Influence of subgrade modulus on fatigue cracking damage of cement concrete pavement under traffic load

Based on finite element software KENSLABS, the damage calculation model of cement concrete pavement under traffic load was established.Based on the foundation seasonal adjustment factor and the zero-maintenance fatigue criterion, the influence of overall decrease of subgrade modulus on the fatigue cracking index of pavement was analyzed, the dependence of equivalent axle load factor and number of repetitions for one passage of multiple axles on subgrade modulus was discussed, and the relationships between the cracking index of pavement and the core parameters of pavement design including slab thickness, modulus of rupture of cement concrete, uniaxial load weight, repeated times of uniaxial load in one day were studied.Analysis result indicates that when the seasonal adjustment factor of foundation decreases from 1.0 to 0.8 and from 0.4 to 0.2, the growth rates of cracking index of pavement are 2.8, 2.9, 1.5 and 49.8, 269.0, 1351.4 under the uniaxial load, biaxial load, triaxial load, respectively, so the fatigue cracking index of cement concrete pavement increases with the decrease of seasonal adjustment factor of foundation, and the smaller the seasonal adjustment factor of foundation is, the faster the increase of the fatigue cracking index is.When the subgrade modulus is 60 MPa and the thickness of slab are 15 cm or 35 cm, the uniaxial load is more destructive than the biaxial load, and the biaxial load is more destructive than the triaxial load.The same is true for 15 cm slab with a subgrade modulus of 20 MPa.But for 15 cm slab with a subgrade modulus value of 60 MPa, the order is reverse.So the equivalent axle load factor and the number of repetitions for one passage of multiple axles are affected by slab thickness and subgrade modulus.When the subgrade modui are 20 MPa and 60 MPa and the thickness of slab increases from 21 cm to 25 cm, the fatigue cracking indexes decrease by 1.18×10 and 1.18×10^-2, respectively.When the modulus of rupture of cement concrete increases from 4.0 MPa to 4.4 MPa, the fatigue cracking indexes decrease by 1.28 and 2.20×10^-3, respectively.When the uniaxial weight increases from 80 kN to 160 kN, the fatigue cracking indexes increases by 5.48 and 7.36×10^-3, respectively.When the repeated times of uniaxial load in one day increases from 50 to 90, the fatigue cracking indexes increase by 2.05×10 -1 and 5.07×10^-4, respectively.So when the fatigue cracking index of cement concrete pavement varies with slab thickness, modulus of rupture of cement concrete, uniaxial weight, repeated times of uniaxial load in one day, there is a direct relationship between its ranges and subgrade modulus.When 15 cm cement stabilized base is added, the fatigue cracking design life of pavement increases by 3.42 years under designing working condition.At the same time improving subgrade modulus, some measures including appropriately adding slab thickness, strictly forbidding overloading, setting cement stabilized base should be taken into account preferentially to control the fatigue cracking damage of cement concrete pavement under traffic load.