Effects of air void content and back‑calculation techniques on bituminous mixture modulus via impact resonance

This study evaluates the effect of back-calculation methods and air void content on the complex modulus of bituminous mixtures using impact resonance tests (IRT). Non-destructive IRT were performed on disk-shaped specimens of hot mix asphalt (HMA) at − 20 °C and + 23 °C to determine their resonance frequencies, which are critical for their viscoelastic characterization. Both an analytical method and a numerical back-calculation method were utilized to determine the complex modulus (dynamic modulus and phase angle) of the specimens based on IRT data. Both analytical and numerical methods yielded consistent dynamic modulus values. At − 20 °C, they ranged from 25,000 to 40,000 MPa. At room temperature, values were between 10,000 and 20,000 MPa. However, the two methods differed in their phase angle estimates, especially at higher temperatures, with discrepancies ranging from − 6° to + 3°. The study identifies limitations in current numerical back-calculation techniques, particularly regarding damping estimation, and suggests improvements in optimization procedures. The results also highlight that air void content significantly affects HMA stiffness (about 1000 MPa per % air void content), with higher air voids reducing the dynamic modulus, particularly at lower temperatures. This research advances the knowledge on application of IRT for characterizing bituminous mixtures, highlighting its potential for cost-effective characterization of HMA properties and fostering future standardization.