We’ve all heard people say that all concrete cracks. That’s not really true, but it is true that concrete slabs frequently have cracks. When talking about cracks it helps to have a common understanding of what kind of crack it is and what caused the crack.

1) Crack width: First, we need to know how wide the crack is. Although there are more sophisticated ways, a simple clear plastic crack comparator is often all that’s needed. For cracks through the concrete, it is also important to know if the crack is “active” or moving.

2) Plastic shrinkage cracks and crazing: These are fine, shallow cracks caused by drying shrinkage of the surface when the concrete is still “plastic”—before it has hardened. Plastic shrinkage cracks are usually parallel while crazing cracks (also called map cracking or checking) are intersecting. Crazing is a result of poor surface curing and only visible in hard troweled slabs. These cracks have little impact on durability.

3) Shrinkage cracks: As the concrete slab dries after placement, it shrinks. Contraction joints (or control joints), when cracked, are really just straight shrinkage cracks. Similar “uncontrolled” or “random” cracks—sometimes due to poor joint layout—meander across the surface. They are not usually detrimental to the performance of the concrete but may be aesthetically objectionable to the owner.

4) Offset cracks: If there is an elevation differential across the crack, that’s usually due to a poorly consolidated base beneath the concrete, or sometimes a tree root growing beneath or expansive soils. The crack may be a shrinkage crack or due to inadequate support from beneath. Typically, these cracks will need to be repaired for safety reasons.

5) Diagonal corner cracks: Cracks at the corners of slabs or at a joint intersection are caused by curling or warping of the panel or by overloading of the slab. As a vehicle drives over the curled up corner, with no support beneath, it breaks off. Technically, curling is caused by a temperature differential between the surface and bottom of the slab while warping is from a moisture differential.

6) D-Cracking: These are a series of cracks parallel to a joint. D-cracks are due to moisture infiltration through the joint and into certain concrete aggregates that will crack under freeze-thaw conditions. Since the concrete near the joint is often saturated, freezing begins to crack the aggregates and the concrete. D-cracking is a terminal condition—there is no cure.