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ABSTRACT Laboratory Determination of Resilient Modulus for Flexible Pavement Design Key Words: laboratory testing, test methods, resilient modulus, asphalt concrete, base, subgrade, diametral test, indirect tension test, repeated load biaxial test, unconfined test, specifications, aggregate, lime/flyash stabilization. Abstract: Extensive diametral repeated load tests were performed on asphalt concrete specimens. Extensive repeated load biaxial tests were perfo aped on base and subgrade materials. New resilient modulus test methods were developed for these materials and presented in the form of AASHTO type test procMures. The repeats 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 SHRP EG 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 EVDT to measure vertical deformation and externally mounted EVDTs to determine horizontal deformation. The value of Poisson's ratio used to determine He 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 an (~) optical extensometer, (2) non-contact proximity gages or (3) small EVDTs supported by lightweight clamps. The optical extensometer is highly recommended. For granular materials a vacuum biaxial 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. v I,