D-cracking, a form of concrete deterioration in portland cement concrete pavement, was first described in the 1930s, but within the last several years it has developed into a serious, widespread problem in certain areas of the eastern and west-central United States and in Canada. D-cracking is associated primarily with the use of coarse aggregates in the concrete that disintegrate when they become saturated and are subjected to repeated cycles of freezing and thawing. The cracking originates in the coarse aggregate particles and then propagates through the mortar matrix surrounding the aggregate. A series of slightly inclined cracks develop in the concrete, usually starting at the bottom of the slab and working upward. Signs of D-cracking on the pavement surface include a series of closely spaced cracks that are generally parallel to transverse and longitudinal joints and cracks and to the pavement free edges and may be filled with a dark deposit at the pavement surface. Identification of D-cracking before signs are evident on the pavement surface can be accomplished only by examination of full-depth cores taken through the pavement near the intersections of longitudinal and transverse joints and cracks. Improvements in resistance of coarse aggregates to freezing and thawing can sometimes be accomplished through aggregate beneficiation techniques, including reducing the nominal maximum size of coarse aggregate particles, mechanical separation, and blending. Reducing the maximum size of coarse aggregate particles is considered the besttechnique and aggregate blending is the least desirable. Any one of the three, however, can produce results ranging from completely satisfactory to completely unsatisfactory. Rehabilitation of existing D-cracked jointed pavement is being accomplished primarily by overlaying it with a new bituminous concrete surfacing. Adequate cleaning and repair of the D-cracked pavement before resurfacing is necessary for good performance. Both asphaltic concrete and portland cement concrete are being used in partial-depth and full-depth patching. Formore heavily traveled highways, the current trend is to use portland cement concrete for full-depth patching and to re-establish load transfer. Consideration should be given to increasing the nominal overlay thickness when resurfacing a D-cracked slab to compensate for the continued loss in slab strength caused by D-cracking.
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