Application of waste recycle tire steel fibers as a construction material in concrete

1 Introduction

Concrete is a brittle material with various uses in structures, foundations, reservoirs, roads, bridges, dams, pre-cast members, and walls [1,2,3]. Accordingly, extensive research is directed toward improving concrete characteristics for more practical utilization [4,5,6,7]. Out of many, one direction was the steel reinforcement in concrete in the form of reinforcing bars positioned at particular locations in the respective structural members for bearing shear and tensile stresses [8,9,10]. On the other hand, the addition of steel fibers is in the form of random dispersion in the mix. The uniformly dispersed fibers in a concrete mix play a vital role in resisting the occurrence and propagation of cracks [11]. Furthermore, the engineering behavior of cementitious mortar and concrete can also be improved by incorporating short discrete fibers. It may be noted that the fiber’s addition may not only always enhance concrete strength but it also has a considerable positive impact on concrete ductility, toughness, and resistance against dynamic loading [12,13,14]. A concrete mix having steel fibers in the dispersed form is called steel fiber-reinforced concrete (SFRC) [15]. Adding steel fibers to concrete may improve flexural behavior, which is highly favorable for pavement applications, resulting in a decreased thickness of the pavement and enhanced resistance for vehicular traffic loading [16]. In hydraulic structures, SFRC can resist erosion due to water movement. Furthermore, the steel fiber shotcrete is highly effective for tunnels and bridges in slope stabilization. In refractory concrete, adding steel fibers can bear higher thermal stress more effectively than conventional refractory concrete [14]. However, the need for sustainable development in parallel with enhanced performance of concrete has directed the attention of researchers to use recycled steel fibers instead of conventional ones [4,17,18,19,20,21,22].

In the past, SFRC has been employed in a range of civil engineering structures, including but not limited to foundations, hydraulic structures, tunnel linings, bridge decks, slabs, refractory, shotcrete, and precast elements [14,23]. The steel fiber addition in concrete significantly impacts concrete cost, as using 1% conventional steel fibers approximately enhances the cost of concrete two times [15]. The economic factor has shifted the focus of researchers to the utilization of steel fibers obtained from discarded tires, thereby drawing their attention. The rapid growth in urbanization and the improved living standard has borne out the interest of researchers in this area as all such parameters have enhanced the usage of vehicles. Hence, it is the need of the hour to create a balance between economic development and environmental protection [24]. Nowadays, researchers are more interested in sustainable supplementary cementitious materials for heading toward sustainable development [25,26,27,28]. Thomas et al. [18] reported 10 years as the average estimated vehicular tire life even after two threadings. The disposal of these waste tires is becoming a threat in many under-developed and developing states in terms of landfill shortage, inadequate facilities, and technical knowledge for recycling them into valued products. Hence, waste tire recycling is of increasing research interest in developing countries. It has been concluded in previous research that waste recycle tire steel fiber (WRTSF)-reinforced concrete has similar properties as in the case of concrete reinforced with conventional steel fibers. It would also promote sustainable construction [19]. Every year, around 1.5 billion waste tires are disposed of in landfills, stockpiled, or burned, which seems to increase up to 5 billion by 2030 [20,29,30,31]. Therefore, imposing an effective method for disposing of waste tires is critical. One of the effective disposal methods is the processing of waste tires to recover their primary constituents [32,33,34]. It was previously reported that tire has almost 13–27% steel in them [35,36,37]. Hence, concrete having WRTSFs is an economical, effective, and sustainable alternative to concrete having conventional steel fibers.

Due to considerable variation in fiber-reinforced concrete mechanical characteristics, their mix designs, and various techniques for extracting fibers from end-of-life tires, a thorough review of WRTSF-reinforced concrete is important. Accordingly, in this study, a systematic review is done to classify and summarize research results for exploring the prospects of WRTSFs in concrete, and its potential for different civil engineering applications is highlighted. This research is aimed to review the characterization of WRTSFs and the mechanical performance of the respective concrete. This information will aid in developing a scheme for researchers to extract necessary literature/material from reliable sources. The summarized information in this study would offer the latest information on the characteristics and composition of WRTSFs for their practical utilization in concrete for improving its mechanical properties. It will encourage scholars to explore its applications further.

2 Research significance

The specific goal of this study is to review the application of WRTSFs in concrete, considering different lengths and contents, and to identify research gaps in this domain. The purpose of this literature review is to examine the use of WRTSFs in reinforced concrete, with the goal of identifying significant achievements and research gaps. By doing so, this study aims to provide direction for future research in this area. The improvement in WRTSF-reinforced concrete properties has been reported in the literature for almost two decades or more [38,39,40]. Compiling recent progression and reviewing outcomes reported by different researchers is necessary. Globally, the researchers focused on various objectives while evaluating the WRTSFs’ effect in concrete. Therefore, the critical compilation of results reported by multiple researchers is necessary for presenting a comprehensive perspective of the existing research. Typical steel fibers and WRTSFs have some comparison in terms of physical and mechanical characteristics due to the same constituents. Thus, the testing mechanisms and research processes may be mutually applied to steel fibers. Generally, the researchers have summarized the characteristics of concrete having conventional steel fibers from time to time [41,42]; however, the dedicated literature review on WRTSFs and their impact on various characteristics of concrete is still limited. In the last decade, sustainable WRTSFs have emerged and developed significantly as potential construction materials. Subsequently, the current study is focused on reviewing the significant and vital advances of past and current efforts of various researchers on WRTSF-reinforced concrete, provides summarized findings of the primary contributions, and discusses the potential of future developments. Hence, the compilation of the fresh and hardened characteristics of WRTSF-reinforced concrete emphasizes the research significance of the current study. Accordingly, the research novelty of this study is the comprehensive literature review on the use of WRTSF in reinforced concrete, which includes the compilation of recent research progress, a review of outcomes reported by different researchers, and the identification of research gaps in this domain. The study aims to provide direction for future research in this area and emphasizes the potential of WRTSF-reinforced concrete as a sustainable construction material. The study additionally emphasizes the process of obtaining steel fibers from waste tires and furnishes recommendations regarding the suitable makeup and attributes of WRTSFs to enhance the mechanical features of the composite material. Overall, the research novelty lies in the compilation of fresh and hardened characteristics of WRTSF-reinforced concrete and the potential for future developments in this field. The objective of this article is to gather and summarize the findings of previous research studies and the techniques utilized to recycle WRTSFs. The ultimate objective is to encourage the adoption of concrete reinforced with WRTSF in diverse construction contexts, including but not limited to pavements, tunnel linings, bridge decks, hydraulic structures, and slope stabilization. The study seeks to provide guidance on the appropriate composition and characteristics of WRTSFs to improve the mechanical properties of the composite material.

3 Scientometric analysis

3.1 Methodology

The scientometric analysis of existing studies on adding WRTSFs in concrete is also performed in the current study. The primary focus of conducting scientometric analysis is to determine the proximity of error in numerous studies conducted by researchers on the mechanical properties of WRTSF-reinforced concrete [43,44,45,46]. Scientometric research provides a more objective and unbiased output since it is not reliant on the viewpoint of any single individual [47,48,49,50]. The quantitative evaluation is conducted to observe the research development with the help of maps and connections of bibliometric data. Numerous studies are published on considered research domains, so highly precise dataset selection is vital. Scopus and Web of Science are effective and comprehensive databases for searching literature [51]. Scopus has the latest bibliometric data over a broader range than the Web of Science [51,52]. Accordingly, Scopus is used to compile bibliometric data on WRTSF-reinforced concrete for analysis. The keywords used for searching in Scopus are “steel fibers,” “recycling,” “concretes,” “compressive strength (CS),” “tensile strength,” and “fiber-reinforced concrete.” Data refinement approaches are also used to eliminate irrelevant studies. Science visualization, a method formulated to analyze literature studies for various purposes, is used in scientometric reviews [53,54]. It describes the challenges the researchers face in conducting manual literature reviews and establishes a link between authors, countries, articles, sources, and keywords in a particular research domain [55]. Accordingly, for science mapping and visualization, VOSviewer (version: 1.6.16) is applied in this research, a highly recommended free visualization open-source tool [56,57]. Following this, VOSviewer is employed to analyze data sourced from bibliographic databases. The analysis involves reading the data from CSV files imported from Scopus and creating a map based on the type of data and data source. Throughout the analysis, care is taken to ensure data consistency and reliability. To conduct a science mapping review, an analysis of the mechanical properties of WRTSF-reinforced concrete is carried out, which includes examining keywords, density mapping, and networking. A detailed flowchart showing all procedural steps, like retrieving of data, software tool and data analysis, etc., in the scientometric analysis, is illustrated in Figure 1.

Figure 1

Procedural flowchart of scientometric analysis.

3.2 Scientific mapping of keyword co-occurrence

Keywords primarily represent important domains for a particular area of research. The frequently occurring keywords widely adopted in considered research domains are already discussed in Section 3.1. The network-based linkage of all keywords and their cluster density is presented in Figure 2. The node’s size for a keyword shows the frequency of a specific keyword, and its position represents the co-occurrence of these keywords in relevant publications. The color visualization is also done in which the extensive research on WRTSFs in concrete to explore different aspects is shown by bigger nodes. The bifurcation of additional keywords is done by using distinct colors, showing keyword co-occurrence in multiple studies. Figure 2(a) shows the clusters of keywords defined by colors like green, blue, yellow, and red. Red nodes show the co-occurrence of the most widely used keywords, i.e., the WRTSF-reinforced composites evaluated for mechanical properties to be used in various applications. All these keywords have frequently been used in publications for multiple aspects of WRTSFs. This trend indicates that the use of WRTSFs leads to enhanced properties of concrete, as demonstrated by the density visualization presented in Figure 2(b). The unique colors used in the visualization represent the maximum and minimum density of keywords, with red indicating the highest density, followed by green and blue that gradually decrease, and ultimately yellow, depicting the lowest density. Moreover, in Figure 3, the linkage of different keywords generally applied to explore the mechanical properties of WRTSF-reinforced concrete with relevant areas, such as recycling, CS, split-tensile strength (STS), crushing, etc., is shown. This scientific observation is helpful for the keyword selection to effectively and conveniently retrieve published research on the WRTSF-reinforced concrete properties research domain for future studies. It may be concluded that improved performance of concrete for WRTSF adaptability is considerably related to WRTSFs, its properties, and various mechanical properties of reinforced composites, etc.

Figure 2

Network-based analysis and linkage: (a) all keywords and (b) cluster density.

Figure 3

Recycling linkage with steel fibers and other mechanical strength factors.

4 WRTSF composition and mechanics

Tires are among the vital parts of automobiles and have a lifespan of almost 5 years based on multiple parameters [58]. Steel fibers extracted from waste tires can be employed for several civil engineering structural applications with economic and environmental benefits [59,60,61]. As superior quality steel is used in the production of tires, the steel fibers extracted and recycled from tires possess higher tensile strength than industrial steel fibers [62]. Waste tires were reported as the main origin for extracting steel fibers due to their bulk presence, i.e., almost 17 million tons per annum [63] and its different by-products, which may be utilized to attain sustainable development [64,65,66]. The useful primary components from tire recycling consist of textile fibers, rubber, and steel fibers. A tire is composed of rubber content (46–48%), carbon black (25–28%), steel insert (10–12%), and oil and vulcanizing agents (3–6%). As a result of the higher carbon content, tires offer more energy than coal, and the calorific of fuel extracted from these end-of-life tires was reported to be more than coal [67], having lesser sulfur content [68]. The rubber content from the tires is extracted as crumb rubber. Using crumb rubber in mortar, concrete, or asphalt as natural fine aggregate replacement is among the methods to conserve natural resources [69]. The primary disadvantage of incorporating crumb rubber in cementitious composites is the generation of complex interfacial relation with the surrounding matrix, i.e., paste, decreasing the CS, due to which its applications are still limited [70]. The waste steel from tires is extracted by the shredding process in which the post-consumer or end-life waste tires are cut/chopped into small pieces, followed by the milling of these chopped small pieces to extract smaller rubber particles that are named crumb rubber out of which the steel is separated magnetically [23] as it is frequently reported that there is approximately up to 30% steel in a tire that can be recycled for its possible applications in concrete. The steel fibers recycled from different methods are shown in Figure 4. The different forms of WRTSF after extraction out of tires are summarized from the literature in Figure 5.

Figure 4

Recycled steel fibers from different methods: (a) Lathe machine-generated recycled steel fibers [71], (b) lathe-recycled steel fibers [72], and (c) waste steel scrap [73].

Figure 5

Summarized forms of WRTSF from the literature: (a) Aiello et al. [74], (b) Farhan et al. [75], (c) Gul et al. [76], and (d) Köroğlu [77].

Yu et al. [78] presented the recycling process of fibers extracted from waste tires (Figure 6). Shredding, anaerobic thermal degradation, and cryogenic methods are usually employed to recover steel fibers from waste tires [23]. The steel fiber quantity extracted from an end-of-life tire varies from tire to tire. The tires of light transport vehicles have almost 15% steel component, whereas the heavy transport vehicle tires have around 25% steel component [23]. In the literature, the shredding process is mainly reported for the extraction of waste steel tires from end-life tires [23,79,80]. Some other research studies consider the readily available recycled steel fibers extracted by the cryogenic method [36,81]. Moreover, the extracted recycled steel fibers from the combined shredded and pyrolysis methods are also considered in some studies [82].

Figure 6

Different product extraction from the recycling of waste tires [98].

WRTSFs are a relatively new type of steel fiber that is produced from recycled tires. These fibers have unique properties that make them suitable for use in concrete. The mechanics of WRTSFs can be described by examining their physical and mechanical properties, their interaction with the concrete matrix, and their behavior under various loading conditions [83,84,85,86]. The physical properties of WRTSFs include aspect ratio, length, diameter, and volume fraction. The aspect ratio is the ratio of the fiber length to its diameter. The length of the fiber can vary depending on the size of the tire, and the diameter of the fiber is typically in the range of 0.3–0.5 mm. The volume fraction of WRTSFs in concrete is typically between 0.5 and 2.5% [84,87,88]. The mechanical properties of WRTSFs are influenced by their composition, processing, and microstructure. WTSFs are composed of steel wires that are twisted together to form a bundle. The processing of WRTSF involves cutting the bundles into lengths, cleaning them, and drying them. The microstructure of WRTSFs is characterized by a complex network of interlocking steel wires. The interaction between WRTSF and the concrete matrix is important for understanding the mechanics of WRTSF-reinforced concrete [89,90]. WRTSFs are randomly distributed throughout the concrete matrix and provide additional reinforcement to the concrete. The interlocking of the steel wires enhances the bonding between the fiber and the concrete matrix, which improves the load transfer between the two materials [91,92]. The WRTSF also acts as a crack arrestor, preventing the propagation of cracks and improving the durability of the concrete [93]. Under various loading conditions, WRTSF-reinforced concrete exhibits different behaviors. The addition of WRTSF to concrete increases its ductility and toughness, which allows it to absorb more energy before failure [94,95]. The utilization of WRTSF also enhances the flexural and tensile strengths of concrete, thereby increasing its ability to resist bending and cracking [19,96]. In addition, WRTSF-reinforced concrete exhibits improved fatigue behavior, which is important for structures subjected to cyclic loading [80,97]. In conclusion, the mechanics of WRTSF is important for understanding the behavior of WRTSF-reinforced concrete. The mechanical and physical characteristics of WRTSFs, the interaction of WRTSFs and concrete matrix, and their behavior under various loading conditions all contribute to the overall performance of the composite material. Further research is needed to optimize the properties of WRTSF and their volume fraction in concrete to achieve the best possible mechanical properties.

5 Geometrical and mechanical characterization of WRTSFs

The geometrical characterization of WRTSFs is a critical aspect of understanding their potential applications in engineering and construction materials. The geometry of WRTSFs, such as their length, diameter, and aspect ratio, can significantly impact the mechanical properties and behavior of the resulting composite materials. The length of WRTSFs is an important geometrical property that can impact their dispersion in concrete or other matrix materials [83,97,99]. Shorter fibers tend to disperse more easily in the matrix, while longer fibers may require additional processing to ensure even dispersion. The length of WRTSFs typically ranges from a few millimeters to a few centimeters, depending on the manufacturing process and desired application [100]. The diameter of WRTSFs is another important geometrical property that can impact their mechanical properties. Smaller diameter fibers tend to have higher tensile strength but lower stiffness, while larger diameter fibers tend to have lower tensile strength but higher stiffness. The diameter of WRTSFs typically ranges from a few micrometers to a few millimeters, depending on the manufacturing process and desired application [83,101]. The aspect ratio of WRTSFs, which is the ratio of the fiber length to its diameter, is also an important geometrical property that can impact the mechanical properties of composite materials. A higher aspect ratio typically results in higher tensile strength and stiffness but may also lead to reduced dispersion in the matrix material. The aspect ratio of WRTSFs typically ranges from 10 to 100, depending on the manufacturing process and desired application. Other geometrical properties that can be characterized for WRTSFs include their shape and surface area. The shape of WRTSFs can impact their dispersion and interfacial bonding with the matrix material, while the surface area can impact their bonding and frictional properties with the matrix material. In a study done by Centonze et al. [37], a sample of 2,000 recycled steel fibers was randomly extracted after the shredding process to determine their geometrical properties. Using a micrometer, the diameter of each fiber was measured at the two extremities and mid-length, and the resulting values were averaged. The diameters of the fibers ranged between 0.10 and 2.00 mm, with an average value of 0.24 mm. To further characterize the geometrical properties of the recycled fiber, eight diameter ranges were defined, and the number of fibers falling within each range was counted. The diameter range with the highest frequency of values was between 0.15 and 0.2 mm, comprising 32.75% of the total fibers. The 0.30–0.35 mm diameter range and the 0.20–0.25 mm diameter range followed, accounting for 27.0 and 24.6% of the fibers, respectively. Figure 7(a) illustrates that the predominant type of fibers employed were those with a diameter ranging between 0.20 and 0.25 mm, constituting about 30.17% of the total fibers utilized, with fibers having a diameter of 0.15–0.20 mm following closely behind at approximately 27.08%. For each specimen, the length of the reinforcement, defined as the distance between the outer ends of the fibers was also measured. The recorded lengths ranged from 3 to 170 mm, with an average of 31.4 mm. To analyze the data, the measurements were divided into 15 classes, and the range with the highest number of fibers was 26–30 mm, comprising 17.20% of the total fibers. The 21–25 mm range and the 31–35 mm range followed, accounting for 15.91 and 14.35% of the fibers, respectively. Figure 7(b) shows that fibers measuring 10–15 mm in length make up the majority at around 40.75%, with fibers ranging from 15 to 20 mm in length following at approximately 22.92%. By comparing the mean values of both length and diameter, the aspect ratio of the recycled fibers used was calculated to be 131. In conclusion, the geometrical characterization of waste tire steel fibers is an important aspect of understanding their potential applications in engineering and construction materials. The length, diameter, aspect ratio, shape, and surface area of WRTSFs can significantly impact their mechanical properties and behavior in composite materials. Therefore, careful consideration should be given to the geometrical properties of WRTSFs during their manufacturing and processing for optimal performance.

Figure 7

Geometrical characterization of WRTSFs in terms of (a) diameter and (b) length [102].

Mechanical characterization of WRTSFs is an important aspect of understanding their potential applications in engineering and construction materials [88,103,104]. The mechanical properties of WRTSFs can significantly impact the performance of the resulting composite materials, and therefore, must be carefully evaluated and characterized. One of the primary mechanical properties of WRTSFs is their tensile strength. The tensile strength of WRTSFs is typically measured using standard tensile tests, which involve applying a gradually increasing tensile load to the fiber until it fails. Research has documented a broad spectrum of tensile strength values for WRTSFs, varying from 500 to 2,500 MPa, depending on factors such as the manufacturing process, fiber geometry, and other relevant variables [101,103]. Figure 8 displays the distribution of the tensile strength ranges for the WRTSFs utilized in previous research studies, as summarized by Zia et al. [102]. In addition to tensile strength, the Young’s modulus of WRTSFs is also an important mechanical property that can be characterized. The Young’s modulus is a measure of the stiffness of the fiber, and it represents the ratio of the stress applied to the fiber to the resulting strain. Studies have reported that the Young’s modulus of WRTSFs typically ranges from 100 to 250 GPa, depending on the fiber diameter, manufacturing process, and other factors [79,80]. The fracture toughness of WRTSFs is another important mechanical property that can be characterized. The fracture toughness represents the fiber’s resistance to crack propagation and can be measured using standard fracture mechanics tests. Studies have reported that the fracture toughness of WRTSF can range from 10 to 60 MPa·m^−1/2, depending on the fiber geometry, manufacturing process, and other factors [84,87,88]. Other mechanical properties that can be characterized for WRTSFs include their fatigue behavior, creep resistance, and impact resistance. Fatigue tests involve subjecting the fiber to cyclic loading and measuring the resulting fatigue life, while creep tests involve subjecting the fiber to a constant load over time and measuring the resulting deformation. Impact tests involve subjecting the fiber to a sudden impact load and measuring the resulting deformation and fracture behavior. In conclusion, the mechanical characterization of waste tire steel fibers is an important aspect of evaluating their potential applications in engineering and construction materials. Tensile strength, Young’s modulus, fracture toughness, fatigue behavior, creep resistance, and impact resistance are all important mechanical properties that can be characterized to better understand the performance of WRTSFs in various applications.

Figure 8

Reported tensile strengths of WRTSFs [102].

The mechanical properties of steel fibers play a critical role in determining the concrete’s reaction with them, and these properties are influenced by various factors, including the fiber material’s characteristics, its interaction with the cementitious matrix, the quantity of fiber in the mixture, and the fiber-reinforced concrete’s production process [105]. Furthermore, the dimensions and geometry of steel fibers have a considerable influence, with the aspect ratio of the fiber being particularly crucial [106]. Table 1 provides a comprehensive list of waste recycled steel fibers, varying in diameter from 0.026 to 1.55 mm, to cater to diverse application requirements. Likewise, the fiber length determines its ability to transfer stress within the material, with longer fibers generally being more adept at transferring stress than shorter ones. Furthermore, the elastic modulus and tensile strength of the fibers dictate their strength and stiffness, respectively. The wide range of values for these properties listed in the table showcases the diverse applications and potential uses of recycled steel fibers. Additionally, fiber density is an essential characteristic that can impact the weight and overall attributes of the material it is integrated with. Lastly, the references included in the table provide additional resources for those who desire to learn more about the properties and applications of such fibers. It may facilitate further research and development of innovative uses for these adaptable materials. Additionally, Figures 9 and 10 depict the distinct morphologies of individual and bundled fibers recycled from various sources. Furthermore, effective fiber anchorage is a vital parameter in mitigating the risk of fiber pull-out. Accordingly, steel fibers having small hooks are preferable in this scenario [107]. The fundamental function of steel fibers is to facilitate bridging action during the onset of cracking, culminating in durable performance characteristics, namely, resistance to crack propagation and plastic deformation in concrete elements [108].

Figure 9

Recycled waste steel fiber in isolation form: (a) recycled and conventional [80], (b) single fiber [44], (c) microfiber [113], and (d) macrofiber [113].

Figure 10

Recycled steel fibers in a bunch from different sources: (a) Caggiano et al. [36], (b) Moghadam et al. [120], (c) Suleman et al. [96], (d) Simalti and Singh [121], (e) Frazão et al. [114], and (f) Sengul [116].

6 Adopted lengths and contents of WRTSFs in concrete

The mechanical characteristics of WRTSFs are comparable with those of industrial fibers and have been proposed in the literature as dispersed fibers in concrete [43,44,45,46]. The enhanced STS and flexural strength (FS), along with improved durability characteristics like resistance against corrosion and thermal and acoustic insulation, were also reported in the literature [122,123]. However, Martinelli et al. [95] reported that conventional steel fibers could not be replaced as a whole by recycled steel fibers. For fibers with enhanced efficiency, the combination of both WRTSFs and conventional steel fibers is also considered in various studies [84,95,100,124]. The incorporation of steel fibers in concrete enhances its stiffness, tensile strength, and ductility [125,126]. The primary function of steel fiber incorporation in concrete is the stress transfer in cracks, providing reinforced concrete beams with a diagonal post-cracking stiffness. Further, it improves cracking resistance and prevention against crack width, hardness, and shear strength. Moreover, adding steel fibers to concrete also changes the failure pattern from brittle to ductile flexural failure [127,128,129,130,131,132]. Tunnel linings, slabs, foundations, bridge decks, hydraulic structures, precast elements, and pavements are among the applications of SFRC [16,23]. However, it is critical to use the steel fiber content economically [133]. The length and adopted contents of WRTSFs are gathered from existing studies in Table 2. The optimized length of WRTSFs for improving concrete properties depends on various factors such as the type of application, fiber content, and the properties of the matrix material. It may be noted from the table that several studies have shown that the addition of WRTSFs to concrete can improve its mechanical properties, such as CS, FS, and toughness. The length of the fibers has been found to play a significant role in determining the effectiveness of the reinforcement. In general, shorter fibers (less than 25 mm) are preferred for concrete applications because they disperse more uniformly in the matrix and provide better bonding with the cement paste. However, longer fibers (up to 50 mm) have also been used successfully in certain applications such as precast concrete products and shotcrete. It is also reported that the optimum length of WRTSFs for improving the mechanical properties of concrete was between 20 and 30 mm. It is important to highlight that incorporating WRTSFs into concrete must comply with the relevant standards and guidelines to ensure the safety and efficacy of the composite material. Similarly, the optimized content of WRTSFs in concrete depends on various factors, including the desired properties of the resulting composite material and the specific application requirements. Generally, the addition of WRTSFs can improve the mechanical properties of concrete, such as its tensile strength, toughness, and impact resistance. The table shows several studies that have investigated the effects of different fiber contents on the properties of concrete and reported that the addition of 1–2% by volume of WRTSFs resulted in the highest improvement in compressive and FSs of concrete. Moreover, it was also found that the optimal content of WRTSFs for improving the mechanical properties of concrete was around 0.5–1.0% by volume. It is worth noting that the optimal fiber content can vary depending on the specific application of the concrete. For example, in high-performance applications such as earthquake-resistant structures, higher fiber contents may be required to provide sufficient ductility and energy absorption capacity. On the other hand, in applications such as pavement or flooring, lower fiber contents may be sufficient to improve durability and resistance to cracking. Overall, the optimized content of WRTSFs in concrete depends on various factors and it is important to conduct thorough testing and evaluation to determine the appropriate fiber content for a specific application. Binglin [134] determined the effect of WRTSF contents on concrete behavior. It was revealed that although CS was slightly impacted, the FS was considerably enhanced. Therefore, the WRTSF was endorsed as a green material for improving concrete behavior [44,74,135,136,137]. Aiello et al.[74] focused on exploring the use of recycled steel fibers in concrete to enhance its mechanical properties. In another study [137], the investigation assessed the influence of recycled steel fibers in concrete on its flexural, splitting-tensile, and post-crack strengths, and ultimately concluded that the strengths were enhanced. The recent literature on mechanical characteristics and durability of recycled SFRCs depicted that its application in concrete results in improved resistance against crack propagation, shrinkage behavior, and impact resistance of respective concretes [46,103,138,139].

Incorporating lathe and steel fibers depicts a higher ultimate load than the reference specimen [72]. The study concluded that the lathe waste fiber has significant potential to be incorporated as steel fibers in concrete. The literature suggests that steel fibers obtained by cutting waste tires can serve as a viable alternative to traditional steel fibers when incorporated into a concrete matrix. Incorporating these waste fibers can be justified for comparable characteristics and less cost [36,95,136,137]. According to the literature, although shorter and thinner fibers extracted from waste tires are notably stiffer, incorporating them into concrete as reinforcement leads to enhanced mechanical properties [105,149,150]. Hence, the metallurgical process may transform steel discards into a valued material. The extracted material may be utilized for producing standard steel fibers. This is a more rational way of using discards [151].

7 Mix design of WRTSF-reinforced concrete

WRTSFs are classified as either slightly deformed or straight. Depending on the extraction method, the WRTSF diameter can range from 0.23 to 1.8 mm [23,152]. The fibers that were extracted via the shredding process were of a diameter of almost 0.23 mm. In contrast, the fibers extracted via the pyrolysis method ranged from 0.8 to 1.5 mm [23]. On the other hand, American concrete institute (ACI) [153] categorized WRTSFs as either a crimped base or a smooth surface. Moreover, these fibers can also be micro or macro depending on the length [154]. The size in the case of macrofibers varies from 19 to 60 mm and provides structural support and an effective bridging mechanism for cracks in the hardened concrete [155]. However, in the case of microfibers, the diameter and length generally range from 0.1–1 to 2–10 mm, respectively. Hence, these WRTSFs are small enough for random and easy dispersion in the concrete mix [156]. Amuthakkannan et al. [157] relate the improved mechanical properties of WRTSF-reinforced concrete mix with a uniform dispersion of WRTSFs while mixing the concrete, as improved properties are attained via the collective effect of friction, adhesion, and mechanical interlocking. The contents of aggregate and cement in SFRC are usually more than the typical concrete mix [158]. The WRTSF component is generally added as a last component during the mixing of concrete [159,160], as also illustrated in Figure 11. In the case of WRTSF as well, the same ACI committee [158]-recommended proportion ranges for normal weight fiber-reinforced concrete are generally adopted, as also given in Table 3. Here, a variation in the composition mix is observed in the case of a change in coarse aggregate sizes. The requirement is to increase aggregate sizes, enhance cement content, and reduce the w/c ratio, steel fiber, and aggregates. The basis for this mix design was that in the case of the smaller sizes of aggregates, the combined surface area would be much more significant compared to that in the case of large aggregates. Subsequently, this mix design is appropriate in the case of both conventional steel fibers and WRTSFs [15,161,162]. But it is necessary to evade unwanted situations like balling that may have resulted due to the presence of fibers in bulk quantity (i.e., more than 2% volumetric content), rapid fiber addition in a mixer, incorrect mixing procedure sequence (i.e., the addition of fibers as a first added component) and the presence of excessive coarse aggregates.

Figure 11

WRTSF-reinforced concrete mixing procedure.

8 Workability of WRTSF-reinforced concrete

Workability is a primary limitation in the case of fiber-reinforced concrete for improved concrete properties. For developing an appropriate concrete mix, its workability is a primary factor considerably affected by the content and type of WRTSF [40,59,163]. Though the slump is the standard test to quantify concrete workability, several researchers have reported that in the case of fiber-reinforced concrete, this test is unable to give sufficient knowledge regarding workability [44]. The higher content of WRTSFs not only negatively influences the workability but, at the same time, also affects the uniformity of the fresh concrete. Moreover, the vertical planetary concrete mixer was reported to be an effective solution for homogenous dispersion of WRTSF without any negative impact on workability, even at high fiber volumetric content [37,74,164]. Further to the fiber content, the geometrical features and aspect ratio of fibers considerably impact the uniformity and workability of fresh WRTSF-reinforced concrete mix [37,165,166]. In addition, it was also observed that the geometry of WRTSF has almost the same effect on uniform dispersion and concrete workability of fibers, as for conventional steel fibers [46]. The interlocking potential of steel fibers during mixing was also observed as one more issue for the reduced workability of WRTSF-reinforced concrete. It was regarded as a “balling effect” in the literature. The inconsistency in shape and size of WRTSFs was observed as a primary reason for balling in fresh concrete [163,167]. The enhancement in the superplasticizer content, decrease in the content and aspect ratio of WRTSFs, and the mixing procedure can effectively solve the balling effect and workability of WRTSF-reinforced concrete. Some of the recommendations for avoiding the balling are the WRTSF addition in increments [87], the limitation of steel fibers aspect ratio and volumetric content up to 200 and 0.5%, respectively [75,168], and the WRTSF scattering post to wet mixing of all components in the mixer [169].

9 Mechanical properties of WRTSF-reinforced concrete

9.1 WRTSF effect on the CS of concrete

The WRTSF-reinforced concrete was explored to enhance the CS, which was reported to be reduced by enhancing WRTSF content [79]. In this study [79], a CS reduction of 1.93% was observed for 2% WRTSF content, 13.5% for 4% WRTSF content, 18.3% for 6% WRTSF content, and 26.5% for 8% WRTSF content. Similarly, compromised compressive behavior in the case of WRTSF-reinforced concrete was observed in different studies [93,104]. Martinelli et al. [95] also reported a reduced CS of WRTSF-reinforced concrete. Caggiano et al. [36] also concluded a 5% decrease in the CS of WRTSF-reinforced concrete upon adding 1.25% volumetric content of WRTSF compared to the reference mix design. But, at the same time, the lower contents of WRTSF resulted in enhanced CS. However, in another investigation [83], the CS was enhanced up to 5% when the WRTSF content was 0.5 % and up to 13% when the WRTSF content was 1% than the reference mix. Similarly, the reported mechanical properties of WRTSF-reinforced concrete having varying contents and lengths of WRTSF in the literature are summarized in Table 4.

Table 4

Mechanical properties of WRTSF-reinforced concrete as per the literature

Sl. no.	WRTSF dosage			Mechanical strengths			Ref.
	WRTSF length	WRTSF content		Compressive	Splitting tensile	Flexural
	(mm)	–		(%)	(%)	(%)
1.	40	1.5 and 3%	By volume	4–35	4	—	Akid et al. [110]
2.	23 & 8.7	0, 0.5, 1, and 1.5%	By volume	—	—	Increased	Chen et al. [101]
3.	30–50	0, 0.5, and 1%	By volume	11	90	122	Moghadam et al. [120]
4.	14.9	1 and 2%	By volume	—	24 & 50	—	Shi et al. [140]
5.	14.9	1 and 2%	By volume	—	Up to 50	—	Shi et al. [141]
6.	NA	0.5 and 1%	By volume	13	18 & 45	23 & 52	Kalpana and Tayu [143]
7.	23	1%	By volume	—	—	116.8	Zhong and Zhang [103]
8.	26	50	kg·m−3	22	43	—	Skarżyński and Suchorzewski [145]
9.	9–15	1.5, 3, and 4.5	By volume	4.43	—	—	Girskas and Nagrockiene [146]
10.	3–28	0, 2, 4, and/ 6%	By volume	—	27%	18	Younis [100]

Carrillo et al. [85] reported the stress–strain behaviors of industrial SFRC and WRTSF-reinforced concrete. The results show that the WRTSF contents of 34.0 and 65.1 kg·m⁻³ depicted more resistance against compressive strains than in the case of industrial steel fiber-reinforced concrete. In another study, upon adding 0.23% WRTSF, 23.3% more of the WRTSF resisted compressive stresses reinforced concrete specimens. In contrast, on adding 0.46% WRTSF, the respective WRTSF-reinforced concrete specimen withstood 25.4% more compressive stresses than the controlled specimens [37]. The reason behind this behavior is the random and irregular shapes of WRTSFs, which are added to delay the propagation of initial cracks, limiting the tensile strain while loading is applied, and ultimately enhancing the compressive stress–strain behavior of the respective concrete. Similarly, the same type of stress–strain behaviors was also concluded in several other studies on WRTSF-reinforced concrete as well [84,144,145]. Al-musawi et al. [144] also considered the WRTSFs in rapid hardening mortars that resulted in their enhanced CS. Abbas [71] incorporated 1 mm thick, 2 mm wide, and 50 mm long WRTSFs in cementitious concrete. The study resulted in enhanced CS of 6.1, 8.8, 9.8, and 9.4% upon incorporating 0.5, 1, 1.5, and 2% volumetric content of WRTSF, respectively, with respect to the control concrete. Aiello et al. [74] suggested utilizing a planetary concrete mixer instead of a traditional one to improve the CS of concrete reinforced with WRTSF, as reported in the literature.

Several researchers have reported the enhancement in compressive toughness of fiber-reinforced concrete [170–173]. Cai and Xu [174] also concluded the disintegration under compressive loading for fiber-reinforced concrete with improved and enhanced compressive toughness index. Both the strength and toughness are accounted for in the structural performance of concrete, as reported by Cai and Xu [174] and Kesner and Billington [175]. Moreover, incorporating discrete and dispersed fibers in concrete considerably alters the brittle compressive failure mode to ductile, ultimately increasing its modulus of elasticity [176]. As per the conclusions made by Papakonstantinou and Tobolski [79] and Bjegovic et al. [124], although the elastic modulus of WRTSF-reinforced concrete was enhanced; however, it has not been considerably increased with enhancing contents of WRTSFs. Subsequently, the considerably improved toughness with slightly compromised strengths of WRTSF-reinforced concrete may lead to better structural performance of the respective WRTSF-reinforced concrete structures. However, at the same time, some researchers also recommended the limited contents of WRTSFs in concrete if intending to enhance the CS [71,83,84,144,146,148,177]. However, it was also reported in some other studies that the CS of concrete can even be reduced upon the incorporation of WRTSFs [44,79]. Moreover, in some studies, only a slight improvement in the CS of WRTSF-reinforced concrete [74,178] was reported. The percentage difference of WRTSF-reinforced concrete CS with respect to reference specimens in light of reported studies [37,40,83,87,93,161,179–181] is presented in Figure 12. For example, Aiello et al. [74] reported that the fiber pull-out behavior in the case of WRTSFs is similar to that of conventional steel fibers but without any considerable variation in its CS due to using varying fiber contents. Further, this mechanism can be better demonstrated by an effective contribution provided by fiber reinforcement, which resists concrete fracture. Hence, it is vital to experimentally investigate the significant quantum of specimens to validate improvement in the CS of concrete having WRTSFs.

Figure 12

Percentage difference in the CS reported for WRTSF-reinforced concrete.

The various experimental studies conducted on the use of WRTSF-reinforced concrete and their effects on CS, stress–strain behavior, and toughness are discussed. The results of these studies are mixed and show that the CS of WRTSF-reinforced concrete decreases with increasing WRTSF content in most cases. For example, a reduction of 1.93% for 2% WRTSF content, 13.5% for 4% WRTSF content, 18.3% for 6% WRTSF content, and 26.5% for 8% WRTSF content has been reported. However, some studies have shown that lower contents of WRTSFs can enhance CS, and adding 0.5 and 1% volumetric content of WRTSF can result in up to 13% enhanced strength. Moreover, the irregular shape of WRTSFs can delay the propagation of initial cracks, limiting the tensile strain while loading is applied, and ultimately, enhancing the compressive stress–strain behavior of the respective concrete. Several studies have reported that WRTSF-reinforced concrete exhibits improved toughness, indicating the potential for better structural performance. However, some researchers have recommended limiting the contents of WRTSFs to concrete if the intention is to enhance CS. Additionally, it was also found that the impact of WRTSFs on the modulus of elasticity and the transition from a brittle compressive failure mode to a ductile one was an increase in the modulus of elasticity. The results of these studies suggest that WRTSF-reinforced concrete has improved toughness with slightly compromised strengths, leading to better structural performance. In conclusion, experimental investigations have demonstrated the potential benefits and limitations of using WRTSFs in reinforced concrete. The findings show that an optimized percentage of WRTSF depends on the intended application and desired mechanical properties, and careful consideration should be given to the experimental parameters to ensure reliable and reproducible results. Gul et al. [76] reported the outcomes of the CS test performed on WRTSFs. The results show that when 7.62 cm long WRTSFs were employed, the increasing strength of concrete was observed with the enhancing WRTSF content. However, the WRTSF content of more than 3% resulted in slightly reduced strength. The same pattern was seen with a 10.16 cm long WRTSF. The lower workability might be linked to the decrease in the CS of WRTSF-reinforced concrete when using higher dosages and greater lengths of WRTSF, resulting in a matrix with undesirable pores and cavities.

In order to achieve optimal CS within a certain range, both the length and diameter of fibers should be taken into consideration. Previous studies have shown that an average increase of 2.53% in CS can be observed when 1% WRTSF with an average diameter of 38.18 mm and a thickness of 0.79 mm are used. There is no decrease in the CS when the WRTSF content ranges from 1.25 to 2%. According to Köroğlu [77], the specimen incorporating 40 mm long and 0.89 mm thick WRTSF showed the highest increase (i.e., 27%) in strength. Upon incorporation of 38 mm long having 1 mm diameter with 2.02% WRTSF content, the CS increased by 9%. It is noteworthy that the majority of researchers examined the compressive properties of WRTSF concrete by utilizing WRTSFs of different sizes in quantities less than 1.5%. Only a limited number of studies analyzed the impact of higher than 2% WRTSFs with short-length fibers.

These findings demonstrate the variation in concrete CS by the addition of WRTSFs. After reviewing the literature, it was found that adding 0.40%, by volume, of WRTSF content can result in 29% enhanced CS. In addition, incorporating 4%, by weight, of 29 mm long and 0.20 mm thick WRTSFs can lead to a maximum improvement of 78% in CS. These findings depict the significantly improved strength of concrete by adding WRTSFs. Moreover, the conducted review also indicates that the most significant improvement in CS was achieved by adding WRTSFs at a concentration of 50 kg·m⁻³. The comparison in percentage for CS of WRTSFs with respect to plain/controlled concrete was also done by Zia et al. [102], as shown in Figure 13. Specifically, the use of WRTSFs with a length of 17.50 mm and thickness of 0.25 mm resulted in the most significant improvement in CS. This finding suggests that the concentration and size of WRTSFs play a crucial role in determining the CS enhancement of concrete. Overall, the explored research findings highlight the potential of using WRTSF to enhance concrete strength. The findings suggest that the concentration, length, and thickness of WRTSF are crucial factors that need to be considered when designing concrete with enhanced CS. However, further research is needed to determine the optimal concentration and size of WRTSFs for achieving maximum CS enhancement.

Figure 13

Reported percentage difference in CS of WRTSF-reinforced concrete and plain concrete [102].

9.2 WRTSF effect on the STS of concrete

In the literature, detailed investigations have been carried out to examine the mechanical properties of WRTSF-reinforced concrete [118,169,182–185]. In the case of splitting-tensile strength, Samarakoon et al. [83] reported that the incorporation of these fibers at 1 and 0.5% volumetric content results in enhanced STS of the respective cementitious by 14.2 and 18.3%, respectively. WRTSFs may play a role in resisting crack propagation in concrete, resulting in improved STS [186]. A similar outcome was reported by Skarżyński and Suchorzewski [145] upon the addition of WRTSF in concrete. In this case, the STS was increased by 43% with respect to the reference specimen, which depicts the more effective splitting-tensile behavior of using WRTSFs. Kakvand et al. [147] reported the impact of THE WRTSF diameter on THE concrete STS. It was observed that the incorporation of 70 kg·m⁻³ WRTSFs resulted in enhanced STS up to 35% with respect to the controlled mix. Thirumurugan and Sivaraja [148] reported the effect of WRTSF content, concluding with 9.1, 22.7, 31.8, and 36.4% enhanced splitting-tensile strengths at 0.5, 1.0, 1.5, and 2.0% WRTSF volumetric contents, respectively, compared to the reference mix.

Incorporating WRTSFs in concrete resulted in significantly improved mechanical properties due to their hardening ability against tensile and flexural loadings [95,135,187]. Furthermore, WRTSFs can replace conventional steel fibers with the same amount without any considerable variation in relevant mechanical characteristics. However, Leone et al. [44] reported a slight decrease in WRTSF-reinforced concrete STS with respect to SFRC. More workability of concrete reinforced with waste tires-recycled steel beads up to 4% fiber content was also reported [79]. Aghaee et al. [161] reported that 0.75% volumetric content of WRTSF and conventional steel fiber in concrete resulted in 28 and 26.33% enhanced STS, respectively, with respect to plain concrete. The concrete brittleness was converted into ductility upon the incorporation of WRTSFs. The additional interlocking mechanism is provided by variable diameter and length of WRTSF, as 50% enhanced STS was reported upon incorporating 0.75% volumetric content WRTSFs [188]. However, Atoyebi et al. [4] concluded their research with 14% enhanced concrete STS having 0.6% WRTSF content. The percentage difference of WRTSF-reinforced concrete STS with respect to the reference specimen in light of the reported literature [83,87,161,179,180] is presented in Figure 14. However, the hybrid formulation of WRTSFs and conventional steel fibers resulted in the maximum increase of STS for the respective concrete due to the better mechanical anchorage and effective bridging mechanism [59,84]. Mastali et al. [189] performed the desirability function analysis and reported that in the case of hybrid formulation, 1% volumetric content of conventional steel fibers and 0.5% of WRTSFs could offer better mechanical properties in terms of STS. Similarly, Mastali et al. [181] reported that the hybrid formulation of 0.15% conventional steel fibers and 1.35% WRTSF content gave a desirability function value of 0.620 upon considering the potential global warming factor in the same desirability function analysis. At the same time, in the literature, some conflicting results have also been observed in the case of using WRTSF in concrete in terms of concrete splitting- tensile strength [79,119,167,190,191].

Figure 14

Reported percentage difference in STS of WRTSF-reinforced concrete.

STS is a measure of the ability of a material to resist cracking or splitting when subjected to tensile stress. The literature findings indicate that incorporating WRTSFs in concrete results in enhanced STS. The increase in STS is directly proportional to the volumetric content of the WRTSFs. For example, incorporating 0.5 and 1% volumetric content of WRTSFs in the concrete results in an increase in STS by 18.3 and 14.2%, respectively. The incorporation of WRTSFs in concrete improves the mechanical properties of the concrete by resisting crack propagation. The variability in diameter and length of WRTSFs provides an additional interlocking mechanism that enhances the STS of the concrete. The literature also reports that WRTSFs can replace conventional steel fibers without any significant variation in relevant mechanical characteristics. However, some studies report conflicting results in terms of the STS of concrete having WRTSF, like a slight decrease in STS with respect to steel fiber-reinforced concrete. The hybrid formulation of WRTSFs and conventional steel fibers results in the maximum increase of STS due to better mechanical anchorage and an effective bridging mechanism. The optimal volumetric content of WRTSFs and conventional steel fibers in a hybrid formulation depends on several factors, including potential global warming factor, desirability function analysis, and mechanical properties of the concrete. In summary, incorporating WRTSFs in concrete improves its STS and mechanical properties by resisting crack propagation and increasing its ductility. The use of WRTSFs provides an additional interlocking mechanism that enhances the STS. The optimal volumetric content of WRTSFs and conventional steel fibers in a hybrid formulation depends on several factors, including potential global warming factor, desirability function analysis, and mechanical properties of the concrete.

To sum up, a reduction in STS is reported with the addition of less than 1% WRTSF content, although some specimens exhibited an increase even with less than 1% WRTSF. Specimens that contained 0.20, 0.46, 0.50, 0.50, and 0.80% of WRTSF exhibited a decrease in STS from 10 to 26%. On the other hand, specimens that contained 0.23, 0.30, 0.40, 0.46, 0.50, and 0.50% demonstrated significant improvement, ranging from 3 to 43%, in the STS. No improvement was observed for 0.60% WRTSF, and a decline of 10% was noted only for 1% WRTSF. The addition of 1–5% of WRTSF considerably enhanced the STS. The impact of diameter and the length of fibers on the STS was noticeable, and even with longer WRTSF at 0.20%, a considerable reduction (13%) was observed when conventional mixers were employed [192]. Generally, incorporating an average dosage of 15.71 kg·m⁻³ WRTSF with an average length of 50.86 mm and a diameter of 0.59 mm was observed to cause a reduction in the STS. Previous studies have shown a noteworthy improvement in the STS for WRTSF within the range of 20–60 kg·m⁻³, despite an overall decrease in the STS upon the addition of 15 kg·m⁻³ having an average length of 50.86 mm and a diameter of 0.59 mm. The highest reported increase in the STS was observed by Pawelska-Mazur and Kaszynska [193], who used WRTSF with a diameter of 0.25 mm and a length of 17.50 mm. Short fibers, with a length of less than 20 mm and a diameter of less than 0.30 mm, were found to be more effective in enhancing STS, even when used in a dosage of 60 kg·m⁻³. For long fibers with a length of 50 mm, an increase in the STS of 34% was observed at a dosage of 40 kg·m⁻³. However, the increase in STS was found to decrease the same-size fibers from 40 to 60 kg·m⁻³, with only a 4% improvement observed. Figure 15 illustrates the STS of concrete specimens incorporated with WRTSF obtained from waste tires as a function of their weight fraction. These findings suggest that the increase in the STS can vary for different sizes of WRTSFs at the same proportion of WRTSF.

Figure 15

Reported percentage difference in STSs of WRTSF-reinforced concrete and plain concrete [102].

Hence, it can be summarized that the impact of adding WRTSFs to concrete on its STS is the resistance of a material against cracking or fracturing under tension. Results indicated that adding 2% WRTSF (62 mm in length) to concrete by volume led to a maximal increase in the STS of up to 149%, demonstrating that WRTSFs can significantly enhance the concrete’s tensile strength. According to the available literature, it was found that the STS can be influenced by the length and diameter of WRTSFs used. The greatest increase of 96% was observed at 6% WRTSF (29 mm in length and 0.20 mm in diameter) when the WRTSF was added to the concrete by weight. Moreover, the WRTSF content (in kg·m⁻³) also had an impact on the STS, with the most substantial improvement of 43% occurring at 50 kg·m⁻³ WRTSF (17.50 mm long and 0.25 mm thick). These results highlight the potential of WRTSFs as an effective means of improving concrete’s ability to withstand tension, with the optimal type, length, diameter, and amount of WRTSF depending on the specific application.

9.3 WRTSF effect on the FS of concrete

The FS test is generally conducted as per ASTM C78/C78M-15b standard in the case of plain concrete beamlets. In this case, the beamlets are tested in the flexural testing machine to study the flexural behavior. For fiber-reinforced concrete, the same testing methods and standards were adopted by several researchers [192,194–197] to explore the properties under flexural loading. Accordingly, the same testing standards were reported to be adopted for WRTSF-reinforced concrete [163,167,172,198]. The 15.9% increased FS of WRTSF-reinforced concrete was reported at 0.23% WRTSF content, whereas a 9% improvement was reported by the incorporation of 0.46% WRTSF content in the concrete with respect to the reference specimen [74]. Similarly, Tlemat et al. [93] investigated the WRTSF-reinforced concrete and achieved significant FS enhancement (i.e., up to 150%) by adding 6% WRTSF content having a length of 50 mm and a diameter of 0.8–1.55 mm, extracted by the pyrolysis process. Furthermore, Sengul [46] reported 21.4, 19.6, and 19.6% enhanced FSs of concrete upon incorporating 20, 40, and 60 kg·m⁻³ WRTSF content, respectively. The literature has reported a similar outcome for different WRTSF sizes and volumetric contents [84,95]. Several studies reported that the 0.5% WRTSF content was optimized to attain the highest FS for the respective concrete [199–201]. The percentage difference of WRTSF-reinforced concrete FS with respect to reference specimens in light of the reported literature studies [37,40,87,93,161,179,181] is presented in Figure 16. Grzymski et al. [151] tested the conventional/reference steel fibers and WRTSF-reinforced concrete beams under the application of flexural loading. The tested specimens of plain concrete with reference fibers and WRTSF-reinforced concrete are shown in Figure 17.

Figure 16

Reported percentage difference in the FS of WRTSF-reinforced concrete.

Figure 17

Tested specimens under flexural loading (a) without fibers, (b) conventional fibers, and (c) WRTSFs [151].

The fiber distribution at cracked faces of WRTSF-reinforced concrete and conventional SFRC is shown in Figure 18 [74]. It may be observed here that the fiber distribution in the case of the WRTSF specimen is homogeneous and similar to the case of conventional steel fibers specimens. Najim et al. [117] reported an enhancement of up to 60 and 20% in the first crack load and the ultimate load, respectively, upon incorporating 60 kg·m⁻³ WRTSFs. An enhancement of 75% in flexural stiffness was also observed [88,117,202]. However, it was also reported that adding WRTSFs in concrete does not significantly impact its post-peak behavior. It might be due to an ineffective fiber reinforcement mechanism due to its surface characteristics and geometry, ultimately eliminating the deflection hardening phase [46,136]. The literature also proposes that 25–45% of additional WRTSFs in concrete would produce toughness performance similar to conventional steel fibers [203–205]. A better flexural response may also be attained by combining WRTSFs and hooked end-shaped industrial steel fibers [181,189,206]. Moreover, the WRTSF’s higher aspect ratio significantly enhances flexural response and ductility [36,66,84,164]. For improving the FS, the length of WRTSFs has more effectiveness and efficiency, consistent results were reported in the literature that with enhancing fiber length, an enhancement in FS is observed [40,46,161].

Figure 18

Fiber distribution: (a) WRTSFs and (b) conventional steel fibers [74].

The inclusion of WRTSFs in concrete has been reported to result in substantial improvements in FS, stiffness, and ductility, according to various studies. The FS of WRTSF-reinforced concrete was found to increase with increasing WRTSF content up to an optimum value, beyond which no significant enhancement was observed. The optimum content was reported to be around 0.5% by several studies. However, the post-peak behavior of WRTSF-reinforced concrete was found to be similar to plain concrete due to an ineffective fiber reinforcement mechanism caused by the surface characteristics and geometry of WRTSFs. The aspect ratio and length of WRTSFs were found to significantly influence the flexural response and ductility of concrete. Studies have reported that increasing the fiber length enhances the FS consistently. The literature also proposes that combining WRTSFs with hooked end-shaped industrial steel fibers can improve the flexural response. The fiber distribution at cracked faces of WRTSF-reinforced concrete was found to be homogeneous and similar to that of conventional steel fibers specimens. The enhancement of up to 150% in FS, 75% in flexural stiffness, and 60% in the first crack load have been reported by incorporating WRTSFs in concrete.

Hence, Zia et al. [102] summarized that the addition of 2–3.5% WRTSF content significantly enhanced (i.e., 162%) the FS compared to plain concrete. The specimen with the highest improvement in FS contained 3.5% WRTSF content, with a length of 100.16 mm and a thickness of 0.94 mm. On the contrary, a reduction in FS was observed when the WRTSF diameter or length was decreased for the same WRTSF percentage, as demonstrated by the results of specimens 12–16. However, studies have shown that adding 2% small-sized WRTSF, with 0.20 mm diameter and 13 mm length, to the mixture resulted in a 35% increase in FS. In contrast, a specimen containing 2% WRTSF of larger size and FS of 4.60 MPa did not exhibit any improvement in FS. This could be due to insufficient cement paste to maintain the mixture’s uniformity. Increasing the content of WRTSF from 3.5 to 4% resulted in a decrease in the improvement in FS for the same-size WRTSF from 162 to 128%. On the other hand, a gradual improvement in FS was observed for smaller-sized WRTSF with a diameter of 7.62 mm and thickness of 0.94 mm, ranging from 103 to 122%. For example, the addition of 4% WRTSF (29 mm long and 0.20 mm diameter) in low-strength concrete led to a 9% increase in FS, which decreased to 4% when the same-size WRTSF content was increased to 6%. The use of small-sized fibers at 6% WRTSF led to a significant enhancement in FS, while less lengthy WRTSF could also produce the same strength in concrete. Additional investigation is required to establish the most effective WRTSF proportion for FS, particularly when adding more than 5% of small-sized WRTSF to concrete.

Thus, the impact of adding WRTSFs to concrete on its FS is the ability of a material to resist bending or deformation under load. The literature findings suggest that adding WRTSFs can significantly enhance the FS. Adding WRTSFs into concrete at a volume fraction of 3%, with a length of 45 mm and a thickness of 0.245 mm, led to a notable enhancement in FS (i.e., 157%). This suggests that incorporating WRTSFs into concrete can significantly enhance its ability to resist bending and deformation under load. At 4 and 5% WRTSF, the literature reported an increase of up to 457 and 429% in FS, respectively. However, additional research is necessary to verify the magnitude of the observed increase in FS. This indicates that the quantity of WRTSF used in the concrete can play a crucial role in determining the resulting FS. However, there may be a threshold for the extent of enhancement that can be attained. For instance, when 6%, by weight, WRTSF was incorporated in concrete, the FS was observed to increase to a maximum of 162%. This indicates that the quantity of WRTSF added to the concrete by weight can have a notable influence on the resulting FS. Additionally, the most significant improvement in FS was observed to be 68% upon incorporating WRTSFs with a length of 50 mm and thickness of 0.60 mm at a dosage of 40 kg·m⁻³ in the concrete. The FSs of the concrete are displayed in Figure 19 [102], with the addition of WRTSFs at varying weight fractions. This indicates that the density of WRTSFs added to the concrete can also impact the resulting FS. In summary, the findings of the studies indicate that incorporating WRTSFs can greatly improve the FS and the resistance of concrete to bending and deformation when subjected to load. The optimal characteristics of WRTSFs, including type, length, diameter, weight proportion, and density, may vary depending on the specific application. However, the degree of improvement in FS may be limited, and further research is needed to explore the potential of WRTSFs for improving the performance of concrete.

Figure 19

Reported percentage difference in FSs of WRTSF-reinforced concrete and plain concrete [102].

9.6 WRTSF – matrix interface properties

Gul et al. [76] performed experiments to examine the concrete beams’ failure modes that were reinforced with WRTSFs. Their research also included an examination of the crack-bridging behavior of the reinforced beams, and the results of their experiments are presented in Figure 20. Crack bridging is a key mechanism by which waste tire SFRC improves its toughness and durability. The addition of WRTSF to concrete reinforces the material by bridging the cracks which may occur in concrete. This can prevent the propagation of cracks and lower the risk of failure. The crack bridging mechanism in WRTSF-reinforced concrete can be explained by several factors, including fiber geometry, fiber–matrix bond strength, and fiber orientation. The crimped shape of WRTSFs provides them with a higher surface area and roughness, which enhances their interfacial bond with the surrounding cementitious matrix. This strong bond helps to transfer the stress across the crack face, thereby bridging the crack and preventing its propagation. The fiber orientation also plays a crucial role in the crack-bridging mechanism of WRTSF-reinforced concrete. Studies have shown that the fiber orientation perpendicular to the direction of crack propagation is more effective in bridging the crack and preventing its propagation. This is because the fibers act as a barrier against the crack opening and closing, providing a mechanism for crack control and reducing the risk of failure. Furthermore, the crack bridging mechanism of WRTSF-reinforced concrete is influenced by fiber content and distribution. Higher fiber content and uniform fiber distribution lead to more effective crack bridging and improved toughness. However, an excessive number of fibers can lead to fiber clustering and reduced workability of the concrete mix, which can compromise the mechanical properties of the concrete. Overall, the crack bridging mechanism in WRTSF-reinforced concrete is a complex interplay among fiber geometry, orientation, distribution, and interfacial bond strength. Understanding these factors is crucial for optimizing the fiber content and distribution for specific applications and achieving the desired mechanical properties and durability of the WRTSF-reinforced concrete. Furthermore, several other studies conducted in various locations have demonstrated that the addition of 0.5% WRTSFs can lead to improved flexibility in concrete. These findings are consistent with the previous research that has observed enhanced mechanical properties and crack resistance in concrete with the incorporation of WRTSFs [83,213–215]. Consequently, the use of WRTSFs in concrete has been deemed a promising solution for improving the durability and sustainability of concrete, as well as providing an eco-friendly waste tire disposal solution.

Figure 20

(a) SFRC beam failure and (b) WRTSF-reinforced concrete crack bridging action at the tensile face [76].

In recent years, there has been increasing interest in utilizing WRTSF to improve the mechanical properties of concrete. However, the effectiveness of WRTSF in improving the properties of concrete is largely dependent on the uniform dispersion of the fibers throughout the concrete matrix. A uniform dispersion of WRTSF in concrete is important because it helps to ensure that the fibers are distributed evenly throughout the concrete matrix. This, in turn, enhances the load-bearing capacity of concrete by increasing its resistance to cracking and improving its toughness and energy absorption properties. Moreover, a uniform distribution of WRTSF in concrete can improve the bond strength between the fibers and the surrounding cementitious matrix. Several studies have investigated the effect of WRTSF dispersion on the mechanical properties of concrete. For instance, Caggiano et al. reported that a higher dispersion of WRTSF in concrete resulted in a significant increase in the CS and FS [36]. This enhancement was due to the enhanced interfacial bonding among the WRTSF and the surrounding cementitious paste, which was facilitated by the uniform dispersion of the fibers in the concrete.

Zhong and Zhang [103] conducted a study that emphasized the significance of uniformly dispersing WRTSF in enhancing the mechanical properties of concrete. This study indicated that incorporating the WRTSF substantially enhanced concrete’s FS and toughness. However, the effectiveness of the WRTSF in improving the properties of concrete was highly dependent on the uniform dispersion of the fibers in the concrete matrix. Michalik et al. [86] found that the uniform dispersion of the WRTSF in concrete, as illustrated in Figure 21, would result in enhanced toughness and energy absorption characteristics. This implies that incorporating WRTSF appropriately in concrete has the potential to enhance the mechanical properties of the resulting material, and this could have significant implications in several engineering and construction applications. In conclusion, the uniform dispersion of WRTSF in concrete is a critical factor in improving the mechanical properties of concrete. A more uniform distribution of fibers throughout the concrete matrix can improve the load-bearing capability, toughness, and absorption properties. Therefore, careful attention should be given to the dispersion of WRTSFs during the concrete mixing process to achieve optimal performance.

Figure 21

Uniform dispersion of WRTSFs in cementitious composite [86].

Marthong and Marthong [199] found that the incorporation of 0.5% WRTSFs showed a 19% enhancement in FS. In another study by Fraternali et al. [216], the addition of 13.4 kg·m⁻³ of crimped WRTSFs led to improved FS, whereas using the same amount of smooth fibers resulted in reduced strength. The bond between WRTSFs and the surrounding cementitious concrete matrix is a crucial factor in determining the mechanical properties and durability of fiber-reinforced concrete. Several studies have investigated this bond and its influence on the performance of WRTSF-reinforced concrete. This could be attributed to the stronger interfacial bond between the crimped fibers and the matrix. Michalik et al. [86] investigated the interfacial bonding between WRTSFs and the cementitious matrix using scanning electron microscopy (SEM) analysis. They observed a strong bond between the fibers and the matrix, with no signs of debonding or pull-out of the fibers. The interfacial bond strength was observed to be due to the presence of a thin layer of calcium silicate hydrate (C–S–H) gel surrounding the fibers. The interfacial bonding of WRTSFs with the cementitious matrix and their crack resistance phenomenon is shown in Figure 22 [86]. It was found that the surface texture of WRTSFs plays a crucial role in their bonding with the cementitious matrix. The rough surface of crimped WRTSFs was found to provide better interfacial bond strength compared to smooth WRTSFs. This is because the irregular surface of the crimped fibers provides more contact points with the cementitious matrix, resulting in stronger bonding. Moreover, the use of chemical treatments to modify the surface of WRTSFs to enhance their bond with the cementitious matrix has also been explored. These treatments can modify the surface chemistry of the fibers, improving their wettability and adhesion to the cementitious matrix. Overall, the interfacial bond strength between WRTSFs and the surrounding cementitious matrix is critical in determining the performance of WRTSF-reinforced concrete. The bonding can be influenced by several factors, such as fiber geometry, surface texture, and surface treatment. Further research is necessary to better understand and optimize the bonding between WRTSFs and the cementitious matrix for specific applications.

Figure 22

Interfacial bonding of WRTSFs with the surrounding matrix [86].

In order to assess the effectiveness of fiber reinforcement in concrete, it is important to evaluate the bond between the fiber and the surrounding grout. To achieve this, microscopic analysis was performed by Michalik et al. [86] on fractured concrete samples, which allowed the observation of the boundary between the fiber and the grout. Through this analysis, it was possible to assess the level of anchorage and adhesion between the fiber and grout, as well as the quality of the fiber surface. The anchorage and adhesion between the fiber and grout are critical factors that determine the effectiveness of fiber reinforcement in concrete. If the fibers are not properly anchored and adhered to the grout, they may not be able to effectively reinforce the concrete and prevent cracks from propagating. Therefore, it is important to evaluate the bond between the fiber and grout through microscopic analysis. Microscopic images of fractures in concrete with MSFs are often used to evaluate the fiber–grout bond. These images can provide insights into the quality of the fiber surface and the level of anchorage and adhesion between the fiber and grout. Figures 23 and 24 show examples of microscopic images of concrete fractures with MSFs, which can be used to evaluate the fiber–grout bond and assess the effectiveness of fiber reinforcement in concrete.

Figure 23

Fractures in concrete reinforced with WRTSFs [86].

Figure 24

WRTSFs coated with cement hydration products [86].

Figures 25 and 26, as presented by Michalik et al. [86], illustrate microscopic images of concrete fractures with WRTSFs. After conducting an analysis, it was found that WRTSFs exhibited strong adhesion to the grout, suggesting that they were well-anchored. In addition, the purification process used to eliminate rubber and textile impurities led to a more refined surface, which improved the adhesion between the fibers and the cement matrix. The interface between the fibers and the grout was appropriately dense and non-porous in both instances, with evidence of cement hydration products visible on the surface of WRTSFs. Additionally, Figure 25 demonstrates the crack-bridging ability of WRTSFs, as a microcrack was bridged by the fibers. This occurs when fibers embedded in the cement matrix distribute the load and bridge the microcracks, effectively enhancing the ductility and durability of the concrete. The results obtained from the microscopic analysis support and validate the mechanical test findings that demonstrate the high quality and purity of the recycled tire steel fibers, their strong adhesion to the grout, and their efficient reinforcement of the brittle cement matrix. These outcomes imply that WRTSFs could be a viable substitute for traditional steel fibers because of their excellent ability to adhere to the grout and bridge cracks.

Figure 25

Concrete fracture having WRTSFs [86].

Figure 26

WRTSFs coated with hydration products of cement.

10 Durability of WRTSF-reinforced concrete

Incorporating WRTSFs in concrete improves impact resistance and shrinkage behavior [117,138,217–219]. Corrosive agents’ ingression in concrete uses the three primary transportation modes, i.e., capillary transport, diffusion, and permeation. The resistance against crack propagation tends to reduce the harmful chemical ingression in concrete, resulting in an overall reduction in concrete deterioration. Restraining the width of cracks up to 0.3 mm would result in aesthetic impact only due to surface fiber’s deterioration in a corrosive environment [220–222]. In the case of a chloride-rich atmosphere, CO₂ is susceptible to fibers’ disintegration because of corrosion and thus shows a reduced performance of fiber-reinforced concrete [38,115,223]. The decay considerably influenced the interaction between the fiber and cement matrix. The pre-treatment of WRTSFs at 350℃ altered the retained austenite to bainite in the microstructure and ultimately enhanced the WRTSF hardness. Electrochemical outcomes depicted that in the case of WRTSF in 3.5 wt% NaCl solution, there was a probability of 90% of corrosion, and WRTSFs were reported with more susceptibility to corrosion than conventional steel fibers. Moreover, the presence of rubber at the surface of the fiber does not considerably influence the corrosion resistance of WRTSFs and has a minor impact on the corrosion resistance of WRTSF-reinforced concrete [115]. In the same manner, after the exposure to chloride alternative cycles, no considerable disintegration was observed in WRTSF concrete [224], and no naked-eye deterioration was noted after accelerated exposure to chloride solution. Embedded fibers were observed to be free from deterioration [220]. It was also observed that a mixture of conventional and WRTSF addition in concrete improves durability [225].

11 Applications of WRTSF-reinforced concrete

SFRC is applicable for several civil engineering structures such as pavements, bridge decks, hydraulic structures, tunnel linings, refractory concrete, slabs, foundation, precast elements, and fiber shotcrete [14,23]. However, the applications of SFRC are based on the engineer’s expertise, as the addition of only 1% conventional steel fibers doubles the cost of the respective concrete material [15]. It has made WRTSF an effective and economical alternative to SFRC. In the literature, it is reported that WRTSF is likely to improve the concrete’s mechanical properties. The expectations are to replace the conventional steel fibers with WRTSFs by maintaining uniformity in the case of WRTSFs by adopting more novel techniques for extraction and cutting. The potential civil engineering structural applications of WRTSF-reinforced concrete are summarized from relevant literature data. For example, the governing parameters in a rigid pavement design are compressive and FSs. It is reported in the literature that the primary two parameters that influence the rigid pavement design and thickness are the elastic modulus and FS of the concrete [102,192,226–229]. Following the above recommendations, Farooqi and Ali [228] recommended 7% less thickness in fiber-reinforced concrete than in plain concrete. Similarly, Khan and Ali [230] recommended fiber-reinforced concrete as a more effective and inexpensive alternative for concrete pavements resulting from 12.4 and 16.2% enhanced CS and FS, respectively, than plain concrete [230]. However, in the case of WRTSFs, the CS and FS were found to be 78 and 162%, respectively, compared with reference plain concrete. Thus, there is an anticipation of improved performance of WRTSF-reinforced concrete with respect to the reduced pavement thickness and cost.

Moreover, the occurrence of cracks in canal lining can be affected by multiple factors, which may include water absorption, shrinkage, permeability, tensile strength, and differential settlement [231,232]. The shrinkage cracking may be limited in the case of lesser tensile stresses due to shrinkage compared to the concrete tensile strength. So, it is evident that the tensile strength of concrete is a governing parameter in the case of shrinkage cracking. Further, the differential settlement is another possible cause for the reduced canal lining serviceability. Bending stresses are caused due to differential settlement; hence, higher FS would effectively control it. A combination of these factors indicates that the use of fiber-reinforced concrete can be a beneficial strategy to mitigate cracking in hydraulic structures. Several researchers suggested nylon and polypropylene fibers to improve hydraulic structures’ durability [233,234]. However, due to its strength properties, the WRTSF-reinforced concrete is reported as an economical and effective solution to replace conventional steel fibers and artificial/synthetic fibers for the applications of hydraulic structures [77,100,182]. Hence, it can be summarized that WRTSF-reinforced concrete has several applications in the construction industry. The improved mechanical properties of WRTSF-reinforced concrete make it suitable for use in various infrastructure projects, such as the following:

• Pavements: WRTSF-reinforced concrete can be used for the construction of durable and long-lasting pavements, as it can withstand the weight and stress of heavy traffic.

• Tunnel linings: WRTSF-reinforced concrete is also suitable for tunnel lining applications, as it can provide improved strength and durability to the structure.

• Bridge decks: WRTSF-reinforced concrete can be used in bridge decks to provide improved tensile strength and durability, which can enhance the structure’s overall performance and lifespan.

• Hydraulic structures: WRTSF-reinforced concrete can be used in the construction of hydraulic structures, such as dams and canals, due to its improved strength and durability.

• Slope stabilization: WRTSF-reinforced concrete can be used in slope stabilization projects, as it can provide improved stability and resistance to erosion.

The suitable WRTSF-reinforced concrete composite for these applications depends on several factors, such as the specific project requirements, the environmental conditions, and the availability of materials. The appropriate composition and characteristics of WRTSFs must be determined based on the specific application and the desired performance characteristics. However, it is generally recommended to use WRTSFs with appropriate length, diameter, and content to achieve the desired mechanical properties in the composite material.

12 Discussion

The incorporation of fibers in concrete is an effective method for enhancing its energy-absorption capacity [235–237]. The incorporation of steel, artificial, and natural fibers in cementitious composites has been explored by several researchers for various applications [196,197,236,238–243]. The SFRC was also investigated to improve the concrete structural performance [244–250]. The CS, STS, and FSs were reported to be enhanced by adding steel fibers in concrete up to 45, 5.3, and 5.5 MPa, respectively [245–247]. Similarly, the reported results for synthetic fiber-reinforced concrete in the case of nylon and acrylic fibers were up to 6.3 MPa FS and 3.9 MPa STS [251,252]; whereas for polypropylene fiber-reinforced concrete, the CS was increased up to 46 MPa, and the STS was enhanced up to 4 MPa [253]. In the case of carbon fiber-reinforced concrete, the reported CS and STS were 26 and 5 MPa, respectively [254]. In the same manner, the reported results for glass fiber-reinforced concrete in the case of CS, STS, and FS were 14.1, 1.73, and 3.31 MPa, respectively [251]. However, various approaches have been adopted to reduce the cost of concrete composites along with improved mechanical properties. The utilization of natural fibers as dispersed reinforcement in concrete is one among them. The CS, STS, and FS of sisal fiber-reinforced concrete are reported to be up to 26, 2.8, and 5.9 MPa, respectively [255–257]. As far as CFRC is concerned, the reported CS was up to 17.3 MPa, STS was up to 2.96 MPa, and FS was up to 4.7 MPa [255]. Similar results were reported for concrete reinforced with jute, wheat straw, rice straw, sugarcane bagasse, and banana fibers [192,258–264]. Keeping in mind the environmental impact and economic factors for progressing toward sustainable development, the addition of WRTSFs in concrete is among the effective solutions. The mechanical properties of WRTSF-reinforced concrete are comparable with other fiber- reinforced concretes, as summarized from previous studies. The CS, STS, and FSs of WRTSF-reinforced concrete were reported up to 39, 7, and 3.44 MPa, respectively [37,46,110,120,145,146]. Hence, in light of mechanical properties, WRTSF-reinforced concrete is reported in the literature as an economical and effective solution to replace industrial steel and artificial/synthetic and natural fibers for various civil engineering applications [77,100,182].

British Standards (BS) and the American Concrete Institute (ACI) define the relationship between the CS ( f c ′ ) of plain concrete and its STS or FS. These standards outline the correlation among CS, STS, and FSs. The following equations show the standardized relations:

Eqs. (1) and (2) represent the possible correlation between the STS and FS. The linear enhancement in the STS / f c ′ ratio by incorporating up to 2% WRTSFs, as shown in Figure 27 [102], depicts the improved tensile strength using WRTSFs. Moreover, upon enhancing the WRTSF content, an increasing STS / f c ′ ratio trend has also been observed. The suggested CS/STS ratio for plain concrete may not be appropriate for WRTSF-reinforced concrete, and hence, it needs to be adjusted accordingly. The acceptable agreement is depicted from modest, i.e., 0.46, R ² value for 2% WRTSF. However, beyond this 2% WRTSF, the correlation trend for this agreement between the WRTSF volumetric content and STS/CS ratio is reported to be reduced [77]. The average ratio for STS/CS ratio is 0.67, which is higher as compared to the ratio suggested in the case of plain concrete. Hence, a more precise STS/CS may be attained by considering a denser dataset regarding experimental outcomes for CS and STS of WRTSF-reinforced concrete.

Figure 27

WRTSF (0–2% content) effect on the STS/CS ratio [102].

The correlation between the FS/CS ratio and the WRTSF volumetric content (0.2–2%) is presented in Figure 28 [102]. It may be noted that the FS/CS ratio increases with increasing WRTSF content, depicting that the incorporation of WRTSF results in enhanced FS. This indicates that the FS/CS ratio that is applicable to plain concrete cannot be directly applied to WRTSF-reinforced concrete, and thus requires adjustment. An increasing linear trend is observed for the FS/CS ratio with an increasing WRTSF volumetric content in concrete. The coefficient of regression (R ²) depicts a lesser correlation, i.e., 0.15R ², between the FS/CS ratio and WRTSF content, representing invalidity of correlation as in the case of plain concrete. Hence, there is a need for more extensive experimental data for devising a specific FS/CS ratio for WRTSF-reinforced concrete. The available experimental data are insufficient for formulating a reliable correlation for FS/CS ratio. In this scenario, the mean FS/CS ratio is 1.39, which exceeds the suggested ratio for plain concrete. To obtain a more accurate correlation between the FS/CS ratio in WRTSF-reinforced concrete, a broader database of experimental results on FS and CS should be considered.

Figure 28

WRTSF (0–2% content) effect on the FS/CS ratio [102].

Recycled steel fibers can be an effective solution for improving the mechanical properties of concrete. Steel fibers can increase the tensile strength and ductility of concrete, as well as provide improved impact resistance and crack resistance. However, the choice of recycled steel fibers for this purpose should be carefully considered. One important factor to consider is the aspect ratio of the fibers, which is the ratio of the fiber length to its diameter. Higher aspect ratio fibers generally provide better reinforcing properties than lower aspect ratio fibers. However, the length of the fibers should also be appropriate for the specific application. Shorter fibers may be more effective in reducing shrinkage cracking, while longer fibers may be more effective in providing post-cracking toughness. The shape of the fibers is also an important factor. Generally, fibers with a hooked or crimped shape are preferred over straight fibers because they provide better bonding with the concrete matrix. The surface properties of the fibers should also be considered, as fibers with a rougher surface can provide better mechanical interlocking with the concrete. The quality of the recycled steel fibers should also be evaluated. The fibers should be free from rust, dirt, and other contaminants that can weaken the bond between the fibers and the concrete matrix. The quality of the steel itself should also be considered, as low-quality steel may lead to fibers that are prone to breakage and may not provide the desired reinforcing properties. In addition to these factors, the quantity of fibers used in the concrete mix should be carefully considered. While a higher fiber content generally results in better mechanical properties, there is a limit to how much fiber can be added before it becomes impractical or leads to a decrease in workability. Overall, the choice of recycled steel fibers for improving the mechanical properties of concrete depends on a variety of factors, including the specific application, the aspect ratio, the shape of the fibers, the quality of the fibers, and the quantity of fibers used in the concrete mix. A careful evaluation of these factors can help ensure that the right type of recycled steel fibers is selected to achieve the desired improvements in mechanical properties.

Choosing the appropriate fiber type for a particular application requires considering various factors such as mechanical properties, chemical compatibility, cost, availability, and processing requirements. The mechanical properties of the fiber such as elastic modulus, tensile strength, and density are crucial in determining the suitability of the fiber for a specific application. For instance, high elastic modulus fibers are desirable in applications where stiffness is essential, while high tensile strength fibers are preferable in applications where strength is crucial. In contrast, low-density fibers are desirable in applications where weight reduction is important. Therefore, the fiber with mechanical properties that meet the application requirements should be chosen. The fiber should be compatible with the matrix material and the environment in which the composite will be used. Chemical incompatibility can result in decreased strength and durability. Therefore, fiber should be selected based on its resistance to the chemical environment of the application. The cost and availability of the fiber are also essential factors to consider. Some fibers may be expensive or difficult to source, making them impractical for certain applications. Therefore, the fiber should be chosen based on its cost-effectiveness and availability. The processing requirements of the fiber, such as the need for surface treatment or compatibility with the manufacturing process, should also be considered. The fiber should be chosen based on its compatibility with the processing requirements of the application.

Utilizing WRTSFs in concrete is a promising sustainable solution to the issue of tire waste and can also improve the mechanical properties of concrete. The most suitable type of WRTSFs for enhancing concrete’s mechanical properties is dependent on several factors, including the aspect ratio, tensile strength, diameter, and length. The aspect ratio is a crucial factor in determining the effectiveness of fibers in reinforcing concrete. It is calculated by dividing the length of the fiber by its diameter. Higher aspect ratios generally result in better reinforcing effects. According to studies, WRTSFs with aspect ratios between 50 and 100 are the most effective in enhancing the mechanical properties of concrete [14,74,83]. Fibers with lower aspect ratios tend to have a lower impact on improving concrete properties. Another crucial factor is the tensile strength of the fiber. WRTSFs with high tensile strength are capable of effectively transferring stresses between the cement matrix and aggregates. Research has shown that fibers with tensile strengths greater than 1,000 MPa are the most effective in enhancing concrete’s mechanical properties [66,84,88,99,101,103]. The size of WRTSFs is a crucial factor in determining how effectively they can improve the mechanical properties of concrete. Studies indicate that fibers with diameters ranging from 0.2 to 0.5 mm are the most effective in improving the mechanical properties of concrete [74,84,89,100]. Fibers with smaller diameters tend to have a limited impact on enhancing the properties of concrete. Another factor that affects the WRTSF’s ability to improve the concrete mechanical characteristics is the fiber length. Studies suggest that fibers with lengths ranging from 25 to 50 mm are the most effective in enhancing the properties of concrete [46,87,88,110]. Longer fibers tend to have a lower impact on improving concrete properties. In summary, for enhancing the concrete mechanical characteristics, WRTSFs with aspect ratios between 50 and 100, tensile strength exceeding 1,000 MPa, diameters ranging from 0.2 to 0.5 mm, and lengths ranging from 25 to 50 mm are the most suitable. Selecting the appropriate type of fibers is crucial for achieving the desired mechanical properties of concrete for specific applications.

13 Conclusions and recommendations

Previously, significant progress in the scientific literature related to communication networking inspired the scientific community to introduce a metric that serves as the foundation for modern bibliometrics. This metric employs statistical analysis techniques to establish reliable value scales. Conducting a research study based on a literature review is a suitable approach for identifying the increasing trend of utilizing WRTSFs in concrete to enhance its mechanical properties. The primary aim of the current research is to conduct a systematic review, including a scientometric analysis of the available literature on different aspects of WRTSFs for their application in reinforcing concrete. The following conclusions are drawn from the conducted review:

• The rapid progress in the sector of transportation results in the end-of-life tires problem, becoming a critical issue in various countries. The end-life tires are ultimately disposed of as waste, consequently enhancing the annual discarding of millions of worn-out tires. The steel fibers extracted from waste tire recycling can be an alternative to conventional steel fibers. Scientometric analysis reveals that the most frequently adopted keywords for searching WRTSF-reinforced concrete-related published documents in Scopus are “steel fibers,” “recycling,” “concretes,” “compressive strength,” “tensile strength,” and “fiber-reinforced concrete.”

• WRTSFs can effectively improve the mechanical properties of concrete but careful consideration of factors such as the aspect ratio, shape, surface properties, quality, and quantity of fibers is necessary for optimal results. Similarly, choosing the right fiber type for a specific application requires considering various factors such as mechanical properties, chemical compatibility, cost, availability, and processing requirements. Utilizing WRTSFs in concrete is a promising sustainable solution, and the most suitable type of WRTSFs for enhancing concrete’s mechanical properties has been found to have aspect ratios between 50 and 100, tensile strengths greater than 1,000 MPa, diameters ranging from 0.2 to 0.5 mm, and lengths ranging from 25 to 50 mm. Proper selection of the appropriate fiber type is essential for achieving the desired mechanical properties of concrete for specific applications.

• There are different influences of WRTSFs’ shape, surface texture, and type on the behavior of concrete. As reported in the literature, the diameter, length, elastic modulus, and tensile strength ranges for WRTSFs are 0.15–1.15 mm, 15–60 mm, 200 GPa, and 1,015–2,577 MPa, respectively. Further, the WRTSF content, as reported in the literature, for WRTSF-reinforced concrete are between 10 and 90 kg·m⁻³, by weight and 0.25 and 6%, by volume. The WRTSF-reinforced concrete possesses significantly improved mechanical characteristics.

• In conclusion, incorporating WRTSFs into concrete can considerably improve its CS, STS, FS, fracture toughness, and energy absorption capacity. The resulting strength and toughness can be influenced by the optimal type, length, diameter, weight proportion, and density of WRTSFs used in the concrete mixture. According to existing studies, incorporating WRTSFs into concrete can lead to significant improvements in various strength properties. For instance, the addition of 0.40% WRTSF by volume fraction has been shown to enhance CS by up to 29%. Adding 2% WRTSF by volume fraction, on the other hand, can result in a maximal increase of up to 149% in STS. Furthermore, including 3% WRTSF by volume fraction can lead to an enhancement of 157% in FS, and improve the energy absorption capacity by up to 82% and fracture toughness by up to 110%. However, further research is needed to determine the optimal concentration and size of WRTSFs for achieving maximum strength enhancement.

• Fiber–matrix interaction in the case of WRTSF-reinforced concrete revealed that the effectiveness of WRTSFs in concrete depends on fiber dispersion, orientation, distribution, and bond strength. A uniform dispersion enhances the load-bearing capacity, toughness, energy absorption, and bond strength. Proper incorporation of WRTSFs can improve mechanical properties, important for construction and engineering. Dispersion should be carefully considered during mixing for optimal performance.

• WRTSF-reinforced concrete possesses enhanced mechanical properties and can be utilized in various construction applications. It is a viable material for constructing long-lasting pavements capable of bearing heavy traffic loads. Additionally, it can be employed in tunnel lining applications to provide increased strength and durability. In bridge decks, WRTSF-reinforced concrete can improve the structure’s tensile strength and lifespan. For hydraulic structures such as dams and canals, WRTSF-reinforced concrete can offer improved strength and durability. Moreover, it can be utilized in slope stabilization projects to enhance stability and erosion resistance. The selection of a suitable WRTSF-reinforced cementitious composite depends on several factors, including project requirements, environmental conditions, and material availability. The appropriate composition and characteristics of WRTSFs should be determined based on the specific application needs and desired performance characteristics. In general, it is recommended to use WRTSFs with appropriate length, diameter, and content to achieve the desired mechanical properties in the composite material.

  However, further studies should be conducted to explore the possible applications of WRTSFs in concrete. For the practical applicability of WRTSF-reinforced concrete in the construction industry, the following recommendations for future studies are made:

• Though in-depth experimental investigations have been made in the literature to evaluate the fresh and hardened properties of WRTSF-reinforced concrete, the studies regarding other characteristics of WRTSF-reinforced concrete are still limited that need to be explored.

• It is essential to conduct a thorough examination of the durability characteristics, such as resistance to water and gas permeability, chloride, and freeze–thaw to ensure comprehensive testing.

• The WRTSF-reinforced concrete performance after exposure to elevated temperatures requires further exploration to understand the fire impact on this sustainable composite.

• Moreover, further research needs to be conducted on developing the relation among the processes for recycling, the environmental effect, and the possibilities for modeling.

• Furthermore, more research may be done to improve the interfacial bonding among all the components.

• Also, the modification of ground tire rubber surface and mixing processes needs to be more cost-effective and environmentally friendly to pursue green chemistry concepts.

• In addition, based on the conducted review, literature is lacking in terms of mixing ground tire rubber with virgin rubber. Hence, it should also be focused on in future research.

• The incorporation of fiber in concrete is made to enhance fracture characteristics. However, as reported in the literature, the incorporation of neither WRTSFs nor conventional steel fibers considerably alters the horizontal splitting force. So, further in-depth exploration is required for fracture characteristics such as characteristic lengths and fracture energy.

• During placing, the fiber alignment may also affect the respective concrete properties, which should be explored in future studies. The top/bottom alignment angles and orientation ratio of fibers should be assessed for better acceptability and applicability.

• The performance of WRTSF-reinforced cement-treated mixtures is promising; however, considering the practical applicability, in-depth investigation is still required on a larger scale.

• Additionally, a thorough investigation regarding the fatigue performance of WRTSF is also an essential aspect of understanding the WRTSF-reinforcement effect on modulus degradation and strain reduction of the respective cementitious mix.

• The time-dependent long-term characteristics of WRTSF-reinforced concrete, like creep and shrinkage, also need to be investigated further.