Experimental study on engineering properties of concrete reinforced with hybrid recycled tyre steel and polypropylene fibres
Introduction
Given the fact that normal concrete is inherently brittle making it unsuitable for certain applications, e.g. structural elements subjected to potential dynamic loadings, fibre reinforced concrete (FRC) incorporating randomly distributed short fibres such as steel, polypropylene and glass fibres is introduced to mitigate the brittleness and potential cracking problem of conventional concrete. Nevertheless, mono fibre reinforcement (i.e. concrete reinforced with only one type of fibre) can only improve either strength or ductility of FRC (Pakravan et al., 2017). For instance, the incorporation of fibres with high modulus and strength such as steel and carbon fibres into concrete can effectively improves its strength while its ductility is not enhanced owing to the brittle nature of the fibres (Pakravan et al., 2017). However, utilizing polymeric fibres such as polypropylene fibre with low strength into concrete can significantly improve its ductility, crack-resistant behaviour and corrosion resistance (Feng et al., 2019; Pakravan and Ozbakkaloglu, 2019). Thus, applying hybrid fibre reinforcement (i.e. concrete reinforced with two or more different types of fibre) in concrete is considered as a promising solution to further improve the mechanical and durability performances of FRC through integrating the function of each reinforced fibre.
Among all the combinations of hybrid short fibre reinforcement in normal concrete, the combination of steel fibre (SF) and polypropylene fibre (PPF) is found to be the most effective one in improving the overall properties of the composite especially strength and ductility. In the past two decades, numerous studies (Afroughsabet et al., 2018; Afroughsabet and Ozbakkaloglu, 2015; Almusallam et al., 2016; Caggiano et al., 2016; de Alencar Monteiro et al., 2018; Feng et al., 2019; Li et al., 2018; Li et al., 2019a; Li et al., 2019b; Qian and Stroeven, 2000; Suhaendi and Horiguchi, 2006; Sun et al., 2001; Yao et al., 2003; Yermak et al., 2017) investigated the effect of hybrid SF and PPF with the total fibre volume fraction (Vf) of 0.5–3.6% on the mechanical properties and durability of FRC. For instance, Yao et al. (2003) examined the mechanical properties of concrete reinforced with hybrid hooked-end SF and PPF at the fibre dosage of 0.5% Vf and found that the incorporation of hybrid SF and PPF could create a synergetic effect in improving both strength and ductility of FRC as compared with the plain mixture, which is in agreement with de Alencar Monteiro et al. (2018) and Li et al. (2018) that both peak and post-peak behaviour of concrete reinforced with appropriate dosages of SF and PPF were significantly enhanced. It was also found that the utilization of both SF and PPF into concrete prevents it from spalling under fire attack mainly owing to the increased permeability (Li et al., 2019a, 2019b). However, a huge amount of raw materials (e.g. fossil fuel) and energy is required to produce the aforementioned fibres (Mastali et al., 2018a, 2018b; Onuaguluchi and Banthia, 2018), where the cost of the resultant composite increases with the reduction of sustainability, i.e. inevitable greenhouse gas emissions during the production of steel (Liew and Akbar, 2020). To mitigate the environmental issues and threat of natural resource shortage, an increasing number of studies have been carried out to utilize the recycled materials as the reinforcing fibre for concrete and among them, recycling the materials from the end-of-life tyres, e.g. steel and polymeric fibres to replace the manufactured fibres is one of the most recent attempts.
It was reported that more than 500 million waste tyres are landfilled (Thomas and Gupta, 2016) while limited land disposal sites would become one of the serious threats to human society (Wang et al., 2019). The accumulated solid tyre waste could also pose several challenges to the environment as it may induce fire or disease (Ramarad et al., 2015; Zhong et al., 2019). However, through certain processing stages, rubber particles, steel and polymer fibres can be recovered (Gigli et al., 2019), which provides a potential way to effectively recycle a majority of waste tyres around the world. It is worth noting that steel is a major component of a tyre which accounts for around 13%–27% (Ramarad et al., 2015), implying that the effective usage of recycled tyre steel materials could significantly mitigate the potential problems caused by the waste tyres. For instance, recycling of waste tyres could stop around 1.52 tons of CO2 emissions yearly (Liew and Akbar, 2020). Up to now, many studies (Aiello et al., 2009; Centonze et al., 2012; Frazão et al., 2019; Grzymski et al., 2019; Leone et al., 2018; Skarżyński and Suchorzewski, 2018; Zamanzadeh et al., 2015) examined the effect of recycled tyre steel fibre (RTSF) content on the physical, mechanical and durability properties of FRC with a primary aim of seeking whether the manufactured steel fibre (MSF) could be replaced by RTSF. The majority of the studies indicated that the flexural behaviour particularly post-cracking performance of concrete reinforced with RTSF was comparable to that reinforced with MSF considering certain fibre content. Nevertheless, Skarżyński and Suchorzewski (2018) concluded that concrete mixture reinforced with RTSF exhibited poorer properties compared to that reinforced with MSF under the same fibre volume fraction because of the irregular dimension of RTSF. Grzymski et al. (2019) reported similar observation that the inclusion of RTSF led to the reduced energy absorption capacity of FRC after cracking in comparison with the addition of MSF, suggesting that the use of RTSF with an appropriate content in normal concrete is essential. On the other hand, an increasing number of studies (Bjegovic et al., 2014; Caggiano et al., 2017; Hu et al., 2018; Martinelli et al., 2015; Mastali et al., 2018a, 2018b; Onuaguluchi and Banthia, 2018) focused on the overall properties of concrete reinforced with hybrid MSF and RTSF, which found that partial replacement of MSF by RTSF could create synergistic effects leading to the improvements in tensile strength, ductility and impact resistance whereas the properties of the resultant FRC were negatively influenced when the content of RTSF was excessive. Frazão et al. (2019) concluded that RTSF was more susceptible to corrosion than MSF, which may hinder the application of RTSF-FRC in aggressive environment, e.g. marine. It was reported that PPF has better performance to resist corrosion and improve the ductility. Thus, integrating PPF with RTSF has potential to mitigate the aforementioned issues drawn by RTSF while maintaining the benefits induced by RTSF, e.g. strength. To the best of the authors’ knowledge, only one relevant study (Mastali et al., 2018a) evaluated the effect of hybrid RTSF and PPF on the mechanical properties of self-consolidating concrete with the total fibre content of 1.5% Vf. However, through assessing the effects of three different hybrid combinations, i.e. 0.5% Vf of PPF +1.0% Vf of RTSF, 0.75% Vf of PPF +0.75% Vf of RTSF, and 1.0% Vf of PPF +0.5% Vf of RTSF, it was observed that excessive content of PPF weakened the mechanical properties of FRC mainly due to the weak bonding between fibre and matrix (Mastali et al., 2018a), implying that PPF was not suitable to be used as primary fibre in hybrid fibre reinforcement, i.e. accounting for the major part of hybrid system. However, durability-related properties (e.g. resistance to chloride migration) were not assessed and more different hybrid combinations are required. In order to further investigate the feasibility of combining RTSF as sustainable material with synthetic PPF in concrete, it is vital to conduct an extensive research on the effect of more hybrid combinations (i.e. RTSF as primary fibre) on the engineering properties of concrete.
The main purpose of this study is to provide a comprehensive understanding of the effect of hybrid RTSF and PPF on the engineering properties of concrete considering five hybrid combinations with a total fibre content of 1.0% Vf. Concrete mixtures without fibre reinforcement and reinforced with 1.0% Vf were considered as the reference mixtures. Firstly, a series of tests were carried out to estimate the effects of mono and hybrid fibre reinforcements on the workability, compressive strength, flexural behaviour and drying shrinkage of concrete. The evolution of full strain field under different loading levels and flexural failure of studied mixtures were explored using a non-contact measurement, i.e. digital image correlation (DIC). The interaction between fibre and matrix at the cracking zone was then observed using a digital microscope. Finally, rapid chloride migration (RCM) test was conducted to estimate the effect of different hybrid fibre reinforcements on the durability-related performance of concrete.
Section snippets
Raw materials
CEM I 52.5N Portland cement was used as the main binder material in this study, the fineness of which was 366 m2/kg and its chemical composition is listed in Table 1. Fine and coarse aggregates were Thames valley sand and crushed granites, respectively, which were used in saturated surface dry (SSD) condition whilst mixing. Modified polycarboxylate-based superplasticisers were used to improve the fluidity of mixtures, where the content was set as 0.55% by the mass of cement content. RTSF (see
Workability
Fig. 5 shows the slump of all mixtures that corresponds to the workability of concrete. It can be observed that the addition of fibres reduced the slump of concrete independent of fibre reinforcement type (mono or hybrid). RS1.0 presented a slump of only 90 mm, which was around 59.1% lower than that of concrete without fibre incorporation (F0). The reduction in workability caused by either RTSF or PPF incorporation agreed well with previous studies on RTSF (Centonze et al., 2012; Baricevic
Conclusions
This study investigated the engineering properties of concrete reinforced with recycled tyre steel fibre (RTSF) and polypropylene fibre (PPF) to evaluate the feasibility of using RTSF as primary fibre in hybrid fibre reinforced concrete with a total fibre content of 1.0% Vf. Based on the experimental results, the conclusions can be drawn as follows:
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Regardless of fibre reinforcement type (mono or hybrid), the incorporation of fibres reduced the workability of concrete. RTSF resulted in more
CRediT authorship contribution statement
Hui Zhong: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft. Mingzhong Zhang: Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that there is no conflict of interest.
Acknowledgement
The authors gratefully acknowledge the financial support from the Engineering and Physical Sciences Research Council (EPSRC), United Kingdom under Grant Nos. EP/R041504/1 and EP/N509577/1 (project reference: 1836739). The financial support provided by University College London (UCL) and China Scholarship Council (CSC) to the first author is gratefully acknowledged. The authors would like to thank Mr. Warren Gaynor, Dr. Shi Shi, and Mr. Raman Mangabhai for their help with experiments.
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