Study On Mechanical Properties Of Damaged Concrete Cylinder Confined By CFRP Under Axial Compression

Due to the influence of material aging, natural disasters, has brought different levels of damaged or even destroyed to a lot of concrete structure. It needs huge amount of money to repair them every year. Therefore, the repair and reinforced of concrete structure has attracted widespread attention by scholars both at home and abroad. Because Carbon Fiber Reinforced Polymer (CFRP) has the advantages of light weight, high strength, corrosion resistance and convenient construction, it has many years of application in the field of aerospace. In recent years, with CFRP materials prices fell sharply, its application in the field of civil engineering reinforcement get rapid development. CFRP confined concrete columns is one of the more important application in CFRP reinforced concrete structure, because CFRP confined concrete columns can restore or improve the strength and the deformation ability of the concrete columns, but at present most of the research is focused on the mechanical properties of ordinary concrete under normal conditions, the proposed model is also applicable of concrete structures with no damage and failure, there is little research about the reinforcement of damaged concrete structure, there is no a suitable Constitutive model can be used as a theory basis of design about CFRP reinforcement of damaged concrete structure.This paper based the uniaxial compression test on CFRP confined of high temperature damaged concrete cylinders and CFRP confined of damaged concrete cylinders by axial compression, focuses on the influence of the mechanical properties of concrete under different temperature damage and different amount degree of damage and different amount confinement of CFRP. Compared to the existing research results, it has discussed the mechanism of the damaged concrete cylinders confined by CFRP, and has made correction to the existing of the strength model and the expression of strain, proposed the suitable strength model and formula of strain for the damaged concrete confined by CFRP. The main research content is as follows:(1)a total of36standard high strength concrete cylinder specimens, each group of three, were subject to high temperature by electric furnace, and obtained specimens of different high temperature damaged, then confined them by CFRP before carrying out the uniaxial compressive experiment. By means of experiment, has analyzed the failure mode of concrete, compressive strength, axial and hoop strain and stress-strain curve under different high temperature damage and different confinements. The test results show that the compressive strength of high-strength concrete have a sharp decline after high temperature damaged, but the compressive strength of high temperature damaged cylinders confined by CFRP has obviously increased. Compared to normal concrete, with the same CFRP layer, the compressive strength of high temperature damage cylinder are only declined3%-6%,the confinement effect of different layer of CFRP has obvious difference for the high temperature damage concrete and the normal concrete, the more the layer of CFRP, the greater in the increase of strength and ductility.(2) a total of54standard cylinder specimens, two strength grade of C40and C70, have damaged by the compression-testing machine, and obtained different damage degree of the cylinder specimens, then confined them by CFRP before carrying out uniaxial compression test, the effect of different degree of damage and different CFRP layers has researched, consist of the failure mode, compression strength, strain and stress-strain relationship et. The test results show that the different damage degree of concrete has a great influence in CFRP confined concrete of axial compression performance, compared with none-damage concrete confined by CFRP, damage concrete confined by CFRP have only2.7%-3.8%decrease in strength, the strength of the damage concrete can basic recovery to the state before damaged by CFRP confinement. But as a result of damage caused by the crack of concrete to strength and stiffness, the strength of the damage concrete can’t fully recover to the state before damaged. The more layers of CFRP, the greater the increase of strength and ductility. But in low confinements, the strength increase value of high damage degree of concrete specimens are lower than low damage degree of concrete specimens, the influence of damaged to confined concrete is reduce at the high amount of constraint, the strength enhance value of high damage degree of concrete specimen is higher than low damage degree of concrete specimens.(3) According to the existing research results, the key parameters of the CFRP confined concrete stress-strain relationship curves are the strength of concrete, the stiffness and strength of CFRP. Based on the strength model and the formula of strain analysis, and combined with the test data, using the method of linear regression analysis to make correction the parameters of the model, proposed the strength model of the high temperature damage concrete confined by CFRP and the formula of strain, the strength model of axial compression-damaged concrete confined by CFRP and the formula of strain. Compare the model results with the results of experiment, it shows that the results of this model and the formula of strain are in good agreement with the results of experiment.