Study On The Cement Concrete Pavement Design Considering Base Erosion And Cavity Beneath Slab

In china, the early damage of cement concrete pavement is more serious. The poor construction quality and the improper late maintenance, with no doubt, could cause this situation. However, the deep reason of this early damage is that, under heavy traffic, the anti-scour ability of base course is poor, the pavement structure design is lack of consideration for the influence of base erosion and cavity beneath slab, and the design thickness of pavement course is too thin. In this paper, the change law of base erosion and cavity beneath slab under the couple action of traffic load and water is deeply studied, and the evaluating indicators of base material for anti-erosion properties are proposed, which are available for the pavement structure design; the prediction model of the erosion and cavity beneath slab, which is available for the pavement structure calculation, is firstly set up; and by the three-dimension finite element calculation and regression analysis of cavity beneath slab pavement structure, the calculation formula of load stress is given; based on the theory of the concrete trabecula indoor fatigue test, the concrete fatigue formula, which is available for cavity pavement calculation under changing and repeating load, is firstly derived; concerning the erosion and cavity beneath slab, the practical calculation formula of load fatigue stress is proposed. Based on the summary of the research conclusions above, the cement concrete pavement design method, based on erosion and cavity beneath slab, is systemly proposed.The analysis of the cement concrete pavement and investigation on the Guilin-Liuzhou expressway show that there are three deep-seated reasons that caused the early damage of cement concrete pavement in our country. First, the evaluation index and standard of the anti-scour ability of the base course material are not proposed quantitatively in the current cement concrete pavement design method. Second, the pavement structure design is not taking into account the influence of base erosion and cavity beneath slab. The last one is that the design thickness of pavement course is too thin.The base erosion and pumping only happen during the period when it rains or the caves beneath the slab is in semi-saturation after the rain. The erosion water pressure beneath the slab has relationship with both the speed of the vehicle and the axle load. The higher the speed, the bigger the water pressure; the heavier the axle load, the bigger the water pressure. Taking into account the times of the load function, the evaluation index of the anti-scour ability of the base course------the anti-scour coefficient k_c, is proposed, and then 50 KPa isrecommended as the standard water pressure in the erosion test.The erosion machine of variable water pressure is researched and developed, the erosion tests of cement soil, two ashes-soil, and two ashes and macadam are carried out, and then the results show that anti-scour ability of two ashes-soil is best, the second is two ashes and macadam, cement soil is worse.The shape of cavity beneath slab is close to cone or pyramid, the cavity height is less than side length. By introducing the anti-scour index of base material, the precipitation weather correction coefficient k_d is proposed, the cavity prediction model of pavement initialuse stage and after unjoint is put forward.The three-dimension finite element analysis model of longitudinal joint middle and corer of slab is built, through calculation, the results show that after the slab bottom appears cavity, the load stress is significantly increased by cavity area. Further analysis show that, as to cavity of longitudinal joint middle, the relation between load stress and radius of cavity is exponential, the relation between load stress and relative stiffness radius is power; as to cavity of corer, the relation between load stress and radius of cavity is linear.Through the regression analysis of finite element calculation result, the load stress fitting formula about cavity of longitudinal joint middle and corer of slab is given. Based on considering the change of base top surface equivalent modulus and axle load, the load stress calculation formula of cavity pavement is proposed.Based on the basic theory of indoor concrete trabecular fatigue test, the concrete fatigue formula, which is available for cavity pavement calculation under changing and repeatingload, is derived. By introducing the precipitation weather correction coefficient k_d, thepractical calculation formula of load fatigue stress is proposed, which considers cavity beneath slab and base erosion.According to the fatigue injury equivalent principle, the load conversion relationship formula of cavity pavement, which is available for the pavement load stress analysis, is put forward. Taking into account the analysis above, the cement concrete pavement design method, based on erosion and cavity beneath slab, is firstly and systemly proposed, and then the pavement design flowchart is given.