Fatigue & Rheology Mechanism Of Asphalt Binder Based On Viscoelastic Characteristic

The ability of asphalt binder to resist accumulation of fatigue damage can have a profound effect on the long term performance of asphalt pavements. So the fatigue of asphalt binder has always been the hot research topic at home and overseas. Many researchers believe that the fatigue of asphalt binder is caused by damage, and some fatigue evaluation indices such as Dissipated Energe Ratio, Dissipated Rate were put forward. However, the healing and thixotropy of asphalt binder are getting more and more attention with the research about binder performance going on, and some researchers notice that the fatigue of asphalt binder is not only related to damage but also related to nonlinear viscoelastic healing and rheological characteristic of asphalt binder. So the researches of fatigue characteristic and fatigue evaluation index for asphalt binder should contain both of the influence of damage and rheology. For the above reasons, the healing and thixotropy of asphalt binder were researched deeply from the angle of viscoelastic characteristic of asphalt binder, the influences of thixotropy and healing on the fatigue characteristic of asphalt binder were analyzed. Based on these analysis the fatigue and rheology mechanism of asphalt binder was researched, and the fatigue evaluation index of asphalt binder was brought forward. The contents and results of the research are shown as follow.The existing research method for fatigue characteristic of asphalt binder and mixture was modified after understanding the method deeply. The new research method is based on the rheological characteristic, and the influence of thixotropy was considered in this method.The healing ability of asphalt binder and the influences of rest period, status before rest, temperature, loading cycles, and loading mode etc were researched by using dynamic shear rheometer. The research shows that the longer the rest period, the better asphalt binder heals. The shorter the loading time, the faster asphalt binder heals. The healing velocity increases with the increase in temperature. The healing ability of asphalt binder is different with different loading modes. Under intermittent loading mode, the fatigue curve of asphalt binder differs from the curve under continuous loading mode when the modulus is bigger than the modulus before rest periods applied. When the modulus is smaller than the modulus before rest period applied, the cuver under intermittent loading mode is the same to the curve under continuous loading mode. By comparing and analyzing the dynamic shear test results and the cross section of the specimens, it is concluded that if the loading stops before microcrack appearing or just at the time when microcrack appearing, the modulus can heal totally with enough rest periods.Three evaluation indice were chosen to evaluate the healing characteristic of asphalt binder. The indice were modulus ratio (HI1), slope ratio (HI2) and cycle number ratio (HI3). The healing characteristics of five kinds of asphalt binder were analyzed. By comparing and analyzing the results, it shows that modulus ratio (HI1) is suitable for evaluating the healing ability of initial modulus of asphalt binder and cycle number ratio (HI3) is suitable for evaluating the long term healing ability of asphalt binder.Using the thixotropy research methods in colliod materials as reference, the hysteresis method, stepwise test method and dynamic modulus method were used to research the thixotropy of asphalt binder and the corresponding thixotropy models were established. The research indicates that thixotropy depends on maximum shear rate applied to asphalt binder by using hysteresis method. When the shear rate is very low, the viscosity increases first then decreases with increase in shear rate. This phenomenon disappears when the shear rate is big enough. With decrease in shear rate, the viscosity of asphalt binder will recover slowly. Exponential function can fit the test results at single shear rate better. Extend exponential function obtained from dynamic modulus method can fit the relationship between storage modulus and time better. And the smaller the characteristic time coefficient, the faster the initial modulus heals in short term. The bigger the healing coefficient in kinetics thixotropy model, the better the healing ability of asphalt binder is. Comparing the research results of thixotropy and the results of healing, it shows that the decrease of modulus and the recovery of modulus after loading stops are caused by thixotropy at the beginning of loading.The fatigue-rheology course of asphalt binder and the influence of thixotropy on binder fatigue were researched by analyzing the curve of normalized modulus versus number of cycles, the curve of phase angle versus number of cycles and the curve of normalized (complex) modulus versus phase angle. The results show that the thixotropy influence phase can be fixed well using the turning point at curve of complex modulus versus phase angle. Based on the curve of complex modulus versus phase angle of fatigue test and stress (strain) sweep test, the dividing point of thixotropy phase for the four kinds of asphalt binder were confirmed. The influences of thixotropy and damage on fatigue were separated based on the thixotropy research results and fatigue test data. The fatigue curve after eliminating the influence of thixotropy is greatly different from the curve obtained by the test.Finally, the most common used fatigue indices of asphalt binder were analyzed, that is the cycles when the modulus gets to 50% of the initial value (50%G*), Dissipated Energe Ratio, Damage Rate and phase angle. All the indice can reflect the fatigue characteristic of asphalt binder in some extent, but they all have some deficiency. 50%G* after eliminating the influence of thixotropy was chosen as the fatigue evaluation index, and it can evaluate the fatigue performance of asphalt binder better.