Validity of testing small asphalt concrete mixture beams in the bending beam rheometer

This work uses three asphalt concrete mixtures with decreasing nominal maximum aggregate size (NMAS) to evaluate the validity of using the bending beam rheometer (BBR) to obtain flexural creep modulus of asphalt concrete mixture beams. The flexural creep modulus of asphalt pavement is an important property used to give insight into the cold property behaviors of the pavement. Previous research has indicated that asphalt mixtures can be tested using small beam samples (12.7-mm x 6.35-mm x 127-mm) in the BBR. Given that some of the dimensions are smaller than the aggregate used in the mixture, there is a concern that significant errors would be introduced due to the influence of these larger aggregate, thus the test is not being conducted on the representative volume element (RVE). This paper evaluates this concern. To accomplish this, three mix designs with NMAS, 12.5-mm, 9.5-mm, and 4.75-mm were developed and tested in the BBR. The 12.5-mm NMAS mixture was an actual mix placed on SR-201 in Salt Lake City; the 9.5-mm and 4.75-mm NMAS mixtures were developed to decrease the ratios of NMAS to beam width and NMAS to beam thickness. This approach is meant to imitate the development of an RVE while maintaining the beam size of 12.7-mm width x 6.35-mm thickness x 127-mm length. The two smaller mixtures were developed to be scaled equivalents of the 12.5-mm NMAS with similar volumetric parameters and gradation shapes. A series of experiments using the BBR were performed to calculate the creep modulus of the asphalt mixture beams. Through statistical analysis it was found that flexural creep modulus data obtained from the BBR testing come from a normal distribution with equal variances across different sample groups. This means that the large aggregate mixtures resulted in no more variability than the smaller aggregate mixtures. Consequently, creep modulus data from asphalt mixture beams collected using the BBR could be used to predict the thermal properties of asphalt mixtures. Thus, based on these results, it is concluded that the small beam samples can be tested in the BBR as they meet the minimum RVE requirements.