**3. Manufacturing of polyolefin fibre‐reinforced concrete**

Macro‐fibre volumes currently used in FRC range from 0.3 to 1.5%. With such volume frac‐ tions, the procedure for mix proportioning can be essentially the same as that used for plain concrete [31]. While the addition of fibres does not affect the nature of the components of the mix, it does affect the mix workability. There are no limitations as regards the types of cement employed, although the most common one is a Portland cement without additions. Regarding the type of aggregates chosen, those rounded and crushed have been success‐ fully used without encountering any disadvantage caused by interaction of the fibres and the aggregates [28]. The reduction of the concrete workability can be compensated with slight variations of the aggregate distribution, increasing the amount of fine fractions or even by adding or increasing the amount of admixtures. In any case, it is advisable to prepare trial mixes to achieve the final proportions. FRC can be manufactured, in general, with the same equipment and similar procedures merely by carefully studying the best mixing sequence to ensure that a good uniform dispersion of each type of fibres avoids segregations and balling of the fibres.

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 of the concrete workability.

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 (SCC) [36].

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

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

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.

Macro‐fibre volumes currently used in FRC range from 0.3 to 1.5%. With such volume frac‐ tions, the procedure for mix proportioning can be essentially the same as that used for plain concrete [31]. While the addition of fibres does not affect the nature of the components of the mix, it does affect the mix workability. There are no limitations as regards the types of cement employed, although the most common one is a Portland cement without additions. Regarding the type of aggregates chosen, those rounded and crushed have been success‐ fully used without encountering any disadvantage caused by interaction of the fibres and the aggregates [28]. The reduction of the concrete workability can be compensated with slight variations of the aggregate distribution, increasing the amount of fine fractions or even by adding or increasing the amount of admixtures. In any case, it is advisable to prepare trial mixes to achieve the final proportions. FRC can be manufactured, in general, with the same equipment and similar procedures merely by carefully studying the best mixing sequence to ensure that a good uniform dispersion of each type of fibres avoids segregations and balling

**3. Manufacturing of polyolefin fibre‐reinforced concrete**

a result of the fibre surface geometry.

embossed surface of a polyolefin fibre (right) [28, 30].

150 Alkenes

of the fibres.

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 and/or aggregates may occur.

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 a sense, some of the most common additives are superplasticizer and viscosity modifiers. Once the plain concrete is prepared, fibres are added to the mix and a thorough mixing is car‐ ried out in order to obtain a homogeneous distribution of fibres within the fresh concrete. This sequence has been altered on some occasions by adding the fibres directly after the aggregate homogenization with satisfactory results being obtained.

In the case of SCC with an addition of polyolefin fibres, due to the difficulty of obtaining such a type of concrete some changes should be made in the aforementioned procedure for obtaining satisfactory results. It is advised that fibres be added gradually during the mixing process. A third of the fibres should be added after the aggregate homogenization, another after adding the cement and lime powder, and the last one after pouring the water with the additives. It should be noted that the influence that the fibres have on the fresh properties of concrete might require supporting a final addition of superplasticizer to obtain the desired results in the fresh‐state tests. Lastly, enough time should be left for the chemical additive to act which would mean that on some occasions the mix should rest for a few minutes in the mixer before emptying. **Figure 5** shows the procedure.

Regarding the placing method, if the mix is properly designed PFRC can be placed by exter‐ nal vibration, pumped or projected to pass through obstacles and with a good performance in hardened state. It is true that compacting FRC might be more difficult to achieve with high fibre contents if at least a descent of 9 cm in the slump test is recommended [28]. On another note, the placing conditions and the formwork geometries clearly affect the final properties of the hardened FRC because they influence the final positioning of the fibres [9]. Therefore, it is important to highlight that VCC and SCC moulds are not usually filled with FRC in the same manner. In such a sense, at the placing stage SCC improves the positioning of fibres in the pouring direction. Conversely, external vibration tends to align the fibres perpendicularly to the direction of vibration. Several test recommendations [38, 39] have fixed the procedure for casting the specimens and filling the moulds. Additionally, the standards establish that in the case of self‐compacting concrete the mould should be filled in a single pour and levelled

off without any compaction. The capacity of SCC to level itself enables the mould to be filled

Polyolefin Fibres for the Reinforcement of Concrete http://dx.doi.org/10.5772/intechopen.69318 153

**Figure 6.** Filling methods for FRC: (a) flow method for SCC; (b) RILEM and EN‐14651 Vibrated Concrete [38].

Once demoulded, as in the case of a conventional concrete, elements should be properly cured. In the case of laboratory specimens, they should be cured at 20°C and with a relative

**4. Fresh and hardened concrete properties of polyolefin fibre‐reinforced** 

In the case of a VCC, the fresh‐state properties are usually assessed by means of the slump test. It is clear that the presence of fibres hampers a normal behaviour of the material. Although it is true that as the amount of fibres grows, the viscosity of the PFRC increases it cannot be overlooked that the influence of the fibres is reduced when compared with that of steel fibres. In such a sense, it has been found that with an increment of around 15% of the superplasti‐ cizer added to the mix, it is possible to maintain at similar values the slump even when adding

Similar to the case of a vibrated conventional concrete, the presence of fibres harms the self‐ compatibility that SCC has. However, the flexible nature of the polyolefin fibres significantly reduces such a decrease. In the case of an SCC, the fresh‐state properties of the concrete are frequently determined by using tests such as the slump‐flow test, the L‐box test and the V‐ funnel tests. **Figure 7** shows the influence of the presence of fibres even if an SCC is limited, in both the slump test and the V‐funnel test. This phenomenon underlines the versatile nature of polyolefin fibre if compared with rigid steel fibres of any kind. In addition, even in the case

that concrete discharged from using polyolefin fibres in ready‐mix trucks maintains a regular

Compressive and tensile strengths of fibre‐reinforced concrete have been thoroughly studied in the last decades with regard to steel and synthetic fibres [42, 43]. Fibres typically enhance

addition of fibres, no hint of balling was noticed. Moreover, there is evidence

from one end to the other [30, 40]. **Figure 6** shows the procedures.

humidity above 95% until the age of testing.

of polyolefin fibres [41, 28].

distribution of fibre along the concrete mass [7].

**concrete**

10 kg/m3

of a 10‐kg/m<sup>3</sup>

**Figure 5.** Mixing sequence of a polyolefin fibre‐reinforced concrete.

a sense, some of the most common additives are superplasticizer and viscosity modifiers. Once the plain concrete is prepared, fibres are added to the mix and a thorough mixing is car‐ ried out in order to obtain a homogeneous distribution of fibres within the fresh concrete. This sequence has been altered on some occasions by adding the fibres directly after the aggregate

In the case of SCC with an addition of polyolefin fibres, due to the difficulty of obtaining such a type of concrete some changes should be made in the aforementioned procedure for obtaining satisfactory results. It is advised that fibres be added gradually during the mixing process. A third of the fibres should be added after the aggregate homogenization, another after adding the cement and lime powder, and the last one after pouring the water with the additives. It should be noted that the influence that the fibres have on the fresh properties of concrete might require supporting a final addition of superplasticizer to obtain the desired results in the fresh‐state tests. Lastly, enough time should be left for the chemical additive to act which would mean that on some occasions the mix should rest for a few minutes in the mixer before

Regarding the placing method, if the mix is properly designed PFRC can be placed by exter‐ nal vibration, pumped or projected to pass through obstacles and with a good performance in hardened state. It is true that compacting FRC might be more difficult to achieve with high fibre contents if at least a descent of 9 cm in the slump test is recommended [28]. On another note, the placing conditions and the formwork geometries clearly affect the final properties of the hardened FRC because they influence the final positioning of the fibres [9]. Therefore, it is important to highlight that VCC and SCC moulds are not usually filled with FRC in the same manner. In such a sense, at the placing stage SCC improves the positioning of fibres in the pouring direction. Conversely, external vibration tends to align the fibres perpendicularly to the direction of vibration. Several test recommendations [38, 39] have fixed the procedure for casting the specimens and filling the moulds. Additionally, the standards establish that in the case of self‐compacting concrete the mould should be filled in a single pour and levelled

homogenization with satisfactory results being obtained.

emptying. **Figure 5** shows the procedure.

152 Alkenes

**Figure 5.** Mixing sequence of a polyolefin fibre‐reinforced concrete.

**Figure 6.** Filling methods for FRC: (a) flow method for SCC; (b) RILEM and EN‐14651 Vibrated Concrete [38].

off without any compaction. The capacity of SCC to level itself enables the mould to be filled from one end to the other [30, 40]. **Figure 6** shows the procedures.

Once demoulded, as in the case of a conventional concrete, elements should be properly cured. In the case of laboratory specimens, they should be cured at 20°C and with a relative humidity above 95% until the age of testing.
