What is "D-cracking"? The term, which was introduced in the 1930's may mean any of several things, depending on who uses it. It may mean almost any crack that has been filled with direct and debris. Or it may mean cracks, filled or unfilled, that have been caused by freezing and thawing or other weathering of concrete. More properly the term identifies a series of closely spaced cracks in a pavement wearing surface at specific kinds of locations. These places are adjacent to joints, adjacent to other cracks or adjacent and parallel to the free edges. The term also describes the initial stages of such cracking, which begins below the wearing surface and later develops into closely spaced surface cracks. Thus D-cracking has a characteristic crack pattern and indicates a particular kind of pavement distress. D-cracking is now understood to begin with distress that develops in porous, absorptive coarse aggregate. Such particles near a joint or slab edge may become critically saturated with water. During freezing and thawing, they undergo progressive micro cracking. As time goes on the cracks extend into the surrounding mortar and into other aggregate particles. A four year study of D-cracking has been completed by the Portland Cement Association. The following recommendations can now be made: aggregates with D-cracking potential can be upgraded by using a smaller maximum size in the concrete; the maximum size that can be used without developing distress can be determined by evaluating concretes in standard freezing and thawing tests; selective quarrying should be done at some source to avoid material that is potentially nondurable; and drainage systems for pavement slabs, although allowable, should not be used as the sole means of eliminating the development of D-cracking.