Experimental And Theoretical Research On Seismic Behavior Of ECC/RC Composite Frame

Engineered Cementitious Composite(ECC)is a kind of high performance fiber reinforced cementitious composite(HPFRCC)which exhibits ultra-high tensile ductility,tensile strain hardening and fine multiple cracking behavior.Due to the prominent mechanical performance and crack control capacity,ECC prefers to be applied to the area in concrete structures where needs huge dissipation of energy,or is subjected to the large deformation and shear load,in order to improve the seismic performance and durability of structures.Considering that the current production cost of ECC is relatively higher than that of concrete,it is uneconomical to use ECC for the entire building structure.This paper proposes to combine ECC and concrete to form a composite member on the section of the critical part or in the longitudinal direction of the component.A systematic study on seismic behavior of ECC/RC composite frame sturcture at material level,component level and structure level is conducted adopting the approaches consisting of theoretical analysis,experimental investigation and numerical modeling.1.Material level:(1)Based on the ECC design theory,an optimal design for the fiber volume content of hybrid PVA-ECC was analyzed,considering physical and mechanical properties of PVA fiber produced by Japan and China.It is suggested that the hybrid PVA-ECC should contains a dosage of 1.0% Japanese fibers and 0.6% domestic fibers in volume,and then five groups of mix proportions were determined for preparing specimens in this study.The four bending test and compressive test for 5 groups of specimen are conducted.All hybrid PVA-ECC specimens can exhibit strain-hardening and multiple cracking behaviors,and the addition of domestic fibers instead of partial oiled PVA reduce the ductility of specimen.The deformation capacity shows a decreasing trend with the increase of age.The addition of fiber can obviously improve the compressive toughness of ECC,so no spalling is observed in hybrid PVA-ECC specimen during the whole compressive test.(2)Based on the existing experimental results,a numerical simulation of multiple cracking process for ECC under uniaxial tension has been presented,which mainly contain micro-mechanical model of multiple cracking,crack spacing model,series spring model and statistical distribution model.The accuracy of the modeling has been verified by comparing the simulated result with the tensile test data in terms of cracking stress,cracking strain,peak stress and strain capacity.2.Component level:(1)Totally eight test beams including ECC/RC composite beams and RC beams with three interface treatments are designed to study the flexural behavior.Results indicate that all ECC/RC composite beams failed in a ductile mode,followed by formation of multiple cracks.Dedonding failure is not observed in composite beams before peak load.Furthermore,the crack width of composite beam is less than 100μm when the applied load is lower than 80% of the peak load,which can significantly improve durability of composite beam.A simplified method was evolved base on the simplified tensile and compressive constitutive model for ECC.According to the method of effective moment of inertia suggested by ACI code,a simplified calculation method of deflection for ECC/RC composite beam at the serviceability limit state is proposed,and the equations for the inertia moment of the full cracked section based on the internal force equilibrium are also obtained.The predicted deflection and flexural capacity based on the proposed simplified calculation method shows good consistence with the experimental results.(2)Experimental observations have been done on six ECC/RC composite columns and one reinforced concrete(RC)column under the combined action of constant axial load and reversed cyclic lateral load.Failure mode,hysteretic behavior,displacement ductility and energy dissipation,stiffness degradation,et al have been studied for different test columns.Experimental results indicate that ECC/RC composite columns show better seismic performance in terms of ductility,energy dissipation capacity,and damage tolerance,compared with RC columns,and increase in shear span-to-depth ratio and confinement effectiveness resulted in increase of energy dissipation capacity and ductility.The ECC/RC composite column is modeled based on OpenSees software,and then the parameter analysis is conducted to study the seismic performance of ECC/RC composite column in terms of shear-span ratio,axial load ratio,longitudinal reinforcement ratio,compressive strength of ECC,ultimate compressive strain of ECC and thickness of ECC.(3)The performance of ECC/RC composite column can be divided into five levels,and drift and Park-Ang damage index are both used for performance indicator of ECC/RC composite column.According to statistical analysis from experimental results and pertinent literature,the limits of drift and damage index of ECC/RC composite column in different performance level are suggested.(4)Based on modified compressive field theory(MCFT),averaged principal stress-strain relationships and local stress conditions at crack locations for cracked ECC are modified,taking the fiber-bridging action of cracks into account.Then,the modified MCFT is presented,which can explain shear mechanism of ECC.Furthermore,the expression for the inclination of ECC strut and the principle tensile strain is explicitly derived,and then the simplified calculation method for shear strength of ECC beam with stirrups is proposed based on modified MCFT.A database is established for the shear tests,and it may be utilized to evaluate the proposed modified MCFT as well as simplified method.(5)A multiple line model was proposed for the restoring force model of ECC/RC composite columns in terms of calculation method of characteristic parameters in skeleton curve,equivalent plastic hinge length,and hysteretic rule.Then,validation of the proposed model was verified with the experimental results of ECC/RC composite column.3.Structural level:(1)Shaking table tests of prefabricated ECC/RC composite frame and a control RC frame were conducted to study the distribution of dynamic properties,acceleration,displacement,residual displacement,joint rotation in different intensity earthquakes.Experimental results indicate that when ECC is used in beam-column joint,or plastic zone of beam and column,the shear capacity,plastic deformation capacity,energy dissipation and damage toleration of ECC/RC composite structure are all expected to exceed these of RC structure.In addition,considering the life cycle assessment,ECC/RC composite structure is also cost-effective,despite that the cost of ECC is considerably higher than concrete.(2)The division standard of performance levels of ECC/RC composite frame structure is presented,and the corresponding performance indicator limits are also proposed in terms of maximum story drift,Park-Ang damage index,maximum residual story drift and maximum story acceleration.Moreover,a ECC/RC composite frame structure and a RC frame structure are designed to assess structural response and seismic behavior by multi-index evaluation based on incremental dynamic analysis.Analysis results indicate that ECC/RC composite frame structure exhibit more outstanding seismic resistance.