Fire Endurance of Fiber-Reinforced Polymer Strengthened Concrete T-Beams

Understanding the performance of fiber-reinforced polymer (FRP)-strengthened members in fire is critical to the widespread application of FRPs as repair materials for infrastructure. An investigation was undertaken to examine and document the performance of FRP-strengthened reinforced concrete T-beams under standard fire conditions. Two full-scale reinforced concrete T-beams were strengthened in flexure with FRP sheets and insulated with a patented two-component fire insulation system. The specimens were subsequently exposed to a standard fire under full sustained service load. Member deflections, strain in the steel reinforcement, and temperatures throughout the section were measured and recorded throughout the tests. A numerical heat transfer model was used to predict temperatures within the section at any time during the fire. The predicted temperatures are compared with those observed during the fire tests and are shown to agree satisfactorily. The results indicate that appropriately designed and insulated FRP-strengthened reinforced concrete T-beams can achieve fire endurances of more than 4 hours.

Keywords: fire endurance; fire tests; FRP strengthening; T-beams.

INTRODUCTION

In recent years, structural engineers have become increasingly familiar with fiber-reinforced polymer (FRP) materials. The resistance of FRP materials to electrochemical corrosion, their high strength-to-weight ratios, their inherent light weight, and their ease of application are advantages that make FRPs attractive for both new construction and repair and rehabilitation of structural members, particularly as externally-bonded repair materials for concrete.

Notwithstanding the generally encouraging structural and environmental durability performance of FRPs in research studies and field applications conducted thus far, their adequate performance in fire remains largely undemonstrated. Fire endurance may not be critical when FRPs are used in bridge members, but as FRP-strengthened members become increasingly used in building construction and repair, this important issue must be addressed.1

FRP material behavior in fire

FRP materials are susceptible to deterioration of mechanical properties and combustion when exposed to elevated temperatures or fire, primarily due to the thermal susceptibility of their polymer matrixes. As the temperature of the polymer matrix approaches its glass transition temperature T^sub g^, the matrix transforms to a soft, rubbery material2 with reduced strength and stiffness.3 Common thermosetting polymer matrixes, such as epoxy or vinylester, typically exhibit a T^sub g^ in the range of 60 to 82 °C (140 to 180 °F).4 Under extreme heat, the polymer matrix may ignite, supporting flame spread and toxic smoke evolution.5 Once a fire is over, FRP materials may have suffered charring, melting, delamination, cracking, and buckling6; and their structural integrity might be questionable. Appropriate temperature limits to ensure adequate residual performance are not currently known. Finally, FRP strengthening is typically bond-critical, and bond is severely deteriorated by high temperature because bond depends primarily on the properties of the adhesive/matrix.

Kodur and Baingo7 and Bisby8 surveyed the literature on FRP properties at high temperature, and suggested relationships for the change in tensile strength of FRP materials with increasing temperature. Figure 1 shows the strength versus temperature relationships as reported in these two studies7,8 as compared with curves for concrete and steel, and illustrates that FRP is highly sensitive to elevated temperatures. Wang et al.9 performed an extensive study in an attempt to define the critical temperatures for glass FRP (GFRP) and carbon FRP (CFRP) reinforcing bars; the temperatures at which only 50% of the original strength remained. Based on 57 tension tests conducted at various temperatures, it was determined that CFRP and GFRP lost 50% of their original strength at 250 and 325 °C (482 and 617 °F) (as measured at midlength of the specimen), respectively. Stiffness appeared to suffer negligible losses up to approximately 400 °C (752 °F), above which it decreased rapidly. These tests measured only strength and stiffness of FRP bars and did not examine the effects of high temperature on bond.

In addition to concerns associated with loss of mechanical properties at high temperature, the bond of FRP sheets to concrete is of concern in externally-bonded FRP applications. Further research is required in this area to quantify the degree of FRP bond loss with increasing temperature.

Related research

While tests have been performed by various authors on FRP materials in isolation at elevated temperatures, these results cannot be extrapolated directly to predict the fire performance of concrete members strengthened with externallybonded FRP sheets. To the authors' knowledge, only two studies have published results in the area of fire endurance of concrete beams strengthened in flexure with externallybonded FRP materials, thus highlighting the need for further testing and analysis in this area.