Dynamic compressive properties of lightweight rubberized geopolymer concrete
Introduction
The rapid development in transportation and growing population have witnessed a significant growth in automotive industry meeting the increasing consumer’s demand, resulting in substantial rubber waste from tyre disposal. In 2017, tyre waste was nearly proportional to tyre production with data revealing the annual tyre production went over 2.9 billion units worldwide [1]. Toxic gases with a detrimental effect on air quality may be caused by burning or using tyre as fuel [2] while the landfill of waste tyres without proper treatments results in environmental issues [3]. Therefore, there is a significant need to dispose of waste tyre rubber in a sustainable way by recycling them into different forms, for example crumb rubber, for other industrial purposes. Rubberized concrete, where coarse or fine aggregates are partially replaced by crumb rubber, has been widely investigated in previous studies [4], [5] due to its recycling potential. Compared with conventional concrete, rubberized concrete exhibits lower value in density [6], compressive strength [7], and modulus of elasticity [8]. Rubberized concrete is categorized as a lightweight material, which also covers rubberized foam concrete that has been developed recently and shows the capability of waterproofing [9] and acceptable mechanical properties [10], [11]. Also, the energy absorption capacity of rubberized concrete against impact loads is significantly improved [12], [13], [14], [15], which is attributed to the increased particle friction between cementitious matrix and crumb rubber particles [16]. The prominent energy absorption capacity has enabled rubberized concrete to enhance the structural elements’ impact resistance against dynamic and impact loads, including railway sleepers [17] and roadside barriers [18].
Ordinary Portland cement (OPC) is the main binding material for either conventional or rubberized concrete. Data shows the production of one tone OPC is associated with around one tonne of CO2 emission [19]. This leads to a public concern over the sustainability issue of concrete production. Geopolymer concrete has exhibited great potential to replace OPC-based concrete due to less CO2 emission. It is a type of synthetic material via the geopolymerization process, where alkaline solution acts as a chemical medium while OPC is replaced by fly ash (FA) and/or ground-granulated blast-furnace slag (GGBS) [20], [21], [22], [23], [24]. Several investigations have been directed at the rheological and mechanical properties of geopolymer concrete with aggregates partially replaced by crumb rubber, hereafter termed Rubberized Geopolymer Concrete (RuGPC) in this paper. Wongsa et al. [25] studied the effect of several parameters on the static compressive and flexural properties of fly ash-based geopolymer in which river sand was completely replaced by crumb rubber. It was demonstrated the 28-day compressive and flexural strengths of rubberized geopolymer were 93% and 78% lower than the control mixture (without rubber), respectively. For an individual geopolymer mixture, the compressive strength increased with curing age, and the rate of increase was primarily influenced by curing temperature, followed by sodium silicate to sodium hydroxide (Na2SiO3/NaOH) and alkaline solution to fly ash (AS/FA) ratio. Park et al. [26] investigated the compressive strength of rubberized geopolymer with different types of fly ash and replacement proportion of crumb rubber. It was found that for geopolymer concrete with various calcium oxide (CaO) contents in fly ash when the proportion of fine aggregates replaced by crumb rubber increased from 5% to 20%, the axial compressive strength of the geopolymer concrete showed a downward trend. Aslani et al. [27] proposed the lightweight RuGPC with the incorporation of crumb rubber and polystyrene simultaneously. It was reported that a 20% replacement of 2–5 mm crumb rubber resulted in a decrease of approximately 30% in both the compressive and flexural strengths at 28 days. Similar research was conducted by Zhong et al. [28] who studied engineering properties of RuGPC reinforced with steel fibers. The results showed the addition of up to 1% steel fibers improved the compressive and flexural strengths of RuGPC significantly. Table 1 provides a summary of previous investigations on the mechanical properties of RuGPC.
Differences in response characteristics of concrete subjected to dynamic and quasi-static loads can be attributed to strain rate effect [30]. Several factors such as inertial effect [31] and viscosity effect [32] have a major influence on the dynamic behavior of concrete. The size and percentage of coarse aggregates in concrete also have an effect on dynamic behaviors of concrete subjected to high strain rate [33], [34]. To quantify the strength improvement of rubberized geopolymer concrete at various strain rates, the ratio of dynamic compressive strength to quasi-static one is calculated, which is referred as dynamic increase factor (DIF). Several empirical formulae were suggested to estimate the compressive DIF for conventional OPC concrete [35], [34], [36]. In addition, Hao et al. [31] found the dynamic compressive strength could be influenced by the lateral inertia confinement and the end friction confinement. The Split Hopkinson pressure bar (SHPB) test is a popular dynamic test apparatus, which is widely used to derive DIFs for concrete-like materials at intermediate-to-high strain rates [37], [38], [39], [40]. However, very few SHPB tests were conducted on the geopolymer concrete incorporated with crumb rubber. While previous studies [25], [26], [27], [28] have built the foundation for understanding the quasi-static compressive strength of RuGPC, investigations on the dynamic compressive strength of the same material are still limited. Therefore, it is important to carry out more experimental studies on the dynamic characteristics of RuGPC to unveil its dynamic performances for various practical applications.
In this study, both coarse and fine aggregates in rubberized geopolymer concrete were partially substituted by crumb rubber (0%, 15%, and 30% by volume). The effects of crumb rubber on the compressive strength of RuGPC were investigated through quasi-static and dynamic tests. The SHPB tests were adopted to investigate the progressive failure, crack development, and dynamic compressive strength of RuGPC at various strain rates from 50 s−1 to 136 s−1. The quasi-static and dynamic strengths, failure process, failure modes, and the energy absorption capacity of mixtures with different rubber contents were compared and discussed. For practical applications and design, empirical formulae for estimating DIF of RuGPC compressive strength at different strain rates were derived.
Section snippets
Materials and mixture design
Rubberized geopolymer concrete is composed of geopolymer mortar matrix, traditional natural aggregates, and crumb rubber. Three RuGPC mixtures were designed with different replacement ratios by volume (0%, 15%, and 30%). The details of RuGPC are given in Table 2, where 15% RuGPC and 30% RuGPC represent 15% and 30% by volume replacement of both coarse and fine aggregates by crumb rubber, respectively. The weight ratio of FA to GGBS was set as 3:2 to obtain a desired compressive strength, as
Quasi-static compressive strength and failure mode
The density of different mixtures was measured before testing and results showed 0% RuGPC, had the highest density of 2340 kg/m3, followed by 15% RuGPC (2125 kg/m3) and 30% RuGPC (1980 kg/m3). The 28-day compressive strength of the standard cylinders of rubberized geopolymer concrete was investigated through the quasi-static compression tests. As illustrated in Fig. 3, RuGPC with 0% and 15% rubber content had the compressive strength of 54.0 MPa (STD = 0.7) and 26.2 MPa (STD = 0.6),
Conclusions
This study investigates the dynamic compressive properties of geopolymer concrete, with up to 30% natural aggregates substituted with crumb rubber, by conducting SHPB tests. RuGPC has exhibited significant sensitivity to strain rate with the strain rate up to 136 s−1. The main findings can be summarized as follows:
- 1.
The impact resistance of geopolymer concrete under high loading rates was improved with the increase of rubber content. Given similar strain rate, RuGPC remained relatively intact
CRediT authorship contribution statement
Thong M. Pham: Data curation, Conceptualization, Investigation, Methodology, Writing - original draft, Supervision, Writing - review & editing. Junli Liu: Methodology, Investigation, Writing - original draft. Phuong Tran: Conceptualization, Writing - final draft, Supervision. Voon-Loong Pang: Data curation, Conceptualization, Investigation, Writing - original draft. Feng Shi: Data curation, Investigation. Wensu Chen: Conceptualization, Writing - review & editing. Hong Hao: Funding acquisition,
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
The authors would like to acknowledge Australian Research Council, ARC Laureate Fellowships FL 180100196. The European Union’s Horizon 2020 Research and Innovation Program (grant no. 777823) is appreciated. The donated rubber aggregates from Tyrecycle are appreciated.
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