Current theories for the formation of alligator crack patterns

Up till fairly recently explanations for the cracking of asphalt pavements were focused upon the stress concentrations that arise from wheel loading. It is widely believed that this cracking has its origins in the effects of heavy localised loading, especially in areas where subgrade supports are either inherently weak or have been weakened by the ingress of water beneath the asphalt sheet (Hoque 2006). Or the cracking could result from a process of fatigue cracking under repeated loading (MATES 1988). In either case the likelihood of cracking is recognised to be increased by any age or strain hardening that might have occurred in the asphalt. It is relatively recent recognition that this cracking is usually initiated from the upper surface of the asphalt that has inspired an increase in research activity to better understand the causes of “top down” cracking.

In a report (AAT 2004) commissioned by the US National Cooperative Highway Research Programme (NCHRP 2004), a review of the literature revealed that the “primary mechanism of top-down cracking” in asphalt pavements is “accumulated damage associated with repeated traffic loading”. This finding reflected the great weight of the reported literature which indicated that it was only “likely that thermal stresses could (also) contribute significantly”. In view of some interesting observations in Kenya (Wamburga et alia 1999), that solar radiation could be a major factor in accelerating surface age hardening which in turn could contribute to top-down cracking, surprise was expressed that “this mode of distress has not been acknowledged earlier” (AAT 2004). Indirect evidence of the potential importance of solar radiation had been given in a Japanese study (Matsuno et al 1992) reporting that in pavements experiencing top-down cracking this cracking was generally absent where the pavements were shaded by overpasses. Others agree that there appears to be a thermal stress component to top down cracking (Lytton et alia 1993; Myers et alia 2001). However, despite the growing evidence to the contrary, the dominant weight of opinion remains that cracking, and alligator cracking in particular, is generally caused by repeated traffic loading.