1. Introduction
The durability aspect of geopolymer concrete (GPC) is well established which includes, but is not limited to, low creep, superior resistance against sulphate and acid attack, reduced drying shrinkage, less water absorption, and higher fire resistance and dimensional stability [
1,
2,
3,
4,
5]. In terms of the tensile reinforcement of concrete, steel is the most commonly used material. The corrosion issue in steel can be eliminated by using fibre-reinforced polymers (FRP) such as carbon-FRP (CFRP), glass-FRP (GFRP), aramid-FRP (AFRP), and basalt-FRP (BFRP) as longitudinal and transverse reinforcements [
6]. Major applications of FRP rebar in concrete include marine structures and bridges exposed to de-icing salts [
7] or any other corrosion-prone areas. FRP bars have a higher strength to weight ratio, are electrochemically neutral, and have excellent fatigue and chemical resistance [
8,
9]. In terms of sustainability, GPC is a viable alternative for concrete that uses alkali-activated by-product materials such as rice husk ash and fly ash [
10]. In addition to the conventionally used aluminosilicate precursors, phosphates were introduced to the activation process; this type of geopolymer concrete is termed as phosphate-based geopolymer concrete or aluminosilicate phosphate cement. While the conventional aluminosilicate geopolymer concrete is ideal for large-scaled constructions, the application of phosphate-based geopolymer is considered feasible in biomaterials [
11]. GPC emits 64% less carbon dioxide in its production life cycle compared to its Ordinary Portland Cement (OPC) counterpart [
12]. Furthermore, it has the potential to reduce cost by up to 30% depending on the use of alkaline liquids [
13].
In terms of the structural properties of GPC, contradictory conclusions were reported in the literature. These differences in results can be attributed to the broad range of mix designs adopted by different researchers in terms of curing condition, slag to fly-ash ratio, and alkali-activated constituents [
14]. GPC is reported to have lower elastic modulus [
5,
15] and exhibit more brittle failure behaviour [
16,
17,
18] compared to Ordinary Portland Cement (OPC)-based concrete. While some studies [
19,
20,
21,
22,
23] reported similar behaviour for GPC beams when compared with OPC beams, others [
15,
24,
25] reported significant differences. Apart from the mechanical properties of GPC itself, the bond strength at the FRP to GPC interface plays an important role for effectively carrying the applied load on an FRP-reinforced GPC structural element. It is reported that the glass FRP (GFRP) bar to GPC interface [
26] and aramid FRP or carbon FRP rebar to OPC interface [
27] have similar bond strengths compared to the similar size steel-to-concrete interface. Nevertheless, due to the lower elastic modulus of GFRP, the serviceability requirement in terms of deflection or flexural stiffness was reported to be lower [
20]. In this regard, the implementation of carbon FRP (CFRP) rebar has the potential of improving flexural stiffness, since CFRP has a similar modulus of elasticity to that of steel and has better bond strength with concrete compared to the FRP rebar. For FRP-reinforced GPC beams, most of the studies focused on geopolymer and conventional concrete beams reinforced with GFRP [
28,
29,
30,
31,
32,
33], while only a few reported on GPC beams reinforced with CFRP [
34,
35]. In the present study, both GFRP and CFRP rebars have been considered for the design of beams.
Since FRP bars behave linearly elastic until failure, most of the design guidelines recommend the design of FRP-reinforced geopolymer beams to be over-reinforced. Accordingly, the failure strain of GPC under compression becomes the governing design factor. The failure strain of geopolymer concrete varies significantly and is reported to be in the range of 0.0015–0.0050. Sarkar [
4] recommended a failure strain value of 0.003, Tran et al. [
36] proposed 0.0035, Ahmed et al. [
37] reported a range of 0.002–0.0033, and Maranan et al. [
20] found the failure strain to be in the range of 0.0029–0.0048. Due to a variation in mix design and curing condition, the compressive strength and strain of GPC also varies, and hence, more research is required to investigate the failure strain of geopolymer concrete considering its mix design.
The present study compares the flexural behaviour of both CFRP and GFRP-reinforced geopolymer beams in terms of ultimate, cracking, and service moment capacity, along with serviceable, ultimate, and residual deflection. In addition, strain distribution and failure behaviour are also reported. For this purpose, GFRP and CFRP-reinforced GPC beams were designed, casted, and tested along with normal steel-reinforced OPC concrete beams. Details of the experimental program are reported in
Section 2. Test results are analysed in
Section 3.
The aforementioned strain of concrete plays a vital role in the flexural performance of a reinforced beam. Therefore, it is vital to investigate the varying types of strain and how they affect the contemporary design standards for geopolymer concrete structures. This study will compare experimental moment capacities against two of the most commonly used design standards—ACI440.1R-15 [
38] and CAN/CSA S809-12 [
39]. However, since these two standards are developed for FRP-reinforced OPC concrete structures, an additional two theoretical models are also taken into account, which consider a parabolic stress block [
34] and equivalent rectangular stress block [
36] for geopolymer concrete under compression. Alongside this, a detailed flexural performance that is inclusive of ultimate and service states has been carried out for all beams.
This article is based on a project that was undertaken in collaboration with industry partner Austeng (Australian Engineering Solutions Pty. Ltd., North Geelong, VIC, Australia)—a local consultation company in Geelong, Australia. The City of Greater Geelong (CoGG) has taken numerous innovative and sustainable initiatives in the construction sector. As a part of the “100 years maintenance-free pedestrian bridge” project, CoGG decided to construct or replace 160 pedestrian bridges around the City of Greater Geelong with minimal maintenance requirements using sustainable and durable materials. Accordingly, geopolymer concrete and fibre-reinforced polymer rebars were selected as the materials for the design of these bridges.
The beams of the pedestrian bridges were designed in accordance with AS5100.2:2017 (Bridge design—Design loads). Alongside AS5100, AS1170.0:2002 (Structural design actions—General principles) and AS1170.1:2002 (Structural design actions—Permanent, imposed and other actions) were used to accurately design the beams by considering the loads that the bridges would be subjected to during their design life at the designated site. Both CFRP and GFRP rebar were considered to design the bridge. However, the aim of this project was to select the most accurate design equations while designing FRP-reinforced geopolymer beams.
5. Conclusions
It should be noted that the conclusions derived from this study are based on testing six beams using fly ash-based heat-cured geopolymer concrete (GPC). The axial rigidity of the CFRP and GFRP bars used were 69 and 84% of the axial rigidity of the conventional steel rebar, resulting in a similar reinforcement ratio for steel and CFRP-reinforced concrete, but the value was 2.5 times higher for GFRP-reinforced GPC.
The present study compared the flexural response of FRP-reinforced geopolymer concrete beams against conventional steel-reinforced concrete. Over-reinforced beams were designed based on both CFRP and GFRP rebars. Accordingly, the failure of RC beams was initiated by steel yielding, whereas the FRP-reinforced beams failed due to concrete crushing. It was concluded that both the FRP and GPC are responsible for the reduction of stiffness by up to 40% while comparing against conventional RC beams. However, the reduction is more for GFRP beams considering the axial rigidity of the bars. While the axial rigidity of GFRP bar was 16% less than the same of steel rebar, stiffness was reduced by 30% in the GFRP-reinforced beam. In contrast, CFRP had 31% less axial rigidity than the steel rebar, but it resulted in a 40% reduction in stiffness.
The flexural strength at the service stage (Ms) is a good indication of the performance of any FRP-reinforced concrete/GPC beam. If the flexural strength at the service stage is defined as 30% of the ultimate moment, a GFRP-reinforced GPC beam yielded 44–48% enhancement in the service moment capacity compared to steel RC beams. This improvement amount was 16–32% for CFRP-reinforced GPC beams despite the fact that the reinforcement ratio of the GFRP beam was 2.5 times higher than the same in CFRP-reinforced beams.
Similar behaviour was observed while comparing the ultimate moment capacity. Both the CFRP and GFRP-reinforced GPC attained a significantly higher ultimate moment capacity than conventional reinforced concrete beams. However, the GFRP-reinforced beam achieved a 17% higher moment capacity than the CFRP-reinforced beam, despite having a 250% higher reinforcement ratio. While comparing the CFRP and GFRP-reinforced geopolymer beams under similar conditions (i.e., same properties of geopolymer concrete and same geometry of the rebar), the CFRP-reinforced GPC beam was found to be 32–37% stronger than the GFRP-reinforced one in terms of ultimate load-carrying capacity.
Four theories/design standards are considered for this comparison—ACI440.1R-15, CAN/CSA S806-12, parabolic stress block theory (for GPC under compression), and equivalent rectangular stress block theory for GPC simplified from the parabolic behaviour of GPC under compression. The failure strain of geopolymer concrete varies significantly within the range of 0.0032–0.0048, and hence, it affects the accuracy of the theoretical prediction. Nevertheless, all theories can predict the capacity satisfactorily, except for the parabolic theory, which was found to be unsafe. ACI and CSA both had a maximum error margin of 17%, while CSA yielded the most consistently accurate Mtheoretical/Mexperimental. The equivalent rectangular theory also attained values similar to the ACI code.
As suggested in CSA for the design of an FRP-reinforced OPC structure, the beams should be designed as over-reinforced because FRPs are brittle. This study suggests that with the nominal failure strain of 0.003, the CSA and ACI codes can predict the capacity of FRP-reinforced GPC beams fairly well, even though they are based on OPC.