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2.1. Concrete As the HDPE fibres were produced from recycled stock with no guaranteed engineering properties, the initial aim was to test their influence on concrete of the low-to-moderate com- pressive strength (near the ”C 25/30” class) using the mix defined in Table 1 with the target slump of the fresh mix 75 mm. Table 1: Details of the plain concrete mix. Material into 1 m3 of concrete volume Cement CEM II/A-L 32.5 R Aggregate 0 ÷ 4 mm (quartz) 4 ÷ 20 mm (quartzite) Water (W/C = 0.62) Air content (estimated) mass volume [kg] [m3 ] 380 0.130 780 0.280 860 0.325 235 0.235 / 0.026 2.2. HDPE fibres HDPE (CAS no. 9002-88-4 [26]) is a synthetic polymer known for chemical inertness when in contact with most acids and alkaline substances. Its molecules are continuous chains of (CH2)n methylene atomic groups with the typical lengths 5 · 105 to 107. These molecular chains are three-dimensional with other chains of (CH2) groups branching from the main line; each ending with a saturated (CH3) methyl group as denoted in Fig. 1. HDPE has the higher strength to density ratio than other polyethylenes due to the longer primary and shorter secondary chains which makes its production more expensive. The origin of the recycled plastic for HDPE fibres available for this work is the mixed stock including various post-consumer waste, mainly home appliances. The fibres were pro- duced with diameters Ø 0.25 mm and Ø 0.40 mm and their aspect ratios (length/diameter) were 92 and 75, respectively. Their chemical purity (with traces of PP) was verified by the X-ray diffractometry using Bruker D8 detector. Fig. 2 shows the natural look and the scanning elec- tron microscope (SEM) image of the extruded sample fibres. The characteristic temperature points of the recycled HDPE were obtained from the differential scanning calorimetry (using Mettler-Toledo DSC 1 calorimeter) and the resulting heat vs. temperature flux graph is shown in Fig. 3. The melting and ignition temperatures for the recycled HDPE, 129◦C and 487◦C are, as expected, somewhat lower than the values typical for PP (Table 2). A complete non-linear tensile elongation curve for recycled HDPE is obtained from the direct tension tests of the continuous strands from which the fibres were cut (Fig. 4-a). The stress-strain values plotted in Fig. 4-b refer to the original cross-section of the strands before it was reduced due to the effect of tensile contraction at higher loads. Characteristic values are also listed in Table 2 alongside the typical corresponding properties of poly-propylene (CAS no. 9003-07-0 [26]). Due to the amorphous nature of the polymer, the transition from elastic into plastic state is gradual with the yield and the ultimate strength of recycled HDPE being noticeably below the usual strengths of the new PP or the engineering grade HDPE. The same 3PDF Image | Mechanical properties of concrete reinforced with recycled HDPE
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