| The low temperature cracking disease of pavement is the main problem faced in North China,which sharply shortens the service life of the road.The lowtemperature performance of asphalt pavement mainly depends on asphalt binder.So improving the low-temperature performance of asphalt is an urgent problem to be solved in cold areas.Numberous studies on the low temperature performance has been in progress.However,due to the composition and structure of asphalt is extremely complex,so the mechanism of the complex molecular structure effect on low temperature performance is not clear in the existing research.And most of the existing evaluation methods ignore the physical hardening of asphalt during the long-term low temperature environment.The measured thermal stress of asphalt is lower than the stress produced in the actual use,which overestimates the lowtemperature performance of asphalt.In view of this,based on the dynamic shear rheometer,the evaluation method of asphalt low temperature performance considering the interaction of physical hardening and stress relaxation was established,and based on the molecular dynamics simulation,the effect mechanism of the micro structure of asphalt on the macro low temperature performance was revealed.Firstly,a test method of dynamic shearing rheometer based on 8mm parallel plate was developed to study the low temperature performance of asphalt binder.The test instrument is universal and can make up for the large sample consumption of bending beam rheological test(BBR).Based on WLF equation,generalized Maxwell model,modified Kelvin Voigt model and Boltzman linear superposition principle,the evaluation method of asphalt low temperature performance considering physical hardening and stress relaxation was established.The hardening stress was taken as the evaluation index to characterize the low temperature cracking of asphalt under long-term low temperature action.Secondly,dynamic shearing rheometer(DSR)and differential scanning calorimeter(DSC)were used to study the low temperature properties.The dynamic main curve and DSC test results show that the order of low temperature performance is ZSH90>HY50>LH50>LH110>LH70.The results of hardening stress show that the hardening stress increases with the increase of time,increases first and then decreases with the decrease of temperature,and there is a peak value.The position and size of the peak value are determined by the combination of physical hardening and stress relaxation.The hardening stress of five kinds of asphalt is LH70> LH50>LH110>ZSH90>HY50.Finally,based on the molecular dynamics simulation and the characteristics of the microstructure,the macro-micro relationship on the low temperature performance of asphalt was established,and the effect mechanism of the asphalt composition and structure on the low temperature performance was revealed.The results show that the content of PAHs or S-compounds with small steric hindrance and molecular weight is more,the non bond energy of asphalt is larger,the it is ease to produce molecular motion.So it has high deformation ability,low glass transition temperature and good low temperature performance.The less the content of saturated components with high temperature sensitivity,the better the temperature sensitivity of asphalt.The change rate of free volume with temperature and the temperature range of glass transition is small,so the hardening stress produced by asphalt is small,that is,the contribution of physical hardening to low temperature cracking is small.In this paper,the evaluation method of low-temperature performance considering physical hardening and stress relaxation was established,which provides the basis for the evaluation of mechanical properties of asphalt under the long-term low-temperature action;the effect mechanism of asphalt composition and structure on low-temperature performance was revealed,which provides the theoretical basis for the research and development of new materials with good lowtemperature performance in the future. |
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