Laboratory Determination of Resilient Modulus for Flexible Pavement Design

Extensive diametral repeated load tests were performed on asphalt concrete specimens. Extensive repeated load triaxial tests were performed on base and subgrade materials. New resilient modulus test methods were developed for these materials and presented in the form of American Association of State Highway and Transportation Official (AASHTO) type test procedures. The repeated load test is not easy to perform and must be carried out on a carefully calibrated testing system including using synthetic specimens. To minimize human errors, a fully automated, closed loop testing and data acquisition system should be used. For asphalt concrete, the Strategic Highway Research Program (SHRP) Load Guide diametral testing device should be employed to minimize rocking. A mountable extensometer provides less variance and hence better repeatability for resilient modulus measurement. Poisson's ratio can be evaluated using a surface mounted linear variable differential transformer (LVDT) to measure vertical deformation and externally mounted LVDTs to determine horizontal deformation. The value of Poisson's ratio used to determine the resilient modulus has an important influence on its value. Poisson's ratio should be measured rather than assumed. To reliably determine resilient modulus, axial deformation of base and subgrade materials should be measured on the specimen using either (1) an optical extensometer, (2) non-contact proximity gages or (3) small LVDTs supported by lightweight clamps. The optical extensometer is highly recommended. For granular materials a vacuum triaxial test offers an excellent approach when an optical extensometer is used. An unconfined repeated load test is recommended for cohesive subgrade soils. For cohesive soils, bedding irregularities at specimen ends, specimen aging and soil structure must all be properly considered to obtain reliable resilient moduli.