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The current means of evaluating the low temperature cracking resistance of HMA relies on extensive test methods that require assumptions about material behaviors and the use of complicated loading equipment. The purpose of this study was to develop and validate a simple test method to directly measure the cracking resistance of hot mix asphalt under field-like conditions. A ring shape asphalt concrete cracking device (ACCD) was developed. ACCD utilizes the low thermal expansion coefficient of Invar steel to induce tensile stresses in a HMA sample as temperature is lowered. The results of the tests of the notched ring shaped specimens compacted around an ACCD Invar ring showed good repeatability with less than 1.0°C (1.8°F) standard deviation in cracking temperature. A laboratory validation indicated that ACCD results of five mixes correlate well with thermal stress restrained specimen test (TSRST) results with the coefficient of determination , r2 = 0.86. To prepare a sample and complete TSRST measurement, it takes minimum 2-3 days. For ACCD, two samples can be easily prepared and tested in a single day with a small test set-up. The capacity of ACCD can be increased easily with minimal cost to accommodate a larger number of samples. Among factors affecting the low temperature performance of HMA, the coefficient of thermal expansion (CTE) of aggregate has been overlooked for years. A composite model of HMA is proposed to describe the low temperature cracking phenomenon. Due to the orthotropic and composite nature of asphalt pavement contraction during cooling, the effects of aggregate CTE is amplified up to 18 times for a typical HMA. Of 14 Ohio aggregates studied, the maximum and the minimum CTEs are 11.4 and 4.0 x 10-6/°C, respectively. During cooling, the contraction of Ohio aggregate with high CTE can double the thermal strain of asphalt binders in the asphalt mix and may cause asphalt pavement thermal cracking at warmer temperature.
This report describes the thermal stress restrained specimen test (TSRST), which was selected to evaluate the low-temperature cracking resistance of asphalt concrete mixtures. The TSRST system includes a load frame, step-motor-driven load ram, data acquisition hardware and software, temperature controller, and specimen alignment stand. An experiment design that considered a range of mixture and test condition variables was developed to evaluate the suitability of TSRST for characterizing low-temperature cracking resistance of asphalt concrete mixtures. Four asphalts and two aggregates were selected for the experiment. The mixture variables included asphalt type, aggregate type, and air voids content; the test condition variables included specimen size, stress relaxation, aging, and cooling rate.
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.
The purpose of the field validation program was to evaluate the thermal stress restrained specimen test (TSRST) as the accelerated performance test to predict low-temperature cracking of asphalt concrete mixtures. Construction histories, cracking observations, and temperature data were collected for five test roads. In addition, a validation program was conducted at the United States Army Cold Regions Research and Engineering Laboratory. The laboratory test program consisted of performing the TSRST on specimens fabricated in the laboratory with original materials from the test roads and asphalt concrete pavement specimens cut from the actual test sections. In addition, the field pavements were monitored for crack history and, where possible, crack initiation. TSRST fracture temperature correlated with field cracking temperature and crack frequency. TSRST results can be used to predict field low-temperature cracking of asphalt-aggregate mixtures. Preliminary models to predict cracking frequency and temperature for the test roads were developed.
Introduction and Research Approach -- Findings -- Interpretation, Appraisal, and Applications -- Conclusions and Recommendations -- References -- Appendixes.
A thermal stress restrained specimen test (TSRST) was developed to determine the thermal, or the low-temperature cracking resistance of asphalt concrete mixes. The test system is capable of cooling an asphalt concrete specimen at a constant rate, while restraining the specimen from contraction and periodically measuring elapsed time, specimen surface temperature, and tensile load. TSRST's were performed on both short- and long-term aged specimens. Statistical analyses were performed on the test results. Rankings of asphalt concrete mixtures based on fracture temperature were compared to rankings based on fundamental properties of the asphalt cement.