Nonlinear rheological behavior of bitumen under LAOS stress

The linear viscoelasticity and steady shear properties of bitumen are established; however, bitumen used in pavement also supports time-varying loading in the nonlinear regime. Consequently, recent research focuses on the nonlinear rheological behavior of bitumen under time-varying flows that is essential for establishing the constitutive model. Here, we propose a protocol to obtain the nonlinear rheological behavior of bitumen under a large amplitude oscillatory shear (LAOS) stress. The LAOS stress rheological response of both a neat bitumen and a modified bitumen is analyzed by Fourier-transform rheology and strain decomposition to an orthogonal set of Chebyshev polynomials. We find that the relative nonlinearity of bitumen increases with increasing stress and decreasing frequency; the relationship between I₃/I₁ and stress amplitude can obey the sigmoidal function; the intrinsic nonlinearity Q₀ decreases with increasing frequency and decreasing temperature. Both the FT analysis results and decomposition results show that the nonlinearity of modified bitumen B is much more significant than neat bitumen A. Bitumen A exhibits stress softening and stress thinning under all the studied test conditions, but bitumen B exhibits stress stiffening under some conditions.