Concrete Cylinders Confined by Carbon FRP Sheets, Subjected to Monotonic and Cyclic Axial Compressive Load

Paper in proceeding, 2003

During last decades, most R&D and field application projects have been concentrated on concrete columns repair and reinforcement using composite materials. Composites as external reinforcement result in considerable enhancement in strength and ductility of concrete when loaded axially. The use of composite materials could upgrade civil engineering structures and be the most effective way to introduce composites into widespread civil engineering use. The purpose of this experimental study is to make an investigation on the compressive behavior of concrete strengthened by CFRP of various qualities for the comparison of confinement effectiveness. The investigation included concrete cylinders with diameter 150mm and height 300mm subjected to compressive load. Five different concrete series with cube strengths 34.2; 60.5; 76.2; 81.4 and 104.1MPa, that cover nearly the whole range in practice, were investigated. The parameters taken under consideration for the confinement were the number of layers (CFRP reinforcement ratio ranging from 0.31% to 0.94 %) for wrapping with carbon sheet with tensile strength 4500 MPa and modulus 234GPa. The friction between steel loading plates and concrete surface was reduced by three layers of teflon sheet. The specimens were fully instrumented and apart from load, axial and lateral strains were measured, with LVDTs and strain gauges. The enhancement in strength and ductility of confined concrete was remarkable. The confined concrete displayed a distinct bilinear stress-strain response with transition zone around the ultimate strength of unconfined concrete. Maximum strength increase by the confinement was measured to be over 300% for specimen of 34.2MPa concrete. It is evident that as the CFRP layer number increases, the strength and ductility in the axial direction are both improved. The slope of the branches after transition zone and confining pressure are depended on the volumetric ratio of the confining device. For higher concrete strength, the ultimate strength increase due to confinement was lower, which indicates that the confining pressure exerted by the composite material on the concrete surface, as a restriction of the lateral expansion of concrete, was not so effective for the high strength concrete, showing more brittle behavior. Cyclic load caused no degradation in confining action. On the contrary sometimes caused improvement, which could be due to the fact that the confinement imposed lateral strain by its interaction with the concrete during unload stage too. So in the next loading cycle the internal concrete cracks were uniformly developed in concrete mass and the load distribution was better. Stress-strain relationships of the monotonic load act like an envelope of that of cyclic load since they have similar loading paths. The confined concrete cylinders failed by fracture of the FRP confinement having lower lateral strain than the uniaxial ultimate tensile strain given by producer. CFRP ring test (ASTM standard D2290) reveal the real ultimate tensile strain of confinement, which only slightly exceeds the ultimate lateral strain. For the satisfactory prediction of strength, the stress analysis of crack growth inside concrete after transition zone should be accompanied by the analysis of confinement and its interaction with the damaged concrete.