developement and growing
use of the central mixing plant for the preparation of con-
crete. Almost all manufactured commodities are now produced
in large units and it is to be expected that the production
of concrete in central plants will be the natural result of
the economic growth of the nation.

Such is in fact proving to be the case. There are now
in use around one hundred.plants selling centrally mixed
concrete in this country. Three new ones are soon to be built
in liohigan.

There are many advantages to be gained from the useidf
centrally mixed concrete. One of these is the economy effected.
The small contractor is not compelled to tie up capital in
mixing equipment of his own. He is saved the cost of install-
ing, Operating, dismantling and moving this equipment. He is
able to concentrate his attention on the real problems of
construction such as excacation and placing forms. He is saved
’expensive waits and delaYs for want of concrete materials.

On some Jobs it is difficult to find space available for
the storage of concrete materials. Here again the central
.plant furnishes a solution to the problem as the concrete
can be brought up in trucks when needed and no storage is
necessary on the job. The contractor is saved the expense
of possible spoilage and theft of cement.

102115

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One of the greatest advantages to be claimed for the
central mixing plant is the ease of controlling the quality
of the output. Engineers and.architects are now coming to
realise the importance of such things as the ratio of water
to cement, grading of aggregates, freedom of aggregates
from dirt, and quality of mixing water as determining the
strength to be expected from the concrete. By using the
central mixing plant these factors can be easily controlled.
One inspector stationed at the plant can be made responsible
for the mix. The mixing is done in a large up to date mixer
which can be operated under constant conditions. The aggregates
can be stored in ample stock piles and.proper precautions can
be taken to keep them free from dirt. Moisture content and
size of aggregate can be easily watched and controlled. The
mixing water can usually be secured from the city supply,
thus insuring purity. The cement can be purchased in bulk
and stored in a specially constructed silo. Unloading of
cars is accomplished by means of a pump which has recently
been placed on the market for this purpose. The use of bulk
cement effects a great saving for it saves the expense of
caring for and return of sacks

. In cold weather the central mixing plant is in a good
position to heat its aggregates and.mixing water and thus
furnish warm concrete to the user. This is important for
many engineers are now demanding that the temperature of
the concrete at the time of placing shall not be less than
seventy degrees in cold weather. Experiment has shown that

the loss of heat in transporting the concrete from the plant

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to the job is slight for the usual hauls.

There is, of course, a limit for which the purchase
of centrally mined concrete is economical. The plant must
make a profit if it is to continue in business. If the size
of the job is such that the contractor is able to mix his
concrete cheaper than he can buy it he will naturally mix
his own. It has been estimated that on Jobs using more than
two thousand.five hundred cubic yards of concrete it will be
cheaper for the contractor to do his own mixing. Of course
this would depend altogether on local conditions such as the
price of labor, materials, equipment and haulage. The con-
tractor should carefully consider the problem from all angles
in deciding which is the cheaper method. Well managed
central mixing plants will have one or more sales engineers
who will assist the contractor in computing his costs.

luch experimental work has been done in order to deter-
mine the actual quality of the concrete resulting from
central mixing. ' Tests of concrete specimens, molded at
various periods after the concrete was mixed furnish ample
evidence to show that considerable latitude may be permitted
in the time elapsed between mixing and depositing the concrete
without damaging the strength. Records are available which
show practically no reduction, and in some cases an increase
in strength of concrete hauled from two to three hours in a
truck equipped with an agitating device.“ Taken from "Facts
and Figures on Central nixed Concrete Industry" furnished by

the Slaw—Knox'ccmpany-

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However little was known of the effects of delayed
pouring and retempering upon shrinkage. Since shrinkage is a
very important consideration in the design of structures it
was suggested that a test be run on blocks which had been
subjected to the above conditions. These tests were made
and are the subject of the second part of the present paper.

The equipment of the central mixing plant includes a set
of gravity bins for supplying the aggregates to the mixer.

The aggregates are fed from the bins to a weighing or meas-
uring device and from there to the mixer. The moisture content
of the fine aggregate is subject to quite wide variation and
since the moisture greatlyaffects the volume it is essential
that the moisture be accurately controlled. This is accomplish-
ed in the Blew—Knox innundation system by completely innundat-
ing the sand.in the measuring hopper. Since innundated sand
occupies the same space as dry sand rodded, the bulking effect
is offset. The moisture of the sand is made constant for the
water contained in the the innundated sand.ie always the

same. The moisture contained in the coarse aggregate is not
subject to such great variation and can be determined.with
sufficient accuracy by drying an occasional sample. The
~extraiwater needed by the mix beyond that furnished by the
innundation of the sand.and that contained by the coarse
aggregate is supplied by an excess water tank. This amount
when once determined can be held constant by an adjustment

on the excess water tank.

In other systems the quantities of ingredients for the

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mix is regulated by means of weighing devices. The cement is
fed to the weighing hepper by means of a screw conveyor.

The sizes of mixer used are one yard, one and one half
yards, two yards and three yards. then more than a three yard
capacity is necessary two or more three yard.mixers are pro-
vided. Separate bins and.measuring devices are provided for
each mixer.

Some means of conveying the aggregates from the stock
piles to the bins must be arranged for. Either a crane with
clam shell bucket or a mechanical conveyor with buckets is
used for this;purpose. Where ever possible gravity is made
to aid in the handling of materials.

The central mixing plant should also provide means for
the testing of aggregates, cement and concrete. The actual
breaking of test cylinders can usually be done in a commer—
cial laboratory.

From fifty to eighty five per cent of the cost of the
central mixing plant is involved in hauling equipment. Trucks
with special bodies are used. Ordinary dump bodies have been.
tried, but where a wet mix is desired or where the haul is
moderately long segregation results and remixing at the Job
is necessary.

_ One type of truck body is equipped with baffles thru
which the concrete runs as it is dumped, the idea being to
Obtain a remix thereby. Another type has a cylindrical body
which is rotated.in transit. One type is equipped with an
agitator driven by the motor of the truck. The agitator has
blades driven from a horizontal shaft parallel to the length
0f the truck. Many advantages are claimed for this last type

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of body among them being ease of control of the agitator by

the driver from the cab, non dependence of the agitator upon
speed of the truck, small amount of extra power necessary and
adaptability of the body to different makes of trucks. lith

this particular body dumping may be accomplished from either

the side of the truck or from the rear end, as the conditions

on the job may requime. The body of the truck can also be raised
and the concrete dumped from a high position if such is desir-
able.

Quite a different method of handling centrally mixed
concrete consists in the use of mixer trucks. In this method
a central batching plant is used much resembling the one
already described except that the mixer instead of being
a part of the central plant is mounted on a truck. The mat-
erials are mixed in transit between the batching plant and
the job. As more than one truck mixer is generally needed
it is easy to see that the cost of equipment in this meth-
od is higher than in the first method. The extra cost has
been estimated by one authority to be fifty per cent.

The truck mixer method also places the control of
the mixing operation in the hands of the truck driver who
may or may not give it the necessary attention. Indeed it

has been suggested that in localities where labor unions
are strong it might become necessary to furnish a mixer

Operator with each truck in addition to the regular truck

driver.
It is not likely that the truck mixer would furnish

as homogeneous a mix as the large mixer located at the central

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‘plant. The mixer truck does, however, completely prevent
segregation.

In considering who is best fitted to enter into the
business of operating a commercial central mixing plant,
the materials dealer seems to be the logical choice. He is
already equipped.with a yard, a fleet of trucks and a sids
ing. He is well acquainted with the source of supply for
materials. He also has a clientelle established.among con-
tracecre and.other possible customers for centrally'mixed
concrete. He is able to shift a part of his help to the

operation of the plant.
The contrwtor fihO 1. Cmipp0d '1th I. yard 1. 31.0 I.

possible entrant into the field. While he has trucks and
other necessary equipment he will encounter greater sales
resistance in selling to other contractor than will the
materials dealer.

. Quite a number of commercial central mixing plants
are Operated by companies specifically organidhd.for the
purpose.

The minimum necessary capital for a company engaged
in this business seems to be around.870, 000. One company
started in with $20, 000 but additional capital was soon
necessary. ‘

A plant capable of turning out a normal output of 300
cubic yards per day with a peek output of 300 yards will
cost about $73, 000.

The equipment will consist of the following items:

l 110 tan: 3 compartment, all steel bin,

1 1 cubic yard concrete mixer,

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Special water measuring outfit,

Triple weighing batcher for two aggregates and.cement,

Bucket elevator for cement,

Bucket elevator for aggregates,

Cement house, two car capacity,

8 2 yard trucks with agitator- high lift bodies.

lhile the above equipment costs but 872, 000 it is
evident that for a new concern to get started in business
considerably more capital would be necessary. A site must
be provided with a siding available. A considerable amount
of working capital would be needed.to pay for materials and
labor. Contractors ordinarily require credit accommodation
to quite an extent.

A plant with a normal output of 900 yards with.a.peak
output of 600 yards would require the following equipment
costing 8144, 000:

l 3.compartment all steel bin of 300 tons capacity,

1 130 ton all steel bin for cement,

Triple 'Oighing batcher for aggregates,

Cement weighing batcher,

Bucket elevator for cement,

Bucket elevator for aggregates,

Cement house, 3 car capacity,

16 3 yard trucks with agitator high lift bodies.

Prices charged for concrete range from 88. 00 a yard
toSlz.OO a yard delivered at the job for a l; 3; 4 mix. The
prices charged by plants situated in different parts of the
country differ greatly. However a net profit of about 33. 00

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per yard seems to be about the usual profit aimed at.

0n the above basis a central mixing plant should prove
to be a very profitable investment. It mist be remembered,
however, that the business is still new and as more plants
are installed and competition becomes leaner, profit margins
will be gradually squeezed down. liven so the fairly large
amount of capital necessarily involved in the business will
keep the growth of the business within reasonable bounds.

The successful plant will be the one which is able to
sell the idea of using centrally mixed concrete. The idea
has not yet been accepted by the majority of engineers and
architects and these men must be sold the idea if the new

method comes into wide use.

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Part II

While several experiments have been made with the view
of finding out the effects of delayed pouring and retempering
on the strength of concrete, little has been done to determine
by experiment the effects of these conditions upnn shrinkage.
Since the shrinkage of concrete is a very important considr
eraticn in the design of structures, it was suggested that
a test he run on blocks which had been subjected to these
conditions. These tests were made and are the subject of the
present and.final part of this paper.

It was decided that a rioh.mix would be the best one to
use for the tests, since the rich mix contains a relatively
large percentage of cement and.water paste. As the aggregates
do not shrink, and shrinkage that occurs in concrete is due to
contraction of the water and.cement paste. Therefore a concrete
in which the proportion of paste to aggregates was high would
give the maximum and.mcst noticable shrinkage.

A concrete was designed for a compressive strength of
3000 pounde;per square inch at 38 days.

The first step in design was the determination of the
fineness modulii of the aggregates both fine and coarse. The
ordinary materials as furnished in the laboratory were used.
The weights of material retained on the various sieves after
15 minutes of shaking in the Rotap machine are given in the
table below.

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Coarse aggregate

Sand

 

Sieve Retained per cent cum.% retained per cent cut-fl:
pan 5.0 .100 99.765 50.1 6.020
100 0.0 .000 99.665 151.5 26.260 ‘ 95.09
50 0.1 .005 99.665 214.1 42.920 66.92
50 0.9 .050 99.662 65.5 12.700 24.00
16 4.2 .140 99.652 46.0 9.200 11.50
9 29.4 .946 99.492 7.5 1.500 2.10
400.0 15.555 99.546 5.1 .600 .60
5/9 1272.5 42.146 95.215 0.0 .000 .00
5/4 905.0 26.955' 443.067 0.0 .000 .00
1 570.7 12.567 16.254 0.0 .000 .00
1 1/2 110.0 5.667 5.667 0.0 .000 .00
2 0.0 .0 .0 0.0 .000 .00
5 0.0 .0 .0 0.0 -.000 .00
Totals 2994.9 744.945 197.90

The fineness modulus of the coarse aggregate was thus

found to be 7.45 and that of the sand 1.98. The maximum size

of the coarse aggregate was 1} inches.

From Pref. Abrams B curve a strength of 3000 pounds per

square inch was found to require a water cement ratio of .7.

The slump-mix diagram gave a real mix of l: 2.8 and a fine-

nees modulus of the mixed aggregate of 5.6 using a slump of
from 6 to 7 inches.

Using the formula r equals w gave r equal to 26.5.
m u
where r equals the ratio ofcvolze of fine aggregate to

sum of the volumes of fine and coarse

aggregates measured separately;

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m equals the modulus of the mixed aggregate,

mf' equals the modulus of fine aggregate and

mo equals the modulus of the coarse aggregate.
The unit weights of the aggregates were then found to

be as given below.

wt. 1 cu. ft. damp loose 933415.. 184.58%?)
wt. 1 cu. ft. dry rodded 108.1 ' 110.0 '
wt. of dry material in 1 cu. ft. damp loose98.4 ' 104.0 '
wt. of water in 1 cu. ft. of damp loose 1.0 ' .5 '

The weight of a cubic foot of aggregate dry rodded and
mixed in the volumetric proportions 86.5% sand and 73.5 coarse
aggregate was found to be 125.50 pounds. The weight of a cubic
foot of these aggregates in the same proportions but unmixed

would be 109.49 pounds.
The mix was then designed according to the form given

below. ,

wt. 1. cu. ft. damp loose 93:41]”, 102:5a§%;,
' ' ' ' dry rodded ~ 108.1 ' 110.0 '

" of'material in 1 cu.' ft. damp locse 93.4 " 104.0 '
I .I “t0: I I I I I I 1.0 I. .5 I
bulking factor 1.17 1.05
fineness modulus 1.08 7. 45
maximum size 1} in.

Desired strength - 3000 lbs. per sq. in. Slump 6-7 in. Real
Mn 1 : 8.8. Gals. water per sack of cement - 5.35. Fineness

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modulus of mixed aggregate - 6.0. Percent eand by vol. - 36.5%.
Per cent coarse aggregate - 73.5%.
Shrinkage factor - 87.23%. Nominal mix - 1 : .85 : 2.3
Field.mix l : .99 : 2.4
Water in aggregate for l eack mix, eand 1.0 lbe.
c,gggl,24 '
total 3.34 lbe.
water of absorption for 1 each mix, eand .93 lbe.
c,;gg§,§9 '
total 3.51 lbe.
The water needed.for a one eack batch would then be in pounde,

(5.25)(7.5) plue 3.51 minue 3.24 equaling 40.64 lbe.

It was decided that twelve blocke would be caet for
teet purpoeee. Since they ehould be ee long as could be
conveniently handled it was decoded to make them 4 by 4 by
36 inches.

For torme e flat box having eidee 4 inchee high wee
made of i inch planed number 3 yellow pine lumber. The plan
of the box wae 36 by 54 inchee ineide measurement. Croes-
wiee divieione were made by.nailing in dividere made of the
4 inch.materiel. The bottom of the box was covered with
eheet tin.

To provide means for accurate neaeurement of the
expansion or contraction of the blocke 3/8 inch round rode
were placed vertically protruding for 2 inches thru the
bottom of the forms. To allow the rode to protrude thru the

bottom the form was eupported.upon piecee of 2 by 4 inch materr

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than coupleted the for-e were coated on the inaide with
crude oil in order to prevent eticking and loee of water frca
the mixture. '

The concrete wae nixed in mixing pane, a trowel being
need for mixing. The cement and water were firat mind together
and the aggregatee were then added. Enough concrete for two
blocka wae nixed in each pan. '

The blocke were caet in auch order that the aix for each
block had atccd after aixing one half hour longer than had the .
mix for the preceding block. Of couree the firat block wae
oaat directly after the aateriale were mixed. In the caee of
the other blocke the material was thorcly remixed before being

Poured. In the case of the last three blocka it waa nacaaaary

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to add extra water in order to obtain a workable mix. Theee
blocke were the one. which were ceet 4-1/3, 5, and 5—1/2 houre
respectively after initial mixing of the ingrediente. It was
noticed that the material for theee blocke had a dead and
brownieh color.

The mix wae epaded in the forme in order to eecure blocke
with smooth eurfaces and.without honeycombing. After pouring
the concrete the rode were placed and supported in an upright
poeition until the block. had hardened.

Twenty four houre after the last blocke were poured the
form. were torn away and the blocke removed. Smooth eurfacee
were obtained in all ceeee and the blocke had the appearance
of cut etone.

On the rods projecting from the concrete pointe were
marked with a file 1 end.l-a/8 inches from the eurface of the
blocke. The eurrace between theee file mark. wan cleaned.with
a file and oiled to prevent ruet. The areee eo marked were the
point. between which.meaeurenente were taken.

For measuring the distance between the rode it was nec-
eesary to make and extension on the etrain guage need. Thin
exteneion wee made from a.piece of 5/8 inch round eteel rod.

A hole to receive the ehatt of the etrain zuege wee drilled
3 inches deep into one end of the rod. A eet ecrew wae pro-'
wided for holding the chart of the etrain guage in the rod.
The opposite end of the rod wee turned down to a deifinite
end.

A neane wee thue provided for obtaining differencee of

IOIenrenent between the rode held in th end: of the concrete

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blocks. The shrinkage of-the concrete could be measured two
opposite parallel surfaces and the readinge averaged. Thue
was error due to poeeible warping or curling of the block.
when drying out eliminated. The blocke were supported thrucut
thqtr entire length in a horizontal position on email wooden
rollere eo that they were free to expand.or contract. The iron
rode were in a horizontal poeition and it wae easy to meaeure
between them . ‘

lhen measurement of the blocke wae etarted they were
removed to an unheated.portion of the basement of the writere
houee, it being thought that here they would have a very
constant temperature and would be in comparatively little
danger of being moleeted. Alec here they cculd.be more convene
iently cared for and.meaaured. '

Readings were etarted on the eecond day after caetingfi
and were recorded an tabulated below. Blankete.made ofburlap
were provided and theee were kept wet to provide moisture
for curing.

The guage ueed wae a standard Amee strain gnage with
full circular dial and graduated in thousandthe of an inch.
Ae a standard for the purpose of checking the guage, two
epikee were driven in a wooden block, the dietance between the
blocke being the name ae that between the rods of the teat
piecee.

It was not thought neceeeary to allow for temperature

Ohtngee in taking readinge of the teet blockeeince the

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purpose of the test was a comparison between the shrinkage
occuring in ordinary concrete poured directly after mixing
and concrete poured at deffinite intervals after mixing, the
mix being retenpered where necessary to obtain a workable mix.
However, since the test blocks were kept on the floor of an
unheated.portion of a basement and the readings taken at
10:30 p.m. the temperature was almost constant.}
On the following page is given a cepy of the original
log sheet on which the data was recorded. On succeding pages
are given curves which represent the differences in measure-
ment of the blocks. Shrinkages in thousandths of an inch are
plotted as positive ordinates time elapsed since casting as
abcieeas.

A comparison of these curves show that the special con-
ditions of the test had little effect upon the shrinkage or '
the time at which the shrinkage occured. It is the writer's
belief that if the test had been carried on over a longer
period of tine and.if nore blocks of each class had been.nade

the differences in shrinkage would have been even less.

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The curve below shows the variation in total shrinkagee
of the blocks. Here again little difference is apparent. As
cupared with the first block cast the last block shrank but
.004 inches more. This additional shrinkage shunts to .168
inches in 100 feet. This is for concrete placed five and one
half hours after initial mixing. For concrete east one and one
half hours after initial mi ring the increased shrinkage is
.143 inches in 100 feet. The increases for the ether blocks
are given in the accompanying table.

The total shrinkage curve given on the next page seene
’50 show quite occlusively that sons extra shrinkage may be
expected from concrete that is nixed and net poured inediat-
1!. However, the shrinkage that eccured when the time elapsed
Its one half hour was equal to the shrinkage resulting when

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77m. in Mar: e k/osea’ ‘ Aria/eel)

Ira/{13 a/m’ pouring.

the pouring was delayed five and one half hours and retemper-
ing was necessary.

The table below gives values of the extra shrinkagee
in inches per one hundred feet.

Block no. Tine Total Extra shrinkage
elapsed shrinkage inches per 100 ft.

1 0 hrs. .0695 in. 0

3 1/3 ' .0735 ' .163
3 1 ' .0730 ' .143
4 1-1/3' .0730 ' .143
5 3 ' ' .0735 ' .133
a 24/3" .0735 - . 133
7 a ' I .0715 ' .081
8 3-1/3" .0710. ' .031
9 4 ' .0735. ' .133
10 4-1/3. .0710 ' .061

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Block.no. Tine elapsed Total Shrink. Extra Shrinkage
inches per 1 ft.
11 5 hrs. .0725 .133
13 5-1/3 hrs. .0735 .163
Conclusion

Ihile this experiment was not as exhaustively carried out as
it should have been if the results were to he considered final
yet it is hoped.that it has resulted in some slight addition
to the store of knowledge of concrete. lb hope that itinay'aid
te a feeling of greater confidence in the use of the output of
the central concrete mixing plant which is operated.en a

commercial basis.

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