INVESTIGATIONS ON FLEXURAL RESPONSE OF FRC BEAMS REINFORCED WITH BASALT FIBER REINFORCED POLYMER REBARS

Abstract

Investigations on the flexural response of fiber reinforced concrete (FRC) beams reinforced with basalt fiber reinforced polymer (BFRP) rebars are presented. The development of durability related issues in traditional steel reinforced concrete (RC) structures due to embedded steel reinforcement corrosion is the biggest problem that is causing the shortening of service life of the steel RC structures. Reinforcing the concrete structures using non-metallic and non corrosive fiber reinforced polymer (FRP) rebars will ensure that the structure remains corrosion-free. BFRP rebar is a newly developed FRP rebar that is emerging as a green construction material, and it has become one of the best alternatives to existing FRP rebars. All FRP rebars have a lower Young’s modulus and higher tensile strength than steel reinforcing bars. As a result, FRP reinforced plain concrete beams become less ductile, experience more deflections, and produce more cracks at higher flexural strengths. The flexural behaviour of steel or FRP RC beams not only depends on the area and type of reinforcement provided but also on the properties of the concrete used. The application of BFRP rebars in FRC can increase its performance as a longitudinal reinforcement, as the drawbacks associated with plain concrete (PC) beams reinforced with BFRP rebars can be decreased by increasing the strength and tensile properties of concrete with the addition of fibers. In this study, an attempt was made to develop an eco-friendly reinforcing system (BFRP rebars + basalt fibers and/or polyvinyl alcohol (PVA) fibers) to investigate the flexural response of BFRP reinforced basalt fiber reinforced concrete (BFRC) and PVA fiber reinforced concrete (PVAFRC) beams. Therefore, the present study was aimed to investigate the flexural response of PC, BFRC and PVAFRC beams reinforced with BFRP rebars. This investigation was carried out using two concrete grades: normal strength concrete (NSC) of M30 grade and high strength concrete (HSC) of M70 grade. The use of pozzolanic materials in concrete manufacturing provides economical, technological, and environmental benefits. In this study, alccofine-1203 and fly ash were used as supplementary cementitious materials (SCMs) to partially replace cement in the development of HSC. 4%, 6%, 8%, 10%, 12% and 14% alccofine-1203 with 20% fly ash combination were used to partially replace cement to find the optimum percentage of alccofine-1203 to be used in the development of HSC. For this, a total of seven binder proportions were prepared and evaluated for mechanical properties, microstructural characteristics and compressive stress strain behavior. From the obtained results, it was found that the use of alccofine-1203 in combination with fly ash was beneficial in the development of HSC. Among all, the i replacement of cement with 10% alccofine-1203 with a 20% fly ash combination attributed superior microstructural characteristics for binder mixes and showed highest mechanical properties and compressive stress-strain behaviour for the concrete. In this study, to improve the flexural performance of BFRP RC beams, BFRC was developed using basalt fibers and PVAFRC was developed using PVA fibers. To determine the optimum percentage of basalt fibers and PVA fibers to use for manufacturing BFRC and PVAFRC, additions of 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% (of the volume of concrete) basalt fibers and PVA fibers were made to the developed NSC and HSC. Consequently, a total of 20 mixes (10 BFRC mixes and 10 PVAFRC mixes) were developed and evaluated for fresh and hardened properties. The workability, compressive strength, flexural strength, split tensile strength, load deflection behaviour, and uniaxial compressive stress-strain behaviour of BFRCs and PVAFRCs were studied experimentally. Young’s modulus, and energy absorption capacity, peak-stress, and strain at peak-stress of BFRCs and PVAFRCs have also been studied analytically. Scanning electron microscopy analysis was performed to examine microstructural characteristics of BFRCs and PVAFRCs. The results indicated that the addition of basalt or PVA fibers reduced the workability of concrete mixes. The maximum compressive strength, flexural strength, split tensile strength and better load-deflection and stress-strain behaviour were obtained with the addition of 0.3% of basalt fibers in two strengths of BFRCs and 0.3% PVA fibers in two strengths of PVAFRCs. The modified constitutive analytical model and relationships between properties of compressive stress-strain curves of BFRCs and PVAFCs such as peak-stress, strain at peak stress and material parameter (𝛽𝑛) with modified reinforcing index values of fibbers were proposed for analytical modelling of stress-strain curves of BFRCs and PVAFRCs of two strengths, and a good agreement with experimental results was observed. Additionally, in the literature, the proposed constitutive analytical model and relationships between the material parameter and reinforcing index for analytical modelling of FRC’s stress strain curves failed to accurately predict the experimental stress-strain curves of BFRCs and PVAFRCs. To test the flexural response of BFRP RC (PC, BFRC, and PVAFRC) beams, 16 single reinforced concrete beams with a pure bending region were cast and tested experimentally. Twelve beams were longitudinally reinforced with BFRP rebars in the tension region and four beams were fully reinforced with steel reinforcing bars. Two grades of concrete- NSC and HSC; two types of FRC- BFRC and PVAFRC; and two types of RC sections- under-reinforced and over-reinforced were parameters considered in the investigation. All the beams were tested for ii load-deflection behaviour, moment-curvature relationships, ductility, cracking pattern and failure mode evaluation. The optimum percentage of basalt fibbers and PVA fibers (0.3%) was taken to prepare BFRC and PVAFRC. The obtained results showed that the load-deflection and moment-curvature response of BFRP-reinforced PC, BFRC, and PVAFRC beams exhibited two phases which bounded the cracking point, whereas steel-reinforced PC beams exhibited three phases which bound the cracking and yielding points. Although BFRP rebar is a brittle material with no clear yielding point, BFRP RC beams exhibited more deformation and curvature prior to failure during testing. The amount of deflection, curvature, stiffness and ductility that were exhibited by BFRP-reinforced PC beams were partially countered by reinforcing PC with basalt fibers and PVA fibers. However, due to the higher bond strength and better strain softening behaviour, PVA fibers improved flexural behaviour of BFRP RC beam better than the basalt fibers. ABAQUS based non-linear finite element numerical modelling was conducted to validate the experimental results of BFRP-reinforced PC, BFRC, and PVAFRC beams. The investigation parameters considered in experimental evaluation were the same as those considered in numerical modelling. All the numerically modelled beams were evaluated for load-deflection behaviour, moment-curvature response, ductility and damage pattern. The results showed that the numerical modelled beams behaved similar to the experimentally tested beams. The percentage of error between experimental and numerical results was found within 10%. This shows good agreement between them. The numerically modelled beams accurately illustrated concrete damage in compression and tension, as well as a cracking pattern in tension, in a manner similar to that of the experimental cracking and the damage patterns of concrete in compression and tension. iii