Newly-installed concrete build up tensile stresses as differences in moisture content and temperature increase during concrete curing or drying. These tensile (pull) stresses are mitigated by cracking 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 which can influence the locations at which cracks will develop, they sometimes appear in areas other than at control joints.
Thermal cracking is characterized by temperature differences within the concrete can contribute to the development of cracks. 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 to 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 which are relieved by cracking of concrete near the surface.
Plastic cracking is a type of cracking which 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 which are relieved by cracking near the surface.
There are several visual clues which help differentiate shrinkage cracks from other types of cracks which 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 occurs at the surface and not continuous through the slab thickness.
Cracking is also caused by chemical reaction, namely corrosion and alkali-aggregate reaction. Corrosion is caused by the oxidation of steel to iron oxide. Risk of corrosion or rusting of reinforcement bars is high when there is not enough concrete cover (the space between the concrete surface and reinforcement bars). Rusted or corroded rebars 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 may be displacement of different parts of a structure, a grid or web of cracks, or closed spalling or closed joints.
Republished from: "Identifying Concrete Cracks”, Green Building, Pilipino Express