Development of ecological strain-hardening cementitious composites incorporating high-volume ground-glass pozzolans
Introduction
Strain-hardening cementitious composite (SHCC) also referred to as engineered cementitious composite (ECC) represents a relatively new class of high-performance fiber-reinforced cementitious composites (HPFRCC). The most salient features of SHCC are its exceptional tensile strain capacity of 2–11% [200–1100 times that of normal concrete or conventional fiber-reinforced concrete (FRC)], tight multiple cracking with crack width below 60 µm at 1% strain, and strain-hardening behavior with relatively low fiber volume content, typically below 2% [1], [2], [3], [4], [5], [6], [7], [8]. The design of SHCC is guided by micromechanical principles which systematically account for the mechanical interaction between the three major composite constituents (fiber, matrix, and interface properties) when the composite is loaded [9]. This design approach ensures that upon first cracking under uniaxial tension, SHCC strain-hardens in contrast to common FRC types which rather tension-soften after first cracking. Thus, SHCC achieves remarkable ductility elevating it to a metal-like composite exhibiting significant fracture toughness approaching that of aluminum alloys [10]. These features motivated the use of SHCC in several structural applications including bridge deck link slabs [11], bridge deck overlays, dam repairs and coupling beams in high-rise buildings [12], [13].
Early generations of SHCC used silica fume as supplementary cementitious material (SCM) and polypropylene fibers for reinforcement [1], [14]. Later, for cost-effectiveness and ecoefficiency measures, SHCC with high-volume fly ash (HVFA) emerged and polyvinyl alcohol fibers (PVA) were utilized [15]. As such, for several years, HVFA has been intensively used for SHCC formulation [15], [16], [17], [18], [19]. The use of FA in SHCC enhances the workability due to the slower reactivity of FA, additional to the spherically-shaped smooth-textured FA particles reducing inter-particle friction and enhancing mixture flowability [20]. The use of FA also reduces the heat of hydration and enhances composite durability through long-term development of mechanical strength by pozzolanic activity [15], [18], [21]. HVFA in SHCC can also dilute the concentration of Al3+and Ca2+ cations [15], [27]. Since the latter has a strong propensity for bonding to the hydrophilic PVA fibers [22], [27], the diluting effect of FA reduces fiber/matrix chemical bonding, increases matrix toughness, and contributes towards higher composite ductility [15].
Recently, however, to further foster the eco-efficiency aspects of SHCC, increasing attention is being paid to developing SHCC with local ingredients. This includes development of SHCC incorporating other SCM such as ground granulated blast furnace slag [23], [27] and metakaolin [24], [27] or SHCC with fillers such as limestone powder [25], [26], [27]. Furthermore, due to recent environmental policies in the energy sector in North America, coal-fired power plants are in decline and so is the supply of FA [27]. Therefore, locally available SCMs are becoming more attractive as recommended by the Canadian Standards Association (CSA) [28]. Valorizing locally available alternative SCMs in SHCC formulation can reduce the CO2 invoice of SHCC and may contribute towards localizing its ingredients, hence spreading its implementation. One such SCM in Québec (Canada) is the ground-glass pozzolan (GP) (alternatively known as glass powder) obtained by grinding waste glass with low recycling value. In Québec, for instance, only 49% of waste glass was recycled (2008) [29], the rest which is not recycled (due to either breaking, colour mixing, or expensive recycling cost) is often landfilled, thereby causing a serious environmental burden [30]. Thus, valorization of post-consumer glass by grinding it into GP produces an alternative SCM while solving an obvious environmental problem.
Several years of research and development on GP at Université de Sherbrooke demonstrated successful partial replacement of cement by GP in laboratory conditions as well as in situ for various types of concrete including, regular concrete, self-consolidating concrete, and ultra-high-performance concrete [31], [32], [33]. These efforts also catalyzed the emergence of an industrial-scale waste glass recycling facility in 2014 at the city of Lachute (QC, Canada). The grinding plant produces 30 000 tons of GP at the desired fineness for different applications including valorization in concrete. Following successful demonstration of the performance of GP concrete in various real-life projects [34], and following the production of GP in industrial quantities, GP has now been accepted as a standard SCM by CSA (CSA3000-18) [28].
Beside the eco-efficiency aspects of using GP in concrete, further salient features of concretes incorporating GP are: (i) the higher strength owing to the intrinsic strength and hardness of GP particles [35], the pozzolanic activity forming pozzolanic C-S-H with very low calcium to silicium (C/S) ratio [36], and the filler effect through which GP particles can contribute to increasing matrix strength by enhancing its compactness (void filling effect) [35], (ii) the increased durability owing to the tortuosity, pore refinement, and microstructure densification via pozzolanic activity [37], [38], and (iii) the reduced alkali-silica reaction (ASR), particularly when finer GP is used [39] or added in conjunction with fly ash or slag [40].
Therefore, it is legitimate to believe that the above various advantages of GP can be leveraged to produce SHCC incorporating high-volume ground-glass pozzolan (HVGP-SHCC) with enhanced performance while promoting sustainability. For this, it is hypothesized herein that (subject to optimizing GP content in SHCC), the filler effect, the pozzolanic activity, and the intrinsic high strength of GP particles may allow producing SHCC with higher strength while valorizing an otherwise waste material contributing to environmental pollution. Thus, the current study aims at producing SHCC with full or partial replacement of FA with GP such that the resulting SHCC exhibits higher strength while satisfying pseudo-ductility requirements.
While GP has been incorporated successfully in the past in various types of concretes and resulted in enhanced mechanical properties and durability as highlighted above, its utilization for micromechanically designed SHCC is a rather new research frontier. Some noteworthy attempts have been made to investigate the microstructure, mechanical performance, durability, and self-healing characteristics of SHCC incorporating GP [41], but without attempting the micromechanical properties nor the uniaxial tensile behavior which are fundamental for this type of concrete [1]. Thus, to the knowledge of the authors, there is no published work addressing the design of HVGP-SHCC using the fundamental micromechanical approach. Research outcomes are expected to contribute to the development of high-performance green cement composites necessary for sustainable and resilient infrastructure systems.
Section snippets
Framework for the development of HVGP-SHCC
Pseudo-ductility performance criteria require SHCC mixtures to be designed such that the three composite constituents (matrix, fiber, and fiber/matrix interface) are tailored to obtain a composite exhibiting a smooth transition from the quasi-brittle behavior (of conventional fiber-reinforced cement composites) to a metal-like behavior [14]. This transition is characterized by: (i) a sustained or even higher load carrying capacity after matrix first cracking associated with the steady-state
Tailoring the formulation of HVGP-SHCC
SHCC mixtures developed in this study went through a two-stage tailoring process as highlighted earlier. The first concerns the optimization of matrix packing density whereby particle packing optimization was employed to carry a guided replacement of FA with GP in order to primarily control matrix fracture properties (elastic modulus, tensile strength, and toughness) and implicitly the distribution of initial flow size. Both parameters are intrinsic determinants of pseudo-ductile behavior of
Experimental program
The experimental program involved two phases. The first phase was conducted on suspended mortars and dealt with the effect of GP on micromechanical parameters [matrix elastic modulus (E), fracture toughness (Km), ultimate tensile strength (σfc), and fiber/matrix interface properties]. The second phase concerned the performance of SHCC at the composite level where the effect of different GP contents on composite compressive strength, flexural capacity, and direct tensile behavior was evaluated.
Optimization of matrix packing density
The packing density for the different SHCC formulations is reported in Table 2. Results indicate that incorporating GP increased the packing density from 64% in the reference SHCC (with no GP) to 66, 70, 72, 73, and 74%, respectively, in the SHCC mixtures with 20, 40, 60, 80, and 100% replacement of FA with GP. This is further substantiated by the combined particle-size distribution (PSD) for the different SHCC formulations depicted in Fig. 10. The figure indicates that at an increasing
CRediT authorship contribution statement
Ousmane A. Hisseine: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. Arezki Tagnit-Hamou: Funding acquisition, Project administration, Resources, Supervision, Validation.
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.
Acknowledgments
This project is jointly supported by a Cooperative Research and Development (CRD) grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Graduate Scholarship (CGS) program (Vanier Scholarship award no: 360284). The authors are grateful to the financial support from all these partners.
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