**2. Fibre pull‐out response of polyolefin fibres when added to concrete**

Regardless of the type of fibre used, reinforcement is effective in concrete when the tensile strength of the fibre is significantly higher than that of concrete (two or three times), when the fibre‐matrix bond strength is in the same order of magnitude of the tensile strength of the matrix, and the fibre modulus of elasticity in tension is significantly higher than that of the concrete [27, 28]. **Figure 2** shows the failure mechanisms of the fibres in PFRC.

At the optimum situation, the crack opening is controlled by 'fibre bridging' which has a por‐ tion of fibre on each side of the crack with enough embedded length and allows fibres to work at 100% without any slipping (number 3 in **Figure 2**). In such a situation, if the crack‐growing process continues it is possible to make full use of the potential of the fibre up to its failure. On the contrary, if fibres slip during the opening processes, the debonding process may occur and the fibre may be pulled out. If one fibre is mobilized by friction shear stresses, it is pos‐ sible that such stresses cause matrix cracking.

In order to determine which of the cases shown in **Figure 2** might emerge, the critical length (*l* c ) 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 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].

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

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

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].

concrete [27, 28]. **Figure 2** shows the failure mechanisms of the fibres in PFRC.

**2. Fibre pull‐out response of polyolefin fibres when added to concrete**

Regardless of the type of fibre used, reinforcement is effective in concrete when the tensile strength of the fibre is significantly higher than that of concrete (two or three times), when the fibre‐matrix bond strength is in the same order of magnitude of the tensile strength of the matrix, and the fibre modulus of elasticity in tension is significantly higher than that of the

At the optimum situation, the crack opening is controlled by 'fibre bridging' which has a por‐ tion of fibre on each side of the crack with enough embedded length and allows fibres to work at 100% without any slipping (number 3 in **Figure 2**). In such a situation, if the crack‐growing process continues it is possible to make full use of the potential of the fibre up to its failure. On the contrary, if fibres slip during the opening processes, the debonding process may occur

strength and Young's modulus [19].

[20, 21].

148 Alkenes

The cement content and the water/cement ratio are as decisive as in plain concrete. However, and in contrast to the cement content for SFRC, there is no general recommendation to increase the amount of cement weight used [32]. Such a difference is based on the bendable nature of polyolefin fibre in contrast to the stiffness of the steel fibres that result in a remarkable reduction

Polyolefin Fibres for the Reinforcement of Concrete http://dx.doi.org/10.5772/intechopen.69318 151

Regarding the fine/total aggregates relation, although there are no general recommendations it would be advisable to increase such a relation and limit the maximum aggregate size. For SFRC, it is usually accepted that the maximum aggregate size should not surpass 2/3 of the fibre length (the use of fibres two to five times longer than the maximum aggregate size is frequent) [33, 34]. Such guidelines should be followed and can be considered a valu‐ able rule of thumb given that they enhance workability without affecting the hardened state properties. Moreover, if PFRC is placed by pumping it is recommended (as in the case of SFRC) that the amount of coarse aggregates employed be reduced by 10% [34]. While in the case of a steel fibre addition the possible effects between the proportion of fine aggregates and the fibre content for a given aspect ratio (*l*/*d*) have been clearly reported, in the case of polyolefin fibre such relations have not yet been clearly established due to the flexible nature of the fibres [35]. However, the use of between 40 and 60% of fine aggregates seems to be a fair option in obtaining satisfactory results. Above all, it should be noted that these recom‐ mendations are considered for fibre volumetric fractions below 2%. Above such values, in all probability the number of fibres added would severely change the fresh‐state properties and obtain a heterogeneous distribution of fibres that would lead to a reduction of the properties of the concrete obtained. These precautions should not give the impression of a great modi‐ fication of the fresh‐concrete properties or even of a limited applicability of polyolefin fibres to high‐performance concretes, such as high‐strength concrete or self‐compacting concrete

In the case of combining an SCC with an addition of polyolefin fibres, certain changes should be added to the proportions of the concrete constituents. Some design criteria [28, 37] focus on targeting a slump‐flow diameter of 600 mm, with a recommended reference mixture being about 700 mm of diameter of the patty without fibres. Such rheology characteristics can be obtained by increasing the amount of cement and/or the proportion of fine aggregates by add‐ ing a fine material such as lime powder and using superplasticizer proportions of over 1% of the cement weight. In any case, due to the difficulty of obtaining an SCC, the aforementioned changes should be tested in laboratory preliminary mixes before in situ production. As may be easily understood, such changes in the concrete formulation have a remarkable impact on the final cost of the material. Similar to what happens in the case of a conventional concrete, the slump flow of SCC decreases with the addition of fibres depending on the type of fibre and its geometry. The addition of fibres in all cases alters the results of the fresh‐state tests. If an excessive amount of fibres is added, obstruction of the flow and clustering of the fibres

The mixing sequence employed for a vibrated conventional concrete (VCC) PFRC starts by carrying out a homogenization of the aggregates. The cement and the other fine components, if used, are then added to the mixer. Later, water and additives are added to the mix. In such

of the concrete workability.

(SCC) [36].

and/or aggregates may occur.

**Figure 4.** Set‐up of pull‐out test of a polyolefin‐based macro‐fibre made by [8] (left); pull‐out test result (centre), typical embossed surface of a polyolefin fibre (right) [28, 30].

In the case of a certain type of polyolefin fibre, the test setup and the pull‐out response obtained in the test can be seen in **Figure 4**. The results obtained depend on two variables: the embedded length and the angle formed between the fibre and the free surface of the sample. The amount of energy absorbed while pulling out of the fibre increases as the embed‐ ded length does. However, the effect of the angle between the free surface and the fibre has a minor effect in the total response of the system [8]. Apart from these two factors, the test results showed that the geometry of the embossed surface of the fibre has a major impact on the results. **Figure 4** shows how the load‐displacement curves swing at a certain load level as a result of the fibre surface geometry.

The results obtained in the pull‐out tests show that polyolefin fibres are apt for concrete reinforcement. How these micro‐mechanisms are transferred into the macro‐scale material behaviour will be explained in the next sections. Similarly, both the influence that the fibres have on the manufacturing process and the fresh state of the material will be shown in too.
