Superpave test methods for asphalt – Procedure for DSR testing (NT TR 538)

Abstract

  Between 1987 and 1993 in the U.S.A., the Strategic Highway Research Program (SHRP) surveyed all aspects of the physical tests currently carried out on bitumen, and it was shown that new performance-based methods, such as Dynamic Shear Rheometer (DSR), Bending Beam Rheometer (BBR) and Pressure Ageing Vessel (PAV), are needed. The new bitumen specification developed in the SHRP was called the Superpave specification. The researchers showed that the normal physical tests, such as penetration or viscosity, did not give enough information concerning the behaviour of the road bitumens in actual road conditions. The purpose of the new methods (DSR, BBR and PAV) was to provide information on how the bitumens actually behave in practice. In the new Superpave binder specification, the most significant advancement on the European CEN standard specification was probably the move from empirical testing to advanced functional testing, where a bitumen can be characterized at a controlled rate and temperature in order to obtain the engineering properties of the binder. The new specification facilitates the purchase of superior quality bitumen. Consequently, the chosen bitumen performs adequately and does not cause pavement failure. The progress and suitability of the new testing methods, for European conditions, have been evaluated since the beginning of 2001 in the evaluation group organized by the CEN TC 336 working group WG1 for bitumens. From this evaluation, it was clear that the DSR and BBR methods in particular could also be adapted to European standards. At present, the Nordic countries are not quite as prepared as they should be for carrying out the Superpave tests. This is due to the recently adopted EN bitumen norm specification, which was based only on traditional test methods. The laboratories in the Nordic countries will probably adapt to the new methods and equipment when the CEN working group gains more experience with the new methods. Descriptions of the new methods – DSR, BBR, DTT (Direct Tension Test) and PAV – are included in this report. Because the DSR method was determined to be the main method for measuring the deformation characteristics of road surfaces in the future, this test has been evaluated in more detail. The description of the DSR method is structured as follows: scope and field of application, basic rheological background, device calibration, measurement by deformation or fatigue criteria, choosing the test temperature, making the specimens, and finally analyzing the performance grade (PG grade) of bitumen based on the Superpave specification. The test procedure used to measure the DSR values of the original bitumen and the PAV and RTFOT test-aged bitumens is presented. Some precision estimates are also shown. Suggestions for future work include a continuation of the evaluation of the methods within the CEN European evaluation group. Precision estimates of the DSR method must be backed up by results from similar/comparable studies. The new DSR method seems best suited to the testing of conventional bitumens. It has been difficult to operate DSR with polymer-modified bitumens (PmB). These testing problems with the PmB thus need further investigation. The details of the BBR and PAV tests should also be examined by a separate Nordtest research team in the future.