Measuring the complex modulus of asphalt concrete using ultrasonic testing

Dynamic modulus mastercurves are essential for the design and modeling of asphalt concrete (AC). One way of improving the accuracy of the upper asymptote of the mastercurve is to test at extremely high frequencies or extremely low temperatures. Ultrasound is used extensively in the nondestructive testing of materials and the work completed here demonstrates the potential for the application of this technology to AC. Since testing at extremely low temperatures is not practical, a new ultrasonic technique is developed for measuring the complex moduli of AC. A theoretical explanation of the measurement process is provided. Two AC specimens were tested using the ultrasonic method and the dynamic modulus method in the indirect tensile test (IDT) mode. Both test techniques were performed at four different temperatures. The mastercurves were constructed using time-temperature superposition on the IDT test data and the upper asymptotes were extrapolated. The ultrasonic data was shifted to the desired reference temperature and the predicted moduli were compared to those of the IDT test. It was found that the moduli predicted using the ultrasound measurement agreed well for the specimen with a lower air-void content and differed more for the specimen with a higher air-void content. The phase angles predicted by the ultrasonic method were higher than those obtained from the IDT test. It is believed that this was a result of wave scattering from air-voids and aggregates. Suggestions are made to further increase the accuracy of the technique.