Recommended Changes to the Precision Estimates of AASHTO T 313 (BBR)

A comparison of precision estimates in the 2019 AASHTO T 313 (BBR) with those of 2012 indicates that the 2019 multilaboratory precision values are notably smaller than those of 2012. This decrease has a significant effect on the percentage disagreements between contractors’ test results and those of state departments of transportation (DOTs). A review of state DOT and contractors’ bending-beam rheometer (BBR) results indicated that 39% of contractors' data were in disagreement with the state DOT’s data, primarily because the allowable difference between the BBR results of two laboratories (d2s) is set too low. This could cause unnecessary additional costs of testing and delay in a project. Another shortcoming of the AASHTO T 313 precision statement is that the current precision values are for creep stiffness (S) and slope (m-value) at the PG+10°C test temperature only. With the increased applications of continuous low-temperature performance grade (LTPG) and delta Tc (ΔTc), asphalt binders are also required to be tested at the lower temperature of PG+4°C. Therefore, addition of precision values for the lower test temperature is necessary. This study aimed at reevaluating BBR precision properties at PG+10°C, preparing new sets of precision values for PG+4°C, and determining precision of LTPG and ΔTc. An interlaboratory study involving seven laboratories testing six binders was conducted. The new and improved sets of precision values at PG+4°C and PG+10°C have reduced disagreements between the BBR results of two laboratories to about 5%, which better represents the 95% reliability anticipated in the analysis of precision.