Experimental And Confinement Mechanism Research Of FRP Strips Confined Concrete

Fiber reinforced polymer(FRP)has been extensively used in civil engineering structure reinforcement because of its light weight,high strength,corrosion resistance and other excellent properties.A large number of tests have proved that FRP can effectively improve the carrying capacity and axial deformation capacity of concrete columns.In addition,FRP reinforcement also has the advantages of corrosion resistance,convenient construction and no increase in component size.There are three classical forms of FRP wrapping strengthening schemes: FRP full wrapping,FRP intermitted helical strips(referred to as FRP strip helical wrapping)and FRP intermitted rings wrapping(referred to as FRP ring wrapping).The international code(FIB 2001)and Italian code(CNR-DT 200 R1)both give the design model for the axial compression of confined concrete under three kinds of wrapping forms.Due to the late start of research on FRP in China,the technical standard(GB50608-2010)for engineering application of fiber reinforced composite materials only provides the calculation formula of FRP full wrapping.In fact,FRP can significantly improve the axial compression performance of concrete columns,but under the working load,the columns are still in the elastic deformation stage,so it is more practical to improve the deformation capacity of columns on the basis of ensuring stress hardening.It is true that full wrapping can greatly improve the axial compression performance of concrete columns,but there are some ineluctable structures such as joint in actual projects,therefore strip helical wrapping and ring wrapping(collectively referred to as partial wrapping)are more widely used.Some studies have proved that partial wrapping can also improve the axial compression performance of concrete columns.The partial wrapping also has the flexibility of adjustment between deformation capacity and carrying capacity,less FRP consumption and more convenient mechanical automation construction.When FRP partial wrapping are used to strengthen columns,The confinement stress under partial wrapping is not uniform along the column height.The confinement effect at the strip spacing is weak and the overall confinement effect is reduced.Most of the existing design models and codes adopted the vertical confinement efficiency ratio proposed by the "Arch effect" hypothesis to determine the reduction ratio of FRP confinement stress,which is not accurate enough.The "Arch effect" is proposed for the steel stirrup confined concrete.In addition,the assumption that the arch angle is equal to 45 ° has never been confirmed by any theory or experiment.Based on this assumption,in order to expand the effective confinement region,a new partial wrapping technique named FRP intermitted helical cross strips(referred to as FRP cross spiral wrapping)is proposed.Through the axial compression test of 16 groups with FRP strip reinforced concrete cylinder,the influence of the research variables such as strip wrapping form,strip width and strip net spacing on the axial compression performance of the cylinder is studied with equal volume ratio and effective coefficient product(that is,the theoretical effective confinement ratio is equal),so as to verify the accuracy of the effective coefficient in the existing codes.The test results show that when the theoretical effective confinement stress is equal,the axial compression performance of the ring wrapping and the cross spiral wrapping specimen is very close,the carrying capacity of the strip helical wrapping specimen is the weakest but its axial deformation capacity is the best.This shows that there is still room for improvement of the vertical confinement efficiency ratio.According to the experimental data of this project and other scholars,a large-scale database has been established,including four kinds of wrapping form,which are FRP strip helical full wrapping,FRP strip helical wrapping / FRP ring wrapping and FRP cross spiral wrapping.Through comparison,it is found that the existing design models and analysis models of FRP full wrapped confined concrete cannot provide accurate prediction for the other three kinds of confined concrete.The built-in parameters of the stress-strain model proposed by different scholars for FRP ring wrapped confined concrete are different,and most of the regression analysis is based on their own test results.This kind of model also cannot be applied to FRP full wrapped confined concrete,and needs further improvement.Based on the database,the analysis model of FRP full wrapped confined concrete is modified,and the parameters are proposed to consider the influence of FRP strip net spacing and fiber angle respectively.A new analysis model which is applicable to four kinds of wrapping forms is derived and tested with a large number of test data.The results show that the prediction accuracy of the new analysis model is very high.Finally,the finite element method is used for parametric analysis,and the distribution rule of axial stress in the specimen is discussed in detail.The shape of invalid confinement area in the specimen is determined to be arch,which provides theoretical support for the assumption of "Arch effect".Through the quantitative analysis of the design parameters and the distribution law of invalid confinement areas,it is found that the arch angle ? is not a constant value.After regression analysis of a large number of data,the influencing factors of arch angle are quantified.It is found that the effective confinement area gradually reduces with the increase of the load.The results of this study are helpful to understand the restraint mechanism of FRP partially confined concrete,to establish an accurate theoretical method of FRP strip confined concrete design,and to promote the popularization and application of this reinforcement method,which also has a certain reference value for FRP stirrup confined concrete.The results of this study are helpful to establish an accurate theoretical method of FRP rings partially wrapping confined-concrete design and promote the engineering application of this reinforcement method,and also have some reference value for FRP stirrup confined-concrete columns.