| Concrete and steel reinforced concrete are materials that widely used in civil structures and protective structures.It is always required that these structures should have the capability of resisting the high-rate loads,e.g.impact,explosion,etc.Concrete is also a particle reinforced composite material with very complicated mechanical behaviors.The mechanical behaviors of steel reinforced concrete are further complicated due to the great differences of mechanical properties between steel and concrete matrix.Recently,the previous scholars have conducted many researches on the dynamic properties of concrete material under high-rate loads.The strain rate effect of concrete,dynamic constitutive relations under different loads and the numerical simulation based on the meso-scale model have been the research hotspot of concrete material.In many studies about the dynamic properties of steel reinforced concrete,the steel reinforced concrete is always studied as a kind of steel reinforced structure.However,as a composite material,there are significant deficiencies in the studies on the dynamic experiment,constitutive relation and numerical simulation of steel reinforced concrete.In this paper,the studies on the experiment,constitutive relation and numerical simulation in meso-scale level of plain concrete and steel reinforced concrete are conducted based on the Split Hopkinson Pressure Bar(SHPB)and 3D meso-scale model.Firstly,with the pulse shaping technique and high-speed photography,the impact compression experiments of plain concrete and steel reinforced concrete are conducted based on the SHPB with the large size of ?120mm.The following material behaviors in the tests have been studied,such as pulse waveform features,specimen failure process,crack propagation,failure modes and strain rate effect,etc.For the plain concrete experiment,the "double peak" phenomenon of reflection wave and the "compression wave" phenomenon in the wave tail are revealed and interpreted.Slightly different from this,in the reinforced concrete experiment,with the increasing load velocities,the waveform of reflection wave behind the first peak gradually changes into keeping rising from nearly constant strain rate loading,and ultimately shows as "double peak" under relative higher impact velocity.Moreover,for the SHPB tests of plain concrte and reinforced concrete under various strain rates,some empirical knowledge of the correspondence relation between waveform features and ultimate specimen failure patterns is also revealed.From the observation of specimen failure process and crack propagation,it is found that the rapid development of crack lags behind the occurrence of peak stress,this maybe the main reason that leads to the sustainable load-carrying capacity of material in thedamage failure stage.Compared to plain concrete,the superiorities of the mechanical properties of reinforced concrete are analyzed,as well as the different protective functions of different steel frames to material.The experimental results show that the residual failure patterns of reinforced concrete specimens will keep better consistence with the configuration of the steel frames.Furthermore,for laying the foundation of proposing constitutive relation of material,the following material behaviors are also comparatively analyzed for plain concrete and reinforced concrete,such as the strain rate effect of dynamic compression strength,energy absorption capacities and the maximum compression strain,etc.Then,with consideration of the meso-scale ingredients in concrete,i.e.random aggregate,mortar and the interface transition zone(ITZ)between them,the 3D meso-scale model of concrete is developed and reliably validated based on SHPB test of concrete specimen.Based on this,numerical predictions of impact compression tests of concrete material under various loading conditions are conducted with HJC model.Associating with waveform features,the violation indicator of the specimen stress equilibrium in the SHPB test is first identified for concrete-like damage softening materials.Other concrete material behaviors for stress non-equilibrium are further explored,e.g.specimen failure extent,dynamic increase factor(DIF)and damage development,etc.In the analysis of numerical results,the conception of "damage failure volume" is introduced for explaining the generation of the "double peak"phenomenon of the reflection wave.Especially,the research results show that,after the violation of the stress equilibrium,the concrete specimen will turn into structural response from material response.Moreover,the ITZ effect on material dynamic behaviors is also explored.Furthermore,based on the meso-scale model of plain concrete,with consideration of the complicated steel frame configuration and the bongding layer between steel frame and mortar,the 3D meso-scale model of steel reinforced concrete is constructed.The reliability of this model is also validated based on the SHPB test of reinforced concrete specimen.The influence of the ITZ between steel frame and mortar on the material dynamic behaviors is emphatically discussed.In the end,based on the experimental results,the concrete damage softening constitutive model(CDS model)is proposed through revising the strength equation of HJC model and borrowing from the description for the strain hardening behavior of metal material in Johnson-Cook model.CDS model can well describe the plastic loading stage and the post-peak segmentation softening behavior of concrete in the impact compression process.Furthermore,based on CDS model of concrete material,the reinforced concrete damage softening constitutive model(RCDS model)is proposed with consideration of the reinforcement ratio and weak interface damage of steel.Both the slightly weaken effects of steel on the material peak strength and the enhancement effect of steel on the post-peak material behaviors can be well simulated by RCDS model.With RCDS model,the ?-? curves of reinforced concrete under various reinforcement ratios and strain rates are reliably predicted. |
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