D-cracking--a series of cracks roughly parallel to joints, edges, or transverse and longitudinal cracks--is typically found in pavements or other flatwork. D-cracking is caused by freezing and thawing of concrete. However, unlike typical freeze-thaw damage, the cracking results from moisture expansion within the aggregate particles rather than within the paste.
D-cracking starts when the aggregate particles become saturated. In most cases, the saturation begins at the bottom of a slab or pavement and works upward. Significant damage may occur in the interior of the concrete before cracks are seen on the surface. In other cases, moisture entering the concrete through a crack or joint may also be involved, making surface cracks appear earlier.