Document 6498485

To prevent cracking, isolation joints are needed between members which are expected to experience differential
movement—as in the column and concrete residential floor shown in the above illustration.
How to Minimize Cracking
. . . some basic safeguards against this perennial problem
T
he very nature of concrete often makes it impossible
to eliminate cracks entirely. The extent and severity
of these cracks, howe ve r, vary widely in the field. Concrete cracks that are infrequent, tight, and do not span
are hardly ever objectionable. On the other hand, when
they become extensive, wide and unsightly, they can
constitute a very real problem. The result can be trouble
and hard feelings for all involved.
Let’s take a quick look at some of the factors which influence cracking of concrete.
Restraint: If conditions set up forces in concrete which
alter its dimensions or position and it is restrained, the
result is a crack. Therefore, a concrete slab which is to
butt against a building or other mass of dissimilar size
should have an isolation joint along all points of abutment to accommodate differential movement. As concrete becomes dehydrated, drying shrinkage forces are
set up within the slab. To accommodate these, contraction joints should be placed about every 20 feet.
Mix Design: The temptation is so strong to call for a
soupy mix that it is easy to see why such mixes are commonplace. Howe ve r, the owner has to pay for the workmen’s convenience for a long time. It’s really not that
much more difficult to cast a slab with 3-inch slump concrete than it is with 6-inch slump concrete. When your
ready mixed concrete producer supplies concrete made
with sound, well-graded aggregates, an adequate cement
content, and the proper water content, your men will
have no problems in placing and finishing the concrete.
And you’ll have no trouble with unhappy owners.
Reinforcement: If reinforcement is used for structural
purposes, the size and position will differ from that used
to control thermal and drying shrinkage cracks. For example, in slabwork, if a structural slab or a slab on a poor
subgrade is to be cast, the reinforcement is usually positioned 1/3 or 1/4 of the way from the bottom of the slab.
Howe ve r, if reinforcement is being used to control
shrinkage or thermal cracks, it is placed 1/3 or 1/4 of the
distance from the top of the slab. In addition, the size of
reinforcement needed for structural use is considerably
greater than that needed to control surface cracking.
Wire mesh placed on the subgrade with concrete subsequently cast over it contributes to neither the structural nor surface crack resistance of the slab. Prying up of the
mesh while the concrete is being cast usually is of no help
since it most often sinks to the subbase from its own
weight. This technique most certainly is to no avail when
workmen walk over the area after the mesh has been
pried up. The concrete can be cast in two lifts with the reinforcement placed on top of the bottom lift. Even better,
place the reinforcement on chairs before the concrete is
cast. These chairs need not be expensive; chunks of discarded concrete or broken concrete blocks can be used.
Placing and Finishing: Because concrete almost al-
Drying shrinkage, a major cause of concrete cracking, is
directly related to unit water content. Always use concrete
with as low a water content as practicable.
ways hardens eventually — despite the many ways it
may be misused—the importance of proper placing, finishing and curing procedures is often overlooked. With
the wide range of vibrators now available, there is no
need to do without this valuable piece of concrete placing equipment on any job. Believe it or not, there are
contractors who use vibrators in placing home basement walls. Of course, when vibrators are used, an app ro p riate decrease in slump should be made or vibration will become a handicap, causing increased
segregation, rather than an aid to quality concrete. Wellcompacted concrete is less likely to crack and develop
honeycombing than a carelessly placed mix.
Slabwork should be worked as little as possible, consistent with achieving required finish. Ove rw o rking the
slab surface brings fines and cement paste to the surface
which almost always results in crazing. Of course, never
broadcast cement over a slab to dry the surface—this is
another prime cause of small surface cracks. When casting concrete on hot, dry, windy days, always protect the
concrete surface from excess evaporation between finishing operations.
Curing and Protection: Inadequate curing can account
for loss of up to one-half the potential strength of conc re t e. It seems a shame, therefore, to throw away this
added strength, durability and crack resistance since
curing is a very small cost item. With the many efficient,
easily applied, low-cost membrane-forming curing
compounds available there is no excuse for slighting this
important phase of concrete construction.
Other Precautions: Alkali-aggregate reaction, experienced in some parts of the country, is a phenomenon
that results in severe cracking. The alkalis in cement react with some types of aggregates to result in expansion
that disrupts the concrete—causing cracking, spalling
and popouts. Sulfate-bearing waters also attack some
concretes and result in cracking and general deterioration.
Aluminum objects buried in concrete, especially concrete containing calcium chloride, sometimes cause
cracking and popouts due to a reaction between the aluminum and chemical constituents of the concrete. In
mass concrete, unchecked buildup of heat in the concrete mass can result in severe cracking. Structural overloads will often cause cracks—sometimes temporary,
sometimes permanent.
Some Rules: Scientists have been at work for decades
trying to develop crack-proof concrete. Theoretically,
prestressing should eliminate cracking. An expansive cement is being tested in this country that is aimed at eliminating cracking. Non-shrink aggregates are available
which eliminate or minimize cracking through controlled enlargement of the aggregate particles.
Howe ve r, until a low-cost, high-performance, crackfree concrete is developed, it is suggested that the following rules be followed:
1. Design the members to adequately handle all anticipated loads.
2. Provide proper shrinkage and isolation joints.
3. In slab-on-grade work, prepare a stable subbase.
4. Provide correct reinforcement at proper level.
5. Order concrete with a low-unit water content.
6. Place and finish according to established standards.
7. Cure the concrete thoroughly.
PUBLICATION #C650153
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