Field Investigation and Performance Evaluation of Interface Bonding between Pavement Layers

Proper bonding between pavement layers is essential to provide a monolithic pavement structure that allows distribution of loading over a larger area. Inadequate interface bonding may cause debonding or shear failure at the interface due to the accumulation of concentrated tensile stresses at the bottom of the overlay. Tack coat, a light application of asphaltic cement emulsion is applied on the existing surface to create a strong adhesive bond between pavement layers. The objectives of this study are to investigate the effects of various factors affecting bond strength between hot-mix asphalt (HMA) overlay and underlying pavement layers in the field. Effect of interface bonding on the short-term pavement performance was also evaluated. To achieve the objectives, a list of candidate HMA field rehabilitation projects was identified across the United States representing different traffic and environmental conditions. The experimental program encompassed ten field projects in six states to quantify the effects of the selected variables. These projects included 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 materials. 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). 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 the short-term cracking performance of field pavements. Furthermore, the predicted ISS obtained from the proposed models were in good agreement with the measurements, and the developed nonlinear model showed better prediction accuracy than the linear model