Abstract
Over the past three decades,externally bonded(EB)fiber-reinforced polymer(FRP)laminates(including wet layup FRP sheets and pultruded FRP plates)are increasingly used for the strengthening and repair of reinforced concrete(RC)structures.The success of the EB FRP strengthening technique is attributed to the superior material properties of FRP composites,including their high strength-to-weight ratio,corrosion resistance,ease of application,tailorable performance characteristics,and minimal alterations to the dimensions of the strengthened member.However,a typical ambient-cure epoxy adhesive for EB FRP strengthening has a low glass transition temperature of approximately 45～80℃[1].When the epoxy adhesive is subjected to elevated temperatures close to this characteristic temperature,it changes from a glassy to a viscous state with severe strength and stiffness degradations[2].Also,the EB FRP laminates may burn of f in fire unless a supplemental insulation layer is provided to separate them from fire.Therefore,fire performance of FRPstrengthened RC members is an important issue that needs to be properly considered during the strengthening design process,especially for the purpose to satisfy the requirements of specified fire-resistant ratings in indoor applications(e.g.in buildings)[3].
Almost all the fire endurance tests in the literature were conducted on flexural FRP-strengthened RC beams[4-6].There is lacking research on fire performance of RC beams shear-strengthened with FRP laminates.This paper presents the firstever fire endurance tests on insulated RC beams strengthened in shear with carbon FRP(CFRP)sheets.A total of seven rectangular RC beams were constructed:Three of them were tested at ambient temperature to determine their loadcarrying capacity while the remaining four were first exposed to ISO834 standard fire for a duration of 2.5 h(Table 1 for more details).Figure 1 shows the geometry and reinforcement details of the tested beams.The shear strengthening system consisted of one or two layers of 0.167 mm thick CFRP U-wraps,which were 80 mm wide with a clear spacing of 100 mm between two adjacent U-wraps.For the protected CFRP-strengthened RC beams,the fire insulation was all the same,which was applied to the bottom and two sides of each beam with a 20 mm SJ-2 layer(Table 1).The fire insulation material(commercial designation of SJ-2),supplied by a local material company,was a lightweight fire-resistant cementitious plaster that could be manually applied(troweled)onto the surface of the structural member.According to the brochure provided by the manufacturer,it had the thermal properties at ambient temperature as follows:dry density of 500 kg/m3,specific heat capacity of 1 000 J/(kg·K)and thermal conductivity of 0.12 W/(m·K).
Table 1 Details of the tested beams
Figure 1 Geometry and reinforcement details(unit:mm)
Figure 2 shows the load versus midspan deflection curves of the three beams tested at ambient temperature.It is seen that the elastic stiffness of these beams are almost the same before the concrete cracking,mainly due to the negligible stiffness contribution of the CFRP U-wraps.The ultimate loads of B1-1 and B1-2,however,were much higher than that of B1-0.The gains in ultimate loads were 24% and 30% for B1-1 and B1-2,respectively,when compared with B1-0.
Figure 2 Load-displacement curves(unit:mm)
Fire endurance tests were conducted on four beams,of which an RC beam without fire protection was tested as reference whereas the remaining FRPstrengthened RC beams were protected with 20 mm SJ-2 layer.Figure 3 shows photographs of the tested beams after fire exposure.The RC beam without fire insulation was characterized by shear failure as a result of the development of diagonal cracks,whereas the insulated CFRP-strengthened beam was well protected and did not fail in fire.Several thermocouples(TCs)were installed over the midspan beam cross-section to record the temperature responses of concrete(TC2&TC3),steel stirrups(TC6&TC7)and the CFRP-to-concrete interface(TC1&TC4&TC5).Taking the beam B2-3 as an example,Figure 4 depicts the temperatures measured at various locations during fire exposure.
Figure 3 Photographs of the tested beams after fire exposure
Figure 5 shows the midspan deflection responses of B2-0 and B2-1 during fire exposure.The un-protected beam B2-0 experienced an abrupt deflection response due to the shear failure,whereas the protected beam B2-1 achieved a satisfactory fire endurance of 2.5 h with the maximum deflection less than 10 mm.The midspan deflection responses of B2-2 and B2-3 were similar to that of B2-1 and thus were not reported herein due to the space limitation.
Figure 4 Measured temperatures at various locations
Figure 5 Midspan deflection versus time curves