Field shear test device for quality control testing of asphalt concrete

The field shear test (FST) was originally developed during NCHRP Project 9-7 as a simple and inexpensive alternative to the Superpave shear test (SST), meant primarily for use in quality control/quality assurance (QC/QA) testing. The first FST prototype, however, did not perform as well as hoped. The primary objectives of NCHRP 9-18 were to refine the FST and evaluate its sensitivity to changes in asphalt concrete composition. The redesigned FST is very compact, rugged and extremely easy to operate. Although capable of performing a variety of tests, best results are obtained for frequency sweep data at high temperatures and frequencies of 5 to 10 Hz. Under these conditions, the FST produces dynamic modulus values about 40% higher and phase angles 5 to 15° lower than those generated using the SST. FST dynamic modulus data were found to be fairly sensitive to changes in asphalt concrete composition. Data repeatability is similar for the FST and other methods for measuring asphalt concrete stiffness, including the SST. Future research efforts should initially focus on reducing the variability inherent in FST data and other asphalt concrete stiffness test results, followed by several field demonstration projects.