Structural behaviour of geopolymer concrete beams and columns reinforced with glass fibre reinforced polymer bars

In Australia, the environments are severe to use steel as reinforcement to concrete structures from the viewpoint of corrosion damage. With the limited resources of the state and the federal governments to maintain existing infrastructures, a new approach for construction of more durable infrastructures is required. As a result, glass fibre reinforced polymer (GFRP) bars have gained considerable worldwide interest for use as internal reinforcement to concrete structures that operate in highly aggressive environments. At the same time, the use of geopolymer cement as an alternative to ordinary portland cement (OPC) is currently attracting increasingly widespread attention because its manufacture does not directly create CO2 emissions. However, there is inadequate scientific research undertaken to substantiate the benefit of the combined use of these materials in actual infrastructure, which has been the key motivation for this research. Therefore, this study investigated the suitability and structural behaviour of geopolymer concrete structures reinforced with GFRP bars to allow the safe and responsible introduction of this technology in construction and civil infrastructure. Firstly, the bond between geopolymer concrete and GFRP bar was investigated as this is a critical factor that influences the strength and long-term behaviour of reinforced concrete structures. The results obtained from the direct pullout test showed that the sand-coated GFRP bars have sufficient bond to geopolymer concrete through the sand-particles coated around its surface that provide the necessary mechanical interlock and friction forces. The bond between GFRP bars and geopolymer concrete was found comparable to deformed steel bars and was higher than GFRP-ordinary concrete bond. Generally, as the embedment length and bar diameter increases, the bond stress between the GFRP bars and geopolymer concrete decreases. The use of anchor heads further enhanced the pullout load resistance of the GFRP bars by as much as 49-77%, owing to the mechanical bearing resistance of the anchor heads. The flexural behaviour of geopolymer concrete beams reinforced with GFRP bars was investigated as the second stage. The results showed that the serviceability performance of the beams is affected by the amount of reinforcement. The beams with a higher longitudinal reinforcement ratio exhibited lower deflection and narrower crack width, than the beams with lower reinforcement ratio. The reinforcement ratio and bar diameter, however, did not significantly influence the flexural strength of the beams. The beams with headed GFRP bars yielded similar flexural strength and serviceability performance as the beam with straight GFRP bars. The GFRP-reinforced geopolymer concrete (GFRP-RGC) beams yielded higher flexural strength than both steel-reinforced geopolymer concrete (S-RGC) and GFRPreinforced concrete (GFRP-RC) beams but inferior serviceability performance to S-RGC beams, owing to the higher tensile strength but lower elastic modulus of GFRP bars compared to steel bars. The shear behaviour of geopolymer concrete beams reinforced with GFRP stirrups at different spacing was investigated in the third stage. The results showed that the GFRP web reinforcement doubled both the shear strength and deflection capacities of the beam without stirrups. The spacing of GFRP stirrups did not directly influence the strength and deflection capacities; however, it did affect the shear-crack width development, wherein the increase in stirrups spacing was accompanied by an increase in crack width. The beams with GFRP stirrups yielded relatively similar strength, deflection capacity, and stiffness as the beam with steel stirrups; however, wider shear cracks occurred in the former beams because of the lower elastic modulus of GFRP bars compared to that of steel bars. In addition, the shear capacity of the tested GFRP-RGC beams was higher than that of the FRP-RC beams. The compression behaviour of circular GFRP-RGC columns subjected to concentric axial loads was investigated at the last stage. From the experimental outcomes, the provision of GFRP ties enhanced the compression performance of the geopolymer concrete column without transverse reinforcements. The columns with closely spaced ties yielded higher strength and deformation capacities and failed in a more ductile manner than the columns with widely spaced ties. The spiral-confined columns showed higher confinement efficiency and ductility compared to their counterpart hoop-confined columns, owing to the continuous nature of the spiral that enables it to distribute the stresses uniformly around and along the height of the column. The short columns yielded higher axial strength and stiffness compared to slender columns. This can be expected since the short columns failed due to crushing, a material type of failure, while the slender columns failed due to buckling, a geometric type of failure, owing to the effects of slenderness ratio. Generally, the GFRP-RGC beams and columns yielded better load-carrying capacity than their counterpart GFRP-reinforced concrete (GFRP-RC) with similar configurations and material properties. This could be attributed to the higher elastic modulus of geopolymer concrete compared to normal concrete of the same grade, resulting in better compatibility in the GFRP-RGC system that in a GFRP-RC system. Prediction equations that reliably describe the structural behaviour of geopolymer concrete structures reinforced with GFRP bars were developed in each stage of the study. Cosenza, Manfredi, and Realfonzo (CMR)-based bond-slip laws were developed to model the ascending segment of the bond-slip curve while an analytical model was proposed to estimate the pullout load capacity of the straight and headed GFRP bars embedded in geopolymer concrete. Similarly, new analytical equations based on equivalent stress block and the parabolic stress block were developed to predict the flexural strength of the GFRP-RGC beams. In these equations, the usable concrete strain is 0.0048 and the compression contribution of top bars are included. The deflection of the beam, on the other hand, was estimated accurately by incorporating the constants βa and βb, both functions of actual and balanced reinforcement ratios, to the effective moment of inertia formula suggested by Branson. In terms of shear capacity, the ACI 318-14 strut-and-tie model yielded the most accurate predictions among the design equations employed in the study. Finally, a strength reduction of 0.90 and the compression contribution of GFRP bars up to a strain of 0.002 were considered in the proposed equation to estimate the nominal capacity of the columns. The comparison and validation of the developed analytical models showed good agreement with experimental results. From this study, it is concluded that the GFRP-RGC system is a promising application. An enhanced understanding of the behaviour of geopolymer concrete beams and columns reinforced with GFRP bars is an outcome of this investigation. The analytical equations developed in this study can be important tools for design engineers permitting the safe design and development of GFRP-RGC system, enabling their increased acceptance and utilisation in the mainstream construction applications.

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