1. Introduction
Typical cementitious concrete is usually excellent under compressive loading but weak under the application of tensile forces. However, steel reinforcement is incorporated if the concrete structure is intended to bear tensile forces. The steel reinforcement overcomes the said deficiency in concrete, and the respective concrete is known as reinforced cementitious concrete. The typical brittle nature of cementitious concrete can be altered by having effective modifications in it [
1,
2]. Therefore, the addition of different types of fibers was proposed with the passage of time to overcome this issue [
3]. Moreover, in the 1960s, the concept of fibers addition in cementitious concrete was initiated in which short dispersed fibers were used for incorporation into concrete to enhance the tensile strength of concrete by decreasing its brittleness, ultimately resulting in a particular concrete type named fiber-reinforced concrete [
4,
5]. The incorporation of fibers such as metallic/steel fibers [
6,
7,
8], natural fibers [
9,
10], synthetic fibers [
11,
12,
13], and mineral fibers [
2,
8,
14] in concrete is performed [
15] for enhancing the toughness behavior of concrete [
16,
17]. The concept of fiber spacing theory for fiber-reinforced concrete (FRC) was initially introduced by Batson [
18]. Among secondary/alternative materials that aid in developing eco-friendly cementitious concrete [
19,
20,
21], natural/agricultural/plant fibers come out as the most effective dispersed reinforcing materials for achieving sustainable [
22] and environment-friendly [
23] development. Gangil et al. [
24] conducted a comparative study of properties of natural and synthetic fibers and concluded that the values for elongation at break (%), failure strain (%), and density (g/cm
3) of Coir, Hemp, Jute, Ramie, and Sisal fibers are in line with the respective properties of E-glass and aramid or Kevlar fibers. Moreover, the availability of natural fibers is worldwide, which cost very little compared to artificial fibers [
25]. In the last several decades, the use of natural fibers has been gaining the attention of an increasing number of scientists, researchers, and academics for having alternative economical and eco-friendly materials compared to artificial/synthetic fibers in the last several decades [
26,
27]. Flax, wheat straw, jute, rice straw, kenaf, sugarcane bagasse, coir, bamboo, banana husk, ramie, and henequen [
28,
29,
30,
31] are some natural fibers.
Among all natural fibers, coconut fiber is a famous one that is extracted from the coconut fruits husks (
Figure 1) and is utilized for developing long-lasting and high-strength products [
32]. Globally, coconut is cultivated in multiple countries, specifically in subtropical and tropical areas that contribute significantly to economic growth. In an urge to enhance the mechanical properties of concrete, coconut fiber, as a dispersed reinforcement, has been the focus of multiple studies [
33,
34,
35]. Coconut fiber (Coir) is mainly classified into four different kinds such as buffering coil, bristle coil, white fibers, and brown fibers. The most general form of coconut fiber is bristle coil, having little or no content of pulp and not less than five inches. Brown fiber is extracted from the mature coconut, which is conventionally highly thick, strong, and abrasive, making it the most suitable and widely used fiber [
36]. Distinct from brown fiber, which is extracted from mature coconut, the extraction of white fiber is conducted from immature coconut and, usually, it is not as durable as the brown fiber is [
37]. The whole stepwise procedure for extracting fibers and other products from coconut is illustrated in
Figure 2. Coconut fiber has a bulk quantity of lignin and a lower quantity of cellulose, making it versatile, strong, and solid [
38]. The length, diameter, and aspect ratio of coconut fibers is 8–250 mm [
39,
40,
41], 0.25–1.00 mm [
39,
42,
43], and 100 [
42], respectively. The reported ranges for the density and water absorption of coconut fiber in the literature are 0.67–10 g/cm
3 [
43] and 130–180% [
44], respectively. Furthermore, the reported ranges for the tensile strength and elongation of coconut fiber in the literature are 15–405 MPa [
40,
42,
44] and 25–75% [
39,
40,
44], respectively. Regarding the mechanical properties of coconut-fiber-reinforced concrete, Khan and Ali [
45] studied the effect of 50 mm long coconut fibers having 2% content, by cement mass, in concrete. The study was concluded with enhanced mechanical properties. Similarly, Khan et al. [
46] reported the enhanced/improved energy absorption and toughness index of coconut-fiber-reinforced concrete with respect to the control specimen.
However, due to the organic/biodegradable nature of coconut fibers, their durability is still a concern. Hence, efforts are made to enhance the durability of coir either by soaking it in hot water or some chemical solutions [
48]. Accordingly, coconut fibers are used and available in the two types, i.e., untreated and treated. Ramli et al. [
41] determined the properties of concrete having untreated coconut fibers under different ageing conditions such as exposure to air and seawater. The chloride penetration, intrinsic permeability, and carbonation depth tests were performed to evaluate its durability. The study concluded with coconut-fiber-reinforced concrete′s enhanced strength and durability. Moreover, it was recommended in the study that treatment should be applied to coconut fibers before incorporating them in concrete for providing it with protection from degradation. According to one more study [
49], upon treatment of coconut fibers, removing pectin, lignin, hemicellulose, and wax from the surface of the fiber would result in parenchyma cells elimination, which enhances the contact area among globular marks and fibrils. As a result, the enhancement in fiber roughness is caused, which ultimately enhances the adhesion among the fibers and matrix [
49]. Furthermore, coconut fibers have a lesser conductivity of heat; however, being stiff and strong, the tensile, compressive, and flexural strengths of concrete enhance but with reduced concrete weight [
50].
The increasing environmental problems have led to the development of research on environmentally friendly construction materials such as coconut-fiber-reinforced concrete. Several researchers studied coconut fiber in concrete instead of artificial/synthetic/steel fibers. However, the research knowledge on coconut-fiber-reinforced concrete is still scattered, and there is no easy way to assess the importance of coconut-fiber-reinforced concrete. The difficulties in creative investigation and scholarly collaboration are aroused due to the information limitations of researchers. For this purpose, establishing and employing a technique for scientists/researchers to acquire the necessary information from reliable sources is essential. To the best of the author’s knowledge, no scientometric review has yet been conducted on the literature regarding the utilization of coconut fibers in concrete. Hence, employment of a scientometric method with the help of a software tool can assist in overcoming this gap. The fundamental purpose of this research is to provide an in-depth literature review on incorporating coconut fibers in cementitious composites, focusing on its mechanical properties, applications, and current state in the construction industry. The research gaps and challenges in applying coconut-fiber-reinforced concrete are also elaborated in the present work. The scientometric analysis of published research in coconut-fiber-reinforced concrete up to 2022 is aimed in the current study. The bulk research database may be assessed quantitatively by undertaking scientometric analysis with the help of a suitable software tool. The conventional review-based research is weak to some extent in its capacity for connecting numerous segments of the literature wholly and precisely. Co-citation, science mapping, and co-occurrence are a few main investigation parameters in the modern era [
51,
52,
53]. The discovery of sources having keywords co-occurrence, the primary authors as per articles and citations, vigorously involved research zones, and the most research publications in coconut-fiber-reinforced concrete may also be made with the help of scientometric analysis. The Scopus database is utilized to extract the bibliometric dataset of 235 relevant research publications. The current study would assist academics of the engineering field belonging to various geographical locations in exchanging ground-breaking novel methods/ideas, creating joint ventures, and forming research alliances due to the graphical and statistical depiction of countries and authors. Furthermore, evaluating and critically summarizing the review data on coconut fibers in concrete through this scientometric analysis, the industrial experts of this field can gain a comprehensive insight and clear picture regarding the coconut-fiber-reinforced concrete. This analysis would further allow the relevant industrial experts to gain an understanding of the available knowledge on coconut fibers in concrete, as well as the limitations and boundaries of this sustainable material prior to its practical implementation.
2. Methodology
The scientometric analysis is performed in the current study for the research database to determine the various features of bibliographic data [
54,
55,
56]. Multiple research studies have been reported and carried out in said area showing the uncertain employment of well-known search engines. Scopus and Web of Science, the two more accurate search engines, are mainly discovered for the stated aim [
57,
58]. We collect research data on coconut fibers in concrete for conducting this research using Scopus, a highly suggested academic search engine [
59,
60]. Today, the search in Scopus for “coconut fiber reinforced concrete” finds 235 documents from 2010 to 2022. Numerous preferences-based filters are employed for evading the data, which is unnecessary. “Journal review”, “journal research article”, “conference paper”, and “conference review” are adopted as the type of documents. “Journal” and “conference proceeding” are taken as “source type”. The period selected as “publication year” is “2022”, and “language” is selected as “English”. Further refinement is conducted by selecting “subject areas” such as “material science”, “engineering”, and “environmental science”. With the application of the above-mentioned refinements, a total of 235 records are taken. In the same manner, several research studies have been performed by applying the same technique [
61,
62,
63].
In the academic field, the bibliometric data are analyzed by developing scientific mapping that is generally applied for analyzing scientometric investigations [
64]. Using an appropriate software tool, Scopus records are saved using Comma-Separated Value (CSV) files for evaluation. The quantitative determination of the scientific visualization and the recovered records literature is developed using VOSviewer (version: 1.6.17). VOSviewer is a highly proposed and widely applied tool in the field of academics over a larger range of research areas, and this open-source mapping has instant accessibility [
65,
66,
67,
68]. Hence, the VOSviewer application in this research justifies its aims. Further assessment is conducted by loading the attained CSV files in VOSviewer for data integrity and consistency. Further, the evaluation is also made for the participation of countries, highly cited researchers with significant publications, bibliographic data, frequent keywords, and sources of publication. The several aspects, along with their relationships and co-occurrence, are also presented graphically; however, the statistics of figures are provided in tables. The procedural flowchart for conducting scientometric analysis is illustrated in
Figure 3.
4. Discussion and Future Perspectives
In this work, the statistical and mapping overview of diverse aspects of the available literature on coconut-fiber-reinforced concrete is presented. The manually conducted conventional review research has limited comprehensiveness and a less accurate inter-relation between different segments of the literature. Further, in this study, the assessment of journals having the most published articles, the most widely used keywords in published articles, the main contributing countries, and articles and authors having the most citations in the coconut-fiber-reinforced concrete research field is carried out. The keywords analysis reveals that coconut-fiber-reinforced concrete has primarily been discovered for its mechanical properties [
42,
45,
72,
73,
75,
76]. Furthermore, using conventional materials consumes the natural resources and energy at a larger scale, and the required processes emit bulk CO
2 emissions [
11]. Accordingly, concerns are rising about saving natural resources from excessive depletion. Thus, incorporating coconut fibers in concrete would decrease the cement and aggregates requirement, ultimately causing sustainable/green construction to have lower CO
2 emissions [
26,
34,
75,
79].
It is well known that concrete is basically a majorly utilized material in the construction industry worldwide. However, in natural/conventional form, concrete lacks in strain capacity and resistance against tensile loading and cracking, and behaves more brittle [
9,
80,
81,
82]. In order to resolve these problems, the incorporation of fibers such as synthetic fibers [
11], metallic/steel fibers [
83], and natural fibers [
45,
84,
85,
86] in concrete is carried out for enhancing its toughness. Among all fibers, the addition of coconut fibers in concrete is beneficial to enhance the ductility under different types of loading (compression, splitting-tensile, and flexural), as required to obtain structural safety [
73]. The addition of coconut fibers can decrease concrete’s brittleness in advance to improve various mechanical properties such as enhanced capability of energy absorption and significantly improved tensile strength. However, the coconut-fiber-reinforced concrete applications are still under development. In-depth studies are still needed before widening its applications toward structural members. Currently, the research on coconut-fiber-reinforced concrete primarily focuses on the insight into establishing the optimum mix design for improved properties. Still, there is a need to explore more horizons of using coconut fibers in concrete. The concrete workability with coconut fibers is lesser, which may be enhanced for an improved slump. Incorporating specific admixtures such as super plasticizers and air-entraining agents and pozzolanic materials such as fly ash should also be studied to enhance the concrete flow. Furthermore, the effect of the above-mentioned additives and silica fume should also be explored for improving the mechanical properties of coir-reinforced concrete under compressive, tensile, and flexural loadings. The manual mixing of coconut fibers in concrete is a tedious job that leads to a nonhomogeneous mix. In this scenario, the addition of certain chemicals may be studied to replace said hand-mixing with machine mixing. It is also important to explore the incorporation of coconut fibers on the cement matrix pore structure, crack abridgement, and chloride and water permeability characteristics of concrete. In addition, it is vital to establish an innovative strategy for developing coconut fibers’ water retention capability for high-performance concrete composites with the help of the internal curing process. Moreover, due to durability concerns due to the biodegradable nature of coconut fibers, the structural applicability of coconut-fiber-reinforced concrete at a larger scale is still limited. The comprehensive information on the life cycle assessment (LCA) of coconut-fiber-reinforced concrete is also insufficient and, hence, demands detailed exploration.