Investigation On Mechanism Of Water Vapor Diffusion In Asphalt Mixtures

Moisture damage of asphalt pavements in desert areas with an arid climate indicates that not only liquid water but also water vapor are major contributors that trigger the moisture damage.The relative humidity differential,which exists between the subgrade below and the atmosphere above the pavement,is a major contributor to the water vapor diffusion.Water vapor diffusion in a newly constructed asphalt pavement consists of Phase I(water vapor accumulating toward the asphalt layer)and Phase II(water vapor passing through the asphalt layer).The Phase I water vapor diffusion is of great significance in water molecules movement and accumulation inside asphalt pavements,which is one of the main factors that triggersmoisture damage.However,the relevant researches on water vapor diffusion in asphalt mixtures are still in their infancy and have some deficiencies in terms of experimental design and theoretical derivation.This dissertation attempts to address the aforementioned deficiencies in regard to experimental design and theoretical derivation.The major conclusions and findings are summarized as follows:(1)An experimental method is designed to investigate the water vapor diffusion inside asphalt mixtures.The Gravimetric Sorption Analyzer(GSA)with high resolution is utilized to perform a series of water vapor diffusion experiments,and the weight of water vapor absorbed by the asphalt mixtures could be obtained.The designedexperimental method overcomes the deficiencies of current experimental methods,and could significantly improve the accuracy,and expedite the diffusion process.(2)Water vapor diffusion in asphalt mixtures isverified to be dominated by Fickan diffusion in this study.Two simple water vapor diffusion models in terms of axial and radial directions for infinite boundaries are firstly established.Based on the boundary and initial conditions determined by the designed water vapor diffusion experiment,a more practical 3-D isotropy diffusion model with rigorous derivations originated from Fick's second law in cylindrical coordinates is further developed.Diffusivity of water vapor inside asphalt mixtures and moisture retention capabilityper unit mass under only water vapor pressure are determined by applying the 3-D isotropy diffusion model to fit the data of the diffusion tests.(3)Properties of water vapor diffusion in the relative humidity constructed only by water vapor in the designed experiment are different from that inthe same relative humidity under 1 atmosphere.So relationshipsare established through rigorous derivations between diffusivityD and moisture retention?under only water vapor pressure and those under 1 atmospherewith the same relative humidity based on this difference.In other words,the two parameters under only water vapor pressure are converted into the corresponding values under 1 atmosphere.(4)Relative humidity differentialis one of the key factors that affecting the water vapor diffusion in asphalt mixtures.This paper designs a laboratory experiment to investigate the water vapor diffusion in asphalt mixtures under a number of relative humidity differentials at a constant temperature.Both of water vapor diffusivity and moisture retention capability per unit mass have a proportional linear relationship with the square of relative humidity differential.(5)Temperature is also an important factor that influences the water vapor diffusion in asphalt mixtures.This paper also designs a laboratory experiment to investigate the water vapor diffusion in asphalt mixtures under a number of temperatures at a constant relative humidity.The Arrhenius equation is selected based on theoretical justification to quantify the effects of temperature on diffusivities and moisture retention capabilities per unit mass.(6)When both of temperature and relative humidity differential are regarded as variables,proportional linear relationshipsare established between relative humidity differential and diffusivity as well as moisture retention capabilities per unit mass,and Arrhenius equations are selected to qualify the effects of temperature on water vapor diffusion parameters.These two relationships are coupled in the form of multiplication.This study designslaboratory experimentwith high accuracy and high efficiency to investigate water vapor diffusion in asphalt mixtures.A 3-D diffusion model isestablishedwith rigorous derivations originated from Fick's second law,which differ from empirical model.Water vapor diffusivity and moisture retention capabilities per unit mass determined by model fitting are selected as important parameters that describe the properties of water vapor diffusion inside asphalt mixtures.Arelationship is formulated between each of the two parameters under only water vapor pressure and under 1 atmosphere based on the differences between experimental conditions and actual environment.After rigorous derivations,the relationships between temperature and diffusivity,temperature and moisture retention capability per unite mass,relative humidity and diffusivity,relative humidity and moisture retention capability per unite massare finally established,which could be used to accurately predict the characteristics of water vapor diffusion inside asphalt pavements influenced by the external environment.The series of research works provide the theoretical and experimental basis for exploring moisture damage of asphalt pavements result from water vapor.