Bitumen is a viscoelastic material and its behavior depends on the temperature as well as the rate of loading that it is subjected to. The response of bitumen is linearly viscoelastic until a certain limit and many of the parameters such as the complex modulus, storage modulus, loss modulus and phase angle are defined only under linear viscoelastic conditions. As per current standards (ASTM D7175, 2015), the LVE region is considered as the range of strains up to which the dynamic modulus is decreased by 10% of its initial value.

Bitumen exhibits a fluid like behavior for temperatures ranging from 20℃ to 160℃ . As a result, it can be subjected to substantial large deformations and still remain in the linear region as long as the rate of deformation is relatively small. Therefore, defining the linear limit with respect to strain is not appropriate. In this study, a new methodology is adopted to define the limits of linearity using strain rate and stress. Strain sweep and stress sweep tests were performed at 50℃ and 60℃ at 4 different frequencies (20, 10, 5, 1 Hz) on 2 binders (PMB 40(P), PMB 40 (E)) using a Dynamic Shear Rheometer (MCR 302). The variation of dynamic modulus with strain amplitude and strain rate amplitude were obtained at different frequencies. It was found that the shear rate amplitude at which the modulus drops is invariant of the frequency of testing while the strain amplitude corresponding to the drop changed from one frequency to another. In view of this, the linear limits were calculated with respect to strain rate and stress. The limits so obtained are applicable at all loading rates for both binders. Steady shear tests were also conducted to see if the limit obtained from oscillatory tests are applicable for all loading conditions. Shear thinning was found to occur at shear rates smaller than the limiting shear rate obtained from oscillatory tests, suggesting that while the linear limit should be defined in terms of the magnitude of shear rate, it should be obtained from steady shear tests and not oscillatory tests.