Study On Fire Behaviors Of Concrete Beams Strengthened With Carbon Fiber Sheet

Carbon fiber sheet (CFS) has received widespread attention around the world as a relatively new material and technology for strengthening and repair of reinforced concrete structures in the past two decades. Compared with traditional strengthening technologies, CFS exhibits several advantages, including good resistance to corrosion, ease of application, and excellent mechanical strength. Research results have indicated that, deterioration of mechanical and bond properties of CFS with increasing temperature could result in the behavior of unprotected CFS-strengthened concrete members decreasing, which posed a significant risk for fire safety of these structures. Thus, a more complete understanding of the fire behaviors of CFS-strengthened concrete structures is required. In this paper, the fire resistance of reinforced concrete beams strengthened with CFS exposed to fire is discussed, through fire test, numerical simulation, parameter analysis and practical calculation method. The main research works are included as follows:1. Test results of five concrete beams in fire are presented in this paper, four of which were strengthened with carbon fiber sheet and protected by passive fire insulation, and the other unstrengthened one was tested as a comparison. The primary objectives of these tests are to investigate the influence of flexural cracks at mid span on the temperatures of tensile reinforcements, and to evaluate the failure mode, deformation and fire resistance of the strengthened beams with relatively thin fire insulation. Test results show that: (1) the deflection and crack width and depth of the strengthened beams increase very slowly for a long duration of the fire, leading the effect of flexural cracks on the temperatures of tensile rebars very limited; (2) spalling and debonding of concrete may cause a change in the failure location of the strengthened beams in fire; and (3) in the case that the actual load ratio is not larger than 0.5, the fire endurance of a strengthened beam with relatively thin fire insulation (e.g., 10~20 mm) can meet the requirement of 2 h in the design code.2. A computer program is developed to calculate the temperature fields of insulated concrete beams strengthened with carbon fiber sheet. This program is validated using experimental results from literatures. The influences of some parameters (e.g., type of insulation material, thickness of insulation, insulation scheme, and sectional size of beam) on temperature distributions of the strengthened beams in fire are analyzed using this program. Based on the numerical results, a simplified formula is proposed to predict the temperature fields of the CFS strengthened and insulated beams in fire. Simulation results show that: (1) the thermal fields of the CFS strengthened beams with minor U-shape insulation are similar to those with insulation at beam soffit, expect for locations close to the top end of minor U-shape insulation at beam sides; (2) the temperatures at beam soffit increase slowly with an increasing of heating time due to the protection of fire insulation, but they reach the failure temperature of epoxy quickly; and (3) the insulating effect of cement motor is much weaker than that of fireproof dope for steel structures.3. It is shown that elevated temperature may cause a change in the failure mode of concrete beams strengthened with CFS, as flexural failure at room temperature can be transformed into shear failure in fire. Hereby, the concept of the critical situation (i.e., flexural failure and shear failure occur simultaneously at high temperature) of the strengthened beams is proposed. In this paper, an analysis procedure for flexural capacity and shear capacity of RC beams strengthened in flexure using CFS at high temperature is discussed and validated by test results from literatures. A parametric study is conducted for the critical situation of the strengthened beams with fire insulation. The influences of some parameters, such as span-to-height ratio, confinement ratio, rich degree of shear capacity, thickness of concrete cover, thickness of fire insulation, and strengthening ratio, on the tensile reinforcement ratio related to the critical situation are examined. Based on the aforementioned analysis procedure and extensive numerical results, an empirical expression between the tension reinforcement ratio and the aforementioned parameters is suggested for the critical situation, which can be used to predict the failure mode of the strengthened beams in fire. Some recommendations for fire safety design of the flexurally strengthened and insulated beams are discussed preliminarily. It is important to recognize that increasing of the thickness of fire insulation is not always good for the fire performance of the strengthened beams. A balance between increasing of the flexural capacity of a strengthened beam and enhancing of the critical tensile reinforcement ratio should be made by appropriately determining the thickness of fire insulation through a trial-and-error process.4. Using the concept of equivalent compressive strength of concrete at high temperature, a simplified method is proposed for calculation of the flexural capacity of concrete beams strengthened with externally bonded carbon fiber sheet and subjected to fire. Then, the influence of some parameters (e.g., insulation condition, strengthening ratio, steel ratio, and thickness of concrete cover) on the flexural capacity of the strengthened beams in fire is discussed. Based on extensive parametric analysis, a regressive formula is suggested for the relationship between the flexural capacity of the strengthened beams and the heating time. Simulation results show that: (1) the fire resistance of the strengthened beams obtained using the aforementioned simplified method is in good agreement with the test result; (2) the insulation height at beam sides should be less than 120 mm, and the flexural capacity of the strengthened beams increases with an increasing of the insulation height within a range of 0～120 mm; and (3) it will likely be very difficult to achieve a 2 h fire endurance rating for an uninsulated CFS-strengthened beam. However, the flexural capacity of strengthened beams insulated with only 10 mm layer of fire insulation is obviously higher than that without fire insulation after 2 h of exposure to the fire.5. The CFS-strengthened beams with elastic axial and rotational restraints at beam ends are selected for numerical parametric study, and the effect of some parameters (i.e., axial/rotational restraint ratio, section size, length, load ratio, strengthening ratio, reinforcement ratio, thickness of concrete cover and thickness of fire insulation, etc.) on the axial force and bending moment at the end in restrained beams are analyzed. Based on the extensive simulation results, practical calculation methods for axial force and bending moment at the end of beams subjected to fire are proposed. Simulation results show that: (1) for axially-and-rotationally end restrained beams in fire, the axial force ratio increases gradually first, then varies gently, and finally decreases gradually; (2) the bending moment at the end of beam increases to the peak first, then becomes gentle in fire; and (3) the variation of internal force of restrained beams strengthened with CFS with time is different from that without fire insulation, even with 10 mm of fire insulation.