Experimental Research On Axial Mechanical Behavior Of Short Concrete Column Strengthened With FRP Grid And ECC

Fiber reinforced polymer(FRP)is widely used in large-span structures,towering structure,special engineering conditions,repair and reinforcement engineering because of its high specific strength,high specific modulus,good corrosion resistance and durability.However,many researches show that the organic bonding materials used in FRP and concrete interface are easy to age,which seriously affects the utilization rate and reinforcement effect of FRP.In recent years,the engineered cementitious composites(ECC)with its advantages of ultra high ductility,small crack width and good durability has been tried to be used as an inorganic adhesive material combined with FRP grid(FRP/ECC)to strengthen the concrete structure,in order to achieve the durable strengthening effect on the concrete structure while effectively using FRP material.In this paper,the mechanical properties of FRP/ECC composite confined concrete columns under monotonic and cyclic loads are researched in the following chapters,which provide reference for engineering design and numerical simulation.(1)Considering the reinforcement layer of FRP Grid and mixing proportion of ECC,an experimental program on FRP grid reinforced ECC with high-volume fly ash/slag was conducted under the uniaxial tensile loading.Based on the experimental result and the Richard-Abbot analytical model,a stress-strain model of this reinforced composite material was established.The test results indicate that ultimate tensile stress of composite specimens can get greatly improvement with the increase of grid reinforcement.The ECC with slag has higher compressive strength and cracking strain/stress than those of ECC with fly ash in the same mixing ratio.The strengthening ECC with slag can get better tension mechanical behavior.The analysis results show that the established model can effectively predict the stress-strain relation and ultimate tensile strength of FRP grid reinforced ECC.(2)Considering different types of fiber reinforced polymer(FRP)grid,reinforcement layer and cyclic loading method,the digital image correlation(DIC)measurement technique and traditional strain gauge measurement method were used to investigate the uniaxial tensile mechanical behavior of FRP grid-engineered cementitious composites(FRP/ECC)material and the availability of DIC measurement technique.Based on the experimental results,a cyclic tensile constitutive model of FRP/ECC composites was proposed.The test results indicate that the characteristic of FRP grid,high ductility and multi-cracking properties of ECC were fully utilized in the basalt grid reinforced ECC(BFRP/ECC)composite system.The ultimate tensile stress of FRP/ECC was improved greatly with the reinforcement of FRP grid.The loading schemes have influence on the behavior of loading/unloading behavior.However,they have no significant effect on the ultimate stress/strain of FRP/ECC material.The DIC measurement can effectively capture the generation and propagation of cracks,observe the failure mode and obtain the whole stress-strain curves of specimens.The calculation results show that the proposed stress-strain relationship matched well with the experimental results and can effectively predict the cyclic tensile mechanical behavior of FRP/ECC composites.(3)In order to improve the analysis ability of three-dimensional lattice discrete particle model(LDPM)for predicting the mechanical behaviour of fiber reinforced concrete under cyclic loading,a corresponding cycling constitutive model was established based on crack closure theory and plastic-damage theory.The effective stress-strain was still used for determining the stress-strain relationship under the monotonic loading.While,the constitutive relationships of the tension and shear stresses were established separately to ensure a smooth transition of concrete from tension state to compression state.The material parameters k_t was introduced to control the stiffness attenuation,residual plastic strain and energy dissipation under cycling tension-compression.The unloading stiffness E_dand parameter k_c were introduced to control the unloading-reloading stiffness and energy dissipation under compression.The proposed constitutive model was embedded into the finite element analysis software to simulate the mechanical behavior of concrete members under cycling tension-compression,hydrostatic pressure and compression.The simulation results showed that the established cycling constitutive model in this paper can effectively simulate stress strain relationships,predicting the residual strain and obtaining the energy dissipation of concrete members under different loading.(4)In order to correct the existing three-dimensional lattice discrete particle model(LDPM)cannot accurately simulate the mechanical properties of concrete under cyclic tensile-compressive loading,such as energy dissipation,residual plastic strain,and unloading stress-strain relationship,an cycling constitutive model suitable for three-dimensional discrete particle model is proposed.At the same time,the mechanical behavior of concrete under cyclic tension,tension-compression,high-pressure hydrostatic and unconfined compression test were simulated.using by the mesoscale discrete particle model analysis software MARS.The simulation results show that the proposed cycling constitutive model can effectively simulate the cycling stress-strain curve,energy dissipation and residual strain and its failure mode are consistent with the experimental results.(5)Considering the cured surface and layers of fiber reinforced polymer(FRP)textile and construction technology of engineered cementitious composite(ECC),the static axial compressive experimental program on FRP textile reinforced ECC confined circular reinforced concrete(RC)column was carried out in this paper to research the loading and deformation capacities of RC column.Based on the test results,the numerical analysis model was also established using the finite element software.The test results show that all strengthened RC column fail by the rupture of BFRP flexible textile.After surface treatment,the bonding behavior and compatibility of FRP textile and ECC interface are significantly improved.The peak compressive strength and corresponding peak deformation of confined RC columns are enhanced with the reinforced ratio of FRP textile increased.Besides,the composite strengthened layer combined with both smeared and sprayed ECC strengthening and FRP textile can provide effective lateral confining stress to the strengthened RC columns and delay the yielding of longitudinal reinforced steel rebar.The analytical results show that the finite element model can effectively simulate the peak load and stress-strain relationship of the strengthened RC columns.(6)Considering the strength grade and geometry of the core column concrete,reinforcement layer of FRP grid,monotonic and cyclic loading conditions,the axial compressive tests of FRP grid/ECC composite strengthened concrete short column were carried out to research the bearing capacity and deformation performance of the FRP/ECC confined columns.The test results show that the failure mode of most strengthened columns is the rupture of embeded CFRP flexible grid.With the increase of reinforcement FRP grid layers,the ultimate bearing capacity and deformation performance of the strengthened columns are improved,but with the increase of the strength grade of the core concrete,the increase range of the ultimate bearing capacity is gradually reduced.At the same time,the test results show that the ultimate bearing capacity of FRP/ECC composite columns under cyclic load is smaller than that under monotonic load.In addition,according to the test results and the stress-strain model of FRP confined concrete,the corresponding stress-strain models of FRP/ECC confined cylinder and prism under both monotonic and cyclic loads are proposed in this paper.The analysis results show that the stress-strain curves predicted by these models matched well with test results.