Study On The Mechanism Of Phase Transformation Of Asphalt Binder At Low Temperatures

Complex effects of multiple factors contribute to the low temperature cracking(LTC)of asphalt concrete pavements,which is the significant distresses in many regions and countries affected by cold weather.The main ways to reduce the LTC of asphalt pavements deterioration are the optimization of pavement structure and asphalt material,regardless of the environmental factors including ambient temperature or duration of low temperature.The performances of asphalt material are determined by its own properties.To have a better understanding of asphalt material,it is helpful to investigate its behaviors of phase transformation and physical aging at low temperature,so that the benefited optimization of asphalt material and related engineering problem can help improve the road performance of asphalt concrete structures.This thesis researches the phase transformation of asphalt at low temperature and the physical aging phenomenon which occurs in the process of glass transition and below the temperature of glass transition,by the means of Molecular Dynamics(MD)Simulation and Modulated Differential Scanning Calorimetry(MDSC).To understand the role of SARA fractions(saturate,aromatic,resin and asphaltene)of asphalt binder in the phase transformation and physical aging at low temperature,these four components are simulated and tested respectively.The main conclusions are drawn as follows:(1)There is certain correspondence between the phase transformation and irreversible heat flow curve of asphalt binder.The phase transformation characteristics are especially distinctive between the base asphalt and the aged one,and the enthalpy changes significantly during the phase transformation.The rejuvenated asphalt mixture shows that the compositional properties has been changed by the additional modifiers.Besides,the transformation endpoint from glass to rubber and the transformation beginning point from rubber to flowing area have been blurred.(2)Asphalt is proved to in the nature of polymer glass transition,no matter from the perspective of microscopic simulation or thermal analysis.Though different thermal history treatments have no effect on the glass transition temperature of the asphalt binder,it will be affected by the ways which changes the ratio or components of the asphalt,such as aging or adding regenerants after aging.The thermal analysis test results show that it is difficult to shot the glass transition temperature of aged asphalt at a determined point,but ranges in a relatively wide bound.Due to the higher glass transition temperature of aged asphalt compared with that of base asphalt and aged rejuvenated asphalt,the aged asphalt starts to transit from rubber state to glass state at high temperatures,which suggests the aged asphalt binder does not function well at low temperatures and will aggravate the LTC.The novel asphalt regenerant used in this thesis can effectively improve the properties of the aged asphalt,low its glass transition temperature,and improve its function at low temperatures.(3)The composition and crystallization behavior of asphalt are closely related to its properties changes during physical aging.According to molecular dynamics simulations,its physical aging results from its internal structure molecules: at low temperatures the reduced molecular mobility restricts the movement area of each molecular chain.Initially,the activity of asphaltene and resin decreases,so that they accumulate with each other,the density of asphalt binders increase rapidly as well as its free volume decrease rapidly.Afterwards,the micro-Brownian motion of saturate and aromatic of SARA fractions helps the density and free volume of the asphalt binder to maintain relatively small fluctuations.Besides,the physical aging phenomenon of aged asphalt is more obvious and its impact is greater.Through the modulated differential scanning calorimetry test,it is found that the thermal history affects the crystallization behavior of the asphalt binder,leading to different irreversible thermal phenomena.It can be inferred that the the crystallization behavior of asphalt binder at low temperatures will be changed by the ambient temperature,thereby affecting its low temperature properties and physical aging behavior under non-equilibrium state.(4)According to the molecular dynamics simulation and MDSC of SARA fractions,asphaltene is the most stable component regarding temperature change.The different MDSC test results between the base asphalt and aged asphalt can also prove that the percentage change of asphaltene content has a greater impact on the low-temperature properties of asphalt binder.It is inferred that the glass transition of aged asphalt is related to the increased percentage in asphaltene content.Saturate is the most active fraction,the most sensitive component to temperature changes,and the key to improving the low-temperature properties of asphalt binders.In addition,the independent four fractions have little connection with the phase transition of the asphalt binder.