Simplification of Linear Amplitude Sweep Test and Specification Parameter

The linear amplitude sweep (LAS) test has recently been approved by AASHTO as a provisional standard and is currently being considered for specification of asphalt binder fatigue resistance. The LAS test is a cyclic torsion test conducted in the dynamic shear rheometer (DSR) that uses increasing loading amplitudes to accelerate damage. The LAS test has been proposed as a surrogate to the time sweep (TS) test, which is a conventional fatigue test consisting of repeated cyclic loading at constant amplitude. The TS test is considered unsuitable for specification because of the uncertainty in the test duration required to induce and quantify fatigue. Recent ruggedness testing efforts have shown that the LAS test cannot be conducted by all DSRs. This study evaluates a simpler amplitude ramping procedure that resolved the difficulties encountered with some rheometers. Additionally, a simple specification parameter is introduced to offer a practical approach to define damage tolerance. The definition of failure is based on a more recent study to explain the mechanisms of fatigue damage under TS testing in the DSR. On the basis of a comparison between TS and LAS test results, this paper demonstrates that the damage evolution in the two tests differs, thus indicating that the LAS test is very complex and is better referred to as a damage tolerance test rather than a true fatigue test. With the simplified analysis approach for the modified LAS procedure, a critical cracking length signifying the start of rapid crack growth is proposed for specification of binder damage tolerance at intermediate temperature.