1 Introduction
Recently, the demand for sustainable construction materials is increasing, particularly with the aim of reducing CO
2 emissions. One of the emerging approaches to this problem is to incorporate plant-based fibers, such as kenaf, hemp, and jute, into polymer composites or concrete. This can enhance the strain capacity, toughness, and crack resistance under tension (Elsaid et al.,
2011; Ramakrishna & Sundararajan,
2005). Natural fibers are affordable and promising renewable resources that can enhance the mechanical properties of concrete (Chin & Yousif,
2009; Roma et al.,
2008). Many studies on natural fiber-reinforced cement composites (NFRCCs) have been performed, and these have examined the mechanical properties (Fan et al.,
2012; MacVicar et al.,
1999), cement hydration (Gwon et al.,
2021; Vaickelionis & Vaickelioniene,
2006), setting time (Choi & Choi,
2021; Gwon et al.,
2021), and internal curing (Dávila-Pompermayer et al.,
2020; Jongvisuttisun et al.,
2018) of composite materials, as well as the use of different surface treatments for natural fibers (Li et al.,
2007; Valadez-Gonzalez et al.,
1999). Specifically, kenaf and jute fibers, which are classified as bast fibers, are the most popular natural materials in this field because of their abundancy in Asia-Pacific countries and the United States (Market Research Future,
2020) and these fibers have relatively high tensile strength compared to other types of natural fibers (Li et al.,
2007; Saheb & Jog,
1999). Many studies have also been conducted on NFRCCs that incorporate hemp (Awwad et al.,
2012; Li et al.,
2004), sisal (de Andrade Silva et al.,
2010; Wei & Meyer,
2014), coir (Li et al.,
2006), banana (Zhu et al.,
1994), and flax (Sawsen et al.,
2014,
2015) fibers.
Due to their hygroscopic characteristics, the water absorption capacity of natural fibers is crucial for the production of concrete (Gwon et al.,
2021; Jongvisuttisun et al.,
2018). If the exact amount of water absorbed by natural fibers is considered as additional water in the mix proportions, the reduction in concrete workability owing to the loss of mix water by the fibers can be avoided (Jongvisuttisun et al.,
2018). However, few studies have quantitatively explored this issue (Gwon & Shin,
2021; Gwon et al.,
2021). Gwon et al., (
2021) estimated the water absorption capacity of kenaf microfibers using vacuum membrane filtration method. They found that mortar mixtures containing kenaf microfibers in the saturated surface dry (SSD) condition had a similar range of flow diameters, albeit with different fiber contents. Thus, the mix water quantity remained unchanged in the fresh mortar mixture, aside from the portion of the microfibers. Furthermore, Gwon and Shin, (
2021) determined that the rheological properties of cement pastes could be modified using kenaf microfibers under SSD conditions. However, aside from that of Yun et al., (
2022), there have been no rheological studies examining natural fiber-reinforced concrete, including coarse aggregates.
One of the possible applications of NFRCCs is shotcrete. Several researchers have used hemp fibers in concrete and shotcrete (Morgan et al.,
2017), and the resulting materials exhibited reduced plastic and drying shrinkage compared to those using steel or polypropylene fibers. Shotcrete is a mortar or concrete pneumatically projected onto a surface using a hose at a construction site (Banthia,
2019). Thus, shotcrete typically needs to have relatively low plastic viscosity and high yield stress. It is assumed that such requirements can be met by incorporating natural fibers in cement composites. When natural fibers are used in shotcrete, the water absorbed by the fibers is assumed to be squeezed out under pumping pressure. In turn, the water released from the fibers induce better pumpability. Once the mixture is cast and freed from pressure, the fibers absorb the released water again, leading to better shootability and less rebound. The absorbed water may eventually be used for internal curing (Ahn & Shin,
2018; Dávila-Pompermayer et al.,
2020; Jongvisuttisun et al.,
2018; Mezencevova et al.,
2012). The effects of internal curing are typically evidenced by reduced autogenous shrinkage and self-desiccation in cement composites, as reported by Gwon et al., (
2022). Therefore, the use of natural fibers has been observed to be advantageous for both the fresh and hardened properties of shotcrete. Among these properties, this study primarily explored the fresh properties of NFRCCs without pumping pressure by applying the rheological theory. This was a preliminary case study prior to the application of NFRCCs as shotcrete in the field.
In this study, the mechanical and rheological properties of NFRCCs were investigated using kenaf and jute fibers of varying lengths and volume fractions. In general, the use of conventional fibers (e.g., steel, glass, carbon, and polypropylene) of longer lengths and at higher volume fractions in cement composites improves their compressive and flexural behaviors (Li & Obla,
1994; Singh et al.,
2019). However, the use of natural fibers of longer lengths at higher content generally causes a reduction in mechanical strength owing to the intrinsic porosity and lumen cavities (Gwon et al.,
2021). Despite these drawbacks, natural fibers have beneficial effects on the rheology and hydration of cement composites and are advantageous for shotcreting because of their hygroscopic characteristics. Considering all these, the mix proportions of the NFRCCs were designed with reference to those of shotcrete. Most previous studies on NFRCCs have employed 5- to 30-mm-long chopped fibers at 0.5–3.0% by volume (Onuaguluchi & Banthia,
2016; Pacheco-Torgal & Jalali,
2011; Yun et al.,
2022). Accordingly, this study limited the length and volume fraction of natural fibers to 5–30 mm and 0.5–2.0%, respectively. Rheometry and air-content tests were performed to evaluate the fresh properties of the NFRCCs, and the results of these tests were correlated. Compression and flexural tests were also performed to analyze the compressive strength, elastic modulus, and flexural strength of NFRCCs. Finally, the degree of fiber dispersion was evaluated by fluorescence microscopy.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.