Frequently Asked Questions
Q: What causes concrete to crack?
A: Unexpected cracking
of concrete is a frequent cause of complaints. Cracking can be the
result of one or a combination of factors, such as drying shrinkage,
thermal contraction, restraint (external or internal) to shortening,
subgrade settlement, and applied loads.
Cracking can be significantly reduced when the causes are taken
into account and preventative steps are
||Crazing is a pattern of fine cracks that
do not penetrate much below the surface and are usually a cosmetic
problem only. They are barely visible, except when the concrete
is drying after the surface has been wet.
||Plastic Shrinkage Cracking: When water evaporates
from the surface of freshly placed concrete faster than it is
replaced by bleed water, the surface concrete shrinks. Due to
the restraint provided by the concrete below the drying surface
layer, tensile stresses develop in the weak, stiffening plastic
concrete, resulting in shallow cracks of varying depth. These
cracks are often fairly wide at the
||Drying Shrinkage: Because almost all concrete
is mixed with more water than is needed to hydrate the cement,
much of the remaining water evaporates, causing the concrete
to shrink. Restraint to shrinkage, provided by the subgrade,
reinforcement, or another part of the structure, causes tensile
stresses to develop in the hardened concrete. Restraint to drying
shrinkage is the most common cause of concrete cracking. In
many applications, drying shrinkage cracking is inevitable.
Therefore, contraction (control) joints are placed in concrete
to predetermine the location of drying shrinkage cracks.
||D-cracking is a form of freeze-thaw deterioration
that has been observed in some pavements after three or more
years of service. Due to the natural accumulation of water in
the base and subbase of pavements, the aggregate may eventually
become saturated. Then with freezing and thawing cycles, cracking
of the concrete starts in the saturated aggregate at the bottom
of the slab and progresses upward until it reaches the wearing
surface. D-cracking usually starts near pavement joints.
||Alkali-aggregate reaction: Alkali-aggregate
reactivity is a type of concrete deterioration that occurs when
the active mineral constituents of some aggregates react with
the alkali hydroxides in the concrete. Alkali-aggregate reactivity
occurs in two forms—alkali-silica reaction (ASR) and alkali-carbonate
Indications of the presence of alkali-aggregate reactivity may
be a network of cracks, closed or spalling joints, or displacement
of different portions of a structure.
||Thermal cracks: Temperature rise (especially
significant in mass concrete) results from the heat of hydration
of cementitious materials. As the interior concrete increases
in temperature and expands, the surface concrete may be cooling
and contracting. This causes tensile stresses that may result
in thermal cracks at the surface if the temperature differential
between the surface and center is too great. The width and depth
of cracks depends upon the temperature differential, physical
properties of the concrete, and the reinforcing steel.
||Loss of support beneath concrete structures,
usually caused by settling or washout of soils and subbase materials,
can cause a variety of problems in concrete structures, from
cracking and performance problems to structural failure. Loss
of support can also occur during construction due to inadequate
formwork support or premature removal of forms.
||Corrosion: Corrosion of reinforcing steel
and other embedded metals is one of the leading causes of deterioration
of concrete. When steel corrodes, the resulting rust occupies
a greater volume than steel. The expansion creates tensile stresses
in the concrete, which can eventually cause cracking and spalling.
in concrete can be reduced significantly or eliminated by observing
the following practices:
1. Use proper subgrade preparation, including uniform
support and proper subbase material at adequate moisture content.
2. Minimize the mix water content by maximizing the size and amount
of coarse aggregate and use low-shrinkage aggregate.
3. Use the lowest amount of mix water required for workability;
do not permit overly wet consistencies.
4. Avoid calcium chloride admixtures.
5. Prevent rapid loss of surface moisture while the concrete is
still plastic through use of spray-applied finishing aids or plastic
sheets to avoid plastic-shrinkage cracks.
6. Provide contraction joints at reasonable intervals, 30 times
the slab thickness.
7. Provide isolation joints to prevent restraint from adjoining
elements of a structure.
8. Prevent extreme changes in temperature.
9. To minimize cracking on top of vapor barriers, use a 100-mm thick
(4-in.) layer of slightly damp, compactible, drainable fill choked
off with fine-grade material. If concrete must be placed directly
on polyethylene sheet or other vapor barriers, use a mix with a
low water content.
10. Properly place, consolidate, finish, and cure the concrete.
11. Avoid using excessive amounts of cementitious materials.
12. Consider using a shrinkage-reducing admixture to reduce drying
shrinkage, which may reduce shrinkage cracking.
13. Consider using synthetic fibers to help control plastic shrinkage
See PCA's publication "Concrete
Slab Surface Defects: Causes, Prevention, Repair" (IS177)
for a full discussion on the causes of types of cracking, how to
minimize cracks and proper procedures for dealing with cracking
that can not be eliminated with the proper use of control joints
Other sources for information on the cracking of concrete
ACI 224R (American Concrete Institute Committee 224).
Although the report does not address the topic of what magnitude
of cracking is acceptable in plain concrete (non-reinforced concrete)
it does give reasonably clear guidance on acceptable crack widths
in reinforced concrete which is more critical than plain concrete.
The tolerable crack width values for reinforced concrete are included
in Table 4.1 of ACI 224.