| Asphalt is prone to aging under the combined action of heat and oxygen,but in high temperature and humid areas,the aging behavior of asphalt is more complicated due to the presence of water.Therefore,it is of great theoretical significance and engineering application value for improving the durability of asphalt pavement in humid and hot areas to study the influence of water-heat-oxygen coupling on the performance and chemical composition of asphalt pavement,and to reveal the action mechanism of hot and humid environment on asphalt aging behavior.In this paper,based on extreme high temperature and annual average relative humidity and other climatic data in China,an accelerated indoor simulated damp-heat aging test of asphalt was designed using the time-temperature equivalence principle.The fatigue properties of asphalt before and after damp-heat aging were studied by DSR test,and the change rule of asphalt performance under damp-heat environment was analyzed.Based on the viscoelastic continuum damage(S-VECD)theory,an aging model of fatigue damage of asphalt under the coupled effect of damp and heat was established.Combined application of electrochemical impedance test and digital image simulation research methods,the water-oxygen coupling effect and the diffusion law of saturated water vapor in asphalt were proved.Combined with the correlation analysis of the road performance of recycled asphalt and indoor aging asphalt under different climatic conditions,the kinetic equations of asphalt aging under different aging conditions were established,and the control index of asphalt aging performance based on the distribution of humid and hot climate characteristics was proposed.The following research results were obtained:(1)The variation law of fatigue properties of asphalt under the coupled aging conditions of heat,oxygen and water was explored through temperature frequency sweep and linear amplitude sweep tests.The research results show that with the increase of aging temperature,humidity and duration,the fatigue damage of asphalt increased significantly,resulting in the decrease of its fatigue resistance,and the change of asphalt damage curve was most obvious when the humidity was 50%.The fatigue parameters A and B in the S-VECD model had a good linear relationship with the complex modulus or fatigue factor,respectively,but the fatigue parameter B was more suitable to represent the damp-heat aging dependence of asphalt.The correlation coefficient of asphalt fatigue life distribution near the equation line was 0.92,showing good consistency.(2)The impedance of asphalt during wet-heat aging was tested by electrochemical impedance,and it was found that the coupled diffusion process of water and oxygen in asphalt was mainly the process of combining water and oxygen with polar groups in asphalt.Due to the dense network structure in the SBS modified asphalt,it hindered the diffusion of water vapor and oxygen into the asphalt,delayed the chemical reaction of moisture,oxygen and polar groups in the asphalt,and reduced the micro-activation center inside the asphalt.The capacitance value of the aged asphalt was much lower than that of the base asphalt.The relative diffusion coefficient of water and oxygen in asphalt shows that there was a significant correlation between the capacitance value of asphalt and aging time under different humid and heat environments,that is,log -0 is linearly related to √.The digital image characterization results of the samples show that the diffusion depth of water vapor in the SBS modified asphalt was shallower than that of the base asphalt,and its mass fraction ratio was only 1E-9,which further confirmed that the network structure in the SBS modified asphalt hindered the water vapor diffusion process.(3)The micro-composition and structure test results of asphalt show that the carbonyl index and sulfoxide index of asphalt increased after wet-heat aging,while the butadiene index decreased significantly.Hydrogen substitution and structural dissimilation occurred during the damp-heat aging process due to the combined effect of oxidation reaction and polymerization reaction.In addition,due to the presence of water vapor,the acids,alcohols and aldehydes in the asphalt increased significantly after damp-heat aging,attributed to the hydroxyl groups in the water vapor,which proved that the water vapor played an auxiliary role in the reaction between the asphalt components and oxygen.(4)Through the grey correlation analysis of the macro-performance and micro-index of asphalt before and after damp-heat aging,it is found that there was a good correlation between the two.Some of the properties of the damp-heat aging equation in asphalt focued on the case when the humidity was 50%,such as the modulus index and carbonyl index in the base asphalt and the modulus index and aliphatic hydrogen nucleus index in the modified asphalt,indicating that the influence of 50% humidity on these performance indicators of asphalt was greater than these of 70% humidity,inferring that humidity was not positively correlated with aging characteristics.(5)According to the meteorological data of the annual average maximum temperature and annual average relative humidity of 344 prefecture-level city meteorological stations in China from 1961 to 2020,the K-means clustering method was used to divide them into five hot and humid climate zones from I to V,it is found that in the areas where Zones III and V were located,the influence of humidity needed to be considered.According to the average annual sustained high temperature cycle and humidity,the Zone V was divided into secondary divisions,and it is found that the continuous high temperature cycle of Zone V-e could reach 88 to 111 days.In addition,combined with the relationship between the natural aging of asphalt and indoor simulated aging,the damp-heat aging index of asphalt in each zone was recommended to provide reference for the construction of asphalt pavement in humid and hot areas. |
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