A.Don Marsh responds:
Concrete cracking is caused by stresses
due to drying shrinkage, temperature change, or applied
Drying shrinkage. Drying shrinkage is an
inherent and unavoidable property of concrete. During the
setting and hardening stages, excess mixing water in the
concrete evaporates, causing the concrete to dry from the
surface inwards. Shrinkage begins near the surface, pulling at
the moist inner portions of the concrete, which are restrained
by friction on the sub-grade, reinforcing steel, and building
connections. This restraint prevents the concrete from
shrinking freely and uniformly, resulting in cracking.
While drying shrinkage and some cracking is inevitable, it
can be reduced by specifying adequate compressive strength,
minimizing the water content, spacing control joints properly,
and adequately curing the concrete.
Compressive strengths are governed by local building codes.
In general, basement walls require a minimum 2,500 psi
concrete, while flatwork ranges from 3,000 to 4,000 psi. For
residential work, recommended slumps range from 3 to 5 inches
for flatwork, and 5 to 7 inches for basement walls. Once the
concrete is ordered to a specified slump, don’t add more
water at the site to speed the pour.
The purpose of control joints is to confine cracking to
predetermined points in a slab, rather than letting them occur
randomly. Control joints should be tooled or sawn to a depth of
one quarter the slab thickness. Joints should be spaced at
intervals not more than 30 times the slab thickness. Driveways
wider than 10 feet require both transverse and longitudinal
Curing helps reduce shrinkage cracking and maintains slab
Typically, curing involves keeping the concrete moist and
covered for five to seven days, or applying a spray-on compound
that forms a membrane on the surface.
Temperature changes. Extreme temperature
changes immediately following, and up to a year after, slab
placement can have the same adverse effects on concrete as
drying shrinkage. Proper control joint spacing is the most
effective method to guard against this.
Applied-load cracking. This occurs when the
weight of an object on a slab stresses the concrete beyond its
tensile strength. Such cracking often occurs, for example, when
a heavy truck drives over a sidewalk designed only for
pedestrian and light vehicular traffic. To prevent load-stress
cracking, make sure a slab is built over a uniformly compacted,
well-drained subgrade, and is thick enough to withstand the
kind of use it will get. In residential concrete, 4 inches is
the minimum thickness for walkways and patios. Garage slabs and
driveways should be 5 to 6 inches thick if any heavy truck
traffic is anticipated, otherwise 4 inches is adequate.
— Don Marsh is the media
services representative for the Portland Cement Association in