Evaluating the Effect of Air Voids and Binder Content in Cold Temperature Testing of Asphalt Mixtures with the Bending Beam Rheometer

Pavement performance in cold temperature regions is a concern to highway agencies. Laboratory methods need to be established to test the cold temperature performance of asphalt mix designs. Research conducted at the University of Utah has shown that testing asphalt concrete mixtures using the bending beam rheometer (BBR) is a viable method to measure low temperature performance. Data collected from field cores were used to develop performance relations. However, testing field cores from already constructed roads does not prevent poor performance mixes from being placed on the road. A better approach is to use laboratory-compacted samples from field mixes that are already used for volumetric quality control. While this still relies on material obtained at the time of construction, it is closer to the ultimate goal of evaluation of materials at the mix design stage for cold temperature performance. Typically, volumetric cylinders are gyratory compacted to an air void content of 4% by mass, while field cores target an air void content between 6% and 7%. The question arises in the variability of BBR results from air content ranging between 4% and 7%. For this research, gyratory cylinders were compacted at varying levels of compaction in order to achieve a range of air voids to evaluate stiffness measurements with the BBR. Different binder contents were also tested, with compaction level held constant and binder content varying. For varying air voids, the 60 and 120 second creep modulus was seen to decrease by as much as 30% within the 6% air void range. The m-value for varying air voids was not affected. Results also showed that, at binder content levels close to optimum, no noticeable change was observed in beam measurements. In addition, the m-value was seen to be independent of binder content.