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Appl. Sci. 2022, 12, 2867 13 of 20 ratio differences between the samples used; the higher aspect ratio they show, the lower the ductility they will have. The differences in the manufacturing process between the fibres and the valorised polyethylene are in the final steps. In the case of the fibres, the polyethylene in its cast state leaves the cylinder nozzle at a pressure of 150 bars and is then pulled. This produces a stretching of the material so that the molecules are forced to move in one direction, causing a slight induced crystallisation of the polymer. In the case of the PR, the molten material is injected into a mould at a pressure of 1000 bar and a post-pressure is applied for a few seconds and then the mould is opened. Knowing that the pressure with which the PR is formed is almost eight times greater than that of the fibres and considering that the PR cools down more slowly than the fibres, it can be deduced that the PR is much more compact, which can explain why the PR is more rigid and less ductile than the fibres. 3.4. Concrete with Recycled Fibres 3.4.1. Concrete Workability Table 4 shows the results obtained from the Abrams cone. From these results, it can be concluded that the presence of fibres in the studied range does not affect the concrete’s workability. Table 4. Concrete workability results. Mix HFC HRFP-2 HRFP-4 Cone (cm) 24 23 23 Based on the results shown in Table 5, it can be seen that the presence of fibres barely changes the concrete workability. However, despite the limited influence of fibres on the fresh state behaviour, a slight reduction in the slump flow can be observed. This reduction in the workability of the concrete due to the presence of fibres agrees with the results obtained by other authors [67]. 3.4.2. Physical Properties Table 5 shows the physical property values of the manufactured concretes. As far as the density of the concrete is concerned, it can be seen that there is no clear difference between control concrete and concrete with fibres. Furthermore, the values obtained are within the density range (2–2.6 g/cm3) established for normal concrete according to EHE-8. Table 5. Concrete physical properties. Mix HFC HRFP-2 HRFP-4 Bulk Specific Gravity (g/cm3) 2.20 2.18 2.26 Apparent Specific Gravity (g/cm3) 2.33 2.28 2.36 Absorption (%) 2.52 2.00 1.91 Porosity (%) 5.55 4.35 4.31 However, the inclusion of 2 kg/m3 and 4 kg/m3 of fibres reduces the absorption by 21% and 24%, respectively. In the same way, the porosity of the concrete with fibres is reduced in both cases by 22% with respect to the control concrete. Figure 14 shows how the properties of porosity and absorption can be fitted to an exponential curve with respect to the volume of fibres contained in the concrete, so that the incorporation of fibres willPDF Image | Recycled Polyethylene Fibres for Structural Concrete
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