Experimental Study On Behavior Of Concrete Beam Strengthened With CFRP Sheets Bonded With Inorganic Matrix

Carbon fibres under anaerobic conditions have good fire resistance and are relatively unaffected by temperatures in excess of 1000oC. But the epoxy matrix will be soft at temperatures close to the glass transition temperature (Tg) for the matrix, which is usually between 65oC. and 82oC. Beyond the Tg, the strength and stiffness of a polymer are greatly reduced, resulting in loss of bond properties between the FRP and the substrate or surrounding concrete. Loss of bond properties between the FRP and the concrete is a primary concern, as is loss of strength and stiffness. An important concern regarding the effectiveness and safety of this method is the brittle debonding failures of CFRP reinforced concrete beams subjected to high temperatures.Due to the glass transition temperature of organic adhesives which are too low, concrete structures strengthened with CFRP sheets bonded with organic epoxy adhesives can not meet the heat resisting requirements. Against this background, slag and water glass were selected to preparation of inorganic matrix. Through double-shear test on pasting CFRP sheets alkali activating slag fire-resistant inorganic adhesive on the surface of concrete, the optimized mix proportion of inorganic matrix was determined. The results indicate that the strength in plane of specimens with inorganic adhesive was about equal to that of with organic adhesive, and large area of concrete separates at the interface. The facture process is: firstly, preparing inorganic matrix by the scale of that the mass of water is 0.42 of slag and the mass of soluble glass is 0.12 of slag, secondly, immersing CFRP sheets in inorganic matrix and pestling them, finally, pasting CFRP sheets with inorganic matrix on the surface of concrete. The microstructure and phase composition of the inorganic matrix at normal temperature and after different high temperature were investigated by means of X-ray diffraction (XRD) and SEM respectively. Fire-resistance performance of the inorganic matrix was studied by testing the nominal compressive strength of inorganic matrix after different elevated temperature. The test results verified that the nominal compressive strength of the inorganic matrix after 600℃was not lower than that at normal temperature.Six simply-supported reinforced concrete beams strengthened with CFRP sheets with inorganic matrix was tested. Thereby the test values of flexural carrying capacity of these beams, crack distribution and growth and the curve load-deflection at mid-span were obtained. The failure modes is that the tensile longitudinal steel bars yield firstly, then CFRP sheets break in tension. And the plane-section assumption is applicable. Therefore, it is feasible to strengthen concrete structural members with CFRP sheets with inorganic matrix. The calculation formulas of stiffness, maximum crack width and flexural load-carrying capacity of concrete simply supported beams strengthened with CFRP sheets bonded with inorganic matrix were established based on the experimental results and theoretical study. The calculated results are in good agreement with the experimental ones.Eight prestressed composite beams and six two-span unbonded prestressed concrete beams after the ultimate limit state strengthened with CFRP sheets bonded with inorganic matrix were tested. Thereby the test values of flexural carrying capacity of these beams, crack distribution, growth and the curve load-deflection at mid-span, varitey of stress of unbonded tendons and the plane section assumption at cractical section were obtained. Two ideal failure modes occurs: the one is that the tensile longitudinal steel bars yield firstly, then CFRP sheets break in tension; the other is that the tensile longitudinal steel bars yield firstly, then the extreme concrete fiber in compressive region reaches the ultimate strain. And the plane-section assumption is applicable. Based on the test results, the calculation methods of flexural carrying capacity were presented with the influence that the original beams had been subjected to loads considered. The calculation formula of stiffness was set up, and the calculation values of which are in good agreement with the test results.To study on the anchorchorage behavior of post-embedded bars with inorganic matrix,108 post-embedded bars with inorganic matrix were tested. Thereby failure modes and anchoring capacity of specimens with different anchorage length were obtained. The cone-bond failure mode and steel bar broken in tension occurs, when anchorage length is 8d,10d,12d,14d,16d or 18d. It can be seen that the anchorage performance of post-embedded bars with inorganic matrix is good and post-embedded bars with inorganic matrix can be used for project construction. The anchorage performance of specimens is determained by anchorage length. And the strength of concrete and diameter of steel bar have little effect on the anchorage performance. The calculation formula of anchoring capacity under cone-bond failure mode was presented, and the calculation values of which are in good agreement with the test results. The recommendation on anchorage length was presented based on reliability analysis.