DEVELOPMENT OF SYNTHETIC SPECIMENS FOR CALIBRATION AND EVALUATION OF M SUB R (RESILIENT MODULUS) EQUIPMENT

Abstract

Laboratory measurement of the deformational characteristics of subgrade materials can be quite difficult because of the small values of stress and strain typically involved and the need to eliminate equipment compliance. Measurement of resilient modulus (M sub R) of subgrades falls into this category. Therefore, synthetic specimens with known stiffness characteristics would be beneficial in evaluating and calibrating M sub R equipment as well as training personnel. Two-component urethane elastomer resins are shown to make good candidates for calibration specimens. They can be made with a wide range of stiffnesses that vary from soft subgrades to stiff uncemented bases. Urethane can be modeled as a linear, viscoelastic material with stiffness characteristics essentially independent of confining pressure, strain amplitude, and stress history for the type of cyclic loading used in M sub R testing. Urethane stiffness is, however, dependent on loading frequency and temperature. Therefore, values of Youngs modulus used to equate to M sub R have to be selected at the appropriate frequency and temperature.