Analysis On The Mechanism Of Moisture Damage And Design For Asphalt Pavement Under The Coupled Moisture-Loading Action

In recent years, there are initial failures such as stripping, flushing asphalt, internal loose, pumping, potholes and so on in asphalt pavement on early use, especially after one or two years use. Some of the damages are caused and aggravated by moisture action directly, while others were related to it. It is shown that because of the coupled moisture-loading action, the asphalt moves from bottom of asphalt pavement structure to top by analysis on core sample. In sum, the water within asphalt pavement affect the performance of the pavement to a certain degree directly or indirectly, and reduce the using life of the pavement. However, the research related to the moisture within asphalt pavement concentrate on asphalt mixture water stability evaluation, the development of resistance to water damage materials and construction quality control, etc. Although researchers universally believe that pore press in asphalt pavement structure is the main cause of the moisture damage, but there is not so much theoretical calculation and quantitative analysis about it. In addition, It is short of systematic study about pavement mechanics calculation under the coupled moisture-loading action.As far as the asphalt pavement structure design calculation is concerned, it is based on single homogeneous material hypothesis, while from the point of view of materials, asphalt mixture is not single homogeneous material but porous medium which contains air voids and cracks fully.At first, based on the porous media theory,assuming the asphalt concrete as saturated porous media material, axisymmetric finite element model was established for the typical semi-rigid base asphalt pavement. The pore pressure variations between different loadings, vehicle speeds and thickness of asphalt surface course were analyzed. The result indicates that the maximum pore press occurs in the centre position of lord. And the maximum pore press value decreases with the increasing of the depth and the radial distance. High vehicle speed and overloading will accelerate the reduction of pavement performance when moisture exists in pavement structure. The asphalt film failure is more easily caused by pore press in bottom layer of typical semi-rigid asphalt pavement, when the asphalt surface course is thinner.Secondly, the calculation and analysis on the distributive regularity of the pore pressure with two different boundaries of fluid and on the sensitivity parameters of asphalt pavement structure were carried out. The change of modulus of surface course and base course affects the pore press within asphalt pavement that have been discussed.Thirdly, the orthogonal experiment method was employed to take numerical simulation and experiment for saturated asphalt pavement. The results show that in the factors which affect pore press, the significance order from big to small of the effects of each parameter is permeability coefficient, load peak, traveling speed, surface course stiffness and thickness.Fourthly, a three-dimensional finite elment model was developed in the assumption that the asphalt pavement is saturated. Then the comparison and analysis of time distribution of the three directional strains and stresses and three-dimensional distribution of the vertical stress between saturated condation and dry condition were carried out. The result indicates that characteristics of the dynamic response for asphalt pavement are different among different conditions which include saturated condition and dry condition in surface course. The structual damage will be occurred early,espacially the fatigue cracking and permanent deformation. In asphalt pavement design mothed it is more reasonable to use the flexural-tensilestrain as index than flexural-tensilestress.Fifthly, two three-dimensional finite element models were established for two typical kinds of asphalt pavements which included the semi-rigid base asphalt pavement, the semi-rigid base asphalt pavement with flexible base. Then the comparison and analysis of the spatial and time distribution for asphalt pavement between the two kinds of structures were carried out which included vertical stress, vertical strain, and pore pressure. The result indicates that the semi-rigid asphalt pavement stucture with flexible base show the best performance of resistance the moisture damage relatively the semi-rigid asphalt pavement structure give the worse performance.Sixthly, an axialsymmetry two-dimensional finite element model for the transient dynamic analysis on saturated asphalt pavement was built. Then the calculation and analysis of change rule was carried out for saturated asphalt pavement with different permeabilities in different layers which include pore press, deflection and the radial strain at the bottom of the asphalt wearing course. The comparsion for the mechanics feature of asphalt pavement between saturated condition and dry condition was implemented, and according to the relation between permeability and voidage, the reasonable comands were put forward for the voidage of asphalt mixture layered design in order to prevent moisture damage.Finally, based on the analysis above, a asphalt pavement structure design method considering moisture was conceived.