Abstract
Modulus or stiffness of granular layers is a fundamental material property needed for pavement analysis and design and is also used as a quality control indicator of a constructed pavement layer. This paper presents results from a laboratory study on repeated load triaxial tests with bender elements (BE) to compare the modulus properties of base, subbase, and subgrade materials in small and resilient strain levels. Three pavement base course aggregate materials and one silty-sand subgrade soil were selected to prepare specimens at target maximum densities and optimum moisture contents. BE pairs were installed at different heights in the specimens to collect shear-wave velocity data. In addition, a pair of piezoelectric disk elements was installed in the subgrade sample to collect the compressional wave velocity. Repeated load triaxial testing was performed following the AASHTO T 307 resilient modulus test procedure. From the 15 loading stages at different stress states, the shear-wave velocities, and compressional-wave velocity (when applicable), the resilient modulus properties were evaluated at the end of each stress state. Comparisons were made among modulus values obtained in different strain ranges such that shear-wave velocities measured by BE were utilized to calculate the modulus in a small-strain range, while the resilient modulus in a larger strain range was estimated by strain measurements. The test results regress to a linear relationship between small-strain elastic modulus and resilient modulus for crushed stone coarse aggregates and silty-sand subgrade soils. The findings of this research prove the possibility of establishing correlations to obtain the resilient modulus of granular materials from BE measurements at smaller strains and enable the utilization of the small-strain modulus measured in situ and instantaneously by BE sensors for pavement analysis and design purposes.