Study Of Dynamic Mechanical Properties Of Concrete Under Triaxial Stress States

Concrete material is widely used in construction of building and bridge structures,nuclear containment structures,and hardened military facilities which might expose to multi-hazard loadings such as blast and impact loads during their service life.Under such dynamic loadings,the stress states of concrete material are very complex owing to the complex stress wave propagations and inertial confinement from concrete structure mass to resist fast dynamic deformations.Despite the widespread use of concrete as a structural material,our knowledge about its exact mechanical properties and physical behavior under complex stress states is still limited.With the development of computer technology and computational mechanics,numerical simulations of concrete structures subjected to high-rate loadings such as impacts and blasts have become more and more common.However,reliable computer simulations depend on,among other things,the accurate concrete material model.Inaccurate material model may lead to erroneous numerical simulations.In this work,a systematic research work is carried out by experimental and numerical study to better understand the concrete behaviour under dynamic complex stress states.The primary work and achievements include the following several aspects:(1)Concrete damage and its mechansm induced by high hydrostatic pressure is studied.As it is well known that in theory the homogeneous metal materials will not suffer any damage in whatever high hydrostatic pressure.The commonly used concrete material models in hydrocodes therefore do not consider the damage of concrete material in hydrostatic pressure although concrete is not exactly a homogeneous material.This study characterizes the damage evolution of 50 mm cubic concrete specimens under hydrostatic pressures varying from 30 MPa to 500 MPa for better understanding of the concrete material properties at complex stress states.A series of uniaxial tests were carried out to evaluate the damage degree of concrete after hydrostatic tests.The results show that the residual strength and Young’s modulus of the concrete specimens decrease obviously and the micro-cracks between the cement matrix and the course aggregates are very clear which indicate that the hydrostatic damage of concrete under high pressure cannot be neglected.A three dimensional mesoscale concrete model with the consideration of randomly distributed aggregates and pores is developed to study the stress distribution inside the concrete specimens subjected to different levels of hydrostatic pressures.The simulation results show that under hydrostatic pressure there are significant deviatoric stresses distributed inside the specimen which leads to the hydrostatic damage of concrete.The stress paths have insignificant effects on ultimate strength envelopes.But if the stress path involves a high hydrostatic pressure that damages the concrete specimens,upon unloading the strength envelope “shrinks” because of the damage to concrete due to high hydrostatic pressure.(2)Strain rate effect of concrete under confining pressure is studied.The strain rate effect of concrete material under multi-axial stress states in most of the current material models is based on the uniaxial impact test results because carrying out multiaxial dynamic impact tests is extremely hard.However,the uniaxial test data might not reflect the true behavior of concrete under multi-axial stress states.Modified SplitHopkinson Pressure Bar(SHPB)system with a pressure vessel filled with pressurized fluid or air is used to test the concrete dynamic properties under confining pressures.Although such tests give concrete material properties under multi-axial stress states,as will be demonstrated in this study,they do not lead to accurate results because the confining pressure under impact tests changes when specimen deforms.A mesoscale concrete model with consideration of randomly distributed aggregates is developed to study the strain rate effect of concrete under confining pressures.The results show that the strain rate sensitivity of concrete decreases with the increment of the confining pressure,indicating that using the uniaxial impact testing data overestimates the strain rate effect of concrete material under multi-axial stress states.An empirical relation is proposed in this study to model the concrete Dynamic Increase Factor(DIF)for the case with pressure confinement,which can be used to more accurately represent the DIF of concrete material under multi-axial stress states.(3)Equation of state(EOS)of concrete under tri-axial impact loadings with equalamplitude is studied.Almost all the available test data of Eo S are based on static triaxial tests and dynamic one-dimensional impact tests(e.g.flyer-plate-impact test)owing to the extremely difficulty in conducting the synchronized tri-axial impact tests.Therefore to derive accurate dynamic material properties under multi-axial stress states,it is important to develop reliable techniques to conduct true synchronized tri-axial impact tests.An innovative three-dimensional Split-Hopkinson Pressure Bar(SHPB)test system which can load the specimen synchronically from the three mutually perpendicular directions was developed in Tianjin University.The results show that the bulk modulus of concrete under high strain rate is higher than that under static loadings.The increase in bulk modulus under hydrodynamic loadings is attributed to the waterpressure because the pore-water in the cement paste cannot be drained in a very short loading time.On the other hand,the resistance of microscopic viscous to the development of the micro-cracks is another reason for strain rate effect of bulk modulus.(4)A computational constitutive model for concrete subjected to dynamic loadings is developed.This model is modified from K&C concrete model,considering the pressure dependent Equation of state,strain rate effect,plastic hardening and damage softening.The model is verified by the single element test.