Mechanical performance of CFRP-confined sustainable geopolymeric recycled concrete under axial compression
Introduction
Owing to massive urban renewal and regeneration, the astonishing increase in the generation of construction and demolition waste (CDW) has been witnessed in the past few years worldwide. This consequently has raised growing concerns over the sustainability of the construction industry. Meanwhile, being that aggregate makes up about 70–80% of the total concrete volume, acute depletion of natural aggregate resources would be encountered as growing demand for concrete. In response to these issues, considerable efforts have been involved in the exploit of artificial aggregates manufactured from CDW, namely recycled aggregate (RA) [1], [2], [3], [4], [5]. By replacing natural aggregate (NA) with RA, it could reduce natural resource consumption, avoid landfill caused by CDW, and even render the waste material with additional value [6], [7].
Recently, researchers have utilized RA in geopolymeric concrete for producing the so-called geopolymeric recycled aggregate concrete (RAC) [8]. Geopolymeric concrete is a type of concrete which is manufactured by mixing aluminosilicate bearing materials with alkaline activators to form the slurry to bind aggregate particles. The sources of raw material vary, e.g., fly ash, slag, and metakaolin. Previous studies have verified that geopolymeric concrete has comparable performance to conventional concrete in the aspects of mechanical and durability properties [9], [10]. Most importantly, geopolymeric concrete involves less CO2 emissions compared with conventional concrete [11], [12]. Therefore, geopolymeric RAC could provide a great opportunity for concrete products to move towards high environmental compatibility, by exploiting the advantages from eco-sustainable aggregates and binders.
Despite the above significance brought by geopolymeric RAC, there are still many obstacles in the way of its applications. One of them is that the performance of geopolymeric RAC, in both the short and long terms, is inferior to that of its counterpart based on NA [13], [14]. The existing studies have demonstrated that the RA incorporation has some unfavorable effects on the resulting geopolymeric concrete, including: (1) degradation of mechanical performance, such as compressive strength, flexural strength, tensile strength, and energy absorption capacity [15], [16], [17], [18]; (2) increase of crack density in interfacial transition zones [15]; (3) inferior durability in terms of the resistance to mass transport, chemical attack, and elevated temperature [17], [18], [19], [20], [21]; and (4) decreased dimensional stability, especially creep and shrinkage [22]. These results will be bound to restrict the application of geopolymeric RAC, and undoubtedly, any technique that can eliminate these weaknesses will greatly enhance the attractiveness of this new concrete.
Among many attempts aiming to enhance the mechanical and long-term performance of RAC or even to qualify RAC with structural purposes, external confinement by confining materials has been validated as an effective strategy [23], [24], [25], [26]. Moreover, using fiber-reinforced polymer (FRP) material, to provide external confinement, has attracted increasing attention because of its superiority, such as high strength-to-weight ratio, commendable thermo-mechanical, and excellent corrosion resistance. In the literature, studies of FRP-confined RAC have been documented well [27], [28], [29], [30], [31], [32], [33], [34], [35]. For instance, Xiao et al. [27] was the first who suggested using FRP composites to enhance RAC performance, in which RAC with different RA replacement percentages was confined by glass FRP tubes. Afterward, Zhao et al. [28] and Chen et al. [29] investigated the effects of the RA replacement ratio and FRP thickness on the compressive behavior of RAC confined by glass FRP and carbon FRP (CFRP), respectively. Xie and Ozbakkaloglu [32] recently compared the performance of FRP-confined RAC with circular cross-section and square cross-section. It was concluded that, under similar confinement levels, the circular specimens showed higher compressive strength but lower ultimate axial strain than the square ones.
Analogously, FRP confinement might also be able to provide beneficial effects to the performance of geopolymeric RAC. However, it should be pointed out here that, to the best of the authors’ knowledge, no study has been reported on the behavior of FRP-confined geopolymeric RAC. The only existing studies related were conducted by Ozbakkaloglu and Xie [36] and Lokuge and Karunasena [37], who studied the behavior of FRP-confined geopolymeric concrete. Based on the test results, it was found that under a given confinement ratio, FRP-confined geopolymeric concrete exhibited a similar strength enhancement to, but a lower axial strain enhancement than the counterpart of FRP-confined conventional concrete [36]. Similar results have also been observed when comparing the FRP-confined RAC and FRP-confined conventional concrete behaviors: specifically, the existing models for FRP-confined conventional concrete exhibited some discrepancies in predicting the behavior of FRP-confined RAC [30], [31], [32], [33], [38]. Therefore, it is evident that the behavior of FRP-confined geopolymeric RAC needs to be properly understood and modeled, before a safe and economical design approach can be developed.
Against this background, this study presents a preliminary study on the axial compressive behavior of sustainable geopolymeric RAC confined by CFRP composites, namely CFRP-confined geopolymeric RAC. Fly ash was utilized as the main geopolymeric binder owing to the wide availability, and also various contents of slag were incorporated to obtain the geopolymeric concrete with different strength [39]. Thus, the major test parameters include: (1) coarse aggregate type (i.e., NA and RA), (2) number of CFRP layers (i.e., 1, 2, and 3 layers), and (3) slag content (i.e., 0, 10%, 20% and 30% of the total binder by mass). Special attention is devoted to the stress-strain relationship, dilation behavior, and ultimate condition. The test results are also compared with the predictions by existing stress–strain models proposed for FRP-confined concrete to examine their applicability to CFRP-confined geopolymeric RAC. This study contributes to the potential use of geopolymeric RAC as structural concrete and also enriches the test database of FRP-confined concrete.
Section snippets
Testing specimen
Forty-eight CFRP-confined specimens were manufactured and tested, which covered two recycled coarse aggregate replacement ratios (i.e., 0% and 100%), four slag contents (i.e., 0, 10%, 20%, and 30%), and three thickness of CFRP jackets (i.e., 1, 2 and 3 layers). In addition, twenty-four unconfined control specimens with the same material and geometric properties to the CFRP-confined specimens were tested to establish the test-day unconfined concrete strengths. The specimen details are given in
Unconfined specimens
For unconfined specimens, the micro-cracks emerged on the surface of the cylinder when the applied load approached the peak stress, and then cracks extended to the central section with the displacement increasing. At the post-peak stage, the cracks developed from micro to macroscopic and crossed throughout the entire specimen. Finally, the cylinder failed with several major vertical cracks and the spallation of the lateral surfaces, as shown in Fig. 4. Besides, a more brittle failure process
Existing models
To realize a reliable and cost-effective design of CFRP-confined geopolymeric concrete, an accurate model for predicting its behavior is prerequisites. In the literature, numerous models have been proposed to predict the compressive strength and ultimate axial strain of FRP-confined concrete. For instance, 88 stress--strain models developed for FRP-confined concrete in circular sections have been reviewed and assessed by Ozbakkaloglu et al. [56]. The experimental results of the current study
Conclusion
Experimental investigations on the axial compressive behaviors of CFRP-confined sustainable geopolymeric RAC were conducted on the stress-strain relationship, the dilation behavior, and the ultimate condition. The feasibility of existing stress-strain models to CFRP-confined geopolymeric RAC was examined by the database collected in previous studies. Based on the results and discussion, the following conclusions can be drawn up as:
- (1)
For unconfined concrete, the RA replacement adversely affects
CRediT authorship contribution statement
Zhuo Tang: Formal analysis, Data curation, Methodology, Writing - original draft, Writing - review & editing. Wengui Li: Conceptualization, Formal analysis, Validation, Writing - original draft, Writing - review & editing, Funding acquisition, Supervision. Vivian W.Y. Tam: Validation, Writing - original draft, Writing - review & editing. Libo Yan: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
All the authors appreciate the financial supports from the Australian Research Council (ARC) (DE150101751; DP200100057; IH150100006), University of Technology Sydney Research Academic Program at Tech Lab (UTS RAPT), and University of Technology Sydney Tech Lab Blue Sky Research Scheme.
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