Simulation And Experimental Study On Impact Depth Of Drop Hammer And Back-Calculation Of Pavement Surface Modulus

The subgrade and pavement structure are typical layered structures.The load is transferred from top to bottom,layer by layer.The modulus reflects the basic mechanical behavior of a material.It is the key of structural calculation analysis and evaluation.However,how to detect the modulus of structural layer has become a big problem in road engineering.Aiming at the problem of modulus detection of road structure layer,a new method of modulus detection and back calculation based on drop hammer impact acceleration is proposed in this paper.It can obtain the layer modulus of the structure which affects the depth below the surface layer.Combined with the deflection measurement data,it is of great significance to accurately evaluate the mechanical properties of the road structure layer.In this paper,by coupling PFC5.0 Suite and FLAC3 D,a discrete and continuous coupled drop hammer impact pavement structure model was established.The law of drop hammer and road surface interaction was simulated.The effects of the weight of the hammer and the impact rate of the drop hammer on the impact depth and impact acceleration were studied.A new method was proposed to calculate the layer modulus of pavement structure from the peak value of impact reaction of drop hammer.According to the simulation results,the drop hammer impact tester was assembled.The modulus of cement concrete and other materials was measured by the acceleration sensor.The experimental results were verified with the simulation results.By analyzing the simulation and test data,the prediction equations of the peak impact reaction and the surface modulus of the drop hammer were established.Through the analysis of simulation and test results,the following results are obtained:(1)For the same road structure,the peak value of impact reaction increases with the increase of drop weight and impact rate.For the same thickness of the surface layer,with the increase of the modulus of the surface layer,the peak of the reaction force obtained by the same weight and impact rate will also increase.(2)The drop hammer impact spreads in the form of energy,and there is a certain range of areas that can be affected.The properties of the material in a certain thickness range can be reflected by the peak value of the impact reaction of the drop hammer.(3)A back calculation method of the impact acceleration modulus of the drop hammer based on this principle is designed.This method is used to test the impact acceleration of drop hammer on common pavement materials.The prediction equations between the peak impact acceleration of the drop hammer and the elastic modulus and dynamic deformation modulus of the cement test block are established respectively.(4)The standard hammer type is defined from the research results.The back calculation of the surface modulus of road structure under lossless condition is carried out by using standard hammer type.The prediction equation between the peak of the impact reaction and the modulus of the surface layer is established,which provides a new idea for the back calculation of the modulus of the surface layer.