1. Introduction
Urbanization results in the depletion of significant natural resources and the production of construction waste due to the erection of new buildings and the dismantling of old ones [
1]. Typically, waste concrete is disposed of via landfilling and stacking, resulting in the substantial wastage of land resources [
2,
3]. The reuse of recycled aggregate obtained from crushed, sieved, and cleaned waste concrete in concrete structures stands as a crucial method for the utilization of construction waste resources [
4,
5,
6,
7].
The surface of recycled aggregate is coated with a certain amount of mortar, resulting in increased porosity and water absorption compared to natural aggregate. Furthermore, recycled aggregate often contains numerous micro-cracks, leading to inferior properties compared to conventional concrete [
8,
9,
10,
11]. Incorporating fibers into recycled concrete can partially offset the deficiencies of recycled concrete by enhancing toughness and crack resistance [
12,
13]. Typical fibers used in concrete include steel, polypropylene, glass, carbon, basalt, and natural fibers [
14].
Steel fibers are effective in impeding crack propagation, while polypropylene fibers are cost-effective and long-lasting; therefore, they are the most commonly utilized types in recycled aggregate concrete [
15,
16]. In their study, He et al. [
17] combined polypropylene and steel fibers in recycled aggregate concrete and found that the combined impact of both fibers on recycled aggregate concrete mechanical properties surpassed that of either fiber alone. Afroughsabet et al. [
18] incorporated 1% double hooked-end steel fibers into recycled aggregate concrete and observed significant enhancements in tensile strength, alongside reductions in shrinkage and water absorption. Zheng et al. [
19] improved recycled aggregate concrete with nano-SiO
2 and basalt fibers, resulting in a 34.28% increase in 28-day compressive strength, a 40.55% increase in splitting tensile strength, and a 54.5% increase in flexural strength post-modification. Carneiro [
20] demonstrated that incorporating steel fibers boosted the mechanical strength and toughness of recycled concrete.
Although the utilization of steel and polypropylene fibers in recycled concrete has been extensively researched, there is a growing need to explore the more efficient integration of high-performance natural fibers with a low carbon footprint and biodegradability. Prior studies have investigated the use of natural fibers like hemp, kenaf, and bamboo in concrete, demonstrating the feasibility of plant fibers in enhancing concrete performance [
21]. Bamboo resources, which are abundant globally, have a short growth cycle and require little energy for harvesting and processing, providing energy-saving advantages. Bamboo has a cellulose content of 40–60%, hemicellulose content of about 20%, and lignin content of about 25%. The mechanical characteristics of bamboo fluctuate based on species, age, and geographical origin. Typically, bamboo exhibits a moisture content (in an air-dry state) of approximately 8%, a dry shrinkage rate ranging from 9% to 15%, a density between 0.8 and 1.2 g/cm
3, a compressive strength of about 90 MPa, a bending strength of about 250 MPa, and a tensile strength of about 300 MPa [
22]. Nie et al. [
23] conducted experimental and analytical examinations of the axial compressive characteristics of recycled aggregate concrete columns confined by bamboo tubes. Their findings revealed that the incorporation of short bamboo strips could enhance the properties of recycled concrete columns. The bamboo tube structure offered robust lateral confinement, while the short bamboo strips augmented both the compressive strength and ductility of recycled aggregate concrete columns. In a similar vein, Noh et al. [
24] employed bamboo fibers to substitute aggregate and compared the compressive strength of bamboo fiber-reinforced concrete with that of standard concrete. They determined an optimal bamboo fiber content of 5% for aggregate replacement in concrete.
Bamboo fiber can be categorized into charcoal bamboo fiber, pulp bamboo fiber, and natural bamboo fiber. Pulp bamboo fiber is produced through the pulping of bamboo material, whereas charcoal bamboo fiber is derived from bamboo that has been carbonized into micro-powder and bonded with viscose material. Both processes completely destroy the structure of the bamboo material and are not environmentally friendly. Natural bamboo fibers (NBFs) are directly separated from bamboo. They are elongated in shape, with stiff fibers, rough surfaces, and shallow grooves. The cross-section is nearly round with irregular jagged edges. They retain the natural physical structure of virgin bamboo and have a high specific modulus and high specific strength, making them high-performance renewable fibers [
25]. Additionally, using bamboo fibers in building materials has excellent environmental characteristics, contributing to carbon emission targets since plant fibers can store carbon.
However, natural fibers exhibit certain limitations in concrete applications, such as insufficient bonding at the fiber–concrete interface, low hydrophilicity, and vulnerability to biodegradation. Concurrently, several studies have demonstrated that alkali treatment can enhance the bonding strength between fibers and the cement matrix, as well as improve hydrophilicity [
26,
27,
28]. Wei et al. [
29] explored the durability of sisal fiber-reinforced cementitious materials under wet and dry cycling, revealing an 86% decrease in the tensile strength of fibers embedded in the cement matrix after 30 cycles. Wang et al. [
30] discovered that flax fiber-reinforced polymer (FFRP) reduced flexural strength by 11.2%, 14.9%, and 15.5% and flexural modulus by 21.3%, 32.3%, and 35.8% after natural aging for 60, 120, and 180 days, respectively. Chakkour et al. [
31] demonstrated that exposing plant fiber composites to a wet environment significantly diminishes their mechanical properties.
Additionally, Chakkour et al. [
32] experimentally determined that the tensile strength and Young’s modulus of bamboo fiber composites decreased by 18.15 ± 0.34% and 18.55 ± 0.4%, respectively, after 120 days of immersion. Akinyem et al. [
33] assessed the impact of 50 cycles of wet/dry processes on plant fiber cementitious composites, noting a slight increase in pore size before and after aging tests, with improved dimensional stability when sodium hydroxide was used at concentrations between 0 and 2.0%. Geremew et al. [
34] investigated the effect of alkali treatment on the surface morphology of bamboo fibers. Their findings strongly suggest that bamboo fibers prepared using these techniques could be employed as reinforcing material in composite production. Durability tests indicated that immersing bamboo culm in water for a month enhances its serviceability. This method could reduce reliance on harmful chemical preservatives [
35].
However, existing studies on the properties of NBF-reinforced concrete are limited, underscoring the necessity for further investigation. While many studies have delved into the impact of fiber admixture and length on concrete, it is imperative to broaden the scope of inquiry to encompass the water–cement ratio, given the substantial influence of bamboo fibers’ high water absorption on concrete workability and mechanical properties.
This study investigates the impact of the water–cement ratio, fiber admixture, and fiber length on the overall performance of NBF-reinforced concrete. It examines how NBF content, NBF length, and the water-to-cement ratio affect the workability, compressive strength, splitting tensile strength, and flexural strength of concrete through orthogonal experimentation, with the goal of determining the optimal NBF proportion. Additionally, all coarse aggregates were replaced with recycled aggregate, and the influence mechanism of NBFs on concrete properties was analyzed using scanning electron microscopy (SEM). The study utilized the orthogonal experiment design method, a scientific approach for investigating multifactorial and multilevel experiments. This method selects representative points from a full-scale experiment based on orthogonality, allowing for the comprehensive analysis of various factors.
The results of this study provide a basis for furthering the practical application of NBF-reinforced recycled concrete. They also serve as a reference for innovative approaches to solid waste reuse and the development and adoption of low-carbon biomass fibers. Ultimately, this can accelerate the utilization of environmentally friendly, low-carbon building materials.
5. Conclusions
This study developed sustainable and cost-effective concrete incorporating recycled aggregates and natural bamboo fibers. The findings indicate that natural bamboo fiber-reinforced recycled aggregate concrete is sustainable, cost-effective, and enhances the mechanical properties of the construction material. The following conclusions can be inferred:
(1) The optimal mixing parameters obtained from the orthogonal experiment were as follows: a fiber length of 20 mm (considering lengths of 10 mm, 15 mm, and 20 mm), a fiber content of 0.4v% by volume (considering 0.2v%, 0.3v%, and 0.4v%), and a water-to-cement ratio of 0.55 (considering ratios of 0.55, 0.60, and 0.65). The primary factors influencing the mechanical properties and workability were fiber content and the water-to-cement ratio, with the effect of fiber length being relatively minor.
(2) The optimal average compressive strength of NBF-reinforced recycled aggregate concrete is 25.2 MPa, representing a 7.3% increase compared to 23.4 MPa for recycled concrete without NBF.
(3) The fibers played a role in toughening and resisting cracks within the concrete, thereby enhancing its resistance to brittleness. Under compression, the predominant failure pattern of NBF-reinforced concrete involved the outward expansion of the entire surface mortar layer, with the fibers effectively binding the concrete together. The specimens exhibited good overall integrity after failure, maintaining their complete form without localized collapse.
(4) At the NBF-hardened cement–paste interface, minimal flaky Ca(OH)2 crystals were observed, while the hydration products primarily comprised needle-shaped ettringite and hydrated C–S–H gel. These products intertwined to enhance structural density and facilitate effective bonding at the NBF-hardened cement–paste interface.
This work exclusively focuses on the types and distribution of hydration products in NBF analysis in cement-based materials using SEM for observing their morphology and identifying and analyzing hydration products. In future research, we plan to integrate SEM, XRD, and FTIR techniques to conduct a more comprehensive and scientifically rigorous analysis of the micro-mechanisms of materials. We will carefully study and address this issue in depth in our follow-up research.
The interfacial debonding of NBF in cementitious materials due to fiber water absorption has not been explored in this paper. Therefore, further research is warranted to evaluate the NBF-reinforced concrete outdoor performance under both ultraviolet rays and humidity variations. Additionally, investigations should be conducted on the mechanism of action of modifiers on the fiber surface and their influence on interfacial bonding to expand the potential application.