Concrete is a ubiquitous construction material, the demand for which continues to increase in line with the ongoing industrialization and urbanization of modern society due to its availability everywhere and ease to produce and uses [1,2]. However, the main drawback of using concrete is its brittleness and susceptibility to crack openings and their propagations, thereby creating many problems in its application. A crack in a concrete structure leaves a negative impression on the strength, toughness and serviceability of the structures. One of the solutions to this problem is the inclusion of small fibers in the concrete matrix [3]. Fiber Reinforced Concrete (FRC) is the composite material obtained after mixing of Portland cement concrete with more or less randomly distributed fibers. Use of Fiber Reinforced Concrete (FRC) dates back to the 19th century and since then, FRC has been widely used as a means of improving concrete specifications. In last three decade, the of development FRC progress was permitted to produce a variety of concrete types including Lightweight Fiber Reinforced Concrete (LWFRC), High-Performance Fiber Reinforced Concrete (HPFRC), Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC) and Self-Compacting Fiber Reinforced Concrete (SCFRC) [4]. Fibers used in FRCs are made out of a variety of different materials such as steel, glass, carbon, polyethylene and nylon and are used for various purposes [5]. Steel fibers are typically employed to enhance toughness, tensile and flexural strength of concrete and also to prevent crack propagation in the post-peak regime [6]. The performance of fibers in concrete and their effect on the mechanical properties are highly dependent on the shape and dimensions of fibers, volume fraction and orientation of fibers [7]. Investigations on the cyclic behavior of FRCs showed that fibers affect the accumulative elastic stiffness and plastic strain in FRCs [8]. Moreover, the addition of fibers to concrete modifies its properties both in plastic and hardened state and therefore results into a more sturdy concrete. The aim of this research is to review the effects of addition of SFs in concrete and investigates the mechanical properties.

Different Types of Fiber Reinforced Concrete

According to the origin of fiber or its modulus of elasticity, there are two ways to classify fibers. With regards to their modulus of elasticity, fibers can be divided into two simple group’s namely hard and soft intrusions.

The hard intrusion possesses elastic modulus greater than concrete mix, while the soft intrusion possesses elastic modulus lesser than concrete mix. In brief, glass, steel and carbon fibers having an elastic modulus higher than the cement mortar matrix, which leads to enhances both the impact resistance and flexural strength of concrete. In contrast, vegetable and polypropylene fibers having elastic modulus lower than the cement mortar matrix, which is contributed to improving the impact resistance of concrete alone and there are no many observation effects on the flexural strength of concrete. Based on the origin of fibers, fibers can be classified into three groups including metallic fibers (i.e., carbon steel, stainless steel and steel, whilst, mineral fibers (i.e., glass fibers and asbestos), as well as organic fibers. Organic fibers can be further divided into natural and man-made fibers. Natural fibers which are including (leaf and wood) as well as (silk and hair) fibers can also be classified into sisal or vegetable and animal origins respectively. Another type of organic fiber namely man-made can be classified into natural polymer including protein and cellulose fibers and synthetic fibers including polypropylene and nylon. The classification of different types of fiber is summarised in Figure 1. Moreover, the physical properties of different kinds of fibers are listed in Table 1.

Figure 1: Classification of Different Types of Fibers [9]

Table 1: Physical and Mechanical Properties of Fibres [10]

Steel Fiber Reinforcement Concrete

Steel Fiber (SF) is the most popular type of fiber used as concrete reinforcement. Initially, SFs are used to prevent/control plastic and drying shrinkage in concrete. Further research and development revealed that addition of SFs in concrete significantly increases its flexural toughness, the energy absorption capacity, ductile behaviour prior to the ultimate failure, reduced cracking and improved durability [12-15]. The utilizing of steel fibers can be lead to improve the mechanical properties of concrete including the flexural and tensile strengths and also increased the durability of structures that exposed to harsh environment by hindering the aggressive species (i.e., Cl⁻ and CO₂) to penetrate the concrete matrix. The fibers have the ability to enhance the bonding of concrete materials. Hence, allowing the fiber-reinforced concrete to resist the stresses during the post-cracking stage [16-19]. Ghalehnovi et al. [16] studied the influence of steel fiber-reinforced concrete on flexural strength and ductility by using coarse recycled aggregate. The results indicated that adding 2% of SF-reinforced led to increasing the strength of the reinforced concrete beam substantially. Zhu et al. [20] studied the behaviours of incorporation of the SF with BFRP (basalt fiber‐reinforced polymer) rebar on cracks and deflections of the concrete beam below static loading. The results demonstrated that the presence of SF in concrete beam reinforced with basalt fiber‐reinforced polymer can be effectively reducing the wide cracks and deflections as well. The behaviour of compressive strength for SFRC exposed to high temperature has been investigated by many researchers [12,13,21,22], the results showed that the compressive strength is decreased linearly for concrete treated with SFRC under high temperatures. However, the loss of strength rate could be affected by the alteration of the concrete mix materials. Figure 2 shows the drop of compressive strength for SFRC under high temperature in the presence of various types of pozzolanic materials with different percentages such as Fly Ash (FA), Silica Fume (SF), slag and metakaolin.

Figure 2: Effect of High Temperature on Compressive Strength with 1% FRC [11]

Polypropylene Fibers

Polypropylene fibers (PPF) are one of the main type of fibers which is used as a reinforcing material and can be divided into two types based on their length namely microfibers and microfibers. Al Qadi and Al-Zaidyeen [23] reported that there is no major improving of compressive and flexural strengths for concretes treated with PPF. In contrast, the flexural toughness and impact resistance of concrete were observed to increase in the presence of PPF. PPFRC is not observed to enhance the compressive strength of concrete, whereas the increment of compressive strength is 6% when adding 0.3% of the fiber [4]. However, some researchers Ahmed et al. [24], Assad et al. [25] and Yap et al. [26] found that the exceeding fiber dosage of more than 0.3% exhibited increasing in compressive strengths at early ages and then decreased. This could be explained by poor workability and increasing of air porosity as well as air content of concrete [15,27]. The effect 

Glass Fibers

Glass Fibers Reinforced Concrete (GFRC) is a lightweight and inexpensive material, so, it is contributed to the technology, aesthetics and economics of the construction industry worldwide for more than 40 year [19]. Other researchers [28] concluded that adding a small amount of GFRC into concrete indicated a reduced in the modulus of elasticity. According to the experiment results that mentioned by Huseien et al. [17], Kene et al. [29] and Löber and Holschemacher [30], GFRC has ability to improve the flexural strength of concrete by increasing the content of fiber in concrete matrix. Regardless of a little increase in the flexural strength of concrete, GFRC rises the load-carrying capacity of concrete and minimize the unexpected collapse of concrete structure due to their resistance to control the crack proportion of concrete matrix [18,31,32]. While, Mirza and Soroushian [33] reported that inclusion alkali resistant glass fibers into lightweight concrete was led to improving the concrete properties such as “ductility, flexural strength, temperature resistance, as well as restrained shrinkage cracking. The increment in flexural strength by adding a variety of GFRC percentages were depicted in Figure 3.

Carbon Fibers 

Due to its benefits, such as high elastic modulus, high corrosion resistance, high tensile strength and low density, Carbon Fibers (CFs) have been extensively utilized in concrete [34,35]. Kizilkanat [36] point out the toughness of high-strength concrete, load-bearing capacity, compressive strength and fracture energy can be enhanced in the presence of CFs. Mastali et al. [37] concluded that CFs have the ability to enhance the impact resistance of self-compacting and mechanical properties of concretes. Nevsky et al. [38] compared the compressive and tensile strengths for concrete treated with and without CFs. They found that both of compressive and tensile strengths were increased by 42.7 and 16.4%, respectively. Rodin et al. [35], Asaad et al. [39], Safiuddin et al. [40] observed that CFs enhanced the flexural toughness, compressive strength and strength, splitting tensile strength of concrete.

Figure 3: Comparison of Flexural Strength Between Control and Treated Concrete by GFRC at Different Ages [3] 

Natural Fibers

Based on their common uses in concrete, natural fiber can be divided into three types namely; bamboo, jute and coconut fibers [3]. Dewi and Wijaya [14] and Zhang et al. [41] studied the influence of bamboo fiber on concrete crack and they concluded that adding bamboo fiber into concrete improved the post-cracking load-carrying capacity as well as decrease the beam deflection and crack width. In contrast, many researchers have found that the inclusion of jute fiber not only improving the fracture mechanism but also increasing the mechanical properties of concrete [31,42,43]. However, the mechanical properties of concrete were found to increase by incorporating coconut fiber into concrete [44].

In general, the ability of conventional concrete to resist the cracks is low, its tensile strength is low with limited ductility, hence, it must be reinforced. In this paper, the behavior of fiber-reinforced concrete on mechanical properties has been reviewed. The research studies demonstrate that the performance of normal concrete can be increase by the incorporation of a small amount of fibers, distributed regularly and close to each other. The fiber in concrete acts to improve the mechanical properties of concrete and decrease the cracks as well. A variety of fibers such as glass, steel, polypropylene, carbon, bamboo, jute and coconut has been used in concrete for over 40 years.

Acknowledgment

The authors are thankful for Iraq University College (IUC) for its financial support for this work.

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