Thin HMA Overlays in Texas: Mix Design and Laboratory Material Property Characterization (0-5598-1)

In this interim report, various cold-laid and hot-mix asphalt (HMA) mixes were designed and/or evaluated based on the balanced mix-design concept and the Texas Department of Transportation (TxDOT) crack attenuated mix (CAM) special specification (SS) 3109 specification for their potential application as very thin overlay mixes. The research methodology incorporated extensive laboratory testing and field experiments including the Hamburg, overlay, and the ground penetration radar. While exhibiting fairly satisfactory field performance, laboratory results indicated that the cold-laid maintenance mixes are potentially susceptible to moisture damage (stripping). In general, their laboratory performance under the wet Hamburg test and the overlay test was very poor. Their excellent laboratory performance under dry Hamburg testing at ambient temperature suggest that these cold-laid mixes are good for application in dry areas, but they may not perform well under wet conditions or if water infiltrates into the mix. More research is recommended with these mixes. With the HMA mixes, promising laboratory results have been obtained with fine-graded (3/8-inch nominal maximum aggregate size) mixes; predominantly composed of Type F rock and screenings and an asphalt-binder content of over 7 percent. Based on the TxDOT CAM SS 3109 specification, high quality clean Class A aggregates, such as granite, exhibited superior laboratory performance and are recommended. However, acceptable laboratory designs were also obtained with good quality sandstone and limestone materials. As described in this report, the initial field performance of these mixes has been very good and this will be further validated in Year 2 of this study. Draft specifications and guidelines for thin HMA overlays will be submitted at a later stage after conducting field performance monitoring and evaluations of selected demonstration/implementation projects.