Three-Dimensional Characterisation of Linear Viscoelastic Properties of Bituminous Mixtures

This chapter focuses on the three-dimensional linear viscoelastic (3D-LVE) behaviour of bituminous mixtures and in particular on the measurement and modelling of the complex Young’s modulus and Poisson’s ratio (PR). In the first part of the chapter, the LVE definition of PR is reviewed and experimental measurements of the LVE PR carried out over the last 40 years are summarised. The second part of the chapter is devoted to the description of a RILEM round robin test (RRT) organized by Task Group 3 (TG3) “Mechanical testing of bituminous mixtures” of RILEM TC 237-SIB. Within the RRT uniaxial cyclic (sinusoidal tension/compression or haversine compression) tests at different temperatures and frequencies were carried out on cylindrical specimens cored from laboratory compacted slabs. Two types of bituminous mixtures, GB3 (continuously graded) and GB5® (gap-graded), were analysed. Five laboratories participated in the RRT, each laboratory measured axial and transverse (or diametral) strains using different sensors and configurations. In particular, transverse strain was measured along two orthogonal directions in order to evaluate the effect of compaction-induced anisotropy on PR. Results confirmed that PR of bituminous mixtures is a complex function of temperature and frequency and that the time-temperature superposition principle can be applied (for absolute value and phase angle). For the studied mixtures the norm of the complex PR ranged between 0.22 (low temperatures/high frequencies) and 0.60 (high temperatures/low frequencies) whereas the phase angle was less than 6°. A small difference (less than 0.05) was found between measurements carried out in two orthogonal directions. This small difference is probably related to measurement accuracy and not to the anisotropic behaviour of the material. Comparison of data between the different laboratories, which could not be performed at exactly the same temperatures and frequencies, was performed using a common reference given by the 3-dimensional formulation of the 2S2P1D linear viscoelastic model. This model provides a good simulation of experimental data. Based on close results from all participating laboratories, it is possible to conclude that cyclic uniaxial test could be a good candidate to become a standard test for evaluating the 3D-LVE behaviour of bituminous mixtures.