Thermal cracking of asphalt concrete pavements is responsible for millions of dollars in annual maintenance and rehabilitation costs in the United States and Canada. Thermal cracking is typically associated with low temperatures in northern climates and at high elevations. However, another form of thermal cracking, known as thermal fatigue cracking, has been proposed by several researchers as a potential mode of distress in regions with relatively moderate climates but significant differences in high and low daily temperatures. The primary purpose of the research reported herein was to evaluate the possibility of occurrence of the thermal fatigue cracking mode of distress. A secondary objective was to identify a suitable laboratory test procedure to facilitate a mechanistic analysis of the thermal fatigue mode of distress. In light of these objectives, several laboratory test procedures were evaluated in the bituminous materials laboratory at Oregon State University (OSU). The test procedures evaluated included the phenomenological Thermal Stress Restrained Specimen Test (TSRST), the Energy Rate Integral Test (ERIT), the Direct Tension Test under constant rate of extension (DTT), and the Direct Tensile Creep Test (DTCT). The TSRST results were used to evaluate the possibility of occurrence of the thermal fatigue mode of distress. The ERIT, DTT, and DTCT procedures were evaluated with respect to the identification of a suitable laboratory test procedure to facilitate a mechanistic analysis of thermal fatigue. The results from the laboratory test program indicate that thermal fatigue distress in asphalt concrete mixtures is not a viable mode of distress in the absence of environmental aging. Based on the data presented herein and the results of previous researchers, it is evident that distress often attributed to thermal fatigue cracking is more likely the result of low temperature cracking of environmentally aged mixtures, and/or subgrade-related distress; fatigue distress due to thermal loading of semi-restrained pavements does not occur. / Graduation date: 1993

Thermal distress in asphalt concrete pavements is a widespread problem around the world. Thermal cracking can be divided into two modes of distress: low temperature cracking and thermal fatigue cracking. Low temperature cracking results from extremely cold temperatures; thermal fatigue cracking results from daily temperature cycles. Low temperature cracking is attributed to tensile stresses induced in the asphalt concrete pavement as the temperature drops to an extremely low temperature. If the pavement is cooled, tensile stresses develop as a result of the pavement's tendency to contract. The friction between the pavement and the base layer resists the contraction. If the tensile stress equals the strength of the mixture at that temperature, a micro-crack develops at the surface of the pavement. Under repeated temperature cycles, the crack penetrates the full depth and across the asphalt concrete layer. The thermal stress restrained specimen test (TSRST) was identified as an accelerated laboratory test to evaluate the thermal cracking resistance of asphalt concrete mixtures. The TSRST system developed at OSU includes a load system, data control/acquisition system and software, temperature control system, and specimen alignment stand. The overall system is controlled by a personal computer. A TSRST is conducted by cooling an asphalt concrete specimen at a specified rate while monitoring the specimen at constant length. A typical thermally-induced stress curve is divided into two parts: relaxation and non-relaxation. The temperature at which the curve is divided into two parts is termed the transition temperature. The temperature at fracture is termed the fracture temperature and the maximum stress is the fracture strength. An extensive number of TSRSTs over a wide range of conditions were performed to investigate the thermal cracking resistance of asphalt concrete mixtures. The TSRST results provided a very strong indication of low temperature cracking resistance for all mixtures considered. A ranking of mixtures for low temperature cracking resistance based on the TSRST fracture temperature was in excellent agreement with a ranking based on the physical properties of the asphalt cements. It is highly recommended that the TSRST be used in mix evaluation to identify low temperature cracking resistance of asphalt concrete mixtures. The TSRST showed very promising results regarding the effect of all variables which are currently considered to affect the low temperature cracking of mixtures. The variables considered to have significant affect on the low temperature cracking resistance of mixtures in this study include asphalt type, aggregate type, degree of aging, cooling rate, and stress relaxation. / Graduation date: 1994