The function of the low-temperature asphalt material strength performance tester.

The function of the low-temperature asphalt material strength performance tester.

The Low-Temperature Asphalt Material Strength Performance Tester is a new type of testing equipment used to evaluate the low-temperature cracking performance of asphalt binders or mixtures. Its primary application is to determine the temperature-strain curve of asphalt binders according to the AASHTO T387 Asphalt Binder Cracking Test Method (ABCD), thereby directly obtaining the low-temperature cracking temperature.

The ABCD test for the strength performance of low-temperature asphalt materials involves inserting an asphalt specimen into a ring-shaped test mold composed of silicone rubber and a steel ring. Strain gauges and temperature sensors are bonded to the inner surface of the steel ring. At the location where the strain gauges are attached, there is a silicone rubber protrusion that penetrates through the asphalt sample. The test mold, filled with the asphalt specimen, is then placed in a temperature-controlled chamber, and the test is initiated. The initial test temperature is 20°C, after which the temperature is uniformly reduced at a constant rate of 40°C per hour within the range of 0°C to 20°C. Within the range of -60°C to 0°C, the temperature is uniformly lowered at a rate of 20°C per hour. During the cooling process, the deformation caused by the contraction of the asphalt specimen due to the low temperature is more than 100 times greater than that of the steel ring, thereby exerting a pressure effect on the steel ring. Strain gauges are used to measure how strain varies with temperature. As the temperature continues to drop, stress concentrations develop in the annular region formed by the silicone rubber protrusion on the asphalt specimen, leading initially to crack formation. When cracks occur, the pressure exerted by the asphalt on the steel ring suddenly decreases, causing a sharp change in strain. Consequently, the cracking temperature of the asphalt can be accurately determined based on the temperature-strain relationship. Therefore, compared to the BBR low-temperature bending beam rheological test and the DT direct tensile test used in Superpave PG grading, the ABCD test method does not require any assumptions or calculations, enabling a more direct determination of the asphalt’s cracking temperature. To validate the effectiveness of the ABCD test method, a total of 23 laboratories across the United States participated in evaluating and verifying its performance during the development of the test method. Among these, 11 laboratories conducted comparative tests against the results of the BBR test, while another 11 laboratories compared their findings with those from two test roads: the Elk Country test road in Pennsylvania, USA, and Highway 17 in Ontario, Canada.

In the comparative tests conducted by 23 laboratories using the Low-Temperature Asphalt Material Strength Performance Tester, asphalt was classified into high (H), medium (M), and low (L) grades according to its modulus. Overall, the consistency of the test results was good. For low-modulus asphalt, the variability in test results was greater during sampling, transportation, and handling; in contrast, high-modulus asphalt was less affected, and its test results were more stable. By comparing the evaluation results from the ABCD and BBR low-temperature tests on SBS-modified asphalt, it was found that the ABCD test results indicated that as the SBS content increased, the low-temperature cracking temperature of the asphalt decreased—that is, its low-temperature performance improved. However, the BBR test results typically yielded the opposite conclusion, with experimental findings contradicting theoretical expectations. Therefore, compared to the BBR test, the ABCD test method is better suited for evaluating the low-temperature performance of SBS-modified asphalt (measuring both the low-temperature cracking temperature and the low-temperature failure strength). The Low-Temperature Asphalt Material Strength Performance Tester can also employ two different methods to assess the low-temperature performance of asphalt mixtures—these methods are known as the “Asphalt Concrete Cracking Device, ACCD Test” and the “Ohio Coefficient of Thermal Expansion/Contraction Device, OCD Test.”

The ACCD test, an instrument for evaluating the strength performance of low-temperature asphalt materials, was developed as a simplified version of the TSRST test (Constrained Specimen Temperature Stress Test). ACCD specimens can be molded, drilled, and cut using a rotary compactor. A steel ring is placed at the center of the specimen. During the cooling process, the steel ring restricts the shrinkage of the asphalt mixture, while the asphalt mixture exerts pressure on the strain gauges mounted on the steel ring. Strain gauges and temperature sensors record the temperature-strain curve of the specimen, enabling the calculation of the low-temperature cracking temperature of the asphalt mixture.