Identifying concrete cracks
by Norman Aceron Garcia
Newly installed concrete builds up tensile stresses as differences in moisture content and temperature increase during concrete curing or drying. Cracking mitigates these tensile (pulling) stresses, and many factors can affect the progression of such stresses.
Control joints are grooves pressed into the concrete during the finishing phase. Control joints are installed in an effort to determine the part at which concrete will crack. Cracks tend to develop in the grooves first because the concrete slab is thinner and weaker at these points. There are many factors that can influence the locations at which cracks will develop, they sometimes appear in areas other than at control joints.
Thermal cracking is caused by temperature differences within the concrete. This condition is intrinsic in continuously-poured concrete slabs. In applications such as concrete slabs and residential foundation walls, cracking is normal and unavoidable. Heat is generated when concrete hardens during the chemical hydration process. As the concrete exterior dries, water evaporates from the void spaces. As water dissipates, the concrete mixture particles move closer together that results in volume shrinkage. Since the concrete exterior is exposed to air but the concrete within is not, concrete close to the surface dries and shrinks at a rate different from that of the underlying concrete. This condition produces tensile stresses that are relieved by the cracking of concrete near the surface.
Plastic cracking is a type of cracking that is due to the differences in moisture content between surface concrete and underlying concrete. This happens when the water from the surface evaporates faster than the moisture migration from the underlying concrete. When this occurs, surface concrete will dry more rapidly than interior concrete. The resulting differences in moisture content yield tensile stresses that are relieved by cracking near the surface.
There are several visual clues that help differentiate shrinkage cracks from other types of cracks that can appear in concrete slabs and foundation walls. These are vertical displacement, linear crack continuity, continuity through the slab, corrosion and alkali-aggregate reaction. Vertical displacement – in which concrete on one side of the crack will be higher than concrete on the other side – is normally caused by soil settlement or heaving. Contrarily, cracks due to shrinkage are usually not linearly continuous. If you look closely, you will see interruptions or gaps along the crack line. Shrinkage cracks only occur at the surface and are not continuous through the slab thickness.
Cracking is also caused by chemical reactions, namely corrosion and alkali-aggregate reaction. Corrosion is caused by the oxidation of steel to iron oxide. A risk of corrosion or rusting of reinforcement bars (rebar) is high when there is not enough concrete cover (the space between the concrete surface and reinforcement bars). Rusted or corroded rebar can crack the concrete surface, which is manifested by visible cracks at the slab surface.
On the other hand, alkali-aggregate reaction is concrete weakening resulting from the reaction of aggregates (sand & gravel) with alkali hydroxides found in concrete. Warnings of this type of deterioration might be: displacement of different parts of a structure; a grid or web of cracks; or closed spalling or closed joints.
Norman is a registered Professional Engineer and a Certified Property Inspector. Please visit www.mrpeg.ca for more information on home inspection and maintenance tips.