Cracks in concrete are extremely common but often misunderstood. When an owner sees a crack in their slab or wall, especially if the concrete is relatively new, they automatically assume there is something wrong. This is not always the case. Some types of cracks are inevitable. The best that a contractor can do is to try to control the cracking. This is done by properly preparing the subgrade, assuring that the concrete is not too wet, utilizing reinforcement where needed, and by properly placing and spacing crack control joints and expansion joints. However, sometimes cracks happen in spite of any precautions taken.
The American Concrete Institute addresses this issue in ACI 302.1–04. “Even with the best floor designs and proper construction, it is unrealistic to expect crack-free floor or slab. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking on every project, and that such occurrence does not necessarily reflect adversely on either the adequacy of the floor’s design or the quality of its construction (Ytterberg1987; Campbell et al. 1976)”.
Shrinkage cracks can happen anywhere in a slab or wall, but one place where they almost always happen is at re-entrant corners. Re-entrant corners are corners that point into a slab. For example, if one were to pour concrete around a square column, he would create four re-entrant corners. Because the concrete cannot shrink around a corner, the stress will cause the concrete to crack.
To combat random shrinkage cracks, control joints are incorporated into the slab. Control joints are actually contraction joints because they open up as the concrete contracts or gets smaller. They are simply grooves that are tooled into fresh concrete, or sawed into the slab soon after the concrete reaches its initial set. Control joints create a weak place in the slab so that when the concrete shrinks, it will crack in the joint instead of randomly across the slab.
Crack control joints should be placed at all re-entrant corners and slab penetrations, and evenly spaced throughout the rest of the slab. A good rule of thumb for four inch thick residential concrete is to place joints so that they separate the slab into roughly equal square sections, with no joint being further than about 10 feet from the nearest parallel joint.
The importance of reinforcement
The use of synthetic fibers, reinforcing wire mesh, or rebar can add some extra support to concrete, but none of them will fully prevent cracking, especially not in Calgary. However, if cracks happen, reinforcement can hold the different sections together. The presence of reinforcement can be the difference between a crack remaining hairline in nature or separating and becoming wider and unsightly. Steel reinforcement can also keep the concrete on both sides of a crack on the same horizontal plane. This means that one side doesn’t heave or settle more than the other, which could cause a tripping hazard. It is sometimes impossible to determine exactly what caused a particular crack. However, proper site preparation and good concrete finishing practices can go a long way towards minimizing the appearance of cracks and producing a more aesthetically pleasing project.