A novel disc bend test specimen for realistic measuring of tensile strength in pavements under transverse cracking condition

The tensile strength of asphaltic mixtures is an important parameter in designing the pavement materials and life assessment of the overlays. This parameter is often determined experimentally by using the indirect diametral tension (IDT) test or sometimes by employing the semi-circular bend (SCB) test. However, neither can provide accurate and true tensile strength values for practical and field situations. While the applied load to the overlay induced by traffic loading is along the compaction direction, there is a 90-degree angle difference between the direction of applied loading to the IDT and SCB samples and the compaction direction of the asphalt mixture manufactured inside the Marshall or Gyratory cylinders. Hence, in this research, a novel test specimen is proposed for the first time to measure the tensile strength of asphalt concrete materials in more realistic conditions. The proposed method utilizes a disc shape sample named Symmetric Disc Bend (SDB), which is a specimen loaded by a conventional three-point bending fixture. The tensile strength value obtained from the SDB specimen was observed to be significantly greater than the results obtained from the SCB and IDT methods for any given test conditions. The main reason for such discrepancy can be attributed to the compaction direction of the asphalt mixture and the applied loading direction to the samples. Accordingly, the tensile cracking resistance of the HMA mixture along the compaction direction is significantly higher than in the other directions. Therefore, the test data often determined from the IDT testing method can provide conservative predictions for the service life of real asphaltic pavements. In addition, due to the bending type load in the SDB testing procedure, the proposed method provides better consistency with the tensile cracking of real flexible pavements subjected to bending type loads applied from the vehicles. Some suitable SDB dimensions and loading span ranges were proposed for conducting the indirect tensile strength tests using the SDB specimen. In addition, the fracture patterns of specimens tested were simulated and predicted using the XFEM method, and stress distributions were reviewed.