A Direct Shear Approach to Assess the Interface Bonding Performance

Abstract

The objective of this study was to investigate the effects of various factors affecting bond strength between hot mix asphalt (HMA) overlay and underlying pavement layers in the field. The effect of interface bonding on short-term pavement performance was also evaluated. A list of candidate HMA field rehabilitation projects was identified across the United States, representing different traffic and environmental conditions. These projects included the rehabilitation of new, existing, and milled HMA pavements, and PCC pavements. Each field project involved at least one slow setting and one rapid setting non-tracking tack coat material, thereby creating one or more pairs of tack coats for comparison. The HMA overlay construction used different types of tack coat materials at various residual application rates. Specimens were cored from the evaluated test sections at different service times to determine the interface shear strength (ISS) according to AASHTO TP 114, “Standard Method of Test for Determining the Interlayer Shear Strength of Asphalt Pavement Layers.” Results of this study showed that ISS was largely dependent on the type of pavement surface (i.e., HMA vs. PCC) receiving tack coat, and pavement surface texture (i.e., milled vs. non-milled). In general, milled HMA surface yielded the highest ISS, followed by new HMA, existing HMA, and PCC surface types. Non-tracking rapid setting tack coats with stiff base asphalt cement exhibited the highest ISS compared to slow setting tack coats. With respect to the effect of service time, ISS increased with service time due to tack coat curing effect and densification of overlays. Laboratory measured ISS values correlated well with short-term cracking performance of field pavements. Results presented herein were part of NCHRP Project 9-40A on the “Field Implementation of the Louisiana Interface Shear Strength Test.”