Evaluation of Bond Strength Between Pavement Layers

The primary objective of this project was to develop a test for measuring the bond strength between pavement layers. The research was also to evaluate tack coat materials and application rates for the Alabama Department of Transportation (ALDOT). The project included a laboratory phase and a field phase. For the laboratory work, the experiment included two types of emulsion (CRS-2 and CSS-1) and a PG 64-22 asphalt binder that are allowed by ALDOT’s specifications. Bond strengths were measured with a shear type device at three temperatures and three normal pressure levels. Three application rates that encompassed the specification range were investigated for each tack coat. Laboratory prepared mixture samples included a coarse-graded blend and a fine-graded blend to represent two different surface textures. The effects of tack coat type, application rate, mixture type, testing temperature and normal pressure on the bond strength were evaluated. In the laboratory phase, it was found that all of the main factors used in the test plan affected bond strength. Testing temperature had the most significant impact on bond strength. As the temperature increases, bond strength decreases significantly. Testing normal pressure affected bond strength differently for high, intermediate, and low temperatures. The PG 64-22 had higher bond strength than the two emulsions, especially for the fine-graded mixture tested at high temperature. For the range studied, tack coat with low application rates generally provided high bond strength for the fine-graded mixture. However, for the coarse-graded mixture, bond strength does not change much when application rate varies. The two mixture types provided different bond strengths. The influences of tack coat type and application rate on bond strength are different for the fine-graded and coarse-graded mixtures. Based on the laboratory work, a draft procedure was developed for determining the bond strength between pavement layers. An easy to use procedure was selected that was believed to provide a good indication of the quality of the bond. The procedure utilizes the simple shear device developed by National Center for Asphalt Technology (NCAT) which is similar to bond strength devices used in several European countries. The draft procedure is based on a test temperature of 77oF and a loading rate of two inches/minute. The draft bond strength procedure was validated in the field phase of the study. Test sections with different tack coat application rates were set up on seven paving projects. For each test section, the actual application rates were measured and cores were taken to measure the bond strength using the draft procedure. On a few projects, the measured tack coat application rates were significantly lower than the range targeted by the specifications. A key finding of the field study was that the bond strength between pavement layers is significantly enhanced for milled surfaces. Good bond strengths were obtained for both emulsion and paving grade asphalt tack coats applications.