Concrete pavement D-cracking is a terminal condition. Once it starts, there's no known cure. The cause is known--coarse aggregates susceptible to freezing and thawing deterioration. But the only way to prevent D-cracking is to avoid using these aggregates in concrete pavements.

CAUSES AND SYMPTOMS

Aggregates that cause D-cracking absorb moisture from the pavement base and from surface water entering through cracks and joints. The cracks usually begin in the lower portion of the slab and move upward. They always start along cracks, joints, or free edges.

FACTORS INFLUENCING D-CRACKING DEVELOPMENT

Coarse aggregates that cause D-cracking are nearly always sedimentary materials: limestone, dolomite, shale, sandstone, and graywacke. But not all sedimentary aggregates cause the problem. Aggregates with low permeability, high porosity, and small pore size are most likely to cause D-cracking. Frequent freezing and thawing cycles are most harmful. The D-cracking problem isn't as prevalent in very cold regions where fewer freeze-thaw cycles occur.

IDENTIFYING SUSCEPTIBLE AGGREGATES

For unproven aggregate sources, identifying poor performers requires testing the aggregate, either in concrete or by itself. Freezing and thawing tests are most commonly used. Changes in specimen weight, length, or modulus of elasticity are used to predict field performance.

MAKING POOR AGGREGATES BETTER

If aggregate from a given source causes D-cracking, is there any way to upgrade the aggregate? Three techniques are being used today: coarse aggregate particle size reduction; heavy media separation of harmful particles; and blending harmful aggregates with more durable aggregates.