Study On The Influence Mechanism Of Physical Hardening On Low Temperature Properties Of Asphalt Materials

Due to the increasing burden of road transportation in northern China in recent years,and the low temperature in winter in the northern alpine region,a large number of cracks caused by cooling and fatigue often appear.In order to prevent asphalt pavement cracks,road workers have developed a variety of test methods and evaluation systems to study the low temperature performance of asphalt and asphalt mixture,but after the evaluation and comparison of the selected asphalt with good low temperature performance into the actual project will still occur relatively serious low temperature cracking.This is because the influence of physical hardening on the low temperature performance of asphalt is not analyzed when evaluating its low temperature performance.Therefore,it is of great significance to fully understand the influence of physical hardening on the low temperature performance,and to carry out in-depth research on the change mechanism of macro-micro performance of physical hardening of asphalt,so as to accurately evaluate the low temperature performance of asphalt and asphalt mixture.First of all,starting from the low temperature fracture performance of asphalt,a self-made test system for the low temperature visco-elastic fracture behavior of asphalt binder is made.The system is composed of a temperature control system,a loading system and a data acquisition and control system.The temperature control system is realized by Pelter technology.The loading system consists of stepping motor,ball screw and test head.The data acquisition system consists of tension sensor,displacement sensor,data acquisition card and corresponding data acquisition program.The stability of all parts can be guaranteed after testing,and the sensor has been calibrated.After SENB parallel test,it is found that the device meets the parallelism.Secondly,the mechanical behavior of asphalt at low temperature under physical hardening is studied,and the constitutive equation based on physical hardening is established.BBR test and SENB test were used to test the relationship between asphalt rheological property and fracture property and physical hardening time.The results showed that the main modulus curve,fracture deflection,fracture deflection and FST of asphalt changed significantly with physical hardening,and the low temperature performance decreased,indicating that physical hardening could affect the evaluation result of asphalt low temperature performance.Based on CAM model,WLF equation and Boltzmann linear superposition principle,the asphalt stiffness modulus and main curve are further fitted and analyzed,and the temperature stress constitutive equation based on physical hardening is proposed.Thirdly,the change of rheological property and fracture property of asphalt is taken as the degree of physical hardening.BBR and SENB test are used to analyze the relationship between the degree of physical hardening and the five influencing factors.In addition,the addition of modifier and the increase of dosage can reduce the effect of physical hardening.Then,based on the physical hardening of asphalt,the lowtemperature performance evaluation system using BBR and SENB test is proposed:(1)short-term aging asphalt is selected as the material;(2)the test temperature is-18?;(3)the constant temperature time is 24h;(4)S(60),m value,fracture deflection,fracture energy and FST are taken as key indexes to evaluate asphalt performance.Finally,the asphalt model is simulated by Materials Studio,a molecular dynamics software,and the mechanism of physical hardening of asphalt is verified.The results show that the mechanism of physical hardening is the reduction of free volume and the aggregation of asphaltenes.DSC test was conducted to analyze the thermodynamic heating process of asphalt,and it was found that physical hardening would lead to the increase of glass transition temperature of asphalt,decrease of specific heat capacity and molecular freedom,decrease of thermal stability and sensitivity of thermal change,change of components,transformation of aggregation state,increase of intermolecular force and decrease of free volume.In addition,by changing the simulation parameters and test conditions,it is found that the variation of asphalt microstructure with constant temperature time,temperature,aging degree and other factors is consistent with the macroscopic mechanical behavior.