Compressive Performance Study On CFRP Wrapped Concrete Columns With Partial Deteriorated Strength

Fiber reinforced polymer(FRP) composites are used in strengthening concrete columns frequently due to their superior advantages.Most applications in the past focused on the entirely wrapping,while in practice,partial strength deterioration of concrete columns occurs occasionally.Frost damage,aging or construction quality might lead to the concrete strength be deteriorated.In these cases,wrapping CFRP is considered as an effective approach to maintain the regular service.However,the whole columns confinement including intact and deteriorated parts has been regarded as a waste.Few literatures involved in partial confinement only to the defect parts.This paper aims to present an effective method to partial deteriorated strength columns,and to investigate the mechanical behavior of the defect columns.For concrete columns and RC concrete columns with partial deteriorated strength, wrapping with CFRP only in the strength deteriorated regions,then axial and eccentric compressive loading were applied to all specimens.Based on generalization and experimental results of many former researchers,this paper analysized the formula to calculated bearing capacity of concrete columns sufferred from axial or eccentric compressive loading,and then derived the procedure how to determine CFRP amount.The research contents and results are as follows:Five groups totally 15 of 150×150 mm plain square columns are cast;each specimen has two different strengths,the lower strength in the middle segment and higher strength in both ends.The lower strength is to simulate the status of partial deteriorated region.Different layers of CFRP sheets have been wrapped just on the lower strength part to gain the reinforcement with CFRP sheets,and to verify the practicability of partial confinement. Specimens are subjected to monotonic axial compression until failure.Axial load,axial and transverse strains are measured to compare the different behaviors between the two parts. Experimental results show that partial confinement can significantly enhance the strength and the ductility of the deteriorated strength part,then,the bearing capacity of the whole column can be increased subsequently.Test data indicate that the ultimate load of the confined column is higher than that of the original column without deterioration;partial confinement on weakness is a feasible approach.Findings of this paper prove that partial confinement on deteriorated parts of concrete columns would develop as an alternative to avoid cost and time consumption in practice.Similar to the plain concrete columns,5 steel reinforced concrete columns with partial deteriorated strength were cast and suffered from axial compressive loading until failure. Wrapped with different amount of CFRP,the columns presents good performance compared with the columns without CFRP reinforcement.Experimental results show that because CFRP effectively limited the concrete' lateral deformation,the ultimate compressive strength and deformation capacity of concrete have been prominently enhanced,the plastic distortion capability of steel bar can be made full use of and has the positive attributes to improving the carrying capacity and ductility of structure.Meanwhile,The experimental results indicate that the ultimate beating capacity and ductility of the whole column increase as the CFRP layer increase,but the increasing rate of beating capacity reduces while CFRP layers increases, which means there was a utilization ratio for different layer of CFRP.Besides,wrapping with CFRP,the ductility of columns with strength deficiency were improved greatly compared with not only the control columns with strength deficiency but also the control colulmns with original strength.Generalized the existed stress-strain models of FRP reinforced concrete presented by the former researchers,the paper analysized the factors influencing the partial reinforcement.And then analysized the formula to calculated bearing capacity of concrete columns sufferred from axial compressive loading,the calculaty results are agree well with the experimental ones, which means that using this formular could predict the bearing capacity of strength deficient concrete columns reinforced by CFRP.Based on the calculation of bearing capacity,then the paper derived the procedure how to determine the CFRP amount.16 partial deteriorated strength RC columns confined with CFRP were tested under eccentric compression.Based on the destructive tests,this paper analyzed the strengthening effect with the parameters such as different CFRP amount,different strengthening mode and different eccentricity.The experimental results indicate that the ultimate bearing capacity and ductility of the specimen have a certain increase as the amount of transverse CFRP increases, but it is not in linear relationship.Actually,the increasing ratio of bearing capacity reduces with CFRP layers increases.The results also show that CFRP can enhance the strength and ductility of columns with small eccentricity effectively,but the enhancement of capacity is very small and ductility is still high for columns with critical eccentricity;the distribution of sectional strain accords with plain section assumption;the tensile strain of external FRP exists distinct strain gradient due to uneven dilation of compressed concrete,and the testing value of axial magnifying coefficient of eccentricity is far different from the calculating value according to concrete code,and so on.Analysized the distribution of sectional strain,introduced the strain gradient factor caused by eccentricity,modified the parameters considered with equivalent rectangular stress distribution,and so on,this paper focused on bearing capacity calculation of CFRP wrapped deficient columns over the eccentrical loading.With comparation theoretical results and the experimental results,the theoretical value has the higher accuracy and was more conservative, which is more favorable for predicting bearing capacity of columns in practice.The research results show very usful for the practical engineering,and the authors hope it can be developed as guide for more comprehensive application.