Softening and local self-heating of bituminous mixtures during cyclic loading

Bituminous materials are heterogeneous media composed of aggregate particles and bituminous binder. The repetition of loading cycles on such materials induces stiffness changes. Those changes are measured during cyclic tension-compression tests by tracking the evolution of the norm and of the phase angle of the complex modulus. Different phenomena may be responsible for the stiffness changes. Some of those phenomena are completely reversible, for example, self-heating, recoverable nonlinearity (complex modulus dependency on stress/strain amplitude) and another bulk phenomenon, which is hypothesised as thixotropy. Another one, known as damage, is irreversible. All stiffness change in bituminous materials is frequently considered as caused by damage. This is not realistic, as careful analysis shows that the other phenomena are responsible for a major part of the stiffness decrease in most fatigue tests. Local self-heating could be a physical explanation for a large part of the observed reversible stiffness decrease. This paper proposes three simplified coupled thermomechanical analytical calculations in order to model the observed stiffness decrease due to local self-heating. The last calculation considers a heterogeneous structure of bituminous mixtures, composed of rigid monodisperse spherical particles and a mastic phase. The model allows calculating the initial slopes of the curves' norm of complex modulus and phase angle versus number of cycles. Aggregate gradation of the mixture, mastic film thickness, volume fraction of binder and thermophysical properties of aggregate particles and binder are taken into account. Simulated and experimental results for one bituminous mixture were compared for tests at different temperatures and frequencies. Results show that local effects could explain the observed initial decrease in the norm of complex modulus during cyclic tests.