Specimen geometry and aggregate size effects in uniaxial compression and constant height shear tests

A study that is part of the National Cooperative Highway Research Program's Project 9-19, "Superpave Support and Performance Models Management," was carried out to determine the least number of test specimen dimensions that would provide measured responses and material properties, regardless of the test specimen size and aggregate size. The study was conducted in two parallel experiments that addressed specimen size and geometry effects on uniaxial compression and constant height shear tests. For uniaxial compression tests, a sample size of 100 mm dia and 150 mm in height was sufficient to accurately characterize basic dynamic modulus and permanent deformation responses. On the other hand, for the constant height shear test, no restricting dimension above which the measured responses were independent of specimen size could be found. The poor diameter to height ratio because of the 150-mm dia restriction imposed by the Gyratory compactor and the non-linear shear strain distribution caused by sample gluing were the two errors that served as the additive influence that rendered shear tests unable to yield true mechanistic parameters for specimens compacted in the Superpave Gyratory Compactor.