Asphalt Concrete Cracking Device to Evaluate Low Temperature Performance of HMA

The current methods of evaluating the low temperature cracking resistance of HMA rely on extensive tests that require assumptions on material behaviors and the use of sensitive loading equipment. Th e purposes of this study were i) to develop a composite model for the low temperature behavior of asphalt pavement for the use in mechanistic analysis and ii) to develop a valid and simple test device to directly measure the cracking resistance of hot mix asphalt under field-like conditions. Among the factors affecting the low temperature performance of HMA, the coefficient of thermal expansion (CTE) of aggregate has been misrepresented for years. A composite model of HMA was developed 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 were amplified up to 18 times for a typical HMA. Of the local aggregates studied, the maximum and the minimum CTEs were 11.4 and 4.0 x 10(-6)/degrees C, respectively. The composite model showed that during cooling, the contraction of aggregates with a high CTE can double the thermal strain of asphalt binders in the HMA and could cause thermal cracking of the pavement to occur at warmer temperatures. A ring-shaped thermal restrained test, named the asphalt concrete cracking device (ACCD), was developed. The ACCD utilizes the low thermal expansion coefficient of Invar steel to induce tensile stresses in an HMA sample as the temperature is lowered. The results of tests using notched ring-shaped specimens compacted around an ACCD lnvar ring showed good repeatability. These tests showed a less than 1.0 degrees C standard deviation in cracking temperature. Laboratory validation indicated that ACCD results of five mixes correlate well with the thermal stress restrained specimen test (TSRST) results showing a correlation coefficient of 0.93. For TSRST, it takes two to three days from the preparation of the sample to the completion of the measurement. For ACCD, two samples can be prepared and tested in a single day with a small test set-up. Moreover, the capacity of ACCD can be easily increased to accommodate a larger number of samples.