Initial Forensic Evaluation of a Dry-Process Rubber-Modified Asphalt Pavement

Abstract

The Additive Group (AG) experiment at the National Center for Asphalt Technology (NCAT) Test Track includes a section paved with a dry-process rubber-modified asphalt mixture (GTRDry) which has displayed severe levels of cracking after 10 million equivalent single-axle loads (ESAL) of accelerated trafficking. In the same period, the styrene–butadiene–styrene (SBS) modified control mixture displayed no cracking. Both mixtures displayed similar laboratory evaluated dynamic modulus |E*| and fatigue properties. The objective of this investigation was to determine the cause of the unexpected cracking observed in the GTRDry test section via forensic analysis, laboratory data, measured structural responses, and structural simulation. It was found that GTRDry had significantly lower foundation support relative to the control, and its average temperature-corrected asphalt concrete (AC) modulus (E₆₈) was lower than expected (based on the results of |E*| testing). Both factors contributed to the high tensile strains at the bottom of the AC layer (double those of the control). This was assumed to be the sole cause of the GTRDry cracking (a traditional bottom-up fatigue cracking mechanism), until forensic coring revealed mid-depth, horizontal cracking, prompting structural simulations via WESLEA. The results indicated that the lower AC and support moduli of the GTRDry section resulted in higher flexural strain responses. Peak shear stresses occurred near the location of mid-depth cracking; however, the magnitudes were found to be independent of the layer moduli, remaining relatively similar between the two sections. It was, therefore, concluded that the paved N1 (GTRDry) mixture was potentially more susceptible to horizontal cracking and bottom-up fatigue cracking distresses than the SBS-modified control.