Axial Compression And Impact Mechanical Behavior And Constitutive Model Of CFRP Confined Ultra-high Performance Concrete

With the improvement of people’s requirements for building functions,construction engineering is developing in a higher,longer,and deeper direction,which puts forward higher requirements on the mechanical properties of concrete and its components.Using Ultra High Performance Concrete(UHPC),combined with lightweight,high-strength,and corrosionresistant carbon fiber reinforced composite materials,the CFRP confined UHPC prefabricated components are developed to meet the needs of small-sized,high-performance compressive components in fabricated building structures.However,domestic and international researches are focused on CFRP confined Normal Concrete(NC)or High Strength Concrete(HSC).Research on CFRP confined UHPC is still very limited,especially the impact properties of CFRP confined UHPC.Therefore,this thesis takes CFRP confined UHPC as the research object,and uses a combination of experimental research and theoretical analysis to discuss CFRP confined UHPC under monotonic axial loading,single-cycle axial loading,repeatedcycle axial loading,impact axial loading,and impact splitting loading.The mechanical behaviors and the corresponding ultimate stress and strain prediction models and stress-train curve model are established.The main contents and partial conclusions are as follows:(1)In order to optimize the preparation method of UHPC,this study used mechanical tests and micro-analysis methods to explore the effect of different mixing ratios and maintenance conditions on the internal microstructure of UHPC.In this study,UHPC fluidity test,axial compression test and scanning electron microscopy test were carried out,and the influences of water binder ratio,sand binder ratio,steel fiber content,defoamer content,curing method and age on UHPC working performance,compressive performance and microstructure were analyzed,and the reinforcement mechanism of UHPC was revealed.The results show that the Interface Transition Zone(ITZ)between C-S-H gel and aggregate is the weak link inside the UHPC.Improving the density and strength of the ITZ is the key to enhancing UHPC;the fluidity of UHPC increases significantly with the increase of the waterto-binder ratio.But its compressive strength increases first and then decreases as the water-tobinder ratio increases.Too high mortar ratio is not conducive to UHPC performance,and it will cause its compressive strength to decrease.Incorporation of defoamers can effectively improve the apparent quality of UHPC,but may reduce the working performance and compressive strength of UHPC.Incorporating 2.5% steel fiber can greatly increase the compressive strength of UHPC and significantly improve its brittleness characteristics,but it will reduce the working performance.High-temperature curing can significantly stimulate the volcanic ash effect of silica fume and slag,and the 4-day compressive strength of UHPC is increased by about 50% compared with ordinary curing,which has obvious advantages of early strength,but there may be a decline in strength at a later stage.(2)The monotonic axial compression test of CFRP confined UHPC investigates the influence of CFRP thickness and UHPC strength on CFRP confined UHPC compressive strength,elastic modulus,Poisson’s ratio of stress-strain relationship,axial-lateral strain relationship,and failure mode.The results show that the ultimate stress and strain of two-layer CFRP confined UHPC under monotonic loading increased 1.4 times and 3.5 times respectively.The stress-strain curve and the axial-lateral strain curve of CFRP confined UHPC can be clearly divided into three stages.The unconfined UHPC specimen has almost no elastoplastic deformation before reaching the ultimate state.With the increase of the FRP layers in CFRP confined UHPC,the slope,ultimate stress and strain of the second ascending stage increased significantly.Moreover,the uneven distribution of hoop strain gradually slows down,but the degree of uneven distribution of axial strain gradually increases.(3)Theoretical analysis of the ultimate state prediction formulas and stress strain model of CFRP confined UHPC under monotonic axial compression is carried out.This study collected the existing ultimate state prediction formulas and stress-strain curve models for CFRP confined concrete and experimental results of CFRP confined UHPC.Then a theoretical model suitable for CFRP confined UHPC was established.The research results show that,like confined NC and confined HSC,the strength enhancement factor and strain enhancement factor of FRP confined UHPC are proportional to the unconfined compressive strength,but the existing models underestimate the axial deformation of CFRP confined UHPC and rupture strain of CFRP.The rupture strain prediction formula based on the nominal confinement force.The CFRP confined UHPC ultimate state prediction formulas and stress-strain curve model agree well with the test results.(4)This study carried out experimental research and theoretical analysis of CFRP confined UHPC under axial single-cycle and repeated-cycle compression.The key parameters such as the stress-strain behavior,residual axial strain,and CFRP fracture strain of CFRP confined UHPC under two cyclic loads were analyzed,and the stress-strain model applicable to CFRP confined UHPC under axial cyclic load was modified.The results show that the ultimate stress and strain of the two-layer CFRP confined UHPC increased by 2% and 0% under single cyclic loading,respectively,and increased by 1% and 7% under repeated cyclic loading,respectively.The ultimate stress and strain of the four-layer CFRP confined UHPC increased by 4% and 7% under single cyclic loading,respectively,and increased by 5% and 6% under repeated cyclic loading,respectively.In addition,the existing models cannot accurately predict the compressive strength,ultimate axial strain and residual strain under cyclic loading;this chapter proposes a new residual strain prediction formula based on the existing formula and the test results.The improved model can accurately predict the cyclic stress-strain curve of CFRP confined UHPC.(5)In order to explore the dynamic compressive performance of CFRP confined UHPC,a separate Hopkinson pressure bar(SHPB)test was carried out to discuss the influence of strain rate and confined thickness on the dynamic mechanical energy such as the failure mode,strain rate and dynamic compressive strength of the specimen.And the dynamic compressive strength prediction formula suitable for CFRP confined UHPC is established with the independent rate of strain rate and constraint rate.The research results show that for every 0.2MPa increase in air pressure from 0.6MPa,the dynamic compressive strength of unconfined specimens is increased by 29%,35%,and 51%,respectively,and for the one-layer confined specimens,they are increased by 9%,27% and 34%.Increasing the number of confined layers can greatly reduce the dynamic increase factor as the strain rate increases.The CEB-FIP model overestimates the dynamic compressive strength of unconfined and CFRP confined UHPC.The prediction results based on the dynamic compressive strength prediction formula in this paper are in good agreement with the test results.(6)In order to simulate the lateral impact resistance of CFRP confined UHPC,this study used SHPB device to carry out CFRP confined UHPC impact splitting loading test.The effects of strain rate and CFRP confined thickness on the dynamic mechanical properties such as failure mode,strain rate,time-splitting tensile stress curve,dynamic splitting tensile strength and dynamic increase factor of the specimen were discussed.The results of the study indicate that for every 0.2MPa increase in pressure from 0.4MPa,the dynamic splitting tensile strength of unconfined specimens is increased by 5%,13% and 38%,respectively;for one-layer confined specimens,respectively 10%,33% and 50%.The dynamic splitting tensile strength increases with the increase of strain rate and the number of confined layers.But increasing the number of confined layers can make DIF decrease with the increase of strain rate.The dynamic splitting tensile strength increases with the increase of strain rate,and also increases with the increase of the number of confined layers.The CEB-FIP model underestimates the DIF of unconfined and CFRP confined UHPC.In this paper,the proposed model based on the parameters of strain rate and confinement rate can accurately predict the dynamic splitting tensile strength of CFRP confined UHPC.