We've got a series of parallel cracks along the joints in our concrete parking lot. An engineer says they're D-cracks. Is this kind of cracking caused by alkali-silica reaction (ASR)?
No. D-cracking is caused by using coarse aggregates that are susceptible to freezing-and-thawing deterioration. The aggregates absorb moisture from the pavement base and from surface water entering through cracks and joints. If aggregate pores are full when freezing occurs, internal pore pressure cracks the particles, causing the mortar to crack as well. More cracks develop with repeated freeze-thaw cycles. D-cracks are roughly parallel to the adjacent joint. Many individual cracks caused by alkali-silica reaction are approximately perpendicular to the direction of the joint. These ASR cracks are also commonly associated with fainter map cracking elsewhere in the pavement slab. Handbook for the Identification of Alkali-Silica Reactivity in Highway Structures, by David Stark, is an excellent aid to detecting ASR damage and distinguishing it from other types of concrete damage. Published by the Strategic Highway Research Program, the 49-page, color-illustrated handbook can be purchased from the Portland Cement Association. Call 800-868-6733 and request publication LT165.