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correlate any reduction in the water permeability and the rate of drying shrinkage of HDPE FRCE with its increased resistance to the plastic shrinkage cracking. 3.5. Free drying shrinkage Drying shrinkage of concrete is associated with the loss of capillary water during harden- ing of the hydrated cement paste. While the effectiveness of fibres in reducing shrinkage and cracking is expected to be greater under restrained conditions during the first 24 h after casting, the free drying shrinkage is also important feature for its implications on the serviceability and maintenance of reinforced and prestressed concrete structures. Five 250 mm long prisms were cast for each mix to monitor free drying shrinkage during a period of 90 days after casting. For every set of 5 shrinkage prisms per mix, three different water curing times (immediately after casting) were: 3 days for one prism, 7 days for the next two and 14 days for the remaining two prisms. Other factors that influence shrinkage such as the indoor temperature (T ≈ 22◦C), environment humidity (Hi ≈ 55%), type of cement and water/cement ratio remained constant. The shrinkage readings were taken daily from two sides of each specimen using the 200 mm Demec analogue gauge with the resolution of 8.1 micro-strains (8.1·10−6). The time-dependent changes in elongations for all three curing regimes are plotted in Fig. 11 until the age of concrete reached 90 days. The graphs show a drying shrinkage behaviour that is typical for normal concrete for which the shrinkage strain rate slows down after 21 days. In comparison to the plain concrete, the free drying shrinkage of HDPE FRC is, on average, lower by the order of 10 ÷ 15%. Whilst modest, this improvement is also typical for PP and other synthetic fibres and is of the same order as the reduction in free drying shrinkage in concrete reinforced with 1% of steel fibres [35]. The duration of water curing seemed to produce little difference between plain and HDPE fibre reinforced concrete while the largest reduction in the free shrinkage (in the order of 20 ÷ 25%) was consistently measured on specimens from mix C7 with the largest amount (1.25%) of Ø 0.40 mm fibres. 3.6. Durability of HDPE fibres in concrete From the fresh concrete mixes for all seven series (Table 1), the average of the pH readings taken about 90 min after the addition of water was 12.4. Assuming that the pH value of hardened concrete would not be lower, the ability of HDPE FRC to preserve any advantageous mechanical properties depends on the resistance of fibres to alkalinity. Fig. 12 shows SEM images of the fractured surfaces of concrete with the Ø 0.25 mm and Ø 0.40 mm HDPE fibres still embedded in 90 days old concrete. The surface of the HDPE fibres themselves is without detectable signs of chemical deterioration and the visible damage appears to be only the result of the surface friction as fibres were pulled out from concrete when the specimens failed during testing. Fig. 12-b shows a fundamental weakness of the simply extruded HDPE fibres: they do not adhere strongly to concrete and, when subjected to tension or other deformation while bridging the cracks, the fibres are easily pulled out. In contrast to the commercially available PP fibres that underwent chemical treatment to develop hydrophilic properties, the bond strength of these recycled HDPE fibres comes only from friction with the surrounding concrete. The initial promising results of this study open the door to several potential applications of HDPE in reinforced concrete civil structures such as on-the-ground slabs, bridge decks and water-retaining walls. However, further experimental research is necessary to confirm that the 9PDF Image | Mechanical properties of concrete reinforced with recycled HDPE
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