Reduced Testing Protocol for Measuring the Confined Dynamic Modulus of Asphalt Mixtures

Research project NCHRP 9–19 identifies the confined dynamic modulus as one of three favorable indicators for evaluating the rutting potential of a mixture. Though important, dynamic modulus testing at multiple confining pressures takes too long for state highway agencies to use it routinely. Therefore, several methods have been suggested to measure and predict confined dynamic modulus values without the need to run numerous tests. Experimental results show that the linear viscoelastic properties of an asphalt mixture are not affected by different confinements and that all confining stress effects are manifest in the elastic modulus at equilibrium, similar to unbound granular materials. The proposed method uses a Prony series representation of the dynamic modulus curve and master curve shift factors obtained from unconfined testing. This method uses the elastic modulus values predicted from a modified version of the universal material model to predict dynamic moduli at different levels of confinement. Beyond the typical AASHTO TP62 testing procedure under an unconfined condition, additional testing is conducted at 54°C at three levels of confinement. This reduced testing protocol provides reasonable results, with most errors less than 20%. The largest errors between the measured confined and unconfined data were generally overpredicted values at 54°C because the universal model overpredicted the elastic modulus. The applicability of this method is verified for the asphalt mixture performance tester as long as three levels less than 250 kPa are used, because 250 kPa is the maximum confining pressure that the tester can handle.