Experimental Study On Ambient Temperature And High Temperature Behaviors Of RC Beams Strengthened With Carbon Fiber Sheet Using Geopolymers As Adhesive Agent

The strengthening technology using fiber reinforced polymers has been widely used in the structural strengthening field. Fiber reinforced polymers have excellent reinforcement effect at ambient temperature,but exhibit great strength degradation at high temperatures, due to the softening of organic adhesive. This disadvantage hinders the application of this technology in the buildings with fire resistance requirement and in the bridges located at hot area, and also reduces fire safety level of the existing structures strengthened using this technology. Geopolymer is a new kind of environmental-friendly inorganic matrix material,which has favorable fire resistance performance and high mechanical properties. Geopolymers have been tentatively proven to be used as adhesive in structural strengthening field.The main objective of this dissertation is to develop formula of geopolymers and to study ambient and high temperature behaviors of RC beams strengthened with carbon fiber sheets using geopolymers as adhesive agent.The following works and achievements are included.(1) Geopolymers were prepared using metakaolin and alkali activator. The mechanical behaviors of metakaolin-based geopolymers at ambient and high temperatures were studied through bending and compressive strength tests and double shear tests. The influence of the water-binder ratio and carbon fiber content on the mechanical behaviors of geopolymers was investigated. Based these test results, an optimal formula A of geopolymers was obtained. Compared with epoxy resin, geopolymers prepared using Formula A(Geopolymer A) exhibit slightly lower bond strength at ambient temperature, but higher bond strength at high temperatures. However, bond strength of Geopolymer A decreases at a high rate after 300 oC, and it waster content losses quickly in large-area construction.(2) Formula A of geopolymers was improved through mixing fly ash with metakaolin. The fluidity, setting time, bending strength, compressive strength and shear strength of geopolymer with different fly ash content was tested at ambient temperature and after exposure to high temperatures. Based on these test results, the improved Formula B with 50% fly ash content was obtained. The bond strength of geopolymers prepared using Formula B(Geopolymer B) is close to that of epoxy resin at ambient temperature, and retains 59.8% ambient strength after exposure to 300 oC. The bending strength of plain concrete short beams strengthened with carbon fiber sheet using Geopolymer B as adhesive was tested at ambient temperature and after exposure to high temperatures. The test results show that it is feasible to strengthen concrete beam using geopolymer B as adhesive, and the applicable temperature should be controlled under 300 oC.(3)Six concrete beams, including three beams strengthened with carbon fiber sheets using geopolymer as adhesive, two concrete beams strengthened with carbon fiber sheet or basalt fiber sheets using epoxy resin as adhesive and an un-strengthened beam, were tested under static loads at ambient temperatures. The test results show that the bearing capacity and stiffness of the beam, strengthened with carbon fiber sheets using Geopolymer B as adhesive and using stirring technology to boost the infiltration of geoplymer into carbon fiber sheets, has been greatly improved, compared to the un-strengthened beam, and also it is not lower than that of the beam strengthened with carbon fiber sheet using epoxy resin as adhesive, but the ductility of the strengthened beams decreases.(4) Five concrete beams were tested in fire, two of which were strengthened with carbon fiber sheets using geopolymer as adhesive, two strengthened with carbon fiber sheet or basalt fiber sheets using epoxy resin as adhesive, and one was not strengthened. The test results show that the reinforcement layer, forming by carbon fiber sheets and geopolymer, can play an important role at a longer duration than that from carbon fiber sheets and epoxy resin. However, if no appropriate fire protection measures is taken to limit the temperature at the reinforcement layer lower than 300 oC, the reinforcement layer forming by carbon fiber sheets and geopolymer would fail, which would induce the quick damage of the beam.