Research On Fatigue Damage Of Asphalt Pavement Structure In Long And Steep Slope Of Mountain Highway

Due to the terrain and the limit of highway cost,inevitably there are many large longitudinal slope sections on the construction of highway in mountainous area in China.Slope length and slope of some longitudinal sections even beyond the maximum prescribed technical standard of highway engineering in existing.Compared with the general highway,the running speed of the vehicle on the long longitudinal section of the asphalt pavement of the mountain highway is low.When the vehicle is overloaded,the climbing speed will drop sharply,which will lead to the early damage of the asphalt pavement,so this section should be specially designed.Since China will face more mountain roads on construction of highways,this paper systematically analyzed the mechanical response and fatigue damage of asphalt pavement structure under different factors under the influence of different factors by means of theoretical analysis.First,the traffic composition and axle load distribution characteristics of the main highway in Chongqing province were analyzed,and the type 115(4 axle)Dongfeng Hercules heavy truck(DFL3310A22)dump truck was selected as the typical heavy vehicle in this paper.According to the theory of vehicle driving,the dynamic characteristics of the vehicle was drawn and the influence of different factors on the climbing speed of the typical heavy load vehicle was analyzed.The results show that the greater the gradient,the greater the loading rate and the higher the altitude was,the faster the running speed of heavy vehicle on the long and steep slope declined,and the smaller the balance speed and the required slope length were.At the same time,a prediction formula for balance speed which contained gradient,loading rate and altitude was established.The result showed that the influence of gradient on the balance speed is the largest,followed by the loading rate and the influence of altitude is the smallest.Secondly,the research on tire-pavement contact characteristics were summarized,which meaned the grounding shape and pressure were studied deeply.According to the equivalent area method and considering the convenience of finite element modeling and dense grids may lead to non convergence,the simplified form of tire grounding model was established.Meanwhile,the mechanical response of the stress and strain of the pavement structure under the unequal distribution load was analyzed,and the most disadvantageous position of each mechanical index was obtained.Then,considering the typical pavement working temperature,Brown method was selected as the asphalt mixture dynamic modulus calculation method,at the same time,the climbing speed characteristics of heavy vehicle and tire ground appearance were combined to study the relationship between the influence factors and the mechanical response index.The results showed that the maximum asphalt layer strain position of semi-rigid base moved from the bottom of the asphalt layer under low temperature up to the depth of 7cm~8cm asphalt layer under high temperature,which could effectively prevent the shear deformation under heavy load,and be more favorable to improve the overall pavement anti-rutting performance.Meanwhile,the bottom of asphalt layer and the upper structure of granular base respectively were more prone to fatigue cracking and rut.It was also discovered that the influence of gradient on stress was less than the effect on strain;the thicker asphalt layer could cut down the cracking,permanent deformation of subgrade,fatigue cracking and shear failure of asphalt layer;the thicker base layer could improve the base bottom tensile stress and vertical compressive strain on top of subgrade;the bottom of asphalt layer will appeared tensile stress and the position of the maximum tensile asphalt layer moved from middle of the asphalt layer to the bottom of the layer,which is unfavorable for the fatigue and shear failure of the asphalt layer.Finally,the statistical data of annual temperature measurement field of asphalt pavement was counted,and the diurnal variation of the temperature field was divided into two stages and the estimate formula are given.Then,per hour and whole month fatigue damage were cumulated according to Miner fatigue law.On this basis,the fatigue damage change law of asphalt pavement structure in large longitudinal slope section of mountain highway in high temperature month was studied.The results showed that the increase of gradient,loading rate and altitude will lead to serious fatigue damage of pavement structure,and semi-rigid base and granular base asphalt pavement structures should pay attention to the upper and bottom of the asphalt pavement structure value respectively? In the analysis,it is found that the fatigue damage results were more responsive to the fatigue failure characteristics of the actual pavement than the tensile strain response.In the large longitudinal slope of asphalt pavement structure design or large modification,without considering the elevation of the limited conditions,the granular base asphalt layer thickness and base thickness could be increased,surface layer modulus could be improved,and semi-rigid base thickness and cement content could be decreased to improve the ability of the fatigue resistance of asphalt layer.