Estimating the Modulus of HMA Samples at the Laboratory Stage through Resonant Tests

The dynamic modulus of asphalt concrete (AC) is a fundamental parameter in the design of asphalt pavements. Consequently, predicting the dynamic modulus of AC samples at a laboratory stage is critical for obtaining a successful mix design, optimizing maintenance and rehabilitation schedules, and saving money. Resonant tests are low-cost and high-speed and allow the derivation of the resonant modulus (ER) based on the detection of two resonant frequencies of the sample under free vibrational conditions caused by lightly striking the specimen with a suitable implement. Unfortunately, for resonant tests, the disc-shaped formulas reported in the current standards are defined for relatively thin specimens (t/r<0.5, where t stands for thickness and r for radius) and their application to common hot mix asphalt (HMA) samples (whose t/r is usually in the range 0.4-2.1) is not possible. Consequently, the main objectives of the study presented in this paper are to: (1) extend the applicability of the resonant-based method to estimate the dynamic modulus at the design stage; and (2) investigate the appropriateness of resonant-based moduli to provide a low-cost and high-speed tool to compare and rank different bituminous mixtures at the laboratory stage. To this end, four different types of bituminous mixtures underwent resonant, indirect tensile, and fatigue testing. Afterwards, test data were analyzed, the coefficients for the application of the resonant method outside the current domain of values were derived, and comparisons were carried out. Results demonstrate that the trend of the coefficients, k1 and k2 (defined in the current standards), herein derived for 2.1>t/r>0.5, follows a quite reasonable decreasing tendency and that the temperature dependence of resonant moduli is usually lower than that of stiffness. Furthermore, resonant-based moduli and stiffness moduli provide quite comparable rankings. The extension of the applicability of the current standards provides an approximate tool to carry out low-cost and high-speed inferences at the laboratory stage on common HMA samples and resonant-based moduli and stiffness moduli provide quite comparable rankings. Results can benefit both researchers and practitioners.