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Indirect tensile test as a simple method for rut resistance evaluation of asphalt mixtures – Polish experience

To assess the resistance of asphalt mixes to permanent deformations, the wheel tracking tests are frequently used, what requires specialized equipment as well as relatively large size samples. For this reason, researchers are seeking for a more simple test that will help to assess the rut resistance of asphalt mixes. According to Christensen and Bonaquist as well as Zaniewski and Srinivasan, this may be the indirect tensile strength (ITS) test. Therefore, this research experimentally examined the relationship of ITS results on rutting parameters, i.e. proportional rut depth (PRDAIR) and wheel tracking speed (WTSAIR). The samples of asphalt concrete (AC), as well as SMA and BBTM, were compacted with the Marshall hammer, but ITS tests were conducted with a typical Marshall press, at a loading rate of 50 mm/min. The test results indicated the need for separate analysis of continuous type mixtures (AC) and gap-graded mixtures (SMA and BBTM). The relationship of ITS results with rutting parameters depends on: the type of mixture gradation (higher correlation coefficients were obtained for gap-graded mixtures) and ITS test temperatures (test results at 40°C achieved better correlations than those obtained at 25°C). Using the regression analyses, the prediction limits as well as confidence limits were calculated, which allowed to develop criteria for classifications of asphalt mixture resistance to rutting categories, on the basis of ITS test.