**1. Why polyolefin fibres?**

The recent advances made in polymer science, chemical composition and engineering have increased the importance of polyolefins in day‐to‐day applications. Polyethylene and poly‐ propylene are widespread polyolefins and the fastest growing polymer family due to the

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lower cost of production compared with the plastics and materials they replace [1]. Polyolefin fibres encompass a spectrum of uses in modern societies. The associated low costs, good resis‐ tance to chemicals, and high strength and toughness have encouraged the use. Their commer‐ cial advantages and disadvantages are listed in **Table 1**, although it should be noted that not all are applicable to the case of reinforcing concrete. In general, polyolefin fibres have good tensile properties, good abrasion resistance and excellent resistance to chemicals.

resistance to chemical attack, and a reduced moisture regain, have emerged as an alternative

In order to consider these newly developed applications, in 2006 the European Committee for Standardization (CEN) approved the European Standard 14889 [13] which classifies the types of fibres that can be added to concrete. Such a recommendation divides the fibres into two groups: the first one deals with steel fibres and the second one polymer fibres. However, not all the characteristics that may be relevant in examining the performance of FRC were addressed. The recommendation defines the possible geometrical shapes and physical param‐ eters of the fibres and establishes the procedures for the measurement of the fibre mechanical properties such as tensile strength or modulus of elasticity. In the case of polymer fibres, they are divided into two groups: non‐structural micro‐fibres and structural macro‐fibres. The cri‐ terion used is their equivalent diameter, with it being classified into two types depending on whether its diameter is greater or smaller than 0.30 mm. **Figure 1** shows the classification of the fibres made by such a reference [13]. Polyolefin fibres with surface bulges and grooves along the fibre surface are produced from homo‐polymeric resin into a mono‐filament form [14] and,

Among the synthetic macro‐fibres that can be employed in concrete, those made of high‐density

and a reduced tensile strength between

Polyolefin Fibres for the Reinforcement of Concrete http://dx.doi.org/10.5772/intechopen.69318 147

to corrosive steel solutions that use steel‐reinforcing mesh or steel fibres.

according to EN‐14889, are classified as Class II macro‐fibres.

polyethylene (HDPE) boast a density of 0.95 g/cm3

**Figure 1.** Fibre classification following EN‐14889 [13].

Regarding their use in concrete, the development of polyolefin‐based synthetic macro‐fibres with improved mechanical properties has extended the use of such plastic fibres beyond a conventional use in shrinkage‐cracking control. Such synthetic macro‐fibres have become an alternative to the traditional use of steel fibres in fibre‐reinforced concrete (FRC) [3], forming what has been termed steel fibre‐reinforced concrete (SFRC). The addition of randomly dis‐ tributed steel fibres to concrete improves its low tensile strength and its brittleness enabling its use in industrial pavements or tunnels [4–6] among others. Based on the existing codes and standards [7–9], the contribution of the steel fibres has been considered in the structural design in recent years [10–12]. However, the recent concern of society regarding the environ‐ mental cost of materials, building processes and infrastructure refurbishment and rehabilita‐ tion has given rise to certain structures having a lifespan of up to 100 years. Therefore, the durability of materials has emerged as a key factor in the choice of materials. In such a sense, the potentially corrodible nature of steel fibres has aroused an interest in fibres that are not only chemically stable but also increase the mechanical performance of concrete. In addition, steel fibres are expensive for both purchase and in terms of storing and handling. Plastic industry in recent years has solved the aforementioned disadvantages allowing the produc‐ tion of a new generation of polyolefin‐based synthetic macro‐fibres that are inert in an alkaline environment and provide concrete with structural capacities to substitute steel reinforcement. Therefore, polyolefin fibres, which have good tensile properties, abrasion resistance, excellent


**Table 1.** Commercial advantages and disadvantages associated with the use of polyolefin fibres [2].

resistance to chemical attack, and a reduced moisture regain, have emerged as an alternative to corrosive steel solutions that use steel‐reinforcing mesh or steel fibres.

lower cost of production compared with the plastics and materials they replace [1]. Polyolefin fibres encompass a spectrum of uses in modern societies. The associated low costs, good resis‐ tance to chemicals, and high strength and toughness have encouraged the use. Their commer‐ cial advantages and disadvantages are listed in **Table 1**, although it should be noted that not all are applicable to the case of reinforcing concrete. In general, polyolefin fibres have good

Regarding their use in concrete, the development of polyolefin‐based synthetic macro‐fibres with improved mechanical properties has extended the use of such plastic fibres beyond a conventional use in shrinkage‐cracking control. Such synthetic macro‐fibres have become an alternative to the traditional use of steel fibres in fibre‐reinforced concrete (FRC) [3], forming what has been termed steel fibre‐reinforced concrete (SFRC). The addition of randomly dis‐ tributed steel fibres to concrete improves its low tensile strength and its brittleness enabling its use in industrial pavements or tunnels [4–6] among others. Based on the existing codes and standards [7–9], the contribution of the steel fibres has been considered in the structural design in recent years [10–12]. However, the recent concern of society regarding the environ‐ mental cost of materials, building processes and infrastructure refurbishment and rehabilita‐ tion has given rise to certain structures having a lifespan of up to 100 years. Therefore, the durability of materials has emerged as a key factor in the choice of materials. In such a sense, the potentially corrodible nature of steel fibres has aroused an interest in fibres that are not only chemically stable but also increase the mechanical performance of concrete. In addition, steel fibres are expensive for both purchase and in terms of storing and handling. Plastic industry in recent years has solved the aforementioned disadvantages allowing the produc‐ tion of a new generation of polyolefin‐based synthetic macro‐fibres that are inert in an alkaline environment and provide concrete with structural capacities to substitute steel reinforcement. Therefore, polyolefin fibres, which have good tensile properties, abrasion resistance, excellent

) Low melting point (120–125°C for PE;

tensile properties, good abrasion resistance and excellent resistance to chemicals.

**Advantages Disadvantages**

Good tensile properties 160–165°C for PP)

Micro‐organisms and insects Poor dyeability Almost negligible moisture regain High flammability Good wicking action Inferior resilience

High insulation Significant degree of creep

**Table 1.** Commercial advantages and disadvantages associated with the use of polyolefin fibres [2].

Excellent resistance to mildew, 100°C

Good abrasion resistance Prone to photolytic degradation Excellent resistance to chemicals Inferior shrink resistance above

Low density (0.90–0.96 g/cm3

146 Alkenes

Avoidance of dermatological problems

In order to consider these newly developed applications, in 2006 the European Committee for Standardization (CEN) approved the European Standard 14889 [13] which classifies the types of fibres that can be added to concrete. Such a recommendation divides the fibres into two groups: the first one deals with steel fibres and the second one polymer fibres. However, not all the characteristics that may be relevant in examining the performance of FRC were addressed. The recommendation defines the possible geometrical shapes and physical param‐ eters of the fibres and establishes the procedures for the measurement of the fibre mechanical properties such as tensile strength or modulus of elasticity. In the case of polymer fibres, they are divided into two groups: non‐structural micro‐fibres and structural macro‐fibres. The cri‐ terion used is their equivalent diameter, with it being classified into two types depending on whether its diameter is greater or smaller than 0.30 mm. **Figure 1** shows the classification of the fibres made by such a reference [13]. Polyolefin fibres with surface bulges and grooves along the fibre surface are produced from homo‐polymeric resin into a mono‐filament form [14] and, according to EN‐14889, are classified as Class II macro‐fibres.

Among the synthetic macro‐fibres that can be employed in concrete, those made of high‐density polyethylene (HDPE) boast a density of 0.95 g/cm3 and a reduced tensile strength between

**Figure 1.** Fibre classification following EN‐14889 [13].

25 and 40 MPa. These reduced mechanical properties hamper their use as a way to improve concrete mechanical properties. Nevertheless, other types that have been recently employed, which are manufactured with polyethylene terephthalate (PET), boast remarkable mechani‐ cal properties. Their tensile strength above 400 MPa might enable a successful use in concrete reinforcement but there are several issues reported [15]. Some that have been thoroughly studied are the difficulties found in their manufacturing process as well as their reduced resis‐ tance to alkaline environments [16, 17]. These two difficulties prevent their widespread use as a concrete reinforcement. Other types of macro‐fibres that deserve being cited are those obtained from virgin and recycled polypropylene (PP). PP fibres have been widely used in the concrete industry, due to its ease of production, high alkaline resistance [18], and high tensile strength and Young's modulus [19].

and the fibre may be pulled out. If one fibre is mobilized by friction shear stresses, it is pos‐

Polyolefin Fibres for the Reinforcement of Concrete http://dx.doi.org/10.5772/intechopen.69318 149

In order to determine which of the cases shown in **Figure 2** might emerge, the critical length

) of the fibres used requires examination. Such a critical length has been defined as the length that allows the tensile strength of the fibre to be used without pulling it out of the matrix. In an ideal situation, when the fibre is being pulled out from the concrete matrix, two types of forces are applied to the fibre, preventing it from being extracted: the chemical adhesion in the inner part of the fibre and the frictional bond in the part of the fibre closer to

sible that such stresses cause matrix cracking.

the crack. A sketch of this can be seen in **Figure 3**.

**Figure 2.** Energy absorbing the fibre matrix mechanisms [29].

**Figure 3.** Pull‐out mechanisms [8, 28].

(*l* c

Polyolefin fibres, which are among those considered PP fibres, enjoy an outstanding mechani‐ cal behaviour, their modulus of elasticity being of great relevance. The common value of such modulus is 9 GPa or even up to 15–20 GPa, which is much higher than certain other plas‐ tics that offer around 2–3 GPa. In addition, polyolefin fibres boast a tensile strength above 400 MPa. These remarkable properties have been obtained by using a bi‐component fabrica‐ tion strategy that combines two polymers: a core of high modulus and a sheath of low modulus [20, 21].

Another reason behind the remarkable performance of polyolefin fibres is the notable bond generated between the fibres and the concrete matrix due to their rough surface. This is pro‐ vided for both the shape of the fibres and the mechanical interaction that takes place when the fibres are loaded. In such a sense, the interface fibre‐matrix becomes rougher due to the damage of the fibre surface produced during the mixing process. Such roughening forms a mechanical interlock opposite to the relative movement of fibres after the cracks are initiated [22–24]. Concerning the fibre shape, the optimum macro‐synthetic fibre geometry has also been sought. This involved exploiting the matrix anchorage fully without fracturing the fibres, and reaching the maximum pull‐out resistance. In terms of bond, the crimped ones were the best among several deformed synthetic structural fibres [25, 26].
