Study Of The Influence Factors Of Water Vapor Passing Through Asphalt Mixtures

Water vapor is one of the important sources of moisture damage in asphalt mixture.It is important to research the relationship between water vapor diffusion characteristics and environment as well as the internal structure of the mixture,thus better evaluating moisture damage in asphalt mixture.In the present study,water vapor movement in diverse service environment and in asphalt pavement with different structures remains unclear,so this paper focuses on the study of the law of water vapor diffusion in asphalt mixture under the above factors.Aiming at the universal process of water vapor passing through the asphalt layer,the test for water vapor diffusion based on gravimetric method was developed to simulate water vapor movement driven by various relative humidity gradients in asphalt pavement,simultaneously considering the influence of different temperature,negative pressure,pavement thickness,void content and so on.Then the penetration of water vapor with time was measured and the diffusion coefficient was calculated by Fick's first law.The penetrating diffusion law of water vapor in asphalt mixtures under various influence factors was uncovered,providing a theoretical basis for further research on the moisture damage mechanism of asphalt mixture.The main conclusions and findings are as follows:(1)Experimental schemes of water vapor passing through the asphalt layer under a variety of RH differentials are developed.A linear relationship is identified between the diffusion flux and the RH differential,which demonstrates that this kind of water vapor diffusion followed Fick's first law of diffusion.The calculated water vapor diffusivity in Phase II diffusion is independent of the variation of the RH differential.(2)The diffusion flux of water vapor in AC-20 and OGFC-13 mixtures tend to increase exponentially with temperature and the relationship between diffusivity and temperature follows the Arrhenius equation.It is observed that the activation energy of AC mixture is higher than OGFC mixture,but the growth rate of diffusivity of AC mixture is obviously lower.It shows that although the trends of diffusion rate with temperature in both types of asphalt mixtures are similar,the sensitivity to temperature varies with the internal structure of mixtures.(3)A test method for water vapor diffusion under negative pressure is proposed,and the water vapor diffusivity of asphalt mixture under different vacuum is measured.It is found that the diffusion rate of water vapor increases with the increase of vacuum.And the penetration of water vapor under negative pressure is significantly higher than that under normal pressure.So the characteristics of water vapor diffusion in asphalt mixture with large thickness and low void which are difficult to measure under normal pressure can be obtained by keeping test apparatus conditioned under negative pressure.In addition,due to the good inverse relationship between the diffusivity and the pressure,the water vapor diffusivity under normal and negative pressure can be converted to each other.(4)The diffusion flux and diffusivity are measured under negative pressure for three types of asphalt mixtures whose thickness is selected as the typical strcutural thickness.The measurement results indicate that OGFC>AC>FAM.It is shown that the characteristics of water vapor diffusion are greatly influenced by the internal structure of the mixture.With the increase of the thickness of the structure layer,the diffusion flux of three kinds of specimens decreases and the diffusivity increases.It reflects inhomogeneity of the asphalt mixture.Besides,thickness will have more significant effect on water vapor diffusivities with the increase of void fraction and compactedness,which demonstrates the increased inhomogeneity of asphalt mixtures.(5)Water vapor diffusivity of AC and OGFC mixtures with different air voids is measured.The relationship between diffusivity and percent air voids and percent air connected voids is established,and the inflection point position of diffusivity in AC mixtures is determined.It is shown that the relationship between diffusivity and percent air voids and percent air connected voids will vary from asphalt mixtures with different gradation types.