Creep Induced Deflections of Concrete Elements Reinforced with Polymer Composite Bars

Abstract

With the increasing costs of steel reinforcement and corrosion related problems, the construction industry is looking for alternative materials to substitute steel reinforcement. This paper studies the application of composite fiber reinforced polymer bars to reinforce concrete flexural elements. As an example, Glass Fiber Reinforced Polymer (GFRP) composite bars have a modulus of elasticity of approximately 1/6 of the modulus of elasticity of steel reinforcement. This fact makes the design of GFRP-reinforced concrete elements governed by serviceability requirements, rather than by strength requirements in most cases. In addition to that, polymer composite bars have been shown to creep under sustained loads. Therefore, this paper presents an analytical method that can be used to compute the deflections induced by creep in polymer composite reinforced concrete flexural elements. This method can be a valuable tool for the design of concrete elements reinforced with polymer composite bars where deflections are the governing design factor. Background A reinforced concrete structure must be designed to meet strength and serviceability requirements. Strength requirements indicate that a structural member must be able to sustain the factored design loads. Regarding serviceability requirements, GFRP-reinforced concrete element deformations under service (unfactored) loads must be limited to maintain the function of the structure, to minimize damage to nonstructural elements, to give the users confidence in the structure, and to prevent the structural behavior from being different from that assumed (Park, 1975). GFRP bars have higher tensile strengths and lower tensile and shear stiffnesses than conventional steel reinforcing bars. As a result of the lower axial stiffness of the GFRP bars, GFRP-reinforced concrete members deform more than typical steel-reinforced concrete members. Consequently, the design of GFRP-reinforced concrete members is typically governed by serviceability requirements. Allowable deflections are thus common governing requirements for GFRP reinforced concrete elements. Deflections of structures are of two types: immediate and long-term. The contribution of creep and shrinkage of concrete, as well as creep of GFRP bars account for the total long-term deflections. Based on the research of Brown (Brown, 1997), Kage et al. (Kage, 1995), and Vijay and GangaRao (Vijay and GangaRao, 1998), the current ACI 440 (ACI 440, 2003)