|The objectives of this research are twofold. The first is to develop and validate an indirect tension stress relaxation test methodology for assessing asphalt binder properties using compacted hot mix asphalt (HMA) samples. The second is to evaluate the influence of adding various percentages of reclaimed asphalt material (RAP) to HMA mixtures using this stress relaxation test. More than 32 different HMA mixtures (two binders, two aggregate sources, two sources of reclaimed asphalt pavement (RAP) used at five different percentages, three replicates) were compacted and tested using indirect tension stress relaxation test at two temperatures (5 and 22oC).
Two relaxation characteristics were used to evaluate binder properties in the compacted HMA samples: 1) the initial stress relaxation modulus (at time=1 second), and 2) the curvature coefficient (exponent) from a power law model fit to the data for each mix.
The stress relaxation test showed that the addition of RAP increases the modulus linearly from 0 to 50% of RAP but that there is no statistically significant change in modulus when increasing the percent RAP above 50%. The curvature coefficient decreases linearly with the increase of the percent RAP from 0 to 50%, but like for the modulus, there is no statistically significant change in curvature coefficient when increasing the percent RAP above 50%. Similar trends are seen at both test temperatures.
Statistical analyses also showed that there are no statistical differences in the power model constants due to changes in the gradation, aggregate source, or RAP source. Since the initial hypothesis was that this test should be primarily a function of binder properties, the lack of significant influence by the aggregate gradation and aggregate type was expected. It was originally thought that RAP sources from different regions of the country (Alabama and Minnesota) would produce measurable differences in the effective mix binder properties since different performance grades (PG) are used in the different regions. However, testing of the extracted RAP binders showed that there was little difference in the recovered, aged binder. Therefore, little change in the effective HMA binder is expected.