United States Department of Transportation
i
A simple test procedure for evaluating low temperature crack resistance of asphalt concrete.
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2009-11-01
[PDF-4.00 MB]
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Edition:Final report.
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Abstract: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.
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