Elsevier

Engineering Structures

Volume 224, 1 December 2020, 111246
Engineering Structures

Mechanical performance of CFRP-confined sustainable geopolymeric recycled concrete under axial compression

https://doi.org/10.1016/j.engstruct.2020.111246Get rights and content

Highlights

  • The RA replacement adversely affects the compressive strength and elastic modulus of geopolymeric concrete.

  • CFRP confinement has a significant enhancement on both strength and ductility of geopolymeric concrete.

  • The increase of RA replacement results in decreased compressive strength and reduced volumetric dilation.

  • The aggregate types and slag contents do not have a significant effect on the CFRP confinement effectiveness.

  • The CFRP confinement effectiveness for the ultimate axial strain is affected by the aggregate type and slag content.

  • The proposed models can accurately predict the ultimate conditions of CFRP-confined geopolymeric concrete.

Abstract

Sustainable geopolymeric recycled aggregate concrete (RAC) by utilizing environmentally-friendly binder-geopolymer and constructional solid waste-recycled aggregate (RA) will facilitate the sustainability in concrete industry. This study investigated the compressive behavior of sustainable geopolymeric RAC confined by carbon fiber-reinforced polymer (CFRP) jackets. A total of 72 cylindrical fly ash/slag-based geopolymeric concrete specimens, including 48 CFRP-confined specimens and 24 unconfined specimens were fabricated and tested. The testing variables included: coarse aggregate type (i.e., natural aggregate and RA), thickness of CFRP jackets (i.e., 1, 2, and 3 layers) and (iii) slag content (i.e., 0, 10%, 20% and 30% of the total binder by mass). The results indicate that the CFRP confinement  remarkably enhances the compressive strength and ultimate strain of geopolymeric concrete, and the enhancement is more pronounced with the increase of CFRP jacket thickness. Moreover, the RA replacement and the inclusion of slag have minor influences on the CFRP confinement performance for the compressive strength, but have obvious effects on the CFRP confinement performance for the ultimate axial strain. Based on the test results, empirical stress and strain models were proposed to predict the ultimate condition of the CFRP-confined geopolymeric concrete.

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.

References (65)

  • M. Sandanayake et al.

    Greenhouse gas emissions of different fly ash based geopolymer concretes in building construction

    J Cleaner Prod

    (2018)
  • C. Shi et al.

    Performance enhancement of recycled concrete aggregate – a review

    J Cleaner Prod

    (2016)
  • N. Kisku et al.

    A critical review and assessment for usage of recycled aggregate as sustainable construction material

    Constr Build Mater

    (2017)
  • J. Xie et al.

    Coupling effects of recycled aggregate and GGBS/metakaolin on physicochemical properties of geopolymer concrete

    Constr Build Mater

    (2019)
  • Z. Tang et al.

    Uniaxial compressive behaviors of fly ash/slag-based geopolymeric concrete with recycled aggregates

    Cem Concr Compos

    (2019)
  • P. Nuaklong et al.

    Recycled aggregate high calcium fly ash geopolymer concrete with inclusion of OPC and nano-SiO2

    Constr Build Mater

    (2018)
  • F.U.A. Shaikh

    Mechanical and durability properties of fly ash geopolymer concrete containing recycled coarse aggregates

    Int J Sustainable Built Environ

    (2016)
  • Y. Hu et al.

    Physical-mechanical properties of fly ash/GGBFS geopolymer composites with recycled aggregates

    Constr Build Mater

    (2019)
  • A. Wongsa et al.

    Use of crushed clay brick and pumice aggregates in lightweight geopolymer concrete

    Constr Build Mater

    (2018)
  • R. Navarro et al.

    Mechanical properties of alkali activated ground SiMn slag mortars with different types of aggregates

    Constr Build Mater

    (2018)
  • J. Xu et al.

    Prediction of triaxial behavior of recycled aggregate concrete using multivariable regression and artificial neural network techniques

    Constr Build Mater

    (2019)
  • Y.-C. Tang et al.

    Study of seismic behavior of recycled aggregate concrete-filled steel tubular columns

    J Constr Steel Res

    (2018)
  • C. Gao et al.

    Strength and ductility improvement of recycled aggregate concrete by polyester FRP-PVC tube confinement

    Compos B

    (2019)
  • J. Xiao et al.

    Mechanical properties of confined recycled aggregate concrete under axial compression

    Constr Build Mater

    (2012)
  • G.M. Chen et al.

    Behavior of CFRP-confined recycled aggregate concrete under axial compression

    Constr Build Mater

    (2016)
  • C. Gao et al.

    Behavior of glass and carbon FRP tube encased recycled aggregate concrete with recycled clay brick aggregate

    Compos Struct

    (2016)
  • T. Xie et al.

    Behavior of recycled aggregate concrete-filled basalt and carbon FRP tubes

    Constr Build Mater

    (2016)
  • B. Yan et al.

    Behavior of flax FRP tube encased recycled aggregate concrete with clay brick aggregate

    Constr Build Mater

    (2017)
  • L. Zeng et al.

    Compressive test of GFRP-recycled aggregate concrete-steel tubular long columns

    Constr Build Mater

    (2018)
  • T. Ozbakkaloglu et al.

    Geopolymer concrete-filled FRP tubes: behavior of circular and square columns under axial compression

    Compos B

    (2016)
  • I. Martinez-Lage et al.

    Sustainability evaluation of concretes with mixed recycled aggregate based on holistic approach: technical, economic and environmental analysis

    Waste Manag

    (2020)
  • C. Thomas et al.

    Micro- and macro-porosity of dry- and saturated-state recycled aggregate concrete

    J Cleaner Prod

    (2019)
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