Fire Endurance of Fiber-Reinforced Polymer-Confined Concrete Columns

The use of fiber-reinforced polymers (FRPs) for strengthening and rehabilitating reinforced concrete structures has been the subject of numerous research projects and has seen widespread implementation in recent years. Very little information is available on the behavior of FRP materials at high temperatures, however, and this is a primary factor discouraging the widespread application of FRP wraps in buildings where fire-related issues are critical design requirements. This paper presents the results of two full-scale fire endurance tests on circular FRP-wrapped reinforced concrete columns insulated with different thicknesses of fire insulation. Test data are compared with the predictions of a numerical fire simulation model, and the model is shown to adequately predict the observed thermal and structural response. It is demonstrated that, while currently available infrastructure composites are particularly sensitive to elevated temperatures, appropriately designed FRP-wrapped reinforced concrete columns are capable of achieving the required fire endurances.

Keywords: confinement; fibers; fire endurance; rehabilitation; reinforcement; strength.

INTRODUCTION

Confinement of reinforced concrete columns by circumferential wrapping with fiber-reinforced polymer (FRP) sheets is now widely recognized for its ease of application and effectiveness (Bisby et al. 2002). This technique can be used to increase both the strength and ductility of concrete columns, and has seen recent widespread use for rehabilitation, repair, strengthening, and seismic upgrading of concrete structures. If FRP materials are to be used for confinement of concrete in building and other applications, then the ability of FRP materials and FRP-confined concrete members to withstand fire must be ascertained, and rational procedures to design for fire safety must be formulated. Fire endurance tests conducted by Blontrock, Taerwe, and Vandevelde (2000, 2001) on carbon FRP-plated reinforced concrete beams and slabs indicated that externally bonded FRP materials perform poorly during fire, and thermal insulation of the FRP reinforcement is required to prevent instantaneous loss of effectiveness upon exposure to fire.

RESEARCH SIGNIFICANCE

Currently, there is no information available on the behavior of loaded FRP-wrapped reinforced concrete columns during fire. This paper presents the results of two full-scale fire tests on circular FRP-wrapped reinforced concrete columns, conducted as part of a larger study investigating the fire endurance of FRP reinforcing systems, and uses the data obtained to validate a previously presented numerical model that can be used to predict the fire endurance of these members (Bisby 2003).

BACKGROUND

FRP materials are extremely sensitive to the effects of elevated temperatures, and severe deterioration in mechanical properties, bond properties, or both can be expected at temperatures approaching the glass transition temperature Tg of the polymer adhesive/matrix (Bisby 2003; Blontrock, Taerwe, and Matthys 1999). Furthermore, all organic polymer matrix materials are combustible and will burn when subjected to a sufficiently high heat. Commonly used matrix materials such as polyester, vinyl ester, and epoxy not only support combustion, but evolve large quantities of dense black smoke (Sorathia, Dapp, and Beck 1992). Thus, there are a number of concerns, both environmental (smoke generation, toxicity) and structural (loss of strength, stiffness) associated with the use of FRP as external reinforcement for concrete members in buildings (Kodur 1999). In the current paper, the focus is on the structural behavior of FRP-wraps during fire, and the consequences of FRP degradation at high temperatures on the fire endurance of FRP-wrapped reinforced concrete columns. Detailed discussions on the deterioration of strength and stiffness of FRP materials at elevated temperatures is presented by Bisby et al. (2002) and Kodur and Baingo (1998).

Design for fire safety

Design for fire safety is concerned primarily with the protection of human life and property during fire (Lie 1992). Thus, one of the primary goals of structural fire engineering is to ensure prevention of structural collapse, at least until all building occupants have had the opportunity to safely evacuate. For building columns, structural fire endurance criteria in North America are defined exclusively in terms of load-bearing capacity because columns do not generally perform fire separation or fire barrier functions (Lie 1992). Columns are essentially required to retain sufficient strength to carry their presumed working (service) load for the required duration during exposure to a standard fire.

While the fire endurance of conventional reinforced concrete columns is generally satisfactory, when a column is strengthened with a circumferential FRP wrap (that is, its axial load capacity is increased), then there is a serious concern that loss of effectiveness of the FRP wrap during fire could lead to sudden collapse under increased service loads. For this reason, ACI 440.2R-02 (ACI Committee 440 2002) requires that FRP wraps be designed under the assumption that they are completely lost during fire. This requirement is based on conservative assumptions and is not founded in research. Furthermore, such a requirement is potentially restrictive when FRPs are contemplated for use in buildings. The engineering community often requires design recommendations developed based on the results of full-scale fire endurance tests (Munley and Dolan 2001) such as those presented in this paper.