Forensic Analysis of Slippage Cracking (05-1422)**
Stephane Charmot, SemMaterials Company
Pedro Romero, University of Utah
Michael Dunning, Clark County Department of Public Works

Premature pavement failure is costly to agencies and disturbing to the traveling public. This paper discusses slippage cracking which occurred shortly after construction affecting about half of the overlay project length. This article offers a comparative analysis based on the review of quality control information, performance testing of the hot mix asphalt using the Superpave shear tester (SST), in situ evaluation of the unbound layers using the dynamic cone penetrometer (DCP) and computed surface deflections. A great emphasis was placed on the evaluation of the mix properties because of the perceived lack of mix cohesion when visually examining the damaged pavement. Performance testing of field cores indicated that the hot mix from the section with cracking performed similarly in terms of stiffness and slightly better in terms of accumulated permanent strain than the hot mix from the section without cracking demonstrating that the cause of the problem was not related to hot mix asphalt properties. Testing of laboratory samples prepared with various binders also confirmed the superior properties of the polymer-modified asphalt used on the project. The results of this study showed that the DCP curves were significantly different between the sections with and without slippage cracking. Estimated surface deflections were higher for the areas which had slippage cracking. An empirical relationship was used to estimate equivalent single axle loads (ESAL’s) to failure from surface deflection showing that it would take less traffic for the section that had slippage cracking to fail when compared to the section without cracking.