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They
give the Nominal Mix and correction must be made to obtain the Field Mix. In
important work it is not enough to take the mix from these tables. The
aggregates should be tested and the proper proportions determined by means of
the designing data given in this article, the results being checked by
compression tests on cylinders. The necessity of waiting 28 days for results is
often a serious difficulty. Seven-day tests are coming into use as a guide to
the 28-day strength. It is possible that comparative tests between Portland
cement concrete and that made with the new alumina cements will eventually be
made, enabling 24-hour tests on aggregates with the quick-hardening cements to
be used as a guide for designing Portland cement concrete mixes. Parallel with
more scientific methods of proportioning concrete there are being developed
more exact methods for the field, notably more exact devices for measuring the
materials. The uncertainty due to the bulking of loose moist sand is now often
met by measuring the sand and water together by the so-called inundation
method, taking advantage of the fact that when there is an excess of water over
that required to fill the voids the sand volume is closely the same as when
measured dry and loose. This is shown by Plate I where the bulking effects of
varying percentages of water are recorded. The procedure in designing a mix is
illustrated by the following example: It is desired to proportion the concrete
for a reinforced concrete building. The city code places the 28-day compressive
strength at 100 percent. "Effect of Moisture in Sands," R. R.
Litehiser in "Concrete," January 1925. "Standard Method- of Test
for Unit Weight of Aggregate for Concrete" (Serial Designation C29-21)
A.S.T.M. Reprinted as Appendix XV of the 1924 Joint Committee Report. The
Nominal Mix is found to be 1-1.56-3.63 and accordingly the sand is 30 per cent
of the mix. The Real Mix is 1-4.5. The fineness modulus of the combined
aggregates is 5.72. The 1916 Joint Committee and many building codes place the
average 28-day strength for ordinary 1-6 or 1-2-4 concrete at 2000 pounds per
square inch, the measurements being made by loose volume. It seems certain the
Joint Committee proportions refer to dry material although it is not so stated.
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By
compacting the aggregates by rodding would reduce the proportions to about
1-1.9-3.8 or 1-5.7Nominal Mix. The 1924 Joint Committee specifies a somewhat
richer mix if this strength of 2000 pounds per square inch is desired. The
table from that report (Appendix A) gives the average proportions of 1-1.7-3.4
or 1-5.1, for ordinary 2000 pounds per square inch concrete, 6-7 inch slump,
made with the usual size materials for reinforced concrete construction. To
make this comparison on the basis of Field Mix account must be taken of the
bulking effect of moisture on the fine aggregate as well as upon the difference
between loose and rodded measurements. The moisture
content of sand on the job is often 5 percent, so for the coarse sand must be
taken to obtain as much sand as there is in 1 cubic foot dry and rodded; for the coarse aggregate of Ex. 2, 105 -~ 98 = 1.07
cubic feet. Assuming this amount of bulking to be possible in the field the
1916 Joint Committee nominal mix of 1-1.9-3.8 is equivalent to a field mix of 1-1.9
X 1.4-3.8 X 1.1 = 1-2.7-4.2 or 1-6.9. In the same way the 1924 Joint Committee
nominal mix corresponds to a field mix of 1-11 X 1.4-3.4 X 1.1 = 1-2.4-3.7 =
1-6.1. For smaller sized aggregates the 1924 Joint Committee calls for a still
richer mix. The actual amount of bulking of wet sand depends on the manner of
handling. The data in Plate I were obtained by pouring sand slowly into the
several containers and the bulking is very much more than occurs when sand is
shoveled from a pile into barrows. So the figures given above are useful only
to emphasize by exaggeration the fact that applying the tabular proportions to
field measurements is on the safe side. It is not possible to say whether or
not the generalization of the Water-Cement Ratio will stand finally as the
unquestioned law of concrete strength but there is no doubt but that it marks a
very long step ahead in our knowledge. In fact it is not an exaggeration to say
that it is working a revolutionary change in concrete making with a great
betterment of product. The theory of aggregate analysis with the Fineness
Modulus as the standard is not at all a matter of the same order. While it has
proved an extremely valuable method of analysis its use is not essential to the
application of Professor Abrams' major theory. Unquestionably there remains
much to be learned concerning the action of aggregates. To many the weak point
in the new theory is the proviso that it holds only with workable mixes.
Workability is a somewhat indefinite standard but one that implies to the
practical man a rather narrow range of possible
variation. There is argument as to whether the slump test really measures
workability which is a function of both the wetness and the grading of the mix.
The fact that quite generally it is necessary to add more sand than called for
by the theory is a real difficulty. The leading investigators in this field
agree in their recognition of the fundamental importance of the proportion of
water used in concrete but in other points there are differences of opinion. Probably
the best place to turn to for information concerning progress is to the series
of the Proceedings of the American Concrete Institute. Other rather random
references are given in the list that follows. Concrete differs from other structural
materials which come to the job as finished products in that it is manufactured
where it is used. Good quality of concrete is the first essential for the
permanence and solidity of the structure in which it is placed and accordingly
its manufacture is a heavy responsibility upon the engineer. Structural steel
is a standardized article of commerce, made under rigid supervision, and it can
be bought in the open market with confidence that it will pass the rigid
requirements of the American Society for Testing Materials. However on all
important work involving large tonnage the engineer provides for careful
inspection and tests of the steel. How much more essential is it that the
engineer select with care the manufacturer of his concrete, the contractor, and
hold him rigidly to the best methods of modern workmanship to ensure that the
structural concrete be of the requisite strength and quality.
The
preceding chapter outlined the best methods of proportioning concrete; the
present chapter is concerned with the best methods of the actual manufacturing
process itself. It consists largely of quotations from the 1924 Joint Committee
report which give an excellent summary of the best practice. It should be
realized that in some particulars these requirements are rather stricter than
can be easily enforced on small jobs. The mixing of concrete, unless otherwise
authorized by the Engineer, shall be done in a batch mixer of approved type
which will insure a uniform distribution of the materials throughout the mass so
that the mixture is uniform in color and homogeneous. The mixer shall be equipped
with suitable charging hopper, water storage, and water-measuring device
controlled from a case which can be kept locked and so constructed that the
water can be discharged only while the mixer is being charged. It shall also be
equipped with an attachment for automatically locking the discharge lever until
the batch has been mixed the required time after all materials are in the
mixer. The entire contents of the drum shall be discharged before recharging.
The mixer shall be cleaned at frequent intervals while in use. The volume of
the mixed material per batch shall not exceed the manufacturer's rated capacity
of the mixer."
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Brookline Massachusetts Concrete Cutting and Core Drilling
Brookline
Massachusetts Concrete Cutting and Core Drilling
