Thermo-stresses And Excess Pore Fluid Stresses Of Flexible Pavement

Besides vehicle load, the environment factors such as temperature and moisture have great effect on flexible pavement. Thereinto, the effect of temperature on pavement performance is generally considered to be a very important factor since it directly affects the pavement structural capacity and thermal cracking is associated with high repair costs and the acceleration of other failure mechanism, such as weakening of subgrade and aggregate layers through water infiltration, stripping in asphalt layers, loss of subgrade support, and so on. Therefore, it has important realistic significance in pavement engineering to lead a further analysis of the interaction between thermal stresses of the pavement and the influencing factors, taking consideration into the real behavior of material characteristics in asphalt pavement, so as to improve asphalt concrete's thermal resistance.Significant advancements have been achieved over the past 40 years in understanding the mechanisms of thermal fatigue cracking and researches on thermo-stresses of flexible pavement are mainly based on multilayered elastic theoretical system. However, during the calculation of thermo-stresses, the effect of temperature on flexible pavement is usually neglected and it is assumed that material characteristics of asphalt mixture are constant at present. In fact, asphalt concrete is a kind of material sensitive to temperature. The modulus of elasticity, Poisson's ratio and the coefficient of thermal expansion of asphalt concrete are not constant at different temperature. This assumption of temperature-independent material characteristics brings convenience in calculation, but leads to the significant difference between calculating results and the real state of flexible pavement, and limits the applicability of the solutions obtained to certain ranges of temperature.This current research presents an analytical study of thermal stresses of asphalt pavement by transfer matrix method and stiffness matrix method, respectively, in which material characteristics are taken as parameters dependent on reference temperature. Then, numerical simulation and analysis of thermo-stresses, prediction of low temperature cracking, and the feasibility of fiber Bragg grating (FBG) sensors in flexible pavement are thoroughly investigated. In addition, analytical solutions of excess pore fluid stresses in flexible pavement is derived by stiffness matrix method. The main contents of the current research are as follows:(1) Based on thermo-elastic theory and material characteristics being regarded as functions of temperature, analytical solutions of thermo-stresses in multilayered elastic system are derived by transfer matrix method and mathematical methods of Laplace and Hankel integral transformations. Distribution of temperature and material characteristics of asphalt layer, effect of temperature-dependent material characteristics on thermo-stresses in flexible pavement are analyzed under daily variation of temperature. Because of the consideration of the effect of temperature on material characteristics, calculating results of thermo-stresses are more close to the real status of flexible pavement.(2) Taking the effect of temperature on material characteristics of flexible pavement into consideration, analytical solutions of thermo-stresses are derived by stiffness matrix method. The analytical solutions are clear in concept because the expression of state vectors is resolved into two parts: one caused by vehicle load and the other caused by temperature. Moreover, because the elements of matrix do not include positive exponential function, the calculation is not overflowed and the shortages of transfer matrix method can be overcome. Based on a calculation example, the effects of temperature-dependent modulus, coefficient of thermal expansion, and Poisson's ratio of asphalt layer on thermo-stresses are analyzed.(3) With reference to relevant literatures, the functional relationship between modulus of elasticity, coefficient of thermal expansion, and Poisson's ratio and temperature are fitted. Hereby, influences of pavement structural thicknesses and material characteristics of base course on thermo-stresses are studied and critical cracking temperature is predicted for a practical flexible pavement, which accords well with the real situation of low temperature cracking.(4) Some FBG sensors packaged and designed by Institute of Earthquake Engineering of Dalian University of Technology are used in a real flexible pavement, and a real-time optical FBG sensor system is built up with these sensors. Strain of pavement structure under static load, temperature and strain distribution of asphalt pavement caused by air temperature changes are monitored with this system for several months and compared with the analytical results. The agreement of experimental observations and calculation results shows the feasibility of FBG sensors in temperature and strain monitoring of flexible pavement structure and the correctness of analytical solutions in this paper.(5) Permeated flexible pavement is regarded as an axial symmetric body of multilayered saturation elastic half space. Based on the fundamental thermal Boit consolidation equations, explicit solution of the excess pore fluid stresses for flexible pavement is obtained by stiffness matrix method and some mathematic methods such as Laplace and Hankel integral transformations. The stresses of the permeated flexible pavement are calculated and analyzed based on a practical flexible pavement.