Performance of concrete beams reinforced with GFRP bars under monotonic loading

Abstract

The use of GFRP bars in concrete structures has emerged as an alternative to conventional RC due to the corrosion of steel in aggressive environments. Although many studies were reported on concrete beams reinforced with GFRP bars, still the full potentiality of GFRP reinforced concrete is yet to be known. This study reports experimental, analytical and numerical investigations carried out on concrete beams reinforced with TMT and GFRP bars under monotonic loading. Experimental studies were carried out on concrete beams of size 100 × 200 × 1500 mm reinforced with TMT (10 mm and 12 mm dia.) and GFRP bars (10 mm and 13 mm dia.). The deflection of GFRP reinforced concrete beams corresponding to service load is found to be 2.75 times the deflection of TMT reinforced concrete beams. Widely used analytical/empirical models were compiled to predict the load deflection behaviour of beams. The analytical models were primarily based on effective moment of inertia to account for beams stiffness after cracking. Nonlinear finite element (FE) analysis has been carried out on concrete beams reinforced with TMT and GFRP bars. FE models were developed by employing the appropriate constitutive relationship of concrete, TMT and GFRP bars. The integrity of all the elements has been ensured by applying the appropriate constraint conditions. Average load–deflection plots obtained from experiments were compared with computed load–deflection of analytical and numerical studies. From the load–deflection behaviour, it is observed that (i) numerical prediction is stiffer and (ii) the analytical models incorporated with tension stiffening aspects predicted the deflections close to experimental observations.

Introduction

Reinforced concrete is very commonly employed material in the construction of civil infrastructure, airport construction, bridges, buildings etc. Due to rapid increase in infrastructure in many countries/cities, high demand for reinforced concrete is expected. Corrosion is a crucial problem in steel reinforcement which deteriorates the material when it reacts with the environment [1], [2]. An experimental Glass Fibre Reinforced Plastic (GFRP) rebar has emerged as a promising alternative to conventional steel reinforcement with excellent results in terms of corrosion resistance. GFRP rebars are usually composed of glass fibres reinforced with a thermosetting resin and manufactured using a pultrusion process. Its advantages include high longitudinal strength and tensile strength, high stiffness to weight ratio, resistance to corrosion and chemical attack, light weight, controllable thermal expansion and damping characteristics and electromagnetic neutrality. Fibre Reinforced Plastic (FRP) bars exhibit linear elastic behavior up to failure (brittleness), which affects the ductility of the concrete members. Further, the failure modes of FRP-reinforced concrete (FRP-RC) members vary significantly with the amount of reinforcement. Most design codes and guides suggest over-reinforcing of FRP reinforced members to ensure plastic deformation of the compressed concrete and to enhance ductility. Although, the GFRP (Glass Fibre Reinforced Plastic) rebar has some advantages with regard to corrosion, its usage in the construction industry is still a big question mark due to its low modulus of elasticity and serviceability aspects of structural elements. Due to low modulus of elasticity, the concrete members reinforced with FRP bars undergo large deflections and wider cracks which affect its serviceability. Hence, the design of such structures should be governed by their serviceability limit state (deflection and cracking) rather than their ultimate limit state. Further, it was observed that surface configuration may play a significant role with respect to cracking performance and consequently crack width. Recently, some studies were reported combining steel bars with FRP bars (hybrid system) in reinforcing concrete structures, which overcome the ductility and serviceability problems of purely FRP reinforced structures. This hybrid system ensures the ductility of structure due to the contribution of additional steel reinforcement and enhanced load carrying capacity due to FRP bars.

For the last two decades, several studies were reported on the flexural response of FRP RC beams [3]. In the case of serviceability, and specifically for deflections, several researchers [4], [5], [6], [7] proposed coefficients to modify Branson’s equation used in steel design codes [8], while others [9], [10] proposed a modified equivalent moment of inertia derived from curvatures. These approaches were adopted in several design guideline proposals for FRP RC [3], [11], [12], [13], [14], [15], [16], [17] carried out several investigations on concrete beams reinforced with steel, FRP, aramid FRP and combination of both steel and FRP or aramid FRP or glass FRP under flexural loading. From their studies, it was reported that (i) the addition of steel reinforcing bars to heavily aramid FRP-reinforced concrete sections significantly enhanced the ductility and reduced the crack widths and spacing, (ii) higher flexural strength was realized for the case of hybrid-reinforced beams (combination of steel and glass FRP) compared to steel or glass FRP reinforced beams, (iii) the combination of steel and GFRP reinforced beam yielded higher ductility, and (iv) all hybrid-reinforced beams (steel + GFRP) failed due to concrete crushing after yielding of steel reinforcement and GFRP bars were effective in maintaining the flexure capacity of the beams and to enhance their serviceability aspects. Bischoff and Gross [18], [19] mentioned that the abrupt loss of stiffness at cracking affects the post-cracking behavior and deflection. Mousavi et al. [20] investigated the deflection of GFRP-RC beams and described that (i) the low elastic modulus of GFRP bars responsible for the abrupt loss of concrete stiffness and (ii) the bond-dependent coefficient and the modulus of elasticity of the FRP bars are the main factors affecting the behavior of the GFRP-RC beams. Refai et al. [21] carried out studies on concrete beams reinforced with hybrid bars (combination of GFRP and steel) by varying the reinforcement ratio and the ratio of steel to GFRP bars. The experimental responses were compared against the codal provisions of ACI-440.1R-06. New bond coefficient was proposed to predict the crack width and new deformability factor was proposed to predict the deformation of hybrid beams. Doo-Yeol et al. [22] performed flexural studies on ultra-high-performance fiber reinforced concrete beams reinforced with GFRP bars and reported that the crack width criteria of ACI 440.1R and CAN/CSA S806 at serviceability limit state and deformability requirement by CAN/CSA-S6 [23] were satisfied. Ibrahim [24] performed nonlinear finite element analysis of concrete deep beams reinforced with GFRP bars for prediction of load–deflection behaviour, failure load, failure mode, crack propagation, GFRP strain and concrete strain distribution. Amr El-Nemr et al. [25] investigated the flexural behaviour and serviceability performance of concrete beams reinforced with different types of GFRP bars by considering the various aspects such as (i) variation in modulus of elasticity (46.4–69.3 GPa), (ii) surface profile (sand-coated and helically-grooved), and (iii) reinforcement ratio. It was reported that the cracking behavior of the tested beams with sand-coating of GFRP bars enhanced the bond performance in concrete more than the helically grooved profile. Further, it was also reported that ACI 440.1R-06 and ACI 440.1R-15 underestimated the deflection, while ISIS M-03 and CSA S806-12 provided conservative deflection values at 0.30 of nominal moment capacity.

GFRP was used as concrete bridge deck reinforcement for the Sierrita de la Cruz Creek Bridge located 40 km northwest of Amarillo, Texas in 2000 by replacing the original bridge which was structurally deficient and corroded [26]. The Crowchild Trail Bridge, located in Calgary, Alberta, utilized ribbed-deformed GFRP C-Bar as reinforcement in the barrier walls and deck slab [27]. The Joffre Bridge, located in Sherbrooke, Quebec, over the St-Francois River, contains GFRP bars as reinforcement in the 45 MPa concrete in the sidewalks and the traffic barriers [28].

Although lot of work has been reported on GFRP reinforced concrete beams, still its usefulness is yet to be recognized. This paper reports the studies carried out on concrete beams reinforced with Thermo Mechanical Treated (TMT) bars and GFRP bars with respect to experimental, analytical and numerical. This paper is organized in three major sections, (i) first section deals with the experimental studies, (ii) various analytical models are compiled in section two to predict the load–deflection behavior of GFRP reinforced concrete beams, and (iii) numerical investigations on concrete beams reinforced with TMT and GFRP in section three. The deflections obtained from experiments are compared with analytical and numerical predictions.