A triaxial linear viscoelastic characterization framework for asphalt concrete based on the 2S2P1D model

Asphalt concrete materials are known to exhibit pressure dependence in response to mechanical loading. However, for conventional dense-graded asphalt mixtures, only weak pressure sensitivity was noted in the relaxation spectrum, time-temperature shift factors, and low-temperature moduli. Given these facts, a triaxial linear viscoelastic characterization framework was developed, which consisted of a convenient experimental protocol for the confined dynamic modulus test and a modeling approach based on the 2S2P1D model. In this framework, the same relaxation spectrum and time-temperature equivalence were applied across all different confining conditions, while the pressure dependence was fully assigned to the equilibrium modulus. The resultant prediction errors in dynamic modulus were in general below or comparable to typical test variability. Numerical implementation of the modeling method in the time domain was accomplished via Prony discretization of the continuous relaxation spectrum, and was demonstrated via application to triaxial relaxation and creep data. The model's satisfactory performance suggested that the confining condition had an insignificant impact on the molecular processes occurring in the asphalt phase during relaxation, owing to the low aggregate mobility and thus minimal asphalt flow as restrained by the dense structure. Limitations of the proposed modeling scheme as well as methods available from the literature were revealed lastly upon asphalt concrete materials with unconventional gap and open gradations. •The test protocol included standard .|E*| test and confined only at the highest temperature•The modeling assumed a single relaxation spectrum and time-temperature equivalence.•Pressure dependence was solely assigned to the equilibrium modulus.•Asphalt molecular relaxation processes were pressure insensitive in the dense structure.•Unconventionally graded mixtures exhibited full-range pressure sensitivity.