Experimental Research On Flexural Performance Of RC Beams Strengthened With Hybrid CFRP/GFRP

Carbon Fiber-reinforced Polymer (CFRP) and Glass Fiber-reinforced Polymer(GFRP) are two widely used strengthening materials in reinforcement projects. CFRPmaterials are of high strength and Young’s modulus (tensile strength is between1200~2250MPa and Young’s modulus is between120~250GPa), but little ductility andhigh production cost; while, GFRP materials are of low strength and Young’s modulus(tensile strength is between400~1800MPa and Young’s modulus is between20~55GPa), but high ductility. Researches focus on hybrid materials show that the interplayhybrid material composed by GFRP and CFRP is of positive hybrid effect in strengthand ductility, which improves the brittleness of single FRP material. Thus, such ahybrid material is an efficient strengthening material.The paper analyzes the strengthening mechanism of single FRP material andHFRP material to reinforced concrete based on fracture mechanics. The flexuralperformance of pre-cracked reinforced concrete beams strengthened with CFRP/GFRPis also investigated in detail based on experiments, and so is the second strengtheningeffect of hybrid fiber enhanced structure. The following conclusions have beenobtained:①The anti-cracking mechanism in fracture mechanics shows that, within theinterplay hybrid material composed by GFRP and CFRP, the GFRP of high ductilitycan prevent the random fracture of CFRP of low ductility, which improves thebrittleness of single FRP material; When the concrete structure is strengthened byCFRP/GFRP, concentrated forces can be provided at crack tip to reduce the stressintensity factor at the tip to resist the development of the crack, which can improvesignificantly the crack resistance, delay the development of the existing cracks andprevent the emergence of new ones. Meanwhile, CFRP and GFRP are with coordinatedeformation, which can improve the interfacial adhesive property, avoids thedebonding failure (interfacial cracks caused by deformation incoordination betweenthe structure and strengthening material) caused by CFRP, and also improve theductility and bearing capacity of the structure.②By simulating the unloading and strengthening of a structure, the flexuralperformances of the pre-crack reinforced concrete beams enhanced by CFRP, GFRP orCFRP/GFRP are studied. Results show that the crack resistance, bearing capacity andstiffness of the FRP reinforced concrete beams are significantly improved; CFRPreinforced concrete beams exhibits good crack resistance at early stage of loading;GFRP reinforced concrete beams exhibits good bearing capacity at later stage ofloading; and CFRP/GFRP reinforced concrete beams exhibits all the said advantagesunder different loading stages.③CFRP/GFRP strengthening beams exhibit positive hybrid effect: Its cracking load, yielding strength and ultimate bearing load are improved respectively by80%,40%and58.6%comparing with that of ordinary beams. In a much bigger deformationrange, the fiber cloth bears load coordinately with the steel bars and concrete, whichimproves the crack distribution pattern and makes the stress of the structure moreuniform. Such a structure exhibits better ductility under unstable failure than astructure enhanced by single FRP.④Secondary loading on the CFRP/GFRP strengthening beams under ultimateload show: After the breaking of the fiber cloth in a CFRP/GFRP strengthening beamunder ultimate load, the bearing capacity of such a beam can reach to that of ordinaryreinforced concrete beam when reloaded. It is concluded that the strengtheningstructure can take in certain amount of safety stock when the fiber cloth is employed,which determines that the structure retains some of its bearing capacity even afterfailure instead of failure instantly.⑤Finite elemental analysis shows that the stress intensity factor at the tip ofCFRP/GFRP strengthening small plain concrete test specimens first increases and thendecreases as the height of cracks develops. When the cracks are resisted at the heightof80%of the beam’s height, the test specimens exhibit obvious crack resistance. Andthe CFRP/GFRP strengthening reinforced concrete beams is of high material usingefficiency, bearing capacity and ductility.