Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > Random concrete
cracks
Q: What causes random concrete
cracks and can they be avoided?
 Random
cracks in new concrete slabs are a common complaint from owners.
What causes random cracks; what are the best strategies to avoid
the occurrence of random cracks; what are reasonable expectations;
and do all random cracks in slabs constitute a failure of the concrete?
A: Random cracks in concrete slabs
are most frequently caused one of the following mechanisms:
- Settlement of the soils supporting the concrete slab
Restraint of horizontal movement due to fixed foundation
elements
Overloading, applying a load larger than the slab was designed
to support
Restrained drying shrinkage of the slab
Settlement cracking takes place when the soils
or fill beneath the slab have not been adequately compacted to provide
a consistent level of support for the slab to limit the bending
stresses which crack the concrete. Settlement can be controlled
with consistent preparation (compaction) of the base supporting
the slab.
Slabs placed against fixed foundation elements
(frost foundations, light standards, etc.) produce cracks caused
by bending forces as the slab moves on the surface while the fixed
foundation does not. This mechanism is controlled by placing isolation
joint material between the slab and the fixed foundation to allow
the elements to move independently, thus limits the bending stresses
and subsequent cracking.
Overload cracking is easily controlled with proper
thickness design of the slab considering the largest load that may
be applied to its surface.
Cracks due to restrained drying shrinkage are
caused by the tensile stresses that build in the matrix of the slab
as the concrete gives up moisture over time and is prevented from
shrinking by the soils beneath it. The most common strategy for
dealing with this type of random cracking is to provide closely
spaced contraction joints
(weakened planes) to predetermine where the concrete should crack.
Smooth dowels and dowel plates are another common material used
to provide good structural performance at working contraction joints;
used more often with slabs greater than 150 mm (6 in.) in thickness
that will receive substantial loads the dowels provide load transfer
across working contraction joints. Slabs with properly spaced contraction
joints should typically limit the occurrence of random cracks to
no more than 3% of the panels in the slab. More information can
be found in Concrete
Floors on Ground.
Keep in mind that a crack of itself is not a failure of the concrete,
but rather the normal behavior of the material. It is also common
to use steel reinforcement in a slab to hold cracks tightly to assure
good structural performance. Tight joints provide good load transfer
and maintain equal elevation across cracks, which is the measure
of the structural performance of any slab. It should be recognized
that the use of steel reinforcement may actually increase the potential
for random cracks to occur because the cracks are held tightly and
thus do not allow for the full relaxation of tensile stresses in
the slab.
More information on
what causes concrete to crack.
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