Study On Dynamic Impact Behavior Of Concrete Through Experimental Tests And Meso-scale Simulation

The behavior of concrete under dynamic loads is strongly rate-dependent and differs significantly from that under static ones because its dynamic strengths are enhanced as strain rates increase. This phenomenon is known as the rate effect of concrete. In the present study, experimental tests and meso-scale numerical simulation are performed to study the dynamic impact characteristics of concrete. The achievements are concluded as follows.1. The meso-scale particle element model(PEM) of concrete is introduced and used to simulate tests which determine the tensile strengths of concrete. The uniaxial tensile, splitting and flexure strengths in the numerical simulation match well with the corresponding experiments. Both the numerical and test results conform to the current statistical relationship of the three strengths. Thus, the meso-scale PEM for concrete are validated.2. The impact behavior of concrete beams is investigated using drop-weight facility. Different tups are used and varying drop weights and heights are applied. The experimental results, including impact loads, mid-span displacements, dynamic strains, cracking processes, inertial forces are measured and analyzed in the study. Finally, the equilibrium of momentum-impulse and energy conversion is examined.3. A complete model of the hammer-transducer-concrete beam system using meso-scale PEM is proposed. The numerical results are compared with those from the drop-weight tests. The strain-fracture energy concept is introduced to particle element modeling and the energy conversion between kinetic and strain-fracture energy components are discussed for different strain rates.4. The unbonded prestressed concrete beams are systemically tested using drop-weight facility. The interactions between the concrete beam and prestressed tendon in the impact tests are analyzed. It is concluded that the deformation and cracking of concrete are eff ectively controlled by the tendons. However, if the tendons are yielded, the concrete beams will be cracked seriously with less residual capacities.5. Based on the complete system model for drop-weight tests of concrete beams, a coupling analytical model is used to simulate the impact tests of unbonded prestressed concrete. The PEM model is used for the heterogeneous and brittle material of concrete, while the finite difference method(FDM) is introduced to analyze the behavior of prestressed tendons. The results of PEM-FDM coupling simulation in regard to the dynamic behaviors of concrete beams and the interactions with tendons all support the experimental tests. The coupling model can be applied to dynamic analyses of other composite structures.6. Based on the meso-scale PEM for concrete, a series of dynamic bending tests with varying static pre-loadings are analyzed. The effects of pre-loading can either be an increase or decrease in dynamic strength depending on the different ranges of the complete static stress-strain curve on which the pre-loadings are imposed. When the pre-loading is imposed within the linear elastic limit, the strain energy in the elastic range produced by the static pre-loading is pre-stored in the beam. Such energy will result in a significant increase in total consumed energy and dynamic strength leading to the failure of the beam. Nevertheless, if the static pre-loadings are imposed in the damaged or strain softening range, a few initial cracks will appear around the mid-bottom of the beam, resulting in a partial release of the pre-stored strain energy. Under this condition, the total consumed energy at the instant of dynamic loading is reduced, leading to the decrease of dynamic strength.