Development of ultra-high performance geopolymer concrete (UHPGC): Influence of steel fiber on mechanical properties

Abstract

This study reports the development of ultra-high performance geopolymer concrete (UHPGC) and overcoming the brittleness feature of geopolymer matrix by using different steel fibers. Four straight steel fibers with different aspect ratios and two different deformed steel fibers were investigated. Flowability, compressive strength and flexural behavior including strengths and deflection, and energy absorption capacity of UHPGC, were systematically evaluated. A deformation ratio of steel fiber was introduced to quantitatively correlate the steel fiber shape and the mechanical performance. The flowability of fresh UHPGC mixtures decreased when the fiber content and length increased, as expected, and was inconspicuously influenced by fiber shape. The increase in fiber content and the decrease of fiber diameter contributed to the improvement of the mechanical strengths of UHPGC. The flexural behaviors of UHPGC improved as the fiber volume and length increased, while the compressive and first crack strengths were affected by both curing conditions and fiber dosages as well. Different from Portland cement-based composites, the corrugated fibers with a higher deformation ratio added in UHPGC, had an inferior strengthening and toughening efficiency, while for straight fibers, those longer and smaller in diameter were more preferred. Finally, based on the previous research, a new one with adjustment and simplification was proposed for that of newly-developed UHPGC, and the fitted results had higher correlation coefficients (r²).

Introduction

Ultra-high performance concrete (UHPC) is a cement-based material with superior characteristics of ultra-high compressive strength over 150 MPa, good tensile ductility, high toughness and superior durability. UHPC are usually manufactured through the use of large amount of Portland cement (PC) and small amount of water, the addition of silica fume, quartz powder, superplasticizer and steel fibers, and thermal curing [1,2]. However, the heavy use of PC decreases the sustainability of UHPC due to high energy consumption and carbon emission of PC manufacturing [3,4]. Compared to normal PC concrete, the PC content in UHPC is about 3-fold, which means embedded carbon emissions of 600–1300 kg/m³. On the other hand, replacing PC with alternative low-carbon binders is considered as an effective way of achieving the sustainability of construction materials [5,6]. Geopolymer binders were clinker-free binders having comparable or superior performance to PC [7,8]. It uses aluminosilicate sources such as ground granulated blast furnace slag [9,10], fly ash [11,12], metakaolin [13] and waste glass [14], with activation alkaline activators such as sodium hydroxide (NaOH) and sodium silicate [15,16]. Zhang et al. [17] pointed out that the unit energy consumption and CO₂ emission of geopolymer binders were about 60% lower than that of PC.

Recently, uses of geopolymer binders to manufacture ultra-high performance geopolymer concrete (UHPGC) with steel fibers, have been attempted [[18], [19], [20]]. This novel material possesses comparable performances to that of UHPC. Ambily et al. [18] reported the successful production of slag-based UHPGC with the highest compressive strength of 175 MPa by incorporating steel fibers under ambient temperature curing. Wetzel et al. [19] also reported that with incorporations of metakaolin and silica fume, slag-based geopolymer concrete could obtain reach 180 MPa through refining the pore structure.

One feature of high strength geopolymer binder is its brittleness, which leads to brittle failure [[21], [22], [23]]. However, it could be solved by using steel fibers [[24], [25], [26]]. Various factors related to steel fibers such as dosage, size, shape, distribution and casting direction, affect their contributions. Numerous researches on UHPC have shown that compressive, flexural and tensile strengths could be improved when steel fiber content and length increase [27,28]. The bond properties and pullout behavior of steel fibers in UHPC are influenced by their shape [29,30]. Yoo et al. [31] reported that the pullout strength of UHPC could be improved by using deformed fibers. On the other hand, many literatures reported that the combination of geopolymer concretes and steel fibers was more effective than traditional binder systems due to strong bond strength between the matrix and fibers [[32], [33], [34]]. Ding et al. [35] reported that the increase of steel fiber content resulted in the increase of the compressive strengths of slag-based geopolymer mortars. Aydin et al. [36] found that length of steel fiber had more significant effect on the flexural performance of high strength slag/silica fume-based geopolymer mortars than its dosage. Bhutta et al. [26] reported that geopolymer composite with length-deformed steel fiber exhibited a lower flexural toughness than that with straight and end-deformed fibers due to the higher brittleness of geopolymer matrix [34]. Koeing et al. [37] noticed although a post-hardening behavior was observed with the use of steel fiber, the compressive strengths decreased owing to the decrease of workability of geopolymer mixtures. However, compared with the extensive researches on the effect of steel fiber on the mechanical performance of UHPC, limited investigations have been conducted on UHPGC. Due to a fact that steel fiber is the most expensive components in UHPC [38], understanding the similarity and difference of the role of steel fiber between UHPC and UHPGC is required.

For the structural design of UHPC in practice, constitutive model is required. There have been many published researches on the relationship between uniaxial compressive, flexural and tensile constitutive of UHPC, which, to a certain extent, can promote the application of UHPC [27,39,40]. However, no constitutive model for flexural load-deflection curve of UHPGC, which can also provide a quantitative comparison with those of UHPC, could be identified so far.

In order to better understand the reinforcing and toughening efficiency of steel fiber on UHPGC, the influences of steel fibers on flowability, compressive strengths and flexural behavior of UHPGC are systematically investigated. Six different types of steel fibers with different volume fractions (0%, 1%, 2% and 3%), lengths (6, 8 and 13 mm), diameters (0.12 and 0.20 mm), aspect ratios (6/0.12, 8/0.12, 13/0.12 and 13/0.20) and shapes (straight, hooked-end and corrugated) will be used and studied. Flexural behavior including first and peak crack behaviors, toughness and toughness indexes will be assessed as well. Finally, it proposes a constitutive model for flexural behavior of UHPGC according to the flexural load-deflection curves.