Fire resistance of FRP-strengthened RC beams : numerical simulation and performance-based design

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Fire resistance of FRP-strengthened RC beams : numerical simulation and performance-based design


Author: Gao, Wanyang
Title: Fire resistance of FRP-strengthened RC beams : numerical simulation and performance-based design
Degree: Ph.D.
Year: 2013
Subject: Fiber reinforced plastics.
Reinforced concrete construction.
Concrete beams -- Design and construction.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xxviii, 330 p. : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: The wide use of externally bonded fiber reinforced polymer (FRP) laminates (including wet layup FRP sheets and pultrued FRP plates) in the strengthening of existing reinforced concrete (RC) structures has been a significant development in structural engineering over the past three decades. However, the technology also suffers from one serious limitation when employed for indoor applications in buildings: FRP composites have a poor resistance to fire as organic polymers (normally epoxies) used both as the matrix material and the bonding adhesive soften quickly around the glass transition temperature. Furthermore, when exposed to a high heat flux, the polymer matrix may ignite, resulting in flame spread and smoke generation. To facilitate a safe and economic use of the FRP strengthening technique in building applications, an in-depth understanding of the fire performance of FRP-strengthened RC members is deemed necessary. Against this background, this dissertation aims to develop a comprehensive approach to simulate the fire performance of un-protected and insulated FRP-strengthened RC beams. The dissertation is composed of three main parts: (a) theoretical analyses on the bond-slip behavior of the FRP-to-concrete interface at elevated temperatures; (b) advanced finite element (FE) modeling of the structural behavior of un-protected and insulated FRP-strengthened RC beams exposed to fire; and (c) design-oriented solutions for predicting the fire resistance of un-protected and insulated FRP-strengthened RC beams. In the first part of the dissertation, a set of closed-form theoretical solutions was developed for tracing the Mode II debonding process of FRP-to-concrete bonded joints subjected to combined thermal and mechanical loadings. In order to represent the behavior of a wide range of bonded joints, five different bond-slip models were considered in deriving the closed-form solutions, including the elastic-brittle, bi-linear, elastic-plastic-brittle, rigid-softening, and exponential models. For each bond-slip model, explicit expressions for the debonding load, effective bond length, interfacial shear stress, interfacial slip as well as the load-displacement response were derived. It was found that the debonding of FRP-to-concrete interfaces may be delayed or accelerated by temperature variations (i.e., thermal loading) during service. The theoretical solutions indicate that, provided the bond length is sufficiently long, the debonding load of the FRP-to-concrete interface depends only on the interfacial fracture energy and the temperature variation. A temperature increase leads to an increase in both the debonding load and the effective bond length, and the rate of increase of the latter depends on the interfacial bond-slip model. Based on the theoretical solutions, a nonlinear local bond-slip model was also developed for FRP externally bonded to concrete at elevated temperatures. Two key parameters of the proposed bond-slip model, the interfacial fracture energy Gf and the interfacial brittleness index B were determined using existing shear test data of FRP-to-concrete bonded joints at elevated temperatures. The proposed bond-slip model provides the first-ever constitutive law for describing the local performance of the FRP-to-concrete interface at elevated temperatures.
In the second part of this dissertation, advanced FE models were developed to trace the thermal and structural responses of RC beams (i.e., equivalent to un-protected FRP-strengthened RC beams) and insulated FRP-strengthened RC beams exposed to fire. In the models, the temperature-dependent thermal and mechanical properties of concrete, steel, FRP and interfaces are all appropriately considered. The thermal and structural responses predicted by the FE models were compared with existing fire test data to examine their validity. For RC beams, the comparison showed that the inclusion of the steel-to-concrete interfacial behavior leads to more accurate predictions of the deflection. Besides, the proposed FE model allows the complex distribution and evolution of stresses in the reinforcing steel and concrete to be examined in detail, leading to a better understanding of the local responses of RC beams exposed to fire. For insulated FRP-strengthened RC beams, FE predictions showed that the assumption of perfect bonding between FRP and concrete as adopted by previous numerical models leads to an underestimation of deflections and thus an unsafe prediction of fire resistance. Unless a very thick insulation layer is provided (usually an impractical solution), it was revealed that the main role of the insulation layer is to minimize the degradation of the original RC beam rather than to protect the FRP strengthening system. Therefore, the fire resistance evaluation of an insulated FRP-strengthened RC beam can be conservatively but closely approximated by that of an insulated un-strengthened RC beam. In the third part of this dissertation, simple design-oriented solutions for predicting the fire resistance periods of un-protected and insulated FRP-strengthened RC beams are presented. For the un-protected FRP-strengthened RC beams (i.e., equivalent to bare RC beams), the validated FE model for RC beams was used for extensive parametric studies to investigate the effects of various influencing parameters on the fire resistance periods and to generate sufficient data for regressing explicit design formulae. For insulated FRP-strengthened RC beams under fire, a two-phase design-orientated approach was proposed to predict their fire resistance. The first phase is the development of explicit solutions for the temperature field analyses of insulated FRP-strengthened RC beams exposed to the standard fire. The second phase is the structural response analysis, for which the degradation of the load-carrying capacity of the beams are assessed using the "500 °C isotherm method" in combination with the temperature predictions of insulated beam sections. The fire resistance periods of insulated FRP-strengthened RC beams are defined to be reached when the fire load action exceeds the loading carrying-capacity of the beams during fire. In order to validate the simple design-oriented method, parametric studies using the advanced FE model were carried out to generate the fire-resistance data for comparisons. The comparisons showed that the simple design-oriented method provides a reliable prediction of the fire resistance periods for insulated FRP-strengthened RC beams.

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