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

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 10 kg/m3 of polyolefin fibres [41, 28].

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 of a 10‐kg/m<sup>3</sup> addition of fibres, no hint of balling was noticed. Moreover, there is evidence that concrete discharged from using polyolefin fibres in ready‐mix trucks maintains a regular distribution of fibre along the concrete mass [7].

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

**Figure 7.** Slump test in an SCC PFRC [28].

the tensile properties of the plain concrete. However, their influence on other mechanical properties is varied depending on the type and shapes of the fibres.

The tests and procedures are easy to perform. In the test, a concrete cylinder similar to the type used for compression tests is placed with its axis horizontal and between the platens of a testing machine. When the load is evenly applied along a generatrix, a near‐constant tensile stress occurs in the central part of the vertical diameter [45]. The indirect tensile strength is related with the load at the first crack corresponding to peak load for plain concrete with brittle behaviour. However, this type of test is not suitable for assessing the residual strength of the materials provided by the fibres due to second‐order effects that add bending stresses to the sample. Even though such second‐order effects do not enable accurate residual strength values to be obtained, these indirect tests provide interesting values for the initial tensile strength. As regards the influence of the fibre content in the indirect tensile strength, as in the case of the compressive strength it could be considered that the influence of the fibre volume is negligible if the amount of fibres remains within the regular ranges (as **Figure 8** shows).

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Regarding the modulus of elasticity (*E*) of the composite material, although theoretically its value should be related with the proportions of concrete and fibres, some other parameters have to be considered such as the fibre orientation and fibre length. Even in that case, the influence of the fibres in the modulus of elasticity is not clear as can be seen in **Figure 8**. In some cases, even when adding fibres with higher elasticity modulus than the matrix, a lower

All the features that were mentioned for the case of conventional vibrated concrete are also

Another point that is worth considering is the durability of the PFRC when placed in poten‐ tially hazardous environments. The capacity of the PFRC to maintain its properties even in

value of the composite material has been obtained.

valid without performing major changes in the case of an SCC.

**Figure 8.** Mechanical properties and compressive strength sample after testing.

Compressive strength, which is the most representative parameter to characterize con‐ crete, provides essential information. The test is performed in a similar way to that of plain concrete [44]. In a conventional concrete, strength is not significantly affected when regular amounts of fibres are added. Nevertheless, the failure is usually less brittle due to the enhancement of the ductility and toughness provided by the fibres. Even a reduced amount of fibres produces remarkable changes in the failure mode, with it losing scarcely any mass (as **Figure 8** shows).

Nonetheless, it should be pointed out that there seems to be a threshold of volume fraction from which compressive strength is reduced even below values typical of plain concrete. This might have taken place due to a worsening of workability and compaction that causes hetero‐ geneities in the concrete bulk and reduces its mechanical properties.

The mechanical explanation of this change in the failure mode is based on the reduction of lat‐ eral deformations above stress values 75% of its compressive strength. Such a change prevents the typical shear bands of plain concrete failure mode from appearing, avoiding the explosive failure of the material without fibres.

In order to assess tensile strength (as is accepted for plain concrete assessments), the indirect tensile‐splitting tests—also named Brazilian tests—can be carried out. It should be clarified that in this subsection, tensile properties refer to initial tensile strength assessed by tensile‐ splitting tests. The residual post‐cracking tensile strength is the keystone of the use of struc‐ tural fibres and deserves a specific subsection focussed on fracture behaviour in tension or under tensile‐flexural tests.

**Figure 8.** Mechanical properties and compressive strength sample after testing.

the tensile properties of the plain concrete. However, their influence on other mechanical

Compressive strength, which is the most representative parameter to characterize con‐ crete, provides essential information. The test is performed in a similar way to that of plain concrete [44]. In a conventional concrete, strength is not significantly affected when regular amounts of fibres are added. Nevertheless, the failure is usually less brittle due to the enhancement of the ductility and toughness provided by the fibres. Even a reduced amount of fibres produces remarkable changes in the failure mode, with it losing scarcely any mass (as

Nonetheless, it should be pointed out that there seems to be a threshold of volume fraction from which compressive strength is reduced even below values typical of plain concrete. This might have taken place due to a worsening of workability and compaction that causes hetero‐

The mechanical explanation of this change in the failure mode is based on the reduction of lat‐ eral deformations above stress values 75% of its compressive strength. Such a change prevents the typical shear bands of plain concrete failure mode from appearing, avoiding the explosive

In order to assess tensile strength (as is accepted for plain concrete assessments), the indirect tensile‐splitting tests—also named Brazilian tests—can be carried out. It should be clarified that in this subsection, tensile properties refer to initial tensile strength assessed by tensile‐ splitting tests. The residual post‐cracking tensile strength is the keystone of the use of struc‐ tural fibres and deserves a specific subsection focussed on fracture behaviour in tension or

properties is varied depending on the type and shapes of the fibres.

geneities in the concrete bulk and reduces its mechanical properties.

**Figure 8** shows).

154 Alkenes

failure of the material without fibres.

**Figure 7.** Slump test in an SCC PFRC [28].

under tensile‐flexural tests.

The tests and procedures are easy to perform. In the test, a concrete cylinder similar to the type used for compression tests is placed with its axis horizontal and between the platens of a testing machine. When the load is evenly applied along a generatrix, a near‐constant tensile stress occurs in the central part of the vertical diameter [45]. The indirect tensile strength is related with the load at the first crack corresponding to peak load for plain concrete with brittle behaviour. However, this type of test is not suitable for assessing the residual strength of the materials provided by the fibres due to second‐order effects that add bending stresses to the sample. Even though such second‐order effects do not enable accurate residual strength values to be obtained, these indirect tests provide interesting values for the initial tensile strength. As regards the influence of the fibre content in the indirect tensile strength, as in the case of the compressive strength it could be considered that the influence of the fibre volume is negligible if the amount of fibres remains within the regular ranges (as **Figure 8** shows).

Regarding the modulus of elasticity (*E*) of the composite material, although theoretically its value should be related with the proportions of concrete and fibres, some other parameters have to be considered such as the fibre orientation and fibre length. Even in that case, the influence of the fibres in the modulus of elasticity is not clear as can be seen in **Figure 8**. In some cases, even when adding fibres with higher elasticity modulus than the matrix, a lower value of the composite material has been obtained.

All the features that were mentioned for the case of conventional vibrated concrete are also valid without performing major changes in the case of an SCC.

Another point that is worth considering is the durability of the PFRC when placed in poten‐ tially hazardous environments. The capacity of the PFRC to maintain its properties even in

**Figure 10**, is rather complex and demands highly trained and experienced personnel. Therefore, as it is somewhat expensive and time‐consuming, it is not considered an appropriate method for practical material testing (only being suitable for research purposes in specialized laboratories). The most economical and practical tests available to determine the post‐crack behaviour and assess the influence of conditions such as fibre types and dosage are bending tests. The three‐ point bending (TPB) test uses beams with a cross section of 150 × 150 mm and a span of 500 mm loaded in the middle of the upper face. A transverse notch of standard dimensions is made in the middle of the lower specimen face, in the same cross section where the load is applied. This setup, as shown in **Figure 11**, ensures that the crack is formed in this predefined position,

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making crack control simpler than in un‐notched beams [30, 39].

**Figure 10.** Uni‐axial tension testing for concrete [30].

**Figure 11.** Test set‐up in [38]. Measures in mm.

**Figure 9.** Permeability under pressure of water of VCC and SCC PFRC.

such environments depends on the action of the chemical compounds that ingress in the con‐ crete bulk through the connected network of pores. In that sense, it should be underlined that the presence of fibres might offer preferential ways for such ingress. As can be seen in **Figure 9**, the permeability of the material under pressure of water is uninfluenced by the presence of fibres as there is no dependency of the penetration depth and the fibre content. Therefore, as happens with plain concretes, permeability is related to parameters such as the paste aggregate ratio and the size distribution of the aggregates used. If the type of aggregates and their pro‐ portion in the concrete mix are adequate, PFRC may be a material that bears the most hazardous of environments considered in some recommendations [12] such as those in direct contact with marine water, erosive materials, freeze‐thaw conditions or even chemical industries.
