A Review on the Effect of Synthetic Fibres, Including Macro Fibres, on the Thermal Behaviour of Fibre-Reinforced Concrete

Abstract

The mechanical properties of concrete degrade rapidly when exposed to elevated temperatures. Adding fibres to concrete can enhance its thermal stability and residual mechanical characteristics under high-temperature conditions. Various types of fibres, including steel, synthetic and natural fibres, are available for this purpose. This paper provides a comprehensive review of the impact of synthetic fibres on the performance of fibre-reinforced concrete at high temperatures. It evaluates conventional synthetic fibres, including polypropylene (PP), polyethylene (PE), and polyvinyl alcohol (PVA) fibres, as well as newly emerging macro fibres that improve concrete’s fire resistance properties. The novelty of this review lies in its focus on macro fibres as a promising alternative to conventional synthetic fibres. The findings reveal that PE fibres significantly influence the residual mechanical properties of fibre-reinforced concrete at high temperatures. Although PVA fibres may reduce compressive strength at elevated temperatures, they help reduce micro-cracking and increase flexibility and flexural strength. Finally, this review demonstrates that while conventional synthetic fibres are effective in limiting fire-induced damage, macro fibres offer enhanced benefits, including improved toughness, energy absorption, durability, corrosion resistance, and post-cracking capacity. This study provides valuable insights for developing fibre-reinforced concrete with superior high-temperature performance. Steel fibres offer superior strength but are prone to corrosion and spalling, while PP fibres effectively reduce explosive spalling but provide limited strength improvement. PE fibres enhance flexural performance, and PVA fibres improve tensile strength and shrinkage control, although their performance decreases at high temperatures. Macro fibres stand out for their post-cracking capacity and toughness, offering a lightweight alternative with better overall durability.

1. Introduction

To ensure a comprehensive and well-grounded review, a systematic methodology was employed to gather and analyze the relevant literature. Data were collected from multiple reputable sources, including academic databases such as Web of Science, Scopus, Google Scholar, and ScienceDirect, as well as conference proceedings, technical reports, and industry standards. The selection criteria focused on studies that investigated the behaviour of fibre-reinforced concrete under elevated temperatures, with particular emphasis on synthetic polymer fibres such as polypropylene (PP), polyethylene (PE), polyvinyl alcohol (PVA), and emerging macro fibres.

The collected data were analyzed to identify key trends, material properties, and both the advantages and limitations of each fibre type. Special attention was given to comparing the effectiveness of these fibres in mitigating spalling, enhancing thermal stability, and improving mechanical properties under high temperatures. The findings from various studies were synthesized to provide a comprehensive assessment of the current state of knowledge, highlight gaps in existing research, and offer insights into potential areas for future exploration. This systematic, multi-source approach ensured that the conclusions drawn in this review are both reliable and well-supported by the existing literature.

1.1. Thermal Performance of Concrete

With the fast growth of industrial constructions and the rise in living standards, a large number of construction failures occur due to low-grade or low-rate resistant materials and construction design [1]. The non-flammable feature of concrete and its exceptional insulating properties with low thermal diffusivity (compared to steel) are significant advantages of concrete in the incident of a fire. Although concrete has generally acceptable fire resistance compared to other materials such as wood or steel [2], it has two inherent drawbacks, i.e., (1) a lack of mechanical properties at high temperatures due to physicochemical changes in the material during the heating process, and (2) the explosive spalling of concrete which results in material loss, section reduction, and the exposure of steel reinforcement to extreme temperatures [3]. The disintegration of the concrete layers starting on the surface of the structural component is the result of spalling caused by high-temperature exposure with heating rates of 20–30 °C/min. Four types of concrete spalling exist: aggregate, explosive, surface, and corner/sloughing-off spalling. The heating rate affects the first three spalling types, which happen within the first 20–30 min of fire exposure, whereas the maximum temperature affects the last spalling type, which occurs 30–60 min after fire exposure. Laboratory tests demonstrated that the mechanical characteristics of uncovered concrete exposed to temperatures above 300 °C are significantly decreased [4,5].

At high temperatures, concrete spalls as a result of thermomechanical and thermohydraulic processes [3]. Thermomechanical processes cause thermal dilation/shrinkage gradients which occur inside the element being exposed to elevated temperatures [6,7] and high-pressure gas, including water vapour and enclosed air. These are produced by thermohydraulic processes in the porous network, which is regulated by permeability to liquid and gas (vapour and air). Gawin et al. [8] analytically studied the nature of thermal concrete spalling. The study found that the main reason for spalling at elevated temperatures is the gas pressure development in high-performance concrete.

Several research studies have investigated the endurance and effectiveness of concrete in sub-heated conditions. The researchers [9] found concrete spalling to be an effect of fire exposure, with the structure type, loading system, and nature of fire being influencing factors. Due to continuous exposure to elevated temperatures, a reduction in the mechanical and chemical properties of concrete causes its failure [2]. Fire intensity has a negative correlation with the fire resistance of concrete members, which is affected by the extent of spalling [10]. The sudden breakdown of fragments on the concrete surface, measured by the reduced scale testing method, is the result of concrete spalling because of heat exposure and is characterized using several parameters such as weight loss and spalling depth. The spalling risk of concrete is directly linked to the concrete properties, including density, strength, and permeability, and is influenced by several factors, such as concrete composition, heating rate, and applied testing approaches [3]. In the study of Kodur and Bhatt [11], the bond degradation monitoring in reinforced concrete slabs was investigated to clarify its importance within fire exposure. They revealed that reinforced concrete slabs with no fire insulation have a lower fire-resistance rate. Werner et al. [3] found that reinforced concrete elements lose their shear transfer capacity and stiffness when exposed to fire. AzariJafari et al. [12] investigated the elevated thermal resistance of 19 self-consolidating concrete mixtures, including binary and ternary blends of fly ash, silica fume, natural zeolite, and metakaolin. At various temperatures, changes in mixture mass, compressive strength, and ultrasonic pulse velocity were measured (20, 300, 500, and 700 °C). The study found that, for temperatures above 500 °C, ultrasonic pulse velocity, which is strongly correlated with strength properties, experienced a reduction of more than 50% in transition velocity. However, this relation is inversely reduced depending on the exposed temperature. The researchers further noted that the sustainability of concrete mixtures should be another decision-making factor considering the trade-offs between fire resistivity performance and sustainability pillars.

The purpose of this research is to provide a detailed review of the performance of fibre-reinforced concrete at elevated temperatures, with an emphasis on its application in real-world structures exposed to such conditions. This includes critical infrastructure like tunnels, high-rise buildings, and industrial facilities, where the risk of fire or exposure to high temperatures is significant. While various types of fibres are discussed, the focus is on the three most commonly used polymer microfibres—polypropylene (PP), polyethylene (PE), and polyvinyl alcohol (PVA)—and newly emerging macro-synthetic fibres such as carbon fibre, Dyneema, aramid fibre, basalt fibre, ultra-high-molecular-weight polyethylene (UHMWPE), biodegradable fibres (PVA, PCL, etc.), and conductive fibres. These macro-synthetic fibres have the potential to replace traditional fibres due to their improved post-cracking capacity and the increased ductility of the concrete composite. By enhancing the fire resistance and mechanical properties of concrete, these fibres offer opportunities to improve the safety and durability of structures exposed to high temperatures, thereby reducing the risk of structural failure during fire events and extending the service life of such constructions [13].

1.2. Fibre-Reinforced Concrete

Throughout early civilisation, humans utilized fibres as reinforcements to improve the tensile properties of materials, such as the strengthening of clay or mudbricks with straw or hair. Asbestos fibres were added to cement in the early 1900s. By the 1960s, some materials such as steel, glass, and synthetic fibres were trialled as concrete additives to improve their material properties [14]. The main beneficial characteristics of fibre-reinforced concrete are increased ductility, strength, force tolerance, energy dissipation, crack-resistance, and largely varied application.

Nowadays, four main categories of fibres are applied to reinforce concrete, including glass, steel, polymer, and natural fibres. Steel fibres are the most common type of metallic fibres. Polymer fibres, which are manmade synthetic fibres, are popular additives and are gaining increased attention due to their many advantageous influences on concrete’s characteristics. In the following section, the features of these fibres are discussed and analyzed.

1.2.1. Steel Fibre-Reinforced Concrete

Steel fibre-reinforced concrete (SFRC) involves the most commonly used fibres for cement-based composites, which have applications dating back to the 1960s. SFRC has significant advantages over traditional concrete, including the availability of various steel fibre types with different lengths, thicknesses, and shapes that can help to achieve different properties [14]. The length-to-diameter ratio of steel fibres has a significant influence on cohesion quality, and ranges from 20 to 100, with the length varying from 6.4 to 76 mm. Furthermore, studies on chloride penetration and electrical resistivity have shown that steel fibres enhance the corrosion resistivity of concrete. The chemical composition of steel fibres, in particular the presence of the element copper (Cu) along with the formation of rust layers, helps to enhance corrosion resistivity [15].

The main advantages of SFRC are its superior mechanical properties, with increased strength, stiffness, and high durability, while its high cost and increased self-weight are limiting factors. The corrosion of SFRC is a further inhibiting factor leading to the breaking of fibres in aggressive environments, strength reduction, and unfavourable appearance in prefabricated structures. Additional consequences of steel fibre corrosion include concrete spalling and the reduction in the fibres’ section area, affecting the concrete’s endurance and efficiency.

Studies have investigated the addition of steel fibres in different applications, from ultra-high-strength concrete to lightweight concrete, revealing various benefits of steel fibre. The following presents the findings of different studies on SFRC behaviour at elevated temperatures. Aslani et al. [16] presented the admixture properties of two investigated fibre categories. In addition, fibrous LWSCC outperformed plain concrete at elevated temperatures, especially in residual strength. Another research study comparing the shear strength of SFRC and plain concrete reported that fibrous concrete with 0.25% to 0.5% fibre content showed an average enhanced shear strength of 8.82% to 13.44% at high temperatures up to 800 °C [17]. Lau and Anson [18] investigated the effect of steel fibres in high-performance SFRC at elevated temperatures up to 1200 °C. The researchers found that the steel fibres have a mitigating role in fire-induced damages, especially at elevated temperatures. Further, it was shown that using 1% steel fibre content positively influenced the mechanical characteristics of concrete composite. Purkiss [19], however, reported that the residual strength is not affected by fibre content. In addition, Lie and Kodur [20], who investigated the thermal features of SFRC, found that the thermal properties of steel fibrous concrete resemble that of plain concrete. However, the vast majority of other studies that evaluated the effects of steel fibres on concrete concluded that these fibres have a decisive impact in the thermal behaviour of different types of concrete. The two main disadvantages of using steel fibres, especially in volume contents higher than 1%, are their sensitivity to chloride corrosion and their high ductility.

1.2.2. Polymer Fibre-Reinforced Concrete

The synthetic polymer fibres typically used as additives in concrete are mircofibres (with diameters under 0.3 mm) and include Acrylic, aramid, carbon, nylon, polyester, polyethylene and polypropylene fibres. These fibres are derived from biopolymers. They are white in colour and are produced in different lengths. They typically have a specific weight of approx. 900 kg/m² and a diameter of 20 µm. Polymer fibres are found with two types of threads, single-thread and multi-thread. Adding these fibres to concrete generally causes a loss of workability; therefore, it is suggested to employ them at a low percentage (w/w). Since polymer fibres are low-density and corrosion-resistant, they are very appealing concrete additives for different applications. However, their low modulus of elasticity and adhesion problems present barriers to benefiting from these fibres in concrete.

Today, commercial polymer microfibres are widely used in construction and building applications. A major advantage of polymer fibres is that they are suitable for alkali environments.

Table 1. The properties of selected synthetic fibre types [21].

Fibre Type	Equivalent Diameter (µm)	Specific Weight (g/cm3)	Tensile Strength (MPa)	Modulus of Elasticity (GPa)	Final Elongation (%)	Ignition Temperature (°C)	Melting, Decay, or Oxidation Heat Point (°C)	Water Absorption (%)
Acrylic	12.7–104.1	1.16–1.18	269–1000	13.79–19.31	7.5–50	-	221–235	1–2.5
Aramid I	11.94	1.44	2930	62.06	4.4	High	482	4.3
Aramid II	10.16	1.44	2344	117.2	2.5	High	482	1.2
Carbon (Pan-HM)	7.62	1.6–1.7	2482–3034	379.9	0.5–0.7	High	400	-
Carbon (Pitch-GP)	9.91–12.95	1.6–1.7	483–793	27.58–34.48	2–2.4	High	400	3.7
Carbon (Pitch-HP)	8.89–17.78	1.8–2.15	1517–3103	151.7–482.7	0.5–1.1	High	500	-
Nylon	22.86	1.14	965	5.17	20	-	200–221	2.8–5
Polyester	19.81	1.34–1.39	228–1103	17.24	12–150	593	257	0.4
Polyethylene	25.4–116	0.92–0.96	76–586	5	3–80	-	134	-
Polypropylene	-	0.9–0.91	138–690	3.54–4.83	15	593	166	-

lists the physical properties of different commercial polymer fibres. The thermal resistance of synthetic fibres, which is defined as fibres’ resistance to any damage in the presence of fire or at elevated temperatures, is typically described by the melting heat point or oxidation point. The melting point occurs when a solid fibre turns to glass or enters a flux state. One of the responsible factors of polymer decay is oxidation, which is a by-product of the chemical reaction between polymer and oxygen leading to instant changes in colour and vapour discharges. As shown in Table 1, aramid fibres and carbon fibres have a significantly higher melting point and tensile strength compared to Acrylic, nylon, polyester, and polyethylene fibres.

1.2.3. Glass Fibre-Reinforced Concrete

Glass fibres are another type of fibre used for concrete reinforcement. They have several advantages, such as being non-flammable and resistant to high temperatures and corrosion; they have a high tensile strength and provide good insulation for heat, sound, and electricity. Glass fibres can be incorporated into concrete for a variety of purposes, including crack control, avoiding the coalescence of fissures, and altering the material’s behaviour via bridging cracks [22]. The increment of tensile strength due to fibres is debatable, as they are perceived to be poorly effective before cracking. The result of a study specifically considering microfibres showed that adding micro-glass fibres to concrete can increase its tensile strength by 10–17%, enhancing its capacity to absorb energy and undergo deformation [23].

1.2.4. Natural Fibre-Reinforced Concrete

In cement paste mortar and concrete composites, natural fibres derived from plants (vegetables, leaves, and wood), animals, and geological processes are employed to increase strength qualities. The advantages of natural fibres are their sustainability, low cost, easy usage/handling, and wide availability in different countries. These natural plant fibres include, sisal, coir, jute, Hibiscus cannabinus, malva, eucalyptus grandis pulp, ramie bast, pineapple leaf, sansevieria leaf, kenaf bast, abaca leaf, vakka, date, bamboo, palm, flax, banana, hemp, cotton, and sugarcane [24]. Reis and Ferreira [25,26] reported a greater improvement in the flexural strength of epoxy polymer concrete reinforced with coconut fibres in comparison to glass and carbon fibres. Moreover, a 25% increase in the flexural strength of coconut fibre-reinforced concrete compared to unreinforced concrete was found. Soroushian and Ravanbakhsh [27] reported a 78% reduction in plastic shrinkage cracking by adding 0.06% volume fraction cellulose fibres. In addition to their strength-enhancing properties, the issue of durability for natural fibres in cement paste mortar and concrete composites is of great importance [28]. Natural fibres derived from plants, animals, and geological processes offer a sustainable and environmentally friendly alternative to synthetic fibres. These fibres, including sisal, coir, jute, and hemp, possess inherent resistance to harsh environmental conditions and exhibit good performance over extended periods of time. They can effectively withstand exposure to moisture, chemical attack, and temperature fluctuations, ensuring the long-term durability of the composite material. Moreover, these natural fibres have been found to reduce cracking and enhance the flexural strength of the concrete, as demonstrated by studies that reported significant improvements in these properties through the incorporation of coconut and cellulose fibres. The use of natural fibres also contributes to the overall lifespan and sustainability of structures, thereby highlighting their potential as a viable option for reinforcing cement-based composites [29].

1.2.5. Basalt-Reinforced Concrete

In recent years, applications of basalt fibres in concrete have risen due to their ecologically benign manufacturing technique and their superior mechanical qualities. There are three commercial types of basalt fibre: filament, bundle dispersion, and minibar fibres. Basalt fibre-reinforced concrete can increase the splitting strength of high-strength concrete; however, when the fibre content increases above 5 kg/m³, a fibre agglomeration phenomenon occurs, leading to a decreased split strength. According to research findings [30], the optimum amount of basalt fibres is about 3 kg/m³. Furthermore, concrete specimens with basalt fibres exhibit excellent performance at temperatures higher than 150 °C. It was shown that basalt fibre-reinforced concrete with an optimal fibre amount of 0.2% enhances concrete strength, ductility, and energy absorption, especially at elevated temperatures [31]. On the other hand, basalt fibres are vulnerable to harsh environments, such as chloride attacks and direct flames, and they are expensive compared to currently used synthetic fibres.

2. Properties of Synthetic Polymer Fibres

Synthetic fibres are distinguished between micro- and macro fibres. While the diameter of synthetic microfibres is under 0.3 mm, they do not provide any post-crack ductility. Macro fibres, on the other hand, have a diameter of over 0.3 mm and can increase the ductility of concrete, improve its post-cracking capacity, and act as a sole reinforcement in many concrete buildings.

The main beneficial characteristics of polymeric microfibres are their low sensitivity to corrosion effects, chemical reactions, acidic water, salt, chlorine, chemical components, and microorganisms. In harsh environments, the incorporation of polymeric fibres to reinforced cementitious building materials improves the material properties and strength of the structure in addition to reducing its weight and manufacturing cost (compared to SFRCs). Polypropylene (PP), polyethylene (PE), and polyvinyl alcohol (PVA) fibres are polymer fibres that have gained researchers’ attention due to their advantageous mechanical and physical features, and they have become the most widely used synthetic fibres in cementitious materials for commercial applications. Figure 1 depicts images of typical PP, PE, and PVA fibres.

Table 2. Review studies on different fibres in fibre-reinforced concrete materials.

Research Study	Number of Research Papers on Polymeric Fibres (and Applications) in Concrete			Notes
	PP	PE	PVA
Afroughsabet et al. [32]	26 (10)	2 (2)	8 (6)	Review study on the use of fibres in high-performance concrete.
Thong et al. [33]	-	-	6 (4)	Review on PVA fibres in concrete.
Mukhopadhyay and Khatana [34]	10 (2)	8 (3)	2 (2)	Experimental review study on fibres, including natural, SFRC, and polymer fibres, in concrete.
Yin et al. [35]	8 (4)	-	-	Review on virgin and recycled fibres in reinforced concrete.
Gu and Ozbakkaloglu [36]	26 (13)	1 (1)	1 (1)	Review of aggregates, as well as virgin and recycled plastic fibres, in concrete.

presents the most important review studies on fibre-reinforced concrete, stating the number of research papers investigating PP, PE, and PVA fibres and their application in concrete mixtures [32,33,34,35,36]. It is noted that PP fibres have been the most widely used and studied type of fibres in comparison to PE and PVA fibres.

Some mechanical and physical properties of common polymeric fibres (and steel fibres for comparison), along with their estimated expenses, are given in Table 3. There is substantial variation in the characteristics of fibres. Steel fibres have the greatest specific gravity, modulus of elasticity, and tensile strength, but the lowest elongation at the break. In addition, their resistance to acids and alkalis is varied at a high range, from low to high. The cost of PPs and SFRCs is lower compared to PEs and PVAs. The elastic modulus of the cementitious matrix is in the range of 10–40 GPa. Energy consumption, fibre production equipment, and raw material value are further factors that contribute to the cost of the fibres. Due to the expensive equipment used to manufacture synthetic fibres with high elastic modulus and strength, the cost of these fibres increases.

As seen in Table 3, compared to SFRC, synthetic fibres have a relatively low density, resulting in small-mass high-volume fibre cement-based materials. PE and PP are subcategories of polyolefins and form the largest group of commodity thermoplastics, with many commercial and research applications. PP fibres have a low elasticity modulus compared to steel fibres and are used to control concrete cracking at early ages, leading to a reasonable level of shrinking resistance [35,37,38,39]. To improve the low tensile and impact strength characteristics of synthetic fibres, combining PP and PE fibres can produce fibre mixtures with optimized properties [40].

Table 3. Mechanical and physical properties of fibres (adopted from [41]).

Types of Fibres	Tensile Strength (MPa)	MOE (GPa)	Relative Density (kg/m3)	Acid/Alkali Resistance	Elongation at Break (%)	Approx. Cost ($ /kg)
PP fibres [14,42,43]	240–900	1.5–12	910	High	15–80	1–2.5
PVA fibres [44,45,46]	1000–1600	20–42.8	1290–1300	High	6–7	1–15
PE fibres [47,48]	80–600	5–100	920–960	High	4–100	2–20
Steel fibres as comparison [49]	500–2000	200	7840	Low to High	0.5–3.5	1–8

Table 3. Mechanical and physical properties of fibres (adopted from [41]).

Types of Fibres	Tensile Strength (MPa)	MOE (GPa)	Relative Density (kg/m3)	Acid/Alkali Resistance	Elongation at Break (%)	Approx. Cost ($ /kg)
PP fibres [14,42,43]	240–900	1.5–12	910	High	15–80	1–2.5
PVA fibres [44,45,46]	1000–1600	20–42.8	1290–1300	High	6–7	1–15
PE fibres [47,48]	80–600	5–100	920–960	High	4–100	2–20
Steel fibres as comparison [49]	500–2000	200	7840	Low to High	0.5–3.5	1–8

To analyze the impact of fibre reinforcement on the material strength, an improvement index is established by calculating the ratio of fibre-reinforced cementitious composites strength to plain composite tensile/flexural strength. Figure 2, Figure 3 and Figure 4 present the calculated improvement indices in the tensile/flexural strength in relation to the fibre volume fraction of PP, PVA, and PE fibre-reinforced concrete. The improvement analysis shows that incorporating PE and PVA fibres results in the largest improvement in tensile and flexural strength because of their greater strength and modulus of elasticity compared to PP fibres. Although PP fibres do increase the concrete’s crack resistance and ductility, they have little impact on the strength of cementitious materials, representing one of their disadvantages.

In general, the compressive strength of the material is reduced by adding synthetic fibres to cementitious materials. For chemical resistivity, the presence of synthetic macro fibres acts as a protection for rebars against chloride penetration, and it delays its corrosion; hence, the PP and PE fibre-reinforcement of concrete mixtures leads to improved corrosion resistivity. Further, PP and PE fibres play an interconnecting role in the concrete matrix, and they maintain the concrete in a stable state for a longer time [50,51,52]. Generally, the benefits of concrete reinforcement with high-modulus fibres are superior strength properties, low deflection capability, and narrow crack size. This is in contrast to low-modulus fibre reinforcement, which has a high deflection capacity, greater crack size, and lower strength properties [40].

Figure 3. The tensile/flexural strength enhancement index/strength of PVA (macro/micro) fibre-reinforced concrete (data adopted from [51,52,53,54,55]).

Figure 3. The tensile/flexural strength enhancement index/strength of PVA (macro/micro) fibre-reinforced concrete (data adopted from [51,52,53,54,55]).

Figure 4. Improvement index/strength of the tensile/flexural strength of PE (macro/micro) fibre-reinforces concrete (data adopted from [56,57,58]).

Figure 4. Improvement index/strength of the tensile/flexural strength of PE (macro/micro) fibre-reinforces concrete (data adopted from [56,57,58]).

Macro fibres are a type of synthetic fibre similar to PP and PE microfibres. Compared to PP and PE microfibres, they possess favourable features such as higher corrosion resistivity, lightweight strings, and a high modulus of elasticity, making them appealing as potential alternatives to current synthetic fibres. Characteristic properties of some macro fibres are represented in

Table 4. Characteristics of different macro fibres (data obtained from [31,40,59].

Base Material	Tensile Strength (MPa)	Geometry	Colour	Specific Density	Length (mm)	Ignition Point (°C)	Chemical Resistance	Elastic Modulus (GPa)	Melting Point (°C)
Modified Olefin	640	Continuously Embossed	White	0.9–0.92	54	>410	Excellent	10	159–179
Polyolefin	550	Continuously Embossed	White	0.9–0.92	48	>450	Fair	10	150–165
Polypropylene	550	Continuously Embossed	Transparent	0.85–0.93	30	>380	Fair	8.2	150–165

. Macro fibres made from modified olefin have the highest tensile strength and excellent chemical resistance compared to polyolefin and polypropylene.

3. Thermal Performance of Different Synthetic Fibres in Concrete Composites

In general, adding fibres to concrete reduces the concrete’s spalling when subjected to high temperatures. Sarvaranta et al. [60] reported that PP fibres are the most effective type to reduce concrete spalling. 61. Pardon [61] evaluated the impact of fibres with various doses of fibre, varying between 0 and 3 kg/m³, and an average strength of concrete between 105 and 110 MPa at high temperatures. The outcomes of the study showed that the mass loss due to spalling decreases with the addition of fibres, even at lower doses, such as 0.9 kg/m³ in the tested situation. Lennon and Clayton [12] reported the same findings. In plain concrete, a plateau-like disturbance ends at around 250 °C; in fibre-reinforced concrete, however, this disturbance begins at temperatures above 100 °C and terminates between 160 °C and 200 °C, which is near to the fibre’s melting point. This phenomenon is related to water vaporization in the porous network that consumes energy [62]. When the fibres are present in concrete and are melting, vaporization and water expulsion occur at lower temperatures. This reduces the post-peak pressure of fibre-reinforced concrete, highlighting the importance of fibres in creating a more permeable network in the cement mix. Further, Liu et al. [63] performed a holistic study on the nature of spalling in ultra-high-performance strain-hardening cementitious composite subjected to 30 °C, 200 °C, 400 °C, 600 °C, and 800 °C. Their findings showed that PE fibres with a melting point of 144 °C could not prevent the explosive spalling of concrete specimens; however, PP fibres with a melting point of 160–170 °C at a dosage of 2 kg/m³ could prevent the explosive spalling.

3.1. Thermal Performance of PP Fibre-Reinforced Concrete

Several researchers have investigated PP fibres as one of the most common thermoplastic polymers, and their usage as an additive to fibre-reinforced concrete has been investigated by numerous studies. Research on their behaviour at elevated temperatures found that they improved the thermal stability of the concrete materials and led to a significant improvement in the residual properties after fire exposure [64,65,66,67]. Xiao and Falkner [5] observed that the melting of PP fibres at elevated temperatures results in water evaporation and the subsequent release of vapour pressure. This phenomenon considerably decreases the tendency of reinforced concrete spalling. The behaviour of the mechanical characteristics of self-consolidating concrete with PP fibres subjected to high temperatures was investigated by Uysal and Tanyildizi [68]. Due to the creation of randomly dispersed micro-channels during the melting of the fibres at elevated temperatures, they discovered that this type of concrete had a lower residual compressive strength than unreinforced concrete. They also found that the mineral compounds type did not impact the decrease in the concrete’s compressive strength. Tanyildizi [69] investigated the mechanical characteristics of lightweight concrete under elevated temperatures. The study examined fibre volume ratios of 0.5%, 1.0%, and 2.0% (with a fibre length of 12 mm and a fibre diameter of 18 µm) and found that raising the temperature to above 400 °C decreases the flexural and compressive strength of lightweight concrete. A larger fibre volume ratio resulted in a larger reduction in strength.

Khoury [70] studied the effect of the significant difference in the elasticity modulus between concrete and PP fibres at elevated temperatures. The researcher observed that this difference increases even further, with the modulus of concrete’s elasticity in sub-heated environments above 100 °C, which is approximately 100 times higher than that of PP samples, and about 1000 times higher at 150 °C. Consequently, heated concrete that is reinforced with PP fibres and subjected to loading shows a significant decrease in strength and modulus of elasticity compared to heated concrete without PP fibres. Zhang and Li [71] studied the fire-resistive performance of spray-applied Engineered Cementitious Composites (ECCs) using high-tenacity polypropylene (HT-PP) fibres. ECCs containing HT-PP fibres have attractive features due to their spraying ability with low thermal conductivity; as a sustainable fire-resistant material, they have a strength capacity of 0.87 MPa and strain of 1.0%. The study found that the novel cementitious composites have more considerable adhesive fracture energy (104.3 ± 15.4 J/m²) than non-combustible materials (11.1 ± 1.4 J/m²) [72].

Pliya et al. [73] evaluated the physical characteristics of fibre-reinforced concrete containing PP fibres exposed to elevated temperatures in the range of 150–600 °C. They found that for volume ratios of 0.1% and 0.2%, the porosity of the concrete composite increased considerably with the temperature rising. Fibre-reinforced concrete has 152% more relative porosity than non-fibre concrete at 300 °C. The splitting tensile, compressive strengths, and elastic modulus of PP fibre-reinforced concrete reduced steadily with increasing temperature.

The melting of the fibres and development of lightweight concrete with extra porosity at temperatures up to 800 °C was reported by Mohamad Hosseini et al. [74] in a microstructural evaluation using scanning electron microscopy analysis. The researchers observed an improvement in the ductility of the concrete, as well as higher energy absorption and proper cracking, due to the addition of PP fibres. Various investigations have revealed that while the behaviour of fibre-reinforced concrete is influenced by the thermal properties of the additive fibres, like glass transition temperature (Tg), the temperature at or above which the molecular structure exhibits macromolecular mobility, melting, and viscosity after melting is not affected by the mechanical properties and geometry of the fibres [59,75].

Hiremath and Yaragal [75] experimentally studied the structural properties of ultra-high strength reactive powder concrete (RPC) with PP at increased temperatures from 200 to 800 °C. The outcomes showed that incorporating 0.1% of PP is the most beneficial dosage to mitigate explosive spalling up to 800 °C. For durability features, like the absorption and absorptivity of water, a dosage of 0.5% fibre performed better compared to other fibre dosages. For the residual mechanical characteristics (such as tensile strength and compressive strength), an increase in strength up to 400 °C was observed; however, after 600 °C, an abrupt decrease in strength was observed.

Aslani and Kelin [76] experimentally investigated the structural behaviour of high-performance fibre-reinforced lightweight self-consolidating concrete containing recycled crumb rubber aggregates, lightweight scoria aggregates, macro fibres, and steel and PP fibres, subjected to elevated temperatures. The researchers reported that both mixtures with SFRC and PP-fibres subjected to high temperatures demonstrated a considerable decrease in elasticity modulus. After exposure to 900 °C, the residual compressive strength was about 20% compared to the control specimen, at 25 °C. Varona et al. [77] evaluated the bonding properties of high-strength concrete reinforced by steel and PP fibres after exposure to high temperatures. The outcomes demonstrated that high strength concrete containing calcareous aggregates and hybrid fibres, which included PP fibres and steel fibres with two different aspect ratios, experienced minor spalling at higher temperatures. Varona et al. [78] worked on high-strength concrete with limestone aggregate reinforced by alloyed steel and PP fibres. It was discovered that adding a hybrid fibre mix has a positive effect by mitigating concrete spalling. However, the researchers acknowledged that further investigations on the fibre’s geometry and aspect ratio are required. Xargay H. et al. [79] examined the impact of fire on the structural characteristics of high-strength self-consolidating concrete reinforced with PP and steel fibres. They indicated that fibre integration has advantages such as structural integrity and the enhancement of force distribution. However, using PP fibres caused some explosive cracking, which should be analyzed in future studies. Hou et al. [80] conducted experimental research on the thermal efficiency of reinforced reactive powder concrete (RPC) and normal strength concrete beams. The authors found that applying 0.2% PP and 2% steel fibres in RPC beams led to the least concrete spalling. In contrast, they reported that the normal strength concrete beam was more fire-resistant in comparison to the RPC beam. In another paper, Hou et al. [81] conducted a research study on the impact of fire insulation on the fire-induced spalling of RPC beams. Again, they indicated that RPC beams including 2% steel and 0.2% PP fibres in combination with fire insulation exhibited the least fire-induced spalling. Moreover, this composition retained a sufficient flexural strength in post-fire evaluations.

Li et al. [82] experimentally studied the residual permeability and micro-cracking of ultra-high-performance reinforced concrete with PP fibres. They reported that using PP fibres increases the residual permeability of concrete. Further, Zhang et al. [83] evaluated the preventative impact of PP fibres on ultra-high-performance concrete spalling when exposed to elevated temperatures. They found that PP fibres have an important effect on mitigating spalling. Ozawa et al. [84] reported the preventive influence of PP and jute fibres on the spalling of ultra-high-performance concrete at high temperatures combined with waste porous ceramic fine aggregate as an internal curing material. They reported that incorporating internal curing material and PP fibres leads to decreased autogenous shrinkage and specific density and increased porosity for the concrete. They further proposed the addition of 0.5% jute fibre as the best dosage to mitigate spalling. Although several papers have investigated concrete spalling, few research investigations have been performed on the fire resistance and efficiency of prestressed reinforced concrete beams.

Table 5. Summary of mechanical and thermal properties of PP fibre-reinforced concrete at elevated temperatures.

Study	PP Fibre Volume (%)	Temperature Range (°C)	Key Findings
Uysal and Tanyildizi [68]	0.1%, 0.2%	Up to 800	- Melting of PP fibres creates micro-channels. - Lower residual compressive strength compared to unreinforced concrete. - Mineral compound type does not impact compressive strength decrease.
Tanyildizi [69]	0.5%, 1.0%, 2.0%	Up to 800	- Temperatures above 400 °C decrease flexural and compressive strength of lightweight concrete. - Higher fibre volume ratios lead to larger strength reductions.
Pliya et al. [73]	0.1%, 0.2%	150–600	- Porosity increases significantly with temperature rise. - At 300 °C, fibre-reinforced concrete has 152% more relative porosity than non-fibre concrete. - Splitting tensile strength, compressive strength, and elastic modulus decrease steadily with increasing temperature.
Hiremath and Yaragal [75]	0.1%, 0.5%	200–800	- A fibre dosage of 0.1% PP is most effective in mitigating explosive spalling up to 800 °C. - Strength increases up to 400 °C and abruptly decreases after 600 °C. - For durability features like water absorption and sorptivity, 0.5% fibre content performs better.
Aslani and Kelin [76]	Not specified	Up to 900	- Mixtures with steel and PP fibres show a considerable decrease in elasticity modulus at high temperatures. - Residual compressive strength is about 20% after exposure to 900 °C compared to controls at 25 °C.
Varona et al. [77]	Hybrid fibres	High temperatures	- High-strength concrete with calcareous aggregates and hybrid fibres (PP and steel) experiences minor spalling at higher temperatures. - Hybrid fibres mitigate spalling effectively.
Xargay et al. [79]	PP and steel fibres	Not specified	- Fibre integration enhances structural integrity and force distribution. - Use of PP fibres caused some explosive cracking, requiring further analysis.
Hou et al. [80,81]	0.2% PP, 2% steel	Not specified	- RPC beams with PP and steel fibres exhibit least spalling. - Normal strength concrete beams are more fire-resistant compared to RPC beams. - Fire insulation combined with fibres retains sufficient flexural strength post-fire.
Li et al. [82]	Not specified	Not specified	- PP fibres increase residual permeability of concrete. - Important effect on mitigating spalling in ultra-high-performance concrete when exposed to elevated temperatures.
Ozawa et al. [84]	0.5% jute fibres	Not specified	- Incorporating internal curing material and PP fibres decreases autogenous shrinkage and specific density, and increases porosity. - In total, 0.5% jute fibre addition is optimal for mitigating spalling.

shows the key findings of the studies based on the temperature and fibre volume of PP-reinforced concrete.

3.2. Thermal Efficiency of PE Fibre-Reinforced Concrete

Polyethylene (PE) is the most common plastic in use today, and its primary usage is in packaging. PE fibres are created through the polymerization of ethane, and their addition to cement-based composites has been explored and applied. As for the thermal characteristics of PE fibre-reinforced concrete, Sukontasukkul et al. [85] studied the flexural behaviour and stiffness of PE fibres at elevated temperatures. From the load–deflection response shown in Figure 5, the toughness of concrete improves as the temperature increases from the ambient state to 400 °C. This is due to the enhanced bonding of the composite material caused by enhancing the molecular heat of the concrete texture and the fluxing surface of the PE fibres [75].

The thermal behaviour of cementitious composites with combined PP-PE fibres of different ratios ranging from 0.3% to 1.2% was evaluated by Çavdar [86]. The researcher found that the flexural deflections of this mortar mix increased significantly compared to non-fibre mixtures. At 650 °C, a 77% reduction in deflection was observed for plain concrete, while for mortar containing 9% fibre, this reduction was only about 13% (e.g., a decrease from 1.40 mm to 1.22 mm). In addition, concrete containing PE fibres was experimentally evaluated in an analogous paper investigating SFRC and nanotube fibre concrete [87]. According to their results, the PE fibre composite performed better compared to the other mixtures regarding the spalling and residual compressive strength degradation. Liu and Tan [63] investigated the ultra-high-efficiency of strain-hardening cementitious composites containing PE and steel fibres subjected to extreme temperatures. The outcomes indicated a drop in the compressive strength and continuous concrete spalling because of the severity of cracking at elevated temperatures. Their research suggested that PE fibres have no effect on reducing concrete spalling under high-temperature conditions.

3.3. Thermal Performance of PVA Fibre-Reinforced Concrete

Polyvinyl alcohols (PVAs) are known as a type of synthetic polymer which is widely utilized in a variety of applications, including food, medicine, and commercial and industry products. In cement-based composites, the concrete matrix is uniformly distributed, which generates a multi-directional fibre network, providing shrinkage control, abrasion resistance, and protection from thermal expansion and contraction. PVA fibres can be employed as primary reinforcements under specific load limitations. Sahmaran et al. [72] investigated the impact of PVA fibre-reinforced ECC at elevated temperatures of up to 800 °C. In their experiments, the original compressive strength of the non-fibre concrete significantly decreased in the temperature range of 200–400 °C, and after that, explosive spalling and severe cracking occurred. For ECCs containing PVA fibres, which have a melting point of about 230 °C, no spalling was observed. Compared to the composite without PVA, the least amount of surface cracking, caused by the loss of compressive strength, was found at 400 °C for a mixture containing 14.6% PVA. When the PVA-ECC was subjected to temperatures above 800 °C and air cooling, spalling did not occur, as shown in Figure 6.

Çavdar [21] observed significant declines of 76% and 87% in the flexural strength of a cementitious composite without fibres when subjected to elevated temperatures of 450 and 650 °C. In the case of composites containing 0.5–2.0% fibre volume, the flexural strength at 450 °C and 600 °C was significantly higher than that of the control specimen. However, at raised temperatures, the deflection of PVA fibre composites was larger in comparison to the non-fibre samples (Figure 7). In case of deflection, the results of da Silva Magalhães et al. [88] showed that when the specimens were heated to 90 °C, there was a slight increase in the first-crack stress and ultimate bending strength, but the deflection capacity and toughness decreased significantly. From 110 °C to 190 °C, the first-crack stress increased, but the ultimate bending strength, deflection capacity, and toughness decreased. Even though the composite still exhibited strain hardening behaviour, its overall performance was diminished. At 250 °C, the composite lost load-bearing capacity due to the melting of PVA fibres. The deflection capacity, toughness, and ultimate bending strength significantly decreased compared to unheated specimens. However, the first-crack stress increased. The crack pattern analysis revealed that specimens preheated at temperatures below 190 °C exhibited a multiple-cracking pattern during inelastic deflection. However, the specimens preheated at 250 °C had only one crack. The crack density also decreased with increasing temperature. Overall, the study shows that preheating the PVA-SHCC composite has a detrimental effect on its mechanical behaviour. Higher temperatures lead to reduced deflection capacity, toughness, and ultimate bending strength. However, the first-crack stress tends to increase with temperature.

Da Silva Magalhães et al. [88] studied the thermal durability of strain hardening in cementitious composites containing PVA fibres exposed to 250 °C. They reported that high temperatures altered the material characteristics of the composite, including decreased stiffness, tensile strength, ductility, and cracking. In their study, a reduction in the number of micro-cracks happened as the temperature increased, which is shown in Figure 8. Even after cracking at 190 °C, the composite’s load bearing capacity was evident, while its strain capacity greatly reduced by 83%. At 250 °C, a considerable decline in the strength of the PVA fibre composite was observed. This strain-softening behaviour after the first cracking was attributed to the melting of the PVA fibres (see Figure 9).

The PVA fibre geometry on the maximum pore pressure of fibre-reinforced high-strength concrete was explored by Bangi and Horiguchi [54]. The outcomes demonstrated that longer fibres with smaller diameters outperformed shorter ones with greater diameters. Yu et al. [89] studied the efficiency of PVA-ECC with a high proportion of fly ash (binder to cement ratio = 4.4) subjected to elevated temperatures. It was found that the interface of fibre/matrix was influenced through moderate temperature increases in the range of 50–100 °C. The temperature increase from 20 to 100 °C improved the chemical debonding energy and frictional stress of unheated fly ash due to an accelerated hydration process. This led to improvements in strain capacity and ultimate tensile strength for heated PVA-ECC composites compared to the compressive strengths of unheated concrete subjected to elevated temperatures above 800 °C. Furthermore, at 400 °C, the compressive and flexural strengths of geopolymer concrete composites containing PVA were 45% and 29% greater than those of the control specimen.

3.4. Thermal Efficiency of Macro Fibre-Reinforced Concrete

In recent years, a new kind of synthetic fibre, macro fibres with diameters over 0.3 mm, have emerged as a potential concrete additive. Macro fibres are attractive for use in concrete due to their several beneficial features, including being lightweight, having high corrosion and impact resistance, and possessing favourable fracture properties. Today, the use of macro fibres to reinforce ready-mixed and precast concrete is fast growing in the concrete admixture market. Recent investigations have demonstrated the remarkable impacts of macro fibre addition on the thermal stability of concrete compositions [90]. However, only limited research has been conducted addressing the effects of macro fibres on thermal conductivity, fire-resistivity, spalling, residual mechanical properties, and other thermal behaviours of macro fibres.

Polymer blends are used to create macro-synthetic fibres, initially developed as an alternative to steel fibres in a wide range of applications, including sprayed concrete. Commercial macro fibres have a fibre diameter of over 0.3 mm and are typically classified in five categories, as shown in

Table 6. Properties of various macro fibres.

Macro Fibre Type	Geometry	Tensile Strength (MPa)	Base Material	Specific Weight (kg/m3)	Length (mm)	Alkali Resistance	Elastic Modulus (GPa)
Macro fibre 48 (fibre class II)	Continuously Embossed	640	Virgin Polypropylene	2.5–5	48	Excellent	12
Macro fibre 54 (fibre class II)	Continuously Embossed	640	Virgin Polypropylene	3–6	54	Excellent	12
Macro fibre 60 (fibre class II)	Continuously Embossed	640	Virgin Polypropylene	4–6	60	Excellent	12
Macro fibre MQ58 (fibre class II)	Continuously Embossed	640	Bi-Component Polymer	2.5–5	58	Excellent	10
Macro fibre R65 (fibre class II)	Continuously Embossed	610	Polypropylene	4–6	65	Excellent	10

. Different types of macro fibres have excellent resistance to alkali environments and similar tensile strength (640 MPa), except macro fibre 65, with 610 MPa tensile strength. Pictures of different macro fibres are displayed in Figure 10.

Shen et al. [91] explored the impact of macro fibres on the cracking reactions of early-age high-performance concrete. Temperature-Stress Testing Machine tests with forced cooling were performed, and the temperature loss of the test specimen was measured throughout the cooling stages. The cracking potential (age) of high-performance macro fibre concrete was calculated as the difference between the highest temperature and the cracking age temperature according to the following equation:

$$T_{td}=T_{ht}+T_{ck}$$

(1)

where $T_{td}$ is the temperature loss at the cracking age, $T_{ht}$ is the highest temperature of the concrete composite before cracking, and $T_{ck}$ is the cracking age temperature. When increasing the macro fibre content from 0 to 4, 8, and 12 kg/m³, the temperature loss increases by about 9.2%, 17.6%, and 0.6%, respectively (see Figure 11). It was found that the macro fibres’ inclusion resulted in a considerable decrease in the high-performance concrete cracking potential. The vital importance of macro fibres in concrete is further attributed to the partial load tolerance before the onset of cracking, as well as through the ability to transmit stresses across bridging cracks after crack initiation [91].

Behfarnia and Behravan [92] investigated the effect of high-performance polymer (HPP) fibres, a type of macro fibre, in concrete composites. They reported that incorporating different volumes of HPP fibres did not significantly change the compressive strength of the concrete mixtures compared to those reinforced with steel fibres (SFRC). However, the macro fibres notably enhanced the toughness, tensile strength, flexural strength, and energy absorption of the concrete composites. Their results demonstrated that the influence of macro fibres on characteristics such as toughness was more pronounced than in SFRC composites.

In contrast, a study evaluating the performance of HPP fibres in the concrete lining of water tunnels [42] found that the compressive strength, flexural capacity, permeability, and corrosion resistance of concrete containing HPP fibres changed significantly compared to SFRC specimens. Furthermore, the endurance and functionality of the HPP fibre-reinforced concrete linings were improved. This discrepancy in the findings may be attributed to differences in the experimental conditions, fibre content, or specific applications studied.

Poorsaheli et al. [50] investigated the influence of endurance factors of concrete reinforced with synthetic fibres in shorelines containing high chloride concentrations. The parameters, including permeability under pressure, electrical resistance, half-cell test, absorption of water, and compressive and flexural strengths of three synthetic fibres and macro fibres with various physical characteristics and different volumes, were investigated. The results found that the best short-term durability was obtained for polyolefin macro fibres compared to the other investigated fibres.

Dawood and Ramli [93] evaluated the mechanical characteristics of hybrid fibre concrete composites with a volume fraction of 2%. The researchers added a hybridisation of steel fibres, palm fibres, and synthetic fibres (macro fibres) to high-strength flowing concrete (HSFC) to investigate its compressive strength density, splitting tensile strength, impact load, static modulus of elasticity, toughness indices, and flexural strength. The results showed that hybridisation improved the flexural toughness and tensile strength of the HSFC. Furthermore, as the content of the hybrid fibres increased, the resistance to impact pressure and first-crack and post-crack strength increased.

3.5. Summary of Material Properties and Thermal Efficiency of Fibre-Reinforced Concrete

This research study reviewed the latest findings on the influence of elevated temperatures on fibre-reinforced concrete efficiency. Different types of fibre additives were discussed and evaluated regarding their thermal behaviour, and other material characteristics.

Table 7. Summary of various material characteristics (including thermal behaviour) of different types of fibre-reinforced concrete.

Fibres	Material Characteristics		Other Features
	Advantages	Disadvantages
Steel fibres	Improved residual strength at elevated temperatures compared to non-fibrous concrete [16].	Increased vulnerability to chloride corrosion, leading to fibre breakages, strength reduction and concrete spalling.	The length-to-diameter ratio of steel fibres has a major impact on the cohesion quality, with fibre lengths varying from 6.4 to 76 mm.
	Decreased fire-induced damage at temperatures up to 1200 °C [18].
	Improved corrosion-resistivity due to the presence of copper (Cu) and the formation of a rust layer [15].
		High ductility, increased self-weight, and high cost.
PP fibres	PP fibres are the most effective type of fibre to considerably reduce spalling in concrete exposed to elevated temperatures, including explosive spalling [5,60,63,75].	Significant loss in modulus of elasticity and strength when subjected to elevated temperatures [70,76].	For the residual mechanical properties (e.g., tensile strength and compressive strength), an increase in strength up to 400 °C was observed; however, after 600 °C, there was a sudden drop in strength [75].
	Increased residual permeability [82].	With increasing temperature, the splitting tensile and compressive strengths of PP fibre-reinforced concrete reduce steadily [63].	The melting of PP fibres at high temperatures leads to water evaporation and the subsequent release of vapour pressure [5].
	Improved shrinking resistance due to reduced concrete cracking at early ages [37,38,39].	Less residual strength at elevated temperatures compared to plain concrete [69].	PP fibres have been the most widely used and studied type of fibre in comparison to PE and PVA fibres.
	Improved corrosion resistivity, crack resistance, and ductility.	Increased porosity [84].
PE fibres	Superior performance on spalling and residual compressive strength degradation compared to steel and nanotube fibre concrete [86].	Decrease in compressive strength and continuous spalling at elevated temperatures [63].	Increased stiffness for temperatures up to 400 °C due to enhanced bonding of the composite material at high temperatures [74].
	Contribution to the interconnectivity of the concrete matrix [50,52].	Low tensile and impact strength [40].
PVA fibres	Improved flexural and compressive strength of concrete [21,89].	Reduction in compressive strength for composites with high PVA fibre content for temperatures up to 600 °C [21].	Longer fibres with smaller diameters perform better than shorter fibres with larger diameters [54].
	Prevention of spalling for temperatures up to 800 °C [73].	Larger deflection of PVA fibre composites at elevated temperatures [53].	Compared to PP fibres, PE and PVA fibres lead to a larger improvement in tensile and flexural strength due to their higher strength and modulus of elasticity.
	Improved shrinkage control, abrasion resistance, and protection from thermal expansion and contraction.	Tensile strength, ductility, and cracking potential are negatively affected by high temperatures [88].
	Reduced number of micro-cracks when exposed to increasing temperatures above 250 °C [88].
Macro fibres	Improved corrosion resistivity, toughness, tensile strength, flexural strength, energy absorption, durability, and serviceability [42,92].	Compressive strength is similar to steel fibre-reinforced concrete [92]	Improved flexural toughness and tensile strength when adding a hybridisation of macro fibres, steel fibres, and palm fibres to high-strength flowing concrete. With an increase in the content of the hybrid fibres, the resistance to impact pressure and first-crack and post-crack strength further improves [93].
	Significant reduction in cracking potential in high-performance concrete [75,91].
	Partial load tolerance of fibres before the onset of cracking [91].
	High modulus of elasticity and lightweight strings [42,50,93].

presents an overview of these findings, listing various advantages and disadvantages of using fibre-reinforced concrete when subjected to elevated temperatures.

3.6. Bibliometric Analysis

To provide an overview of the research landscape, a bibliometric analysis was conducted on studies related to synthetic polymer fibres in concrete. The analysis focused on publication trends, the most influential works, and emerging research topics in the field. The results indicate a growing interest in the use of polypropylene (PP), polyethylene (PE), and polyvinyl alcohol (PVA) fibres for enhancing concrete performance, with an increasing number of publications in recent years. Key research gaps include the need for more in-depth studies on the long-term durability of fibre-reinforced concrete and the development of new composite materials with improved fire resistance. This analysis helps contextualize our review within the broader research trends and highlights areas for future exploration. Meanwhile,

Table 8. Trends in research publications [94,95].

Macro Fibre Type	Number of Publications	Focus Area
2015	12	PP and PE fibre applications
2016	18	PVA fibre properties
2017	22	Fire resistance of fibre composites
2018	30	Hybrid fibre use in construction
2019	35	Long-term durability studies
2020	42	Emerging macro-synthetic fibres
2021	50	Sustainability and eco-friendly fibres
2022	58	Application in high-performance concrete
2023	65	Advances in fibre-reinforced composites
2024	72	Novel applications and hybrid systems

provides an outlook of key trends in fibrous concrete publications from 2015 to 2024 [94,95]. Figure 12 illustrates the increasing trend in research publications from 2015 to 2024, highlighting the growing interest in synthetic polymer fibres in concrete. This trend underscores the significance of continued research in this area, especially in addressing the challenges related to fire resistance and sustainability.

4. Recycling Synthetic Fibre-Reinforced Concrete: End-of-Life Solutions

In addition to exploring the mechanical properties and durability of synthetic fibre-reinforced concrete, it is crucial to address its end-of-life solutions [96]. The increasing use of synthetic fibre-reinforced concrete has raised concerns about the sustainability of disposal and recycling options for these materials at the end of their service life. This section explores potential end-of-life solutions for synthetic fibre-reinforced concrete, focusing on its recycling potential as aggregate in new concretes and the consequential effects on the mechanical properties and durability of recycled aggregate concrete (RAC). The disposal of synthetic fibre-reinforced concrete poses several challenges, including landfill restrictions, accumulation in waste streams, and environmental concerns [97]. The accumulation of such materials can lead to increased waste volumes, straining landfill capacities. Moreover, environmental concerns arise from the potential release of non-biodegradable synthetic fibres into the environment, which can persist and contribute to microplastic pollution. To mitigate these challenges and conserve natural resources, there is an urgent need for sustainable disposal strategies.

Recycling synthetic fibre-reinforced concrete as aggregates in new concretes offers a sustainable solution with various ecological benefits [98]. This approach reduces the demand for virgin aggregates, decreases disposal volumes, and diminishes the overall ecological footprint associated with landfilling. Adopting sustainable disposal strategies can be instrumental in safeguarding the environment and promoting a circular, resource-efficient economy [99]. In the pursuit of sustainable construction practises, it is crucial to assess the viability of incorporating synthetic fibre-reinforced concrete waste as recycled aggregate in new concretes, particularly in the production of recycled aggregate concrete (RAC). An examination of the recycling processes, techniques, and technologies is necessary to effectively segregate and process synthetic fibre-reinforced concrete waste into suitable aggregates [99]. This entails investigating methods that ensure the proper separation and treatment of the synthetic fibres for reuse while transforming the remaining concrete into high-quality recycled aggregate.

By utilizing synthetic fibre-reinforced concrete aggregates in RAC, several potential benefits can be realized, including a reduced environmental footprint through the diversion of waste from landfills, the conservation of natural resources, and decreased material costs. However, it is equally important to consider the limitations, such as the possible impact on the structural strength and durability of RAC when compared to traditional concrete, and the need for comprehensive quality control measures. A thorough analysis of the feasibility, benefits, and limitations of incorporating synthetic fibre-reinforced concrete aggregates in RAC will provide valuable insights for sustainable construction practises [100]. Research studies have shown that incorporating synthetic fibre-reinforced concrete aggregates in RAC can enhance its durability [101]. These aggregates offer improved resistance to abrasion, freeze–thaw cycles, alkali–silica reactions, and chemical attacks [102]. The inclusion of synthetic fibres positively influences the durability performance of RAC, resulting in increased longevity and reduced maintenance costs. However, challenges such as achieving proper fibre distribution and optimizing mix design need to be addressed. Best practises include using appropriate fibre volumes and implementing proper quality control measures to enhance the long-term durability of RAC with synthetic fibre-reinforced concrete aggregates [103,104].

5. Role of Fibres in Zero-Cement and 3D-Printed Concrete

The incorporation of fibres into zero-cement concrete and 3D-printed concrete is pivotal in enhancing the performance and sustainability of these innovative construction materials. In zero-cement concrete, fibres such as steel, polypropylene, basalt, and natural fibres improve mechanical properties by increasing tensile strength, ductility, and crack resistance [105]. They mitigate the brittleness associated with alternative binders like geopolymers, leading to more durable and resilient structures. Additionally, using natural or recycled fibres aligns with environmental goals by reducing carbon footprint and promoting resource efficiency. In 3D-printed concrete, fibres address challenges like interlayer bonding weaknesses and anisotropic mechanical properties inherent in the printing process. The addition of fibres enhances interlayer adhesion and overall structural integrity by bridging gaps between printed layers. This results in improved load-bearing capacity and allows for more complex and reliable architectural designs. Future research focuses on optimizing fibre types and orientations for better printability and performance, as well as developing standardized testing methods to facilitate the wider adoption of fibre-reinforced, sustainable construction technologies. Huang et al. (2024) [106] and Wang et al. (2022) [105] showed that fibres improve void distributions in 3D-printed concrete and enhance bond performance in marine environments, respectively.

6. Temperature Effects on Bond Performance and Post-Fire Characteristics of Fibres in Concrete

The bond between fibres and the concrete matrix is a critical factor influencing the mechanical performance and durability of fibre-reinforced concrete (FRC). Temperature variations, especially elevated temperatures and fire exposure, can significantly affect this bond, leading to changes in structural integrity. This section discusses how different temperatures impact the fibre–matrix bond and examines the post-fire characteristics of fibres within the concrete.

6.1. Variation in Bond Performance at Various Temperatures

Temperature fluctuations can lead to the thermal expansion or contraction of materials, affecting the interfacial bond between fibres and the concrete matrix. The differential thermal expansion coefficients of fibres and concrete can induce stresses at the interface, influencing bond strength.

• Steel Fibres: Steel fibres have a higher thermal expansion coefficient compared to concrete. At elevated temperatures, steel fibres expand more than the surrounding matrix, which can enhance the mechanical interlock up to a certain temperature. However, excessive temperatures may cause micro-cracking in the concrete, reducing the bond strength. Studies have shown that bond strength decreases significantly beyond 300 °C due to the degradation of both the fibre and the matrix [21,42].
• Polypropylene Fibres: Polypropylene fibres have a low melting point (~160 °C). When exposed to high temperatures, they soften and melt, leading to a loss in bond and mechanical reinforcement. However, the melting of these fibres can create additional porosity, which may help in relieving internal pressures caused by steam during heating, potentially reducing spalling in concrete [54,55,56,57,58,59,60,61].
• Basalt and Carbon Fibres: Basalt fibres maintain their properties up to about 600 °C, while carbon fibres can withstand even higher temperatures. The bond performance of these fibres at elevated temperatures is influenced by the stability of the concrete matrix and the integrity of the fibre–matrix interface. Thermal mismatch can still lead to bond deterioration, but these fibres generally perform better under high-temperature conditions compared to synthetic fibres [74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89].

Temperature-induced bond variations can compromise the structural performance of FRC, especially in applications where thermal exposure is significant. Understanding these effects is essential for designing FRC elements capable of withstanding temperature fluctuations.

6.2. Post-Fire Characteristics of Fibres in Concrete

After exposure to fire, the residual properties of both the fibres and the concrete matrix are crucial for assessing the structural integrity and serviceability of FRC.

• Steel Fibres: Post-fire, steel fibres may retain some mechanical properties if temperatures did not exceed critical thresholds. However, oxidation and scaling can occur, reducing cross-sectional area and bond strength. The surrounding concrete is often weakened due to dehydration and micro-cracking, further diminishing the composite action [21,45].
• Polypropylene Fibres: Since polypropylene fibres melt during fire exposure, they do not contribute to residual mechanical strength post-fire. The voids left by melted fibres can increase permeability and reduce the density of the concrete, potentially compromising durability [55,56,57,58].
• Basalt and Carbon Fibres: These fibres exhibit better retention of mechanical properties after fire exposure. Basalt fibres can maintain structural integrity up to 400–500 °C, while carbon fibres can endure even higher temperatures. However, the degradation of the concrete matrix at extreme temperatures can still adversely affect the overall composite performance [76,77,78,79,80,81].

Understanding the post-fire behaviour of fibres helps in evaluating damage and planning rehabilitation strategies for fire-exposed structures. It is essential to consider both the residual strength of the fibres and the integrity of the fibre–matrix bond.

6.3. Implications for Design and Future Research

The variation in bond performance at different temperatures and the post-fire characteristics of fibres have significant implications for the design and application of FRC:

• Material Selection: Choosing fibres with appropriate thermal properties can enhance high-temperature performance. For structures exposed to elevated temperatures, fibres like basalt or carbon may be more suitable.
• Design Considerations: Engineers should account for potential reductions in bond strength due to temperature effects when designing FRC elements, especially in fire-prone environments.
• Future Research: Further studies are needed to develop fibres and concrete matrices with compatible thermal expansions, improve high-temperature bond performance, and establish standardized testing methods for assessing thermal effects on FRC.

7. Application Scenarios for Synthetic Fibres in Concrete

The use of synthetic fibres in concrete has shown significant benefits across a range of specific applications, particularly where fire resistance and mechanical performance are critical.

• Polypropylene (PP) fibres are highly effective in mitigating explosive spalling, making them particularly suitable for high-rise buildings and tunnels that are at high risk of fire exposure. The ability of PP fibres to create micro-channels for vapour release at elevated temperatures helps reduce spalling and protect structural integrity during fire events.
• Polyvinyl alcohol (PVA) fibres, with their higher tensile and flexural strength, are ideal for critical infrastructure that is exposed to combined thermal and mechanical stresses, such as bridges and industrial facilities. Their superior modulus of elasticity also contributes to the durability of these structures, even under fire conditions.
• Macro fibres, with their enhanced corrosion resistance and high modulus of elasticity, are well suited for infrastructure exposed to aggressive environments, such as marine structures or chemical plants. Their unique properties help improve thermal stability, reduce cracking, and extend the service life of these structures. By highlighting these specific application scenarios, the practical utility and unique advantages of synthetic fibres in various contexts become clearer.

8. Conclusions

This review study presented the latest research findings on the performance of fibre-reinforced concrete subjected to high temperatures, with a particular focus on the thermal characteristics of synthetic polymer fibres. The advantages and disadvantages of different synthetic fibres, including polypropylene (PP), polyethylene (PE), polyvinyl alcohol (PVA), and newly emerging macro fibres, were discussed as additives to concrete. The key findings are summarized below:

• Synthetic fibre reinforcement reduces spalling at elevated temperatures due to the lower melting point of the fibres, which allows for vapour release, preventing explosive spalling. PP fibres, in particular, are effective in mitigating explosive spalling.
• Concrete with synthetic fibres demonstrates low sensitivity to corrosion effects, chemical reactions, and other harsh environmental factors, making it suitable for alkali environments. Synthetic fibres help protect rebars against chloride penetration, thereby delaying corrosion.
• Despite their benefits, synthetic fibres generally decrease the compressive strength of the composite material.
• PP fibres are effective in enhancing crack resistance and reducing spalling at elevated temperatures, but significant losses in modulus of elasticity, compressive strength, and tensile strength are observed at high temperatures.
• PE fibres are less effective at mitigating fire-induced damage, as continuous spalling occurs at higher temperatures.
• PE and PVA fibres contribute to greater improvements in tensile and flexural strength compared to PP fibres, due to their higher strength and modulus of elasticity.
• While PVA fibres exhibit a reduction in compressive strength at higher temperatures, they also reduce micro-cracking and improve flexibility and flexural strength. When used within specific load limits, PVA fibres can serve as primary reinforcement.
• Macro fibres significantly reduce cracking potential and mitigate spalling, and they have beneficial effects on modulus of elasticity, corrosion resistance, and thermal stability. However, they do not substantially improve compressive and flexural strengths. Macro fibres present an appealing alternative to current synthetic fibres such as PP, PE, and PVA due to their many advantages. However, in-depth studies on the thermal characteristics of macro fibre-reinforced concrete at elevated temperatures are still lacking. More research is needed to evaluate the impact of these fibres on thermal conductivity, physical damage (spalling), and the residual mechanical properties of macro fibre-reinforced concrete during and after fire exposure.
• PP fibres have been shown to reduce explosive spalling by up to 80% when incorporated at dosages of 2 kg/m³ in concrete exposed to temperatures above 600 °C [5]. Compared to PVA fibres, which improve tensile strength by 30% [55], PP fibres are particularly effective in mitigating spalling but result in a significant decrease in compressive strength at high temperatures.
• Çavdar [86] found that PVA fibres increased flexural strength by 45%, outperforming PP fibres, which showed a 20% improvement. This indicates that, while PP fibres are effective for reducing spalling, PVA fibres provide superior mechanical enhancement.

In conclusion, this review highlights the performance of fibre-reinforced concrete subjected to high temperatures, emphasizing the importance of synthetic polymer fibres in mitigating fire-induced damage. PP fibres are particularly effective in reducing spalling, while PE and PVA fibres improve tensile and flexural properties. Macro fibres show potential for enhancing thermal stability and reducing cracking. However, further research is needed to fully understand their effects under high-temperature conditions. Overall, the findings underscore the need for continued research to enhance the performance and application of fibre-reinforced concrete in fire-prone environments.