Q: What causes D-cracking in concrete pavements and what can be done to prevent it?

A: D-cracking refers to cracks in a concrete pavement caused by freeze/thaw deterioration of aggregates in the concrete. It has been seen in some pavements after three or more years of service. D-cracks are closely spaced cracks that begin to form parallel to longitudinal and transverse joints, and then later proliferate outward away from the joints toward the center of the pavement panel. As water accumulates naturally in the base and subbase of pavements, the aggregate eventually can become saturated. Then, through a number of freezing and thawing cycles, the concrete begins cracking near the saturated aggregate at the bottom of the slab, and the cracking continues upward until it reaches the wearing surface.There are a few steps you can take to reduce the likelihood of D-cracking. One is to select aggregates that are known to perform well in freeze/thaw cycles, as determined either by previous field experience or by laboratory freeze/thaw testing of concrete samples according to ASTM C-666 “Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing.” If you have to use marginal aggregates, reducing the maximum aggregate particle size can help reduce the problem. Providing an effective drainage system to move free water out from under the pavement also may be helpful.