Development of a tensile relaxation modulus test for identifying the tensile relaxation spectrum of asphalt binder

The tensile properties of asphalt binder such as strength, ductility, and stress-relaxation (relaxation) modulus are not only closely related to binder performance, but the modulus is also a necessary input for accurate material characterisation, modelling, and simulation. The linear behaviour of a homogeneous isotropic material can be fully characterised by both the tensile and shear relaxation moduli or their corresponding relaxation spectra. The shear properties of asphalt binders are often studied through dynamic shear tests. However, its tensile relaxation modulus and spectrum are seldom determined. Instead, they are often derived from the shear relaxation modulus with a prescribed Poisson ratio. However, it remains a question whether the ratio is material-, time- or even loading-independent. A tensile relaxation test is thus developed for determining the tensile relaxation modulus and spectrum directly. The continuous tensile relaxation spectrum is numerically derived based on the integral equation that connects the modulus to the spectrum. Spectrum regularisation and L-curve are used for solving the overfitting issue of the ill-posed equations. Two binders are used to evaluate the experimental method and the computational approach. The data suggest that the Poisson ratio may not be regarded as a constant for different asphalt binders. Hence, it is necessary to characterise asphalt binders using the tensile relaxation spectrum. Cross-checking between tensile and shear test data and between experimental relaxation moduli and the back-calculated ones from the tensile relaxation spectra indicate that the developed methods are robust and reliable.