Why Concrete Cracks and How to Prevent It
Control joints in concrete help prevent excessive cracking in driveways and large slabs. (Photo courtesy of Angie's List member Cheryl L. of Bellevue, Washington)
It is very common for concrete to have cracks, scaling, crazing and pitting. All concrete has a tendency to crack, and it is not possible to produce completely crack-free concrete. Concrete expands and shrinks with changes in moisture and temperature. The overall tendency is to shrink, and this can cause cracking at an early age of the concrete. Irregular cracks are unsightly and difficult to maintain but generally do not affect the integrity of concrete.
Control joints in concrete driveways or slabs are simply pre-planned cracks. Control joints create a weak place, so that when concrete shrinks, it will crack in the control joint instead of randomly across the slab. The American Concrete Institute addresses this issue: “Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such an occurrence does not necessarily reflect adversely on either the adequacy of the floor’s design or the quality of its construction.”
The most popular explanation for the concrete cracking is the freeze-thaw cycle in the winter. It is recommended to use sand on all concrete instead of salt. Salt will facilitate the freeze and thaw cycles that will then create the problem with pitting and cracking. When the temperature reaches 18 degrees, the concrete will freeze. At 35 degrees, it will melt and cause the cracking, pitting, crazing and scaling.
Concrete also cracks in the extreme sun, as it heats the concrete and causes it to expand. During the day, the sun heats, expands and moves the concrete. At the end of the day, the concrete contracts and regresses to its original, unbaked position. A homeowner can help to prevent cracking, scaling, crazing and pitting by yearly applying a good acrylic silicone solvent-based sealer to their concrete. A concrete contractor can also repair cracks with an epoxy injection, dry packing, or routing and sealing techniques to stabilize the cracks.
Editor's note: This is an updated version of an article originally published on Aug. 5, 2011.