Adhesive and cohesive properties of asphalt-aggregate systems subjected to moisture damage

The bond strength between asphalt and aggregate plays a fundamental role in evaluating the moisture sensitivity of HMA Mixtures. In this study the effect of water on adhesive and cohesive properties of asphalt-aggregate systems was investigated using a modified version of the PATTI. The device was used to measure the pull-off strength on different asphalt-aggregate combinations and to evaluate the influence of water immersion at two different temperatures. In particular, six asphalt binders were employed in combination with two aggregate types, having different asphalt affinity. The effect of the aggregate surface temperature during specimen preparation was also tested. In the first phase of the study the within-laboratory repeatability of the test procedure was investigated. The results showed the PATTI test is able to evaluate with good precision the pull-off strength and that its repeatability depends on the failure type (adhesive or cohesive). In the second phase of the study a full factorial experiment was employed to verify the reliability of the test for routine use in determining the adhesive and cohesive properties of asphalt-aggregate combinations and the effects of moisture damage. The results showed that, in the dry condition, the test was able to measure the internal cohesion of the asphalt binders. The results also showed the effects of water damage on the pull-off strength and the decisive role of asphalt-aggregate affinity was clearly highlighted. Using wet conditioning of the PATTI samples it was proven that water affects the adhesive bond between asphalt and aggregate much more than the asphalt cohesion. Moreover, the results indicate that aggregate temperature during sample preparation has only a limited effect on the adhesive strength.