Equivalent Damage and Fatigue Testing of the HB 2060 Pads (PSR 0-5327-S)

Texas currently has approximately 17,000 miles of load-zoned roads, the majority of which are posted at 58,420 lbs. These roads consist predominantly of thin pavement structures constructed in the 1940s and 1950s, originally designed to carry the lighter loads of those times. In 1989, the 71st Texas Legislature passed House Bill 2060 (HB 2060), which established a $75-per-county permit that allows truckers to carry legal loads (80,000 lbs plus a 5 percent GVW tolerance = 84,000 lbs) on load-zoned roadways and bridges. Previous research has indicated that the HB 2060 permit fee does not provide sufficient revenue to compensate for the damage caused by these heavy loads. This conclusion was based on an evaluation of the damage models in the 1993 American Association of State Highway and Transportation Officials (AASHTO) guide Design of Pavement Structures, Chapter 4, “Lowvolume Road Design,” and a field study. Although this information was compelling, the HB 2060 permits are still available to truckers who choose to operate on the state’s load-zoned road network. Accelerated Pavement Testing (APT) technology has the potential to provide valuable information that, in a relatively short time, could demonstrate the damage caused by heavily loaded trucks to the load-zoned road network. Accelerated Pavement Testing of pavements is widely accepted as an important aid in decisionmaking for material characterization and pavement design, analysis, and performance. APT technology is perfectly suited to address the problem described in this research and to obtain visible short-term results. For this reason, the Center for Transportation Research (CTR) at The University of Texas at Austin proposed to conduct controlled, accelerated pavement tests to evaluate the increased damage to thin pavement structures caused by increasing axle loads. The research project was planned under the assumption that the Texas Mobile Load Simulator (TxMLS) would be available as the APT device. The first objective of the research was to quantify the response and performance of a relatively light pavement structure with increased traffic load applications under the TxMLS. The second objective of the series of four tests was to compare the performance of the same pavement structure under different axle loads. In turn, this activity could serve as an experiment to validate currently used approaches to estimate equivalent damage, specifically the value of the exponent of the so-called powerlaw, which postulates that the damaging effect of an axle increases exponentially with its load (an exponent of four is generally accepted). The third objective was to develop a methodology to estimate the reduction of expected pavement performance as a result of increased axle load. The development of this methodology was to be based on the data collected during the course of Project 0-5372 and was to be used to facilitate the development of guidelines to estimate load equivalency factors for similar pavement structures to those tested under the TxMLS, expanding the test results to a bigger inference space.