A Review on the Performance of Fibers on Restrained Plastic Shrinkage Cracks

Abstract

Multiple studies have investigated the use of steel, synthetic fibers, and natural fibers to reduce plastic shrinkage cracks in concrete, which are mostly caused by water evaporation from the surface of the material. This review used original published research articles from the Web of Science and Scopus database to evaluate the performance and relationship between the fiber volume, aspect ratio, compressive strength, and plastic shrinkage cracking. This review also discussed the most widely used technique for evaluating plastic shrinkage cracking, the ASTM C 1579, with two bottom restraints and a central stress riser to induce cracking, and its modified version with additional reinforcement for further restraining the ASTM C 1579 mold. Longer fibers function better than shorter fibers because of their larger surface area, which allows them to bridge fissures. It was also observed that crack initiation time is delayed when fibers are added to concrete. In addition, as the volume proportion of the fibers increased, the plastic shrinkage cracks decreased, but the compressive strength declined. Furthermore, the volume fraction of the fibers had a greater effect on reducing cracking than the aspect ratio. It was also concluded that a fiber volume inclusion below 1% is best.

1. Introduction

1.1. Plastic Shrinkage

Drying induces plastic shrinkage, which is primarily controlled by the complex interplay between internal and external factors [1,2,3]. The evaporation of water from a freshly placed concrete surface before the hardening or suction of drying are the two main causes of plastic shrinkage. Plastic shrinkage occurs when evaporation outpaces bleeding at the surface. The plastic shrinkage of concrete increases as the amount of water evaporates [4]. The voids between the particles in the fresh concrete are filled with water. A complex network of menisci is created when external factors, such as evaporation at the surface, remove water from the paste.

This, in turn, results in a negative capillary pressure, which causes contraction of the paste volume [5]. Tensile stress builds up in the surface layers during contraction as a result of the non-shrinking constraint of the inner concrete. Owing to its limited strain capacity and poor tensile strength, concrete is vulnerable to cracking [6]. Plastic shrinkage can result in specific visible cracks, particularly in concrete with varying depths, or when constraints such as reinforcing bars, studs, or imperfections on the substrate base are encountered [7,8,9].

Stress concentrations, called cracks, cause the concrete to deteriorate [10,11,12]. As shown in Figure 1, when concrete is exposed to a rapid loss of moisture due to low humidity and wind, high temperature, or both, it can create plastic shrinkage cracking, also known as early-age cracking in concrete, most typically on the exposed surfaces of recently laid floors, slabs, or other elements with significant surface areas. Cracks typically develop a few hours after concrete placement [13]. However, they can happen at any time when surface evaporation surpasses 0.5 kg/m²/h and are more frequent on hot days [5].

These fissures were typically 3 mm broad at the surface. They are positioned anywhere, from a few centimeters to as much as 3 m apart, and their lengths vary from a few millimeters to several meters (from approximately 50 mm to 3 m). They can reach the full depth of the elevated structural slabs or diminish to 20–50 mm within the concrete. The fractures often run parallel to one another and emerge at an angle of approximately 45° in the casting direction [14,15,16,17]. The most prevalent scenario is surface cracking caused by water evaporation from the surface of the concrete or drawn out of the concrete by subbase or formwork components. Other factors that can contribute to cracking include the temperature of the concrete, cement fineness, slump, air content, surface finish, mixing techniques, aggregate gradation, and volume [18,19].

1.1.1. Autogenous Shrinkage

Autogenous shrinkage is a type that occurs internally during the early hours of concrete placement. It is a form of shrinkage not caused by changes in temperature, loss or ingress of substances, or the application of external force or constraint, but lowers the durability of concrete [20]. The capillary tension theory provides the best explanation for this phenomenon, which is brought on by further cement hydration following the development of the cement matrix’s initial structure [21]. When cement paste continuously hydrates in a constant temperature and humidity environment, it causes chemical shrinkage and self-desiccation, reducing the macroscopic volume of the concrete [22,23,24]. This process is known as autogenous shrinkage. Usually, microcracks are formed in mostly high-performance concrete (water-to-cement ratio < 0.4) during the first 24 h (vulnerable in the first 12 h) following mixing with water, this is mostly a result of inadequate internal moisture to hydrate the cement particles completely [25,26,27]. Autogenous shrinkage in high-performance concrete (HPC) is a multifaceted phenomenon that is impacted by multiple factors such as cement type, fineness, SCMs, aggregate, fiber, water-to-cement ratio, admixtures, and curing [20,24,28,29,30,31,32]. Ordinary Portland cement (OPC) concrete experiences autogenous shrinkage and drying shrinkage during the hardening process, which causes variations in the volume of the concrete as the hardened OPC paste shrinks [33,34]. Drying and autogenous shrinkage can happen concurrently if the concrete is not properly cured in dry conditions [35].

1.1.2. Autogenous Shrinkage

While the cementitious materials dry, the volume changes as a result of moisture being removed from the surface of the gel pores. This phenomenon is known as drying shrinkage [36]. The loss of internal moisture in the porous structure to achieve hydrological balance with the external relative humidity of the environment causes the drying shrinkage of a porous medium, such as cementitious materials [37,38]. Their primary causes are the water loss in C-S-H and the variation in internal moisture that occurs as concrete dries. A primary reason for crack formation is drying shrinkage, which happens when constrained concrete parts go through the hardening process [39,40]. The ratio of water to cement is a significant factor in the drying shrinkage rate. The product of hydration and the physical structures of the porosity network are the main factors that influence drying shrinkage. This might be explained by the variations in the kinds of water lost during the different drying phases, which impact the skeleton of the concrete rigidity and porosity network. OPC concrete with a high water content has a poor modulus of elasticity and strength, which increases the likelihood of significant shrinkage. Conversely, as the water-to-binder ratio drops, the drying shrinkage increments decrease as well [36,41]. The degree of restraint imposed by aggregates is a significant component that affects the drying shrinkage of concrete. The volumetric fraction of paste in the concrete mixture and the degree of restraint increases with the elastic modulus of the aggregates [37].

1.2. Fibers

In high or low load-bearing elements, such as industrial floors, rigid pavements, slabs, and beams, fibers can effectively replace traditional reinforcements, partially or completely [42,43,44,45,46]. This is because they enhance the mechanical strength of concrete composites. Fiber-reinforced concrete (FRC) is a better-performing concrete with various applications in civil and environmental engineering. It is created by combining fibers with a concrete matrix. Although fibers are known to possess other functions in concrete, shrinkage crack control and tensile strength enhancement are the major factors [47,48]. The main variables influencing the parameters of FRC are the fiber type, geometry, content, dispersion, and orientation [49].

1.2.1. Natural Fibers

Various plants, including jute and cotton, and minerals and animals, such as silk and wool, can be used to produce natural fibers [50]. Most natural fibers used to control plastic shrinkage in concrete are plant-derived. Over time, this eco-friendly substance has become increasingly favored as a remedy for concrete cracks caused by plastic shrinkage and to enhance the mechanical characteristics of concrete [50,51,52,53]. Natural fibers are less expensive, flexible, and resistant to impact, less abrasive to machinery, less hazardous to health, easier to manufacture, have reduced greenhouse gas emissions, are recyclable, and are carbon dioxide neutral [54].

Table 1. Common fibers used in concretes.

Reference	Group	Types of Fibers
[50,51,55,56,57,58,59]	natural fibers	flax, sisal, hemp, jute, kenaf, cellulose, coconut (coir), bamboo, ramie
[60,61,62,63,64]	synthetic fibers	polypropylene, nylon, polyethylene, polyvinyl alcohol, glass, carbon, polyester
[65,66,67]	steel fibers	hooked-end, straight-end, recycled, crimped, deformed

presents common fibers used in concrete.

1.2.2. Synthetic Fibers

Building panels, aircraft parts, automotive components, and fiber-reinforced polymer (FRP) tanks are examples of high-performance polymer matrix composites composed of synthetic fibers [68,69]. All artificial fibers were made from petroleum derivatives and engineered in a chemical lab to obtain the best possible quality at a competitive price [70,71]. These short fibers have been the subject of decades of concerted research, and their application in reinforcing cement composites is growing rapidly [47,72,73,74]. Depending on their characteristics, they may exhibit strength, flexibility, elasticity, and varying water absorption levels. In general, artificial fibers are more resistant to heat, stains, water, and chemical damage than natural fibers [75,76]. It should be mentioned that they are not always gentle on the skin and occasionally might cause allergic responses [76].

1.2.3. Steel Fibers

Steel was used to create what is known as steel fibers via mechanical procedures. Steel wires must be drawn or cut to obtain the required length. Strands are typically small and discretely sized. Among other things, steel fibers are mostly utilized to improve poor tensile strength, early age, drying shrinkage crack resistance during service duration, resistance to impact, and to reduce spalling [77,78,79]. Steel fiber forms, steel types, and aspect ratios frequently affect their quality [80]. However, industrial manufacturing of steel fibers worsens the depletion of natural resources and produces significant carbon dioxide. Several academics have concentrated on substituting retrieved steel fibers from waste tires with industrial steel fibers to comply with green environmental protection regulations. The mechanical properties of concrete are strengthened by recycled steel fibers, which may be employed in some technical applications instead of manufactured steel fibers [81]. However, it should be noted that steel fibers corrode when exposed and can be very costly when fibers with small diameters are required [82].

1.2.4. Environmental Impacts of Fibers

The cultivation, harvesting, extraction, production, and processing of natural, synthetic, and steel fibers have impacts like greenhouse gas emissions and climate change [83], ozone layer depletion [84], soil degradation, energy consumption, air, water, and microplastic pollution, waste generation, landfill accumulation, and water usage [85,86,87]. For instance, cotton, the most common natural fiber used in clothing manufacturing, has a big impact on a lot of different categories because it uses a lot of energy, water, land, fertilizers, and pesticides [85,88]. Compared to cotton, flax has far less of an impact on the environment in terms of greenhouse gas emission emissions, energy utilization, and water usage [89]. It has also been discovered that, when it comes to raw material extraction, flax has less of an impact on the environment than other fibers like hemp, jute, polyester, and silk [90]. The manufacturing of silk, a luxuriant and biodegradable natural fiber, has a notable influence on the environment [91]. In Ref. [90], the authors claimed that, with GHG emissions of 52.5 kg CO₂ equivalent/kg, energy consumption of 1467.3 MJ/kg, and water consumption of 26,700 L/kg, Indian silk has the largest environmental effect during the raw material extraction phase [85,92].

One of the most widely used synthetic materials in the world is polyester. Polyester is produced using energy-intensive methods and non-renewable resources like petroleum. In order to produce blocks for PET and other industrial uses, crude oil must be extracted and refined [93]. The production of polyester had a major influence on the impact categories for global warming and terrestrial ecotoxicity, but the impact on stratospheric ozone depletion was negligible. Polyester’s synthetic origin contributes to its lower impact value in the ozone depletion category, in comparison to natural fibers, which are impacted by anthropogenic activities during cultivation and harvesting [85,94]. Microfibers likely alter the geochemistry and biophysical atmosphere of terrestrial ecosystems, by their interactions with the biota, leading to toxicity in the environment. Due to their widespread distribution and abundance, synthetic microfibers are posing a serious threat to aquatic life and the environment, and concerns about their potential harm to human health are growing [95].

Presently, there are steel industries worldwide, and, in the process of producing steel, each of them emits greenhouse gases, a result of their high energy and fossil fuel consumption [96]. Carbon dioxide emissions result from the industrial manufacture of steel fibers [97,98]. The mining of iron ore, coal, and limestone is the first step in the manufacturing of steel fibers. Significant environmental damage, such as habitat loss, soil erosion, and water contamination from mine drainage, can result from mining operations [99,100,101]. Blast furnaces or electric arc furnaces are used in the process of turning raw materials into steel which causes air pollution and climate change and releases greenhouse gases (GHGs) such as carbon dioxide (CO₂), sulfur dioxide (SO₂), and nitrogen oxides (NOxs) into the immediate environment [102,103,104]. While fibers have found usefulness in industries, their impact on the immediate environment is a thing to be considered.

1.3. Properties of Concrete

Mechanical properties are attributes that help describe the behavior of concrete [105]. They serve as a source of information for forecasting the behavior of concrete structures under different loading scenarios. Compressive strength is one of the most crucial and frequently measured characteristics of concrete [106]. Concrete is typically employed in compression-loading scenarios in civil engineering because its compressive strength is significantly greater than its tensile or bending strengths [107]. In addition, because it has a direct bearing on the composition of the hydrated cement paste, the compressive strength is frequently used as a gauge of the quality of concrete [107]. For these reasons, choices regarding the strength and serviceability of concrete elements and structures are often based on compressive strength [108,109,110,111,112]. However, the mechanical properties of concrete vary widely. This study focuses on compressive strength, an important consideration in the description and design of concrete structural members.

1.4. Objective of Study

The significance of preventing or reducing plastic shrinkage cracks in concrete cannot be overstated, because they pose a substantial threat to structural integrity and durability [5]. Although numerous studies have explored the use of various fiber additives to mitigate these cracks, it is crucial to conduct an in-depth examination and assessment of single fibers (excluding hybrids) to reduce the appearance of early-age cracks in concrete.

Therefore, this study aims to synthesize available research data, evaluate the effectiveness of single fibers, and analyze specific variables (aspect ratio and volume) that influence the prevention of cracks in restrained early-age shrinkage cracking of cementitious concrete. The insights obtained are expected to guide future research, increase general awareness, and enhance practical techniques for improving concrete durability.

The objective of this study is to review and analyze the published literature on the effects of natural, synthetic, and steel fibers on plastic shrinkage cracking; identify common methods of examining restrained plastic shrinkage cracking; establish a relationship between the effect of fiber aspect ratio and volume on crack reduction, cracking time, and compressive strength; and utilize predictive analysis to determine which variables (aspect ratio or volume fraction) have a greater influence on crack reduction and compressive strength. A materials and methods table was also created to make this review comprehensive.

2. Review Methodology

Research papers were sourced from the Web of Science and Scopus databases using a cross-sample approach to generate 361 unique research publications after the removal of duplicates [113], as shown in Figure 2. The keywords “plastic shrinkage cracking on concrete + fibers”, “early-age cracking on concrete + fibers”, “surface cracking on concrete + fibers”, and fiber-reinforced concrete were used in the selection of articles.

The 361 articles selected were screened for titles and abstracts to eliminate studies that did not meet the inclusion criteria or were irrelevant to the research questions. Furthermore, the articles were thoroughly examined to exclude meeting papers, letters, proceedings papers, and early access papers [114]. To ensure that there was no bias and to achieve the objective of this study, articles that investigated the development of cracks due to “drying shrinkage”, “free shrinkage”, and “autogenous cracking” were also excluded through manual skimming, only journals that evaluated surface cracking that occurred during the early hours of concrete placement were retained. During this stage, a total of 217 articles were excluded in order to achieve the purpose of this review which is to examine the effectiveness of independent fibers in mitigating early-age cracking. The remaining 144 articles were then screened for those that utilized other additives (supplementary cementitious materials, admixtures, hybrid fibers). In this phase, 120 articles were excluded to achieve the predefined goal. Altogether, a total of 337 articles was screened out leaving 24 articles behind.

The remaining 24 full-text articles were examined for the eligibility stage. In this phase, the quality of research, the comprehensiveness of the research, and alignment with the objective of this study were strictly considered. After the completion of this in-depth assessment, 9 articles were excluded leaving a total of 15 articles remaining.

The 15 articles were selected to draw a decision table, and 11 articles were used to assess the efficiency of natural and synthetic fibers. Four articles, including 3 articles from the initial 11, making a total of 7 articles, were used to assess the performance of steel fibers. For the decision table, priority was given to recently published studies from 2020 to 2023, and the search was extended backward, as shown in Figure 2, to explore more articles.

2.1. Concrete Materials

Natural resources play a major role in concrete construction. Fine and coarse aggregates, and cement (binder), primarily ordinary Portland cement (O. P. C), constitute conventional concrete matrices. In a mechanical mixer, all three components are mixed with water to create a versatile substance [115]. Additives contribute to the quality of concrete and provide a sustainable environment. High-strength concrete is often fabricated with a high binder concentration and low water-to-binder ratio. Superplasticizers are added to concrete to increase its workability and produce stronger and more durable concrete by lowering its water content [116]. Using short, randomly distributed fibers in a concrete mix is an effective way to mitigate concerns regarding brittleness [49,117]. Various types of fibers, including steel, synthetic fibers, and natural fibers, are used to increase tensile and flexural strengths [49]. Given the growing demand for its use, the issue of carbon dioxide generation from cement and concrete has been addressed by partially replacing the additional cementitious components in concrete. Additional cement-like substances (silica fume, fly ash, and slag) have been added as binders to reduce the quantity of cement. This reduces the carbon dioxide released into the atmosphere and significantly enhances the performance and durability of concrete [118,119].

2.2. Common Experimental Methods for Evaluating Early-Age Shrinkage Cracks

2.2.1. ASTM C 1579

The ASTM C1579 [18,19] method, as shown in Figure 3, is a standard test method for evaluating plastic shrinkage cracking of restrained fiber-reinforced concrete (using a steel form insert). It is a bottom restraining method which is the most frequently applied testing technique for assessing plastic or early-age shrinkage cracks [120]. The results of this test can be used to compare the performance of concretes that include and do not include fibers, concretes with and without different mixture proportions, concretes with and without different amounts of numerous kinds of fibers, and concretes with various types and quantities of admixtures [121,122]. The ASTM C1579 mold, whose internal measurements are 560 × 355 × 100 mm, is rectangular. The mold consists of two ridges that act as internal constraints and a central stress riser. According to ASTM C1579, cracks in concrete samples are created by the stress riser, and always appear in the middle of the specimen [18,19,123,124].

Following the specifications of this method, after the concrete is prepared according to a desired mix design, two control specimens and two fiber-reinforced or admixture concrete specimens with the same mixture proportion should be prepared, finely finished, and placed simultaneously in a controlled environmental chamber at a temperature of 36 ± 3 °C, relative humidity of 30 ± 10%, and a wind velocity of above 4.7 m/s, maintaining an evaporation rate of 1.0 kg/m² [18,19,124]. The samples are expected to be left in the chamber till the final setting (from about 4 to 8 h after placement). After the final setting period, before crack measurement, the specimens are kept in the lab at 23 ± 2 °C under a plastic sheet to reduce evaporation. [18,19,65,125]. A crack comparator or a microscope should be used to precisely measure the crack width of the specimens parallel to the stress riser at intervals of 10 ± 1 mm along the length of the crack, starting at 24 ± 2 h after mixing. To eradicate the influence of the panel walls on the crack, the crack measurement at 25 mm from each side of the panel is disregarded [10]. The average crack width of the specimens is calculated to the nearest 0.05 mm and compared using the crack reduction ratio formula in Equation (1). The test is deemed invalid and it is advised to raise the evaporation rate and repeat the experiment if the average crack width on the control specimen is less than 0.5 mm [18,19].

2.2.2. Modified ASTM C1579

As stated in the ASTM C1579, the test is rendered invalid if the average crack width of the control specimen is less than 0.5 mm and the environmental condition of the chamber was advised to be raised for subsequent experiments. However, a temperature of 36 ± 3 °C, relative humidity of 30 ± 10%, a wind velocity of above 4.7 m/s, and an evaporation rate of 1.0 kg/m² is critical enough, a further increase in the environmental conditions will be unreasonable. Therefore, when a few researchers encountered the challenge of a crack width of less than 0.5 mm, they improvised by reinforcing the mold instead, to establish a sufficient average crack width for their experiments.

Consequently, reinforcing steel, bolts, and nuts have been used as additional restraints to supplement the bottom restraints of the ASTM C1579. As shown in Figure 4b [124], the ASTM C1579 mold was modified by adding six 10 mm corrosion-free end bolts and nuts at both shorter spans of the mold. It was arranged in three columns and two rows on each side. In [126], the authors included two circular steel bars for alteration to strengthen the constraint, which increased the size of the plastic shrinkage cracks. The two reinforcing bars were placed horizontally, parallel, and in a defined position relative to the shorter span at both ends of the mold. As shown in Figure 4a, in [127], minimal or no cracking was observed during the early tests. Hence, two horizontal steel bars were positioned 60 mm from the bottom of the mold and 27 mm from the walls of the shorter span to the center of the steel bars at both ends. Steel bars were added to assist the two smaller triangles in restraining the concrete laterally.

2.2.3. Kraai Method

The Kraai approach [128] utilizes a built-in edge constraint attached to the wall of the mold. The slab dimensions are typically reported to be around 900 mm long, 600 mm wide, and 19 mm deep. It should be mentioned that several researchers altered the slab dimensions (length, breadth, and depth). This test specimen exhibits early-age cracking due to its high surface area-to-volume ratio [128,129,130,131]. At the end of the experiment, cracks were discovered to be evenly distributed across the surface of the specimen. Furthermore, due to its tiny height, this approach has been primarily employed to examine the plastic shrinkage cracking of cement and mortar samples [128,132]. Figure 5 shows the percentage of methods for the decision table.

2.2.4. Materials and Methods for Decision Table

All plastic cracking experiments follow the same procedure discussed in the ASTM C1579 method. However, the environmental chamber condition and mold dimensions distinguish one method from another.

Table 2. Comparative details of materials and methods used for natural and synthetic fibers.

Ref.	Binder	C:F.A:C.A:W	Fiber Additive	Length (mm)	Aspect Ratio	Tensile Strength (MPa)	Volume (%)	Other	Experimental Description
[133]	Type 1 cement	1:2.09:1.46:0.62	Coconut	40	NM	NM	0.50%	none	A wind tunnel to produce hot wind in a controlled environment. Specimen size—750 × 400 × 25 mm. Time—6 h. Measurement—pictures were analyzed on AutoCAD Env. Cond.—T—60 °C, RH—NM, W—1.34 m/s, E—NM
							0.75%
							1.00%
[134]	CEM II OPC	1:2.2:1.8:0.55	Recycled PET Deformed—D Straight—S	50 S	55	NM	0.50	SP—0.01% of cement	ASTM C1579-06. Time—6 h. Measurement-nm Env. Cond.—T—33 ± 3 °C, RH—30 ± 10%, W—>4.7 m/s, E—1 kg/m2/h
				30 S	33		1.00
				50 S	55		1.00
				50 S	55		1.50
				50 D	70		0.50
				30 D	42		1.00
				50 D	70		1.00
				50 D	70		1.50
[135]	CEM II 32.5R	1:2.69:2.77:0.55	Polypropylene	40	53.3	338	0.3%	SP—0.76% of cement	Specimen size—NM Time—NM Measurement—Image analysis on Adobe Photoshop, ImageJ, and Auto stitch. Env. Cond.—T—25 ± 1 °C, RH—50 ± 2%, W—2.2 m/s, E—1 kg/m2/h
				54	66	481
				40	93	620
			Polyvinyl alcohol	50	76	800
			Polyethylene	52	81	238
[136]	CEM II 52.5N	1:1.62:2.17:0.5	Polypropylene	12	300	nm	0.13%	none	ASTM C1579-06 with two additional horizontal steel rods for restraint. Time—6 h. Measurement—digital image correlation Env. Cond.—T—28 °C, RH—29%, W—22 m/s, E—0.86 kg/m2/h.
[137]	Grade 53 IS:12269-1987	1:1.8:3.4:0.5	Bagas	NM	NM	NM	0.03%	none	Specimen size—500 × 350 × 50 mm with bottom steel mesh of 50 × 50 mm and diameter of 3.0 mm for restraint. Time—6 h Measurement- magnifying lens and hand lens microscope. Env. Cond.—T—37.5 ± 3.5 °C, RH—NM, W—NM, E—NM.
							0.06%
							0.09%
			Coconut				0.03%
							0.06%
							0.09%
			Polypropylene				0.03%
							0.06%
							0.09%
[64]	ASTM type II Portland cement	1:2.73:2.24:0.47	Glass	15	1250	2450	0.1%	none	ASTM C1579-06. Time—24 h. Measurement—images were processed in MATLAB environment Env. Cond.—T—36 °C, RH—25%, W—>4.7 m/s, E—2 kg/m2/h
			Polypropylene	12	545	350
[138]	P.II 52.5R Portland cement	1:1.24:2.21:0.33	Polypropylene	42	NM	550	1.56% of cement. cement—(512 kg/m3)	SP—0.6% of cement	The experiment was conducted in a Temperature Stress Test Machine (TSTM). Specimen size—150 × 150 mm at the center, and 150 × 280 mm at two ends (dog bone shape) restrained and one end. Time—nm Measurement—nm Env. Cond.—T—NM, RH—NM, W—NM, E—NM.
				54		640
				60		640
[55]	CEM I 52.5N	1:6.13:_:0.55	Flax	12	681.4	NM	0.3%	Bentonite (b/c)—2.7 SP—1.05% Lime—22.5% of cement	ASTM C1579-06. Time—6 h. Measurement—digital image correlation Env. Cond.—T—34 ± 1 °C, RH—33 ± 6%, W—6 ± 1 m/s, E—1 kg/m2/h.
							0.6%
				24	1362.9		0.3%
							0.6%
				50	2839.3		0.3%
							0.6%
[139]	PO 42.5 Chinese standard GB 175-2007	1:2.60:3.68: 0.57	Polypropylene	15	333.33	400	0.6%	Slag—8.6% FA.—27.5% SP—1.4% of cement	The experimental methodology was carried out through GB/T 50082-2009 with a crack inducer parallel to the short side of the mold. Specimen size—800 × 600 × 100 mm Time—24 ± 0.5 h. Measurement- images were processed with ZBL-F101. Env. Cond.—T—NM, RH—NM, W—5 ± 0.5 m/s, E—NM.
							0.9%
			Polyvinyl Alcohol	12	307	1620	0.6%
							0.9%
			Cellulose	2.5	166.67	750	0.6%
							0.9%
[140]	OPC CEM I 42.5 R	1:1.96:2.26:0.59	Date palm	10	100–12.5	NM	0.1%	Limestone filler—100%, SP—1%, of cement	Kraai method of estimation. Specimen size—450 × 300 × 50 mm Time—A week. Measurement—NM. Env. Cond. 1—T—20 ± 2 °C, RH—50 ± 5%, W—3 m/s, E—<1 kg/m2/h.
							0.2%
				20	200–25	290 ± 20	0.1%
							0.2%
[141]	O.P.C IS 12269 + Silica fume	1:2:3.06:0.43	Polypropylene	20	200	450	0.5%	SF—7.5%, SP—2.15%, of cement	Comparable to ASTM C1579-06. Specimen size—500 × 250 × 75 mm. The slab’s longitudinal movements were restricted, and additional restraints were applied using bolts and nuts. Time—24 h. Measurement—a curve tracing tool on image analysis software was used. Env. Cond.—T—35 ± 1 °C, RH—40 ± 1%, W—6 m/s, E—>1 kg/m2/h
			Polyester	12	240	970
			Glass	6	600	2280

and

Table 3. Comparative details of materials and methods used for steel fibers.

Ref.	Binder	C:F.A:C.A:W	Fiber Additive	Length (mm)	Aspect Ratio	Tensile Strength (MPa)	Volume (%)	Other	Experimental Description
[125]	CEM II 42.5	1:2.53:3.05:0.55	Recycled Tire	49	140	2500	0.5%	SP—0.45% of cement	ASTM C1579-06. Time—24 h. Measurement—digital image processing Env. Cond.—T—45 °C, RH—37%, W—4.7 m/s, E—1.8 kg/m2/h
[65]	CEM II 42.5	1:2.53:3.05:0.55	Hooked-end	50	50	1150	0.13%	SP—0.45% of cement	ASTM C1579-06. Time—24 h. Measurement- digital image processing Env. Cond.—T—36 ± 3 °C, RH—30 ± 10%, W—>4.7 m/s, E—1 kg/m2/h
							0.26%
							0.38%
			Recycled Tire	49	140	2300	0.13%
							0.26%
							0.38%
[142]	CEM I 42.5N + limestone filler	1:3:2.35:0.55	Recycled Tire	10–40	60–210	2560 ± 550	0.74%	Limestone filler—47% SP—1% of cement	ASTM C1579-06. Time—18 h. Measurement—digital microscope. Env. Cond.—T—36 ± 3 °C, RH—30 ± 10%, W—>4.7 m/s, E—1kg/m2/h
							1.47%
							3%
			Hooked-end	50	45	1115	1.47%
							2.21%
[64]	ASTM type II Portland cement	1:2.73:2.24:0.47	Hooked-end	35	64	1100	0.10%	none	ASTM C1579-06. Time—24 h. Measurement—MATLAB programming software to process images. Env. Cond.—T—20 ± 1 °C, RH—30 ± 3%, W—>8.5 m/s, E—1 kg/m2/h
[131]	O.P.C class 52.5	1:1.68:2.05:0.54	Hooked-end	NM	55	NM	0.05%	SP—0.8% of cement	Kraai method of estimation. Specimen size—840 × 540 × 40 mm Time—5.5 h. Measurement—hand-held microscope and planimeter. Env. Cond.—T—25 ± 1.5 °C, RH—60 ± 5%, W—18 m/s, E—>1 kg/m2/h.
							1.00%
							1.50%
					65		0.50%
							1.00%
							1.50%
					80		0.50% 1.00%
							1.50%
[131]	O.P.C class 52.5	1:1.26:1.55:0.43	Hooked-end	NM	55	NM	0.50%	SP—0.8% of cement	Kraai method of estimation. Specimen size—840 × 540 × 40 mm Time—5.5 h. Measurement- hand-held microscope and planimeter. Env. Cond.—T = 25 ± 1.5 °C, RH—60 ± 5%, W—18 m/s, E—>1 kg/m2/h.
							1.00%
							1.50%
					65		0.50%
							1.00%
							1.50%
					80		0.50%
							1.00%
							1.50%
[141]	O.P.C IS 12269	1:2:3.06:0.43	Hooked-end	30	60	1700	0.5%	Silica fume—7.5% SP—2.15% of cement	ASTM C1579-06 with bolts and nuts used to provide additional restraints. Time—24 h. Measurement—MATLAB programming software to process images. Env. Cond.—T—35 ± 1 °C, RH—40 ± 1%, W—6 m/s, E—>1 kg/m2/h

Env. Cond.: environmental conditions, T: temperature, RH: relative humidity, W: wind, E: evaporation, NM: not mentioned, C:F.A:C.A:W: cement:fine aggregate:coarse aggregate:water, SP: superplasticizer, SF: silica fume.

present the materials and methodology of the articles considered for the decision table, the table also mentions the differences necessary to differentiate each method.

3. Decision Table

The length, width, and total area of a plastic shrinkage crack are typically used to quantify the cracks. In this review, because length and width are considered in the area formula, the total crack area was preferred over crack width or length. However, either crack length or width is selected if the values of the total crack area are not available. The value of compressive strength and cracks were computed as percentages to enable a thorough understanding of the fiber efficiency.

Table 4. Natural and synthetic fibers decision table.

Reference	Fiber	Length (mm)	Volume (%)	Crack Reduction (%)	Cracking Time (Minutes)	Compressive Strength (%)
[133]	Coconut	40		crack length	NM
			0.50	54.43		+12.40
			0.75	72.40		+2.50
			1.00	95.93		−9.00
[134]	Recycled PET Straight—S Deformed—D			crack width	ref—[90]
		50 S	0.50	33.09	90	−7.09
		30 S	1.00	25.29	95	−5.35
		50 S	1.00	54.95	105	−7.14
		50 S	1.50	60.65	120	−8.66
		50 D	0.50	37.78	105	−0.46
		30 D	1.00	49.72	100	−1.69
		50 D	1.00	60.40	120	−2.34
		50 D	1.50	68.69	135	−8.00
[135]			0.3	crack area	NM
	Polypropylene	40		54.61		37 ± 1 MPa at 28 days
		54		65.62
		40		81.73
	Polyvinyl alcohol	50		68.14
	Polyethylene	52		81.55
[136]				crack area
	Polypropylene	12	0.13	no crack		+7.57
[137]				crack area
	Bagas	nm	0.03	86.14
			0.06	79.03
			0.09	91.01
	Coconut		0.03	79.03		NM
			0.06	91.01
			0.09	92.51
	Polypropylene		0.03	88.76
			0.06	91.01
			0.09	79.02
[64]			0.1	crack area	ref—[90]
	Glass	15		59.00	120	−6.77
	Polypropylene	12		43.00	110	−6.51
[138]	Polypropylene		1.56	crack width
		42		Reference
		54		14.9	NM	−2.00
		60		28.4		−3.30
[55]
	Flax	12	0.3	38.81		0.00%
			0.6	53.73		+18.03
		24	0.3	47.76	NM	−2.30
			0.6	55.22		+8.20
		50	0.3	53.73		−1.64
			0.6	62.68		+21.31
[139]				crack area
	Polypropylene	15	0.6	no crack
			0.9	19.83
	Polyvinyl Alcohol	12	0.6	74.02	NM	NM
	Cellulose	2.5	0.6	43.02
			0.9	20.11
[140]				crack area
	Date Palm	10	0.1	72.00		−10.53
			0.2	78.00	NM	−2.63
		20	0.1	76.00		−6.58
			0.2	68.00		−7.89
[141]				crack area
	Polypropylene	20	0.5%	98.80	NM	NM
	Polyester	12		99.87
	Glass	6		96.72

NM: not mentioned, +: increase, –: decrease.

and

Table 5. Steel fibers decision table.

Reference	Fiber	Length (mm)	Volume (%)	Crack Reduction (%)	Cracking Time (Minutes)	Compressive Strength (MPa)
[125]	Recycled Tire	49	0.5	no crack	NM	+0.3
[65]				crack width
	Hooked-end	50	0.13	28.57		+2.5
			0.26	54.61		+2.6
			0.38	69.10	NM	+2.1
	Recycled Tire	49	0.13	30.65		+1.5
			0.26	62.30		+1.9
			0.38	no crack		+2.4
[142]				crack area	ref—[143]
	Recycled Tire	10–40	0.74	15.67	298	NM
			1.47	26.95	301
			3.00	78.36	389
	Hooked-end	50	1.47	25.39	324
			2.21	86.83	331
[64]				crack area	ref—[90]
	Hooked-end	35	0.10	53.00	125	+2.08
[131]				crack area	ref—[25]
	Hooked-end	H1	0.50	7.12	30	−6.91
			1.00	11.37	23	−2.90
			1.50	37.44	12	+0.53
		H2	0.50	22.05	15	−2.24
			1.00	34.66	15	−2.19
			1.50	57.11	25	−7.00
		H3	0.50	19.67	25	−4.45
			1.00	36.55	20	−2.32
			1.50	73.95	28	−3.67
[131]				crack area	ref—[50]
	Hooked-end	H1	0.50	41.82	33	−7.17
			1.00	47.48	37	−5.79
			1.50	71.10	30	−3.75
		H2	0.50	47.10	40	−2.80
			1.00	53.89	22	−0.90
			1.50	71.29	35	−0.34
		H3	0.50	40.53	48	−5.80
			1.00	65.80	35	−8.53
			1.50	69.97	41	−8.53
[141]				crack area	ref—[144]
	Hooked-end	30	0.5	48.58	280	NM

NM: not mentioned, +: increase, −: decrease, H1: aspect ratio 55, H2: aspect ratio 65; H3, aspect ratio 80, ref: reference.

presents the decision table.

% crack reduction = [(cc − fc)/cc] × 100

(1)

where cc is the number of cracks in the control concrete or the compressive strength of the control concrete, and fc is the number of cracks in the fiber concrete or the compressive strength of the fiber concrete.

4. Statistical Interpretation

Data from the decision table were interpreted using Pearson’s correlation coefficient ‘r’ from equation 2 to determine the degree of relationship between the variables. Figure 6 shows the interpretation of “r” values ranging from “−1 to +1”. An “r” value of “0” is interpreted as no linear relationship, from “0.1 to 0.3” is referred to as a weak positive linear relationship, from “0.4 to 0.6” is interpreted as a moderately positive linear relationship, and from “0.7 to 0.9” is considered a strong positive linear relationship, and vice versa. A correlation coefficient “r” of “+1 or −1” is a perfect positive or negative linear relationship. Additionally, using equations 3 and 4, predictive analysis (simple linear regression and multiple linear regression) was carried out, and the line of best fit was drawn to show the relationship between the explanatory variable (x) and response variable (y). The R-squared (R²) value was also computed to show the measure of the strength of the regression model. The R² value is used to determine the suitability of the regression model to predict the response variable. An R² (R-squared) value is measured from 0 to 1, where “0” means no relationship, from “10% to 30%”, from “40% to 60%”, and from “70% to 90%” are regarded to have a weak, moderate, and strong explanatory power, respectively. A “100%” value is interpreted to have a perfect explanatory power.

Table 6. Natural and synthetic fibers interpretation table.

Ref.	Variable	Regression Line
[133]	Volume
		Compressive Strength y = −42.8x + 34.067 R2 = 0.9981 r = −0.9990	Crack Reduction y = 83x + 12.003 R2 = 0.9941 r = 0.9970
[134]	Volume
		Compressive Strength y = −1.57x − 6.06 R2 = 0.7739 r = −0.8797	Crack Reduction y = 27.56x + 22.003 R2 = 0.8972 r = 0.9472
[55]	Aspect ratio (@ 0.3% volume fraction)
[55]	Relationship equation	Compressive Strength = −20.21 + 0.061AS + 57.20VF R2 = 0.8319 r = 0.9121	Crack Reduction = 27.58 + 0.305AS + 34.81VF R2 = 0.9433 r = 0.9712

AS: aspect ratio, VF: volume fraction of fiber, CR: crack reduction, CS: compressive strength, r: correlation coefficient, R²: R square (a measure of the strength of the relationship of regression equation).

and

Table 7. Steel fibers decision table.

Ref.	Variable	Regression Lines
[131]	Volume (H3)
		Compressive Strength y = −2.73x − 4.89 R2 = 0.75 r = −0.8660	Crack Reduction y = 29.44x + 29.327 R2 = 0.8538 r = 0.9240
[65]	Volume
		Compressive Strength y = 3.5928x + 1.0112 R2 = 0.9924 r = 0.9962	Crack Reduction y = 276.11x − 9.1149 R2 = 0.9607 r = 0.9801
[131]	Aspect ratio (@ 0.5% volume)
		Compressive Strength y = −0.1411x + 4.3337 R2 = 0.211 r = −0.4594	Crack Reduction y = 0.7376x + 6.5479 R2 = 0.9968 r = 0.9984
[131]	Relationship equation	Compressive Strength = −3.781 − 0.013AS + 1.153VF R2 = 0.0465 r = 0.2156	Crack Reduction = −69.279 + 0.941AS + 39.87VF R2 = 0.8655 r = 0.9303

AS: aspect ratio, VF: volume fraction of fiber, CR: crack reduction, H1: aspect ratio of 55, r: correlation coefficient, R²: R square (a measure of the strength of the regression equation relationship).

presents the statistical interpretation.

r = n(∑XY) − (∑X) × (∑Y)/√ [n∑X² − (∑X)²] × [n∑Y² − (∑Y)²]

(2)

y = a + bx

(3)

y = a + b_1×1 + b_2×2 + b_3×3 + ……+ b_nx_n + ε

(4)

where “r” is the correlation coefficient, and “n” is the number of data points. “∑XY” is the sum of the product of “X” and “Y” values. “∑X” is the sum of “X” values. “∑Y” is the sum of “Y” values. “∑X²” is the sum of squares of the “X” values. “∑Y² ” is the sum of squares of the “Y” values. “y” is the response variable. “a” is the intercept. “b”, “b₁”, “b₂”, “b₃”, and “b_n” are the slopes. “x”, “x₁”, “x₂”, “x₃”, and “x_n” are the explanatory variables. “ε” is the error term.

5. Discussion

This section establishes the relationship between fiber volume, fiber aspect ratio, and plastic or early-age shrinkage cracking, and analyzes the similarities and differences. This study aims to offer a thorough summary of the current state-of-the-art of plastic shrinkage in concrete and make recommendations for future research.

5.1. Fibre Volume

Previous research indicated that the amount of fiber can be expressed as a percentage of the weight of cement or binder, or as a fraction of the entire concrete mix. This follows the pattern of findings in the decision tables (Table 4 and Table 5) in conjunction with the statistical tables (Table 6 and Table 7). With a positive correlation coefficient (“r”) value, it was proven that a strong positive relationship exists between fiber volume fraction and crack reduction, this indicates that increasing the volume of fiber content will lead to a reduction in plastic concrete cracking. The same trend was observed in a study in [145] when polypropylene fibers of length 12.70 mm were added to a concrete mix at varying volumes of 0.05%, 0.10%, 0.15%, 0.20%, and 0.40%. It was reported that the average crack width of the plastic shrinkage cracks reduced progressively from 0.130 mm to 0.127 mm, 0.080 mm, 0.097 mm, and 0.000 mm, respectively. They furthered their experiment by adding polyvinyl alcohol fibers of length 12 mm to the concrete mixture at volumes of 0.10%, 0.20%, 0.30%, and 0.40%, results showed that the average crack width reduced from 0.036 mm to 0.080 mm, 0.040 mm, and 0.015 mm. Likewise, Table 4 [133] included coconut fibers of length 40 mm at volumes of 0.50%, 0.75%, and 1.00%, it was confirmed that the crack length was reduced by 54.43%, 72.40%, and 95.93%, respectively. Furthermore, ref. [65] in Table 5 added both hooked end steel fibers of length 50 mm and recycled tire fibers of length 49 mm to different concrete mix both at varying volumes of 0.13%, 0.26%, and 0.38%, it was observed that the hooked end fibers reduced cracks at 28.57%, 54.61%, and 69.10%, respectively. In contrast, the recycled tire steel fiber recorded crack reductions of 30.65%, 62.30%, and 100%, respectively. In [146], the authors investigated plastic shrinkage cracks in 3D printed concrete using short polypropylene fibers of 0.3% volume of concrete. The experiment was monitored for 180 min. The results showed that the reference samples had a crack width of 1.41 mm and the polypropylene sample eradicated the cracks from the concrete. However, the compressive strength declined by 4.62% when compared to the reference concrete. Increasing the proportion of fiber in concrete increases its quantities, providing more reinforcing and improving other mechanical qualities to resist fracture development [147]. Also, a greater amount of fiber allows them to span and be present at different positions and orientations through the concrete matrix, allowing good interaction and more crack-bridging [143,148].

However, the relationship between the fiber volume fraction and the compressive strength of concrete shows a negative correlation when fiber content is high, and a positive correlation when the volume of the fiber content is low. In [149], the authors investigated the impact of steel fiber at different fiber contents (0.5%, 1.0%, 1.5%, and 2.0% by volume of concrete) on the compressive strength of high-strength concrete, with 1.5% fiber added, the maximum compressive strength was reached. In [150], the authors added short steel fiber of 13 mm in length and a diameter of 0.2 mm in a volume of 0.25%, 0.50%, 0.75%, and 1.00% of cement weight to 3D-printed concrete. Results showed that the compressive strength experienced increases of 1.05%, 4.69%, 18.75%, and −0.50% at 0.25%, 0.50%, 0.75%, and 1.00% fiber volumes, respectively. In [147], the authors reported that, when up to 2% of natural cellulosic fibers were added to the designed concrete samples, this resulted in a progressive increase in the tensile and compressive strengths of the specimens, and the mechanical qualities deteriorated with increasing fibers above 2%. From Table 4, in [133], the compressive strength of concrete declined from 12.4% to 2.5%, and −9.00% when coconut fibers were added at volumes of 0.5%, 0.75%, and 1.0%, respectively. The trends remained the same in [135], where a reduction in the compressive strength was noticed as straight recycled pet fiber of 50 mm length was added to the concrete at 0.50%, 1.00%, and 1.50%, the compressive strength decreased from −7.09% to −7.14% and −8.14%, respectively. In [151], the authors incorporated shredded face masks of volumes 0.5%, 1.0%, 1.5%, and 2.0% of cement weight in cement mortar. The compressive strength results showed a gradual increase in strength with 1.0% as the optimum, the strength then showed a progressive decline at 1.5% and 2.0% fiber volume addition.

In Table 4 and Table 5 we have proven that including fibers will mitigate or completely eradicate plastic shrinkage cracking in concrete. It was visible that increasing the volume of fiber had a high positive slope on the crack reduction coefficient and a moderate negative slope on the compressive strength coefficient. It is, however, impossible to be specific about the percentage of mitigation. The effect of fibers on early-age cracking and compressive strength of concrete will vary based on the type of fibers, concrete mix ratio, the volume and aspect ratio of fibers included in the concrete matrix, and the severity of the environmental condition.

5.2. Aspect Ratio of Fiber

The aspect ratio of a fiber is a dimensionless quantity used to express its length-to-diameter ratio. A longer, more slender fiber indicates a higher aspect ratio, whereas a shorter, thicker fiber indicates a lower aspect ratio.

The aspect ratio of the fibers showed a strong positive correlation with early-age cracking and a negative correlation with the compressive strength. However, when examined thoroughly (Table 6 and Table 7) with a multiple linear regression equation, it was observed that the rate at which an increase in the aspect ratio affects plastic cracking and compressive strength is lower for every case compared to the effect that will be experienced if the volume fraction is increased.

The experiments [135] that used polypropylene fibers with aspect ratios of 53.3, 66, and 93 showed the best performance of 81.73% crack reduction at an aspect ratio of 93. It was recorded that polypropylene fibers with aspect ratios of 40 and 54 had results of 54.61% and 65.62%, respectively. Likewise, the study of [131] shows that hooked-end steel fibers at distinct aspect ratios of 55, 65, and 80 performed better at an aspect ratio of 80. It was observed that the plastic shrinkage cracks in each specimen improved as the aspect ratio of the steel fibers increased. At a fixed volume of 1.5%, aspect ratios of 55, 65, and 80 recorded a crack reduction of 37.44%, 57.11%, and 73.95%, respectively. Consequently, hooked-end steel fibers with aspect ratios of 55, 65, and 80 resulted in a +0.53%, −7.00%, and −3.67% decrease in compressive strength, respectively. In [134], the authors compared the performance of deformed pet fiber at a volume of 1.00% with an aspect ratio of 70 and 42. The report showed that a deformed fiber with an aspect ratio of 30 reduced cracks by 49.72% while the deformed fiber with an aspect ratio of 50 reduced cracks by 60.40%. However, the compressive strength of concrete decreased. Deformed fibers with aspect ratios of 50 and 30 resulted in −1.69% and −2.34%, respectively, when compared to the control. In [138], the authors experimented on polypropylene fibers of three distinct lengths of 42 mm, 54 mm, and 60 mm with the specimen of 42 as control, and the plastic shrinkage crack of concrete with polypropylene fibers with 54 mm and 60 mm reduced cracking by 14.9% and 28.4%, respectively, when compared to the control. Furthermore, compressive strength decreased by −2.00% and −3.30%, respectively, when compared to the control specimen. In [152], the authors utilized 10 × 5 mm and 20 × 5 mm polypropylene fiber extracted from surgical face masks on cement mortar in volumes of 0.1%, 0.15%, 0.20%, and 0.25%. The 10 × 5 mm sample recorded reductions in compressive strength of 16.67%, 12.50%, 27.08, and 37.50%, when compared to the control sample, and the 20 × 5 mm sample recorded reductions in compressive strength of 20.38%, 16.67%, 31.25%, and 39.58%, respectively, when compared to the control specimen. This indicates that increasing the aspect ratio of the fiber will yield a small increase in crack reduction and have a slight negative impact on the compressive strength of the concrete examined under rapid evaporation. All the studies in Table 4 and Table 5 indicate that longer fibers with a reasonable length (approximately 50 mm) perform better than their shorter counterparts but also harm the compressive strength of the concrete.

Traditional concrete has a weak tensile strength and is prone to cracking at an early age [153,154]. Fibers are materials with a reputation for their tensile strength and, when added to concrete, they serve as reinforcements [155,156] and enhance the durability, toughness, and overall mechanical properties of concrete [157]. They are also helpful in mitigating crack formation in the plastic state [57]. Fibers are a medium to evenly distribute the tensile loads across cracks to prevent crack formation or widening [158]. Longer fibers interact more effectively within the concrete matrix [159]. They can bridge cracks because their high surface areas enable them to span larger distances between aggregates [148,160]. Due to their high aspect ratios, they possess improved mechanical properties like greater tensile strength and modulus of elasticity compared with shorter ones [144]. Also, longer fibers are less susceptible to segregation and clumping [161]. Well-dispersed fibers will foster good interaction with the concrete and contributes to their effectiveness in concrete mixtures [161]. In addition, fibers with large surface areas may weaken the cohesive forces that hold the binder, fine aggregate, and coarse aggregate, which would lessen the compressive strength and efficiency of load transfer [162]. As seen in Table 6 and Table 7, an increase in aspect ratio only had a negligible negative slope effect on the compressive strength coefficient and a weak positive slope on the crack reduction coefficient.

5.3. Cracking Time

In [132], the authors recorded the cracking time of ordinary concrete at approximately 146 min. After 298 min, the recycled tire steel fiber concrete with a 0.74% volume fraction began to crack, and, after 301 min, the 1.47% volume fraction concrete experienced cracking. The 1.47% hooked-end fibers cracked at 324 min. According to [64], the reference specimen cracked at 90 min; however, further experiments using 0.1% hooked-end fibers extended the cracking duration to 125 min. In [141], the authors exhibit an experience similar to that of [64,142]. However, in [133], a distinct pattern of reduction in cracking time was linked to the unusual occurrence of a wind speed of 18 m/s, whereas, in other studies, the wind speed was less than 10 m/s. Therefore, steel fibers can delay the plastic cracking of concrete.

According to Table 5, the use of recycled tire steel fiber and hooked-end steel fiber results in a delay in crack appearance when compared to the control specimen. These studies have demonstrated that adding fibers to concrete can delay the crack initiation time when concrete is in the plastic state. The major cause of plastic cracking has been attributed to continuous evaporation [163] and, once the tensile capillary pressure surpasses the failure limit of the plastic concrete, plastic shrinkage cracking begins. Fibers added to concrete act as a barrier to the surface, reducing quick moisture loss. Fibers also promote internal curing by capturing extra moisture in the concrete matrix. The absorbed moisture reduces suction and early-age shrinkage while also preventing cracking [164]. Furthermore, well-dispersed fibers in the concrete matrix reduce surface tension while increasing cohesion and friction. A lower surface tension causes less moisture to travel from the inside to the surface of the concrete because of the smaller capillary channel. In [1], the authors agreed that lowering the surface tension is an effective way of reducing plastic shrinkage. The cohesiveness helps to limit the movement of cement particles during the early phases of hydration, minimizing the likelihood of plastic shrinkage cracking. Delaying the cracking time for a few minutes or hours will help eliminate or reduce the rate of cracking and would result in fewer, shorter lengths, and smaller widths cracks compared to the traditional control specimen.

5.4. Type of Fiber

A portion of the load is transferred to the fibers by the matrix during loading, preventing the initiation of macrocracks. It has therefore been demonstrated that adding fibers of higher elastic modulus than the matrix to the concrete can boost the strength of the concrete [160]. It is safe to say natural, steel, and synthetic fibers all play a significant role in reducing the development of plastic shrinkage cracking in concrete. From the results in Table 4 and Table 5, we can deduce that the performance of the fibers varies with the volume, aspect ratio, shape, and type of fiber.

In the experiment of [145], polypropylene of 12.70 mm and polyvinyl alcohol of 12 mm were used to reduce cracking at a volume of 0.1% and 0.2%. Results showed that polypropylene fibers reduced cracking by 0.127 mm and 0.097 mm, respectively. However, for polyvinyl alcohol fibers, the authors recorded values of 0.085 mm and 0.073 mm. In [141], the authors compared synthetic fibers (glass, polyester, and polypropylene) and hooked-end steel fibers of the same volume fraction of 0.5%. The results show that all synthetic fiber performances result in approximately 100% crack reduction, whereas steel fiber was 48.58% compared to the control specimen. In [134], the authors used RPET (Recycled PET Fibers) of a deformed and straight shape. Both fibers had a length of 50 mm and a volume of 0.50% and, when they were added to a concrete mix, it was recorded that the deformed fiber resulted in a 37.7% crack reduction and compressive strength decreased by −0.46% while the straight fiber resulted in a 3.09% crack reduction and a −7.09% decrease in compressive strength. It is also obvious, in [65], that when hooked-end and recycled tire steel fibers of approximately the same length (50 mm) and same volume (0.26%) are used this results in 54.61% and 62.30% crack reduction. The difference in performance of both [65,134] can be attributed to the difference in the shape of the fiber. However, in [142,146], this difference was due to the type of fiber. We can then say that natural, synthetic, and steel fiber performances will vary based on the fiber type and their distinct mechanical properties.

5.5. Limitations and Challenges of Using Fibers in Concrete

The addition of fibers to the concrete mix is a task that should be carefully performed. Fibers are lightweight and if they are not well dispersed or there is too much of them in the concrete matrix this might lead to clumping [165] which contributes to the non-uniform distribution of fibers across the concrete and, as a result of poor compaction and bonding, this can have consequences for the compressive strength [165,166]. The most effective way is to feed fibers continuously into the mixer during concrete production to ensure the fibers are evenly distributed [167,168].

For every piece of construction work, a concrete target strength is designated which then determines the mix ratio and the physical and mechanical properties of the concrete. The addition of fibers disrupts the uniformity of concrete and reduces the workability [169] and the air content prescribed in the design stage. Trying to maintain these predefined properties after the addition of fibers might require increasing the water-to-cement or binder ratio, the addition of admixtures like superplasticizers, and the use of entraining agents which, if added in excess, can lead to a reduced strength of the cement paste and increased porosity and voids, which will produce a weaker concrete and increase the cost when compared to the control. This is similar to [170], where the addition of polypropylene fibers lowered the compressive strength from 45 MPa (control) to 41 MPa (0.3% fiber volume). In the experiment of [171], it was reported that carbon fiber samples showed an increase in porosity of 5.22% for long fibers and 7.67% for short fibers. Increasing the fiber volume affected the porosity of the cement mortar samples. There is a direct relationship between porosity and fiber volume percent. Deterioration may result from this since it may lessen compressive strength and increase permeability, allowing chemicals and water to enter the cement mortal sample [171,172,173]. Also, compressive strength samples experienced an increase in strength at 0.4% and 0.8%, and a decrease in strength at 1.2% fiber volume addition of carbon fiber [171].

The cost of fibers is another factor to be considered because it significantly increases the production cost of fiber-reinforced concrete [174,175]. In comparison, the use of synthetic and steel fibers offers benefits related to technological advancements; nonetheless, the expenses associated with production and the environmental effects of these methods remain significant. However, because natural fibers are abundant almost everywhere in the world, their usage, provided they are treated, has great potential, is suitable for technical requirements, and is inexpensive. The use of natural fibers in novel composite materials is intriguing because they are often regarded as waste in many nations [54,176,177].

Furthermore, the production of fiber-reinforced concrete requires the presence of experienced personnel [178,179] to oversee and ensure uniformity of the concrete mixing phase. Though fire-reinforced concrete is costly to produce, the longevity, durability, and reduced maintenance outweigh the cost in the long run [174].

According to Table 4 and Table 5, to strike a balance between plastic shrinkage cracks and the compressive strength of concrete, fiber addition of less than 1% volume fraction performed better.

6. Future Research Direction

• While a few researchers considered crack initiation time, many neglected it. Future research should take the cracking time of concrete into consideration. This will give more insight into the performance of the additive used for mitigation.
• Steel and bolts have been used to further restrain the ASTM C1579 mold. Future research should consider using fiber-reinforced polymer (FRP) composites since the use of these composites is fast spreading in the construction industry due to their high strength and corrosion-resistant behavior. Therefore, there is a need to understand their effect in enhancing early-age cracking. Furthermore, past researchers primarily considered the crack length, width, and area. Therefore, research in the future should consider estimating the value of the crack depth. Doing this will provide a full description of the crack formed.
• Laboratory studies of constrained plastic shrinkage cracking have been conducted using a variety of techniques, including Kraai, Modified ASTMC 1579, and ASTMC 1579. To validate laboratory results and offer useful guidelines for construction methods, real-world case studies and field applications of fiber-reinforced concrete must be recorded and examined.
• For the most past research, only virgin natural, synthetic, and steel fibers have been used to mitigate plastic shrinkage cracking in concretes. Future research should consider the use of recycled fibers to reduce environmental impact, carbon footprint, and cost (recycled fibers are cheaper compared to virgin fibers).
• The role of fiber-reinforced concrete in capturing and managing microplastics in construction and the built environment should be examined. In particular, it is important to assess how the fibers interact with the concrete when they crack to either retain or release microplastics. This study will shed light on how fiber-reinforced concrete in urban infrastructure affects the environment and how sustainable it is.

7. Conclusions

This review examines the interaction and impact of steel, synthetic, and natural fibers on the plastic shrinkage cracking of concrete. The fiber volume, aspect ratio, compressive strength, and cracking time were also investigated. Based on these observations, the following conclusions were drawn.

• Plastic shrinkage cracks in concrete can be partially or completely mitigated by adding steel, synthetic, or natural fibers. Fibers assist in the distribution of environmental loads and improve the ductility of concrete by acting as reinforcements, owing to their tensile strength.
• The inclusion of either hooked-end or recycled tire steel fibers can delay the appearance of cracks when the concrete is in a plastic state. This delay allows the concrete to develop tensile strength, enabling it to withstand harsh environmental conditions and preventing cracking.
• Natural, synthetic, and steel fibers with a greater aspect ratio, longer length (approximately 50 mm), and higher tensile strength outperformed shorter fibers in eliminating plastic shrinkage cracks but also harmed the compressive strength of the concrete because of the high rate of water loss. Because of their increased surface area, they can bridge fractures. Furthermore, their higher tensile strength compared to shorter ones can prevent cracks from widening. The aspect ratio is weakly positively correlated with crack reduction and weakly negatively correlated with compressive strength. In addition, the fiber volume fraction had a greater impact on the aspect ratio for reducing plastic cracking.
• A strong positive relationship existed between the volume fraction of fibers and crack reduction, whereas a strong negative correlation was observed with the compressive strength of concrete. Therefore, increasing the volume percentage of fibers in a concrete matrix minimizes plastic shrinkage cracking but lowers the compressive strength. A volume percentage of no more than 1% of natural, synthetic, and steel fibers is recommended to strike a compromise between plastic shrinkage cracks and compressive strength.
• The type of fiber and characteristics play a significant role in their performance.
• The absence of large data points in the predictive analysis can limit the accuracy of the equations provided. However, it is more accurate when used to predict intermediate data points between the range of the original data points.