What is the underlying cause of 'D-cracking' in concrete pavements?

The underlying cause of 'D-cracking' in concrete pavements is the freeze-thaw deterioration of coarse aggregate particles near joints and cracks. D-cracking is a distress mechanism specific to concrete pavements, characterized by a series of closely spaced, crescent-shaped cracks that typically develop parallel to joints, cracks, and free edges of the pavement. The primary culprit is coarse aggregate that is susceptible to freeze-thaw damage. These aggregates often contain small pores that become saturated with water. During freezing cycles, the water expands, creating internal pressures that cause the aggregate particles to fracture and disintegrate. This disintegration weakens the concrete matrix around the aggregate particles. The resulting distress manifests as cracking patterns near joints and cracks, where water infiltration is most prevalent. These cracks propagate and eventually lead to spalling and disintegration of the pavement surface. Unlike scaling, which affects the surface paste, D-cracking is related to the internal deterioration of the aggregate. Using durable, freeze-thaw resistant aggregates and ensuring proper drainage to minimize water infiltration are key strategies for preventing D-cracking. Air entrainment, while beneficial for overall freeze-thaw resistance, does not fully mitigate D-cracking caused by susceptible aggregates. For example, pavements constructed with certain types of limestone or chert aggregates are known to be prone to D-cracking in regions with significant freeze-thaw cycles.