Design and Performance Evaluation of Very Thin Overlays in Texas (0-5598-2)

Very thin overlays, 1-inch thick or less, were placed as surface layers on five major highways in Texas. There mixes were designed in the laboratory to have a balance of good rut resistance as measured by TxDOT's Hamburg Wheel Tracking test and good reflection cracking resistance as measured by TTI's Overlay Tester. These Crack Attenuating Mixes (CAM) were designed and constructed based on a new special specification SS 3109. In the design phase the optimal asphalt content was initially determined using the Superpave Gyratory Compactor selecting the asphalt content that achieved 98% density after 50 gyrations. This approach worked well with stiff binders and top quality granite aggregates. However design problems were encountered with the transition to softer binders and locally available materials. The researchers proposed an alternative design procedure where the performance tests are run first at a range of asphalt contents and a window defined where both rutting and crack resistance requirements are met. The CAM mixes designed have 100% passing the 3/8-inch sieve and binder contents ranging from 7 to 8.3% asphalt. Few construction problems were identified; these fine mixes are easy to compact and finish. However, on one project thermal segregation problems were identified that caused low density pockets and areas of raveling. Initial performance even over jointed concrete has been good, and skid resistance measurements looked very reasonable. The CAM mixes cost approximately 25% per ton more than the traditional mixes, but as they are placed as 1-inch thick mats rather than 2-inch thick, there is a clear economic advantage of using these high quality materials. TxDOT is in the process of updating SS 3109, and a statewide specification is scheduled for release in early 2009.