Forensic Investigation of Debonding in Rich Bottom Pavement

Rich bottom flexible pavements have become more prevalent as state agencies have begun building perpetual pavements to meet the growing demand for high-performing long-life pavements. The rich bottom uses higher asphalt contents, typically 0.5% above optimum, to achieve greater density and resistance to fatigue. In this way, bottom-up fatigue cracking and consequential full-depth repairs can be eliminated. A rich bottom pavement was included in the 2003 research cycle at the National Center for Asphalt Technology test track to evaluate its effectiveness in controlling fatigue. Although it was expected to perform better than comparable sections, it failed first, in fatigue. An investigation was made to find the cause of the failure and to recommend further action regarding rich bottom pavements. In the course of the study, it was determined that slippage between layers (a construction issue), rather than the increased asphalt content, led to the early failure of the section. Theoretical and measured strain responses, forensic trenching, and laboratory shear strength testing support this conclusion. It was also concluded, as in all pavement construction, that interface bond strength plays a crucial role in pavement performance.