Durability Cracking (D-cracking) is the progressive deterioration of Portland cement concrete (PCC) and is normally caused by winter freeze-thaw cycling. The PCC coarse aggregate source has been identified as causing well designed mixes to develop D-cracking. A common rehabilitation procedure for D-cracked PCC pavements is to overlay the PCC with asphalt concrete (AC). This renews the surface, but little is known about the long term effect of AC overlays on D-cracked PCC pavements. Specifically, will an AC overlay stop the progression of D-cracking, or accelerate it. The primary climatic factors responsible for D-cracking are moisture and temperature. Finite-difference transient flow computer moisture movement modeling as well as field instrumentation and laboratory measurements indicated that AC overlays have negligible effect on the PCC pavement moisture regime. This is due to the extremely low hydraulic conductivity of PCC. Evaporative drying of the PCC extends less than 2 in. into the PCC pavement. The effect of AC overlays on the PCC temperature regime was evaluated by finte-difference heat transfer computer modeling. AC overlays were found to decrease the number of freeze-thaw cycles and the rate of cooling in PCC pavements. The cooling rate at the top of an 8-in. PCC pavement in the St. Louis, Missouri area was decreased from 1.19 to 0.24 F°/hr by a 4-in. overlay.