Tack coat emulsion, comprised of asphalt binder, water and emulsifying agents, is an interlayer membrane applied before asphalt pavement construction, which helps pavement layers bond together and permeate stresses and strains induced by heavy wheel loads through the entire thickness of the pavement structure. Proper interlayer bonding aids in preventing localization of stresses and strains at the surface asphalt layer, where premature fatigue cracking can develop causing early pavement failure. In Oregon, CSS-1H tack coats have been used conventionally; however, newly engineered tack coats are purported to improve interlayer bond quality. Proprietary blends of polymer modifiers used in engineered tack coats are used to improve interlayer bond characteristics. Engineered tack coats are often specific to overlay surfaces with low macrotexture or milled pavement surfaces with high surface macrotexture. However, they are not yet widely adopted due to their increased cost.
Pavement construction can inherently produce adverse conditions that can compromise tack coat bond quality and exacerbate failure mechanisms that stem from poor interlayer bonding. These conditions are prevalent in Oregon pavement construction. Milling of existing aged pavements creates high amounts of dust, some of which is retained on the pavement surface after cleaning. Tack coats readily bond to dust particles instead of the pavement layers, resulting in reduced bond quality. Problems with tack coat distributor equipment can create nonuniform tack coat application due to clogged applicator nozzles, often termed “streaking”, effectively reducing the pavement surface area covered by tack coat. Rainfall during construction can wash away the applied tack coat and reduce the ability of the two asphalt layers to bond together.
In this study, engineered tack coats were evaluated against conventional tack coats under different application rates, pavement surface types (overlay and milled) and in the presence of adverse conditions commonly experienced during Oregon pavement construction. A novel laboratory sample preparation method was developed using a hydraulic laboratory roller compactor and laboratory milling to closely mimic field conditions. Monotonic Direct Shear Testing (DST) was employed for laboratory evaluation of tack coat performance. The Oregon Field Torque Tester (OFTT), developed at Oregon State University, was used to evaluate tack coat bond quality in the laboratory and in the field. Statistical analyses were performed to compare tack coat performance. Comparisons between monotonic DST and OFTT results were also made to identify the suitability of the OFTT for evaluating tack coat bond quality in the field. Field OFTT results were compared to laboratory tests to highlight bond quality issues that exist with pavements in Oregon.
Results showed that engineered tack coats perform better than conventional tack coats in general. Adverse conditions such as dust, nonuniform coverage/streaking and rainfall significantly affected tack coat performance. Comparisons showed that OFTT tests were generally well-correlated with laboratory monotonic DST results, indicating that OFTT testing captures the interlayer bond quality of both engineered and conventional tack coats. Field performance of tack coats as measured by the OFTT were inferior when compared to laboratory results, due to lack of control over extraneous factors during construction. This disparity proves that better control over construction practices needs to be sought in order to obtain proper tack coat bond quality in the field.