Perpetual Pavements in Texas: State of the Practice

As of December 2005 TxDOT has four perpetual pavement sections in service and another four under construction. Project 0-4822 was initiated to perform a structural assessment of these thick asphalt pavements, to identify strengths and weaknesses in the existing structures, and to provide guidance for future designs. This Year 1 report provides an evaluation of the existing sections. It is based on extensive nondestructive testing with Ground Penetrating Radar (GPR), Falling Weight Deflectometers (FWD), field coring, and limited laboratory testing. On a positive note, the stone-filled mixes used in these structures are considerably stiffer than the dense graded mixes traditionally used in Texas. Design moduli values of 750 ksi and 1000 ksi are recommended for future designs with the Stone Matrix Asphalt (SMA) and 1-inch stone-filled (SF) layers. However, three major problems were identified. Firstly, the 1-inch stone-filled layers are prone to vertical segregation. Several of the sections were found to have severe honeycombing at the bottom of the lifts. These mixes are excessively coarse with low asphalt binder contents around 4 percent. Mix design procedures must be modified to eliminate this problem. Secondly, all but one of the projects was found to have de-bonding occurring between layers. This will severely impact the fatigue life of these pavement sections. Thirdly, better guidelines need to be developed on what constitutes a foundation layer for perpetual pavements. Several of the current foundation layers are not thought to be permanent.