Considerations for laboratory resilient modulus testing of unbound pavement base materials

This paper explains equipment related considerations on differences between the lab and field conditions that can lead to variations in resilient modulus (M-r) values of unbound materials as determined in accordance with AASHTO T307 standard. Three different granular materials and an elastic polyurethane (control) sample were tested using commercially available laboratory test equipment. A field test bed was also constructed to measure vehicle (Class 3) induced loading conditions at the bottom of an unbound granular layer underlying new hot-mix asphalt pavement to determine stress pulse duration as a function of vehicle speed. Various interpretation issues were identified within the framework of the testing methods and equipment including: (1) insufficient laboratory sensor sampling rate (per standard): (2) the laboratory specified 0.1 s load-pulse duration and haversine shape are not matching the field stress pulse duration and shape; (3) the need for careful tuning of the load system gain settings: (4) the number of LVDTs used in the vertical strain calculation; (5) limiting quality control and quality assurance to deformation ratio values in the preconditioning sequence: and (6) limiting the load step calculations to the last 5 of 100 load cycles. The goal of this paper is to provide equipment users and specification developers with user knowledge concerning laboratory resilient modulus determination. (C) 2018 Elsevier Ltd. All rights reserved.