Study On Water Migration Behavior Of Large-void Asphalt Concrete Under Different Humid And Hot Conditions

In asphalt pavement,water transport and erosion are important causes of pavement water damage.Large porosity asphalt concrete is more susceptible to the action of water due to its porous characteristics,and the effect of water on asphalt concrete includes two stages of water entry and discharge,therefore,revealing the internal migration behavior of water is the basis for studying the water damage of large void asphalt concrete.In this paper,the internal water migration behavior of large-void asphalt concrete under different humid heat conditions is studied and analyzed,and the migration laws and influencing factors of water in the three stages of infiltration,diffusion and drying in asphalt concrete are clarified.Firstly,the asphalt concrete specimens in the three compacted states were scanned and imaged by X-CT scanning technology and image processing technology to obtain binary images of their internal voids.According to the scanning void image to obtain the void two-dimensional parameters,the void aspect ratio,section area,porosity,number of voids,equivalent diameter and fractal dimension and other void parameters were compared and analyzed,and the results showed that the void aspect ratio,equivalent diameter and fractal dimension can better represent the void shape,cross-sectional area,porosity and number of voids.With the increase of compaction,the internal voids of large-void asphalt concrete are gradually refined,and the number of voids increases.Then,the CT scan images of three specimens with different compaction degrees are reconstructed in three dimensions by Aviro software,the appropriate seepage calculation unit body is selected,and the absolute permeability analysis module in the software is used to calculate the permeability coefficient of the asphalt concrete specimen with different compaction degrees.The correlation between the permeability coefficient and the effective porosity,tortuosity and connectivity index of the unit body was analyzed,and the results showed that the greater the degree of compaction,the smaller the permeability coefficient of the unit.The effective porosity is best correlated with the permeability coefficient of the cell body,and when the effective porosity of the cell body is close,the greater the curvature,the greater the permeability coefficient.The correlation between the connectivity index and the permeability coefficient of the unit body is the weakest,but the overall positive correlation,the larger the connectivity index,the greater the permeability coefficient of the unit.The flow rate streamline diagram inside the specimen shows that not all connected voids have streamline distribution,and the distribution of flow velocity flow lines is related to the seepage direction,entry port conditions and void morphological characteristics,and the flow rate at the pore throat of the void channel is the largest.Then,through the self-made water vapor diffusion test device,the water vapor diffusion coefficient inside the asphalt concrete specimen under different humid heat conditions was calculated.The test results show that the void determines the upper limit of water vapor diffusion,and when the void is small,when a certain temperature is reached,even if the temperature is further increased,the diffusion coefficient changes little.Temperature determines the lower limit,and at lower temperatures,molecules move more slowly,even if the porosity varies greatly,but it does not bring a significant difference in diffusion coefficient.Under the same porosity,the higher the temperature,the more violent the movement of water molecules,and the easier it is for water molecules to penetrate from the specimen,the less water vapor accumulation inside the specimen;The larger the porosity,the greater the penetration of water vapor,the larger the contact area between water vapor and the mixture,and the greater the diffusion coefficient of water vapor.Finally,by simulating the rainfall and evaporation test,the humidity sensor is arranged inside the specimen to obtain the change of internal humidity during the evaporation process of the specimen under different humid heat conditions,and at the same time,based on the actual test,the evaporation model of asphalt concrete specimen is established through COMSOL Multiphysics software,which changes the external conditions of the specimen: humidity,temperature,wind speed,and height of the specimen respectively,and explores the changes of the internal temperature and humidity of the specimen with time under different influencing factors.The results show that the evaporation process of large void asphalt concrete can be divided into four stages,in the evaporation process internal temperature and humidity gradient,humidity with time first fast and then slow from the maximum humidity to the ambient humidity began to decrease,temperature first decrease and then increase phenomenon,ambient temperature,humidity changes will change the internal temperature gradient and humidity gradient distribution of the specimen,while increasing the temperature change range,wind speed on the surface of the specimen is more obvious,the lower part of the specimen evaporation has no effect,the larger the specimen height,the slower the evaporation process,the greater the distribution range of temperature and humidity gradients inside it.