Reduced confined dynamic modulus testing protocol for asphalt mixtures

The dynamic modulus (E*) of asphalt mixtures correlates well with field performance. Studies have shown that in comparisons of dense-, gap-, and open-graded mixtures, confined E* (C-E*) tests are recommended to rank and compare the expected field performance of the different mixtures. Unfortunately, C-E* tests are harder to perform than unconfined E* (U-E*) tests. Factors such as time, availability of equipment, technical complexity, and financial needs must be carefully considered when a confined laboratory-testing program is being proposed. The objective of this study was to develop a simple method by which C-E* values could be predicted from U-E* results with a reduced number of C-E* tests. The methods analyzed were developed by using E* test results conducted on nine asphalt rubber mixtures at different confinement levels and were validated by using three conventional densegraded mixes previously tested at Arizona State University. The U-E* and C-E* test results generally showed a linear relationship with the applied bulk stress and were almost parallel regardless of the confinement level. An approach to predict a C-E* relationship from U-E* test data was developed by using one (or two) confined test(s) instead of the five required by current test protocols. The methods investigated potentially would provide a decrease in laboratory testing time of at least 50%. Because some level of laboratory confinement is recognized for gap- and open-graded mixes, the methods investigated in this study also may prove helpful in the development of more accurate pavement distress prediction models.