Recently poured concrete develops tensile stresses as differences in temperature and moisture content develop in the drying concrete. These stresses are relieved by cracking. A number of factors can factor into the advancement of these stresses.
Control of Crack Locations
Control joints or cracks are sometimes cut into the cement pad in order to determine the areas where cement will crack. Control joints are grooves pressed into the concrete during the final poring and finish process. The cement slab is thinner and weaker at these cut and grooved areas, it tends to crack in these grooves first.
Sometimes cracks in cement floors can develop in other areas besides in the control joists.
Restraint to Shrinkage
Shrinkage is the most common cause of cement cracking. Shrinkage occurs in Large poured cement slabs. In applications such as concrete slabs and residential foundation walls, cracking is expected.
As the surface of the cement dries, water evaporates from the spaces between particles. As this water dissipates, the particles move closer together, resulting in shrinkage of the cement. Because the surface of a concrete slab is exposed to air but the underlying concrete is not, concrete near the surface dries and shrinks at a rate different from that of the underlying concrete. The underlying concrete acts as a restraint to shrinkage, resulting in cracking of the surface layer.
Types of cement cracking
Thermal cracking:
Temperature differences can contribute to the development of cement cracks.
The chemical hydration process through which concrete hardens produces heat which causes concrete to expand. At the same time, concrete at the surface of the slab is exposed to air and loses water through evaporation. Both of these conditions contribute to cooling and shrinking of the concrete near the surface.
The hot, expanding underlying concrete acts as a restraint to shrinkage of the cooling, shrinking surface concrete. This condition produces tensile stresses which are relieved by cracking of concrete near the surface.
Shrinkage cracking:
When concrete is mixed, more water than is needed for hydration is mixed with the dry components, such as sand, cement and an aggregate. Most of the water will eventually evaporate, causing shrinkage of the concrete slab.
Since water evaporates from the surface, which is exposed to air, at a rate different from the underlying concrete, this differential shrinkage rate produces tensile stresses which are relieved by cracking of concrete near the surface.
Shrinkage cracks are very normal in cement pours, and foundation walls. While performing your home inspection in New Jersey we will be evaluating the various cracks that we encounter.
