Page 100 - New Trends in Eco efficient and Recycled Concrete

74 New Trends in Eco-efficient and Recycled Concrete

Figure 3.12 SEM micrographs of concrete containing PP aggregates with replacement level
of (A) 15% and (B) 30% (Yang et al., 2015). PP, Polypropylene.

The air/water permeability of recycled plastic concrete is significantly influenced
by the porosity of concrete (Wang and Meyer, 2012). It was reported that the water
permeability of LDPE coarse aggregate concrete increases by 45.6% with an
increase in RPA% from 50% to 100% (Ali et al., 2018). This observation was a
direct consequence of an increased porosity of concrete with an increase in the
RPA% (Ali et al., 2018). However, a study by Wang and Meyer (2012) reported
that PA concrete containing 50% HIPS fine aggregates exhibits 13.7% lower water
permeability than that of the conventional concrete (Wang and Meyer, 2012). As
was discussed in relation to the water absorption of concrete, this can be attributed
to the fact that polystyrene fine aggregates (e.g., EPS, UEPS or HIPS) have higher
hydrophobicity than natural sands. The air permeability of recycled plastic concrete
has also a similar trend to the water permeability. It was shown that the air perme-
ability of concrete containing PUR foam coarse aggregates was significantly higher
than that of the conventional concrete (Fraj et al., 2010), which is attributed to the
higher porosity of PUR foam coarse aggregates than that of natural coarse aggre-
gates. It was also reported that the air permeability of concrete with PP fibres was
lower than that of the conventional concrete (Bagherzadeh et al., 2011; Bhogayata
and Arora, 2018), which is because of the ability of fibres to prevent the develop-
ment of cracks in the concrete by creating bridges across the cracks.

3.5.2 Degradation in alkaline environment

Fig. 3.13 shows SEM images of PET fibre in the alkaline environment of concrete
at curing ages of 150 days and 1 year. As can be seen in the figure, PET fibre
exhibited a high level of degradation in the alkaline environment. This observation
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is because PET is easily influenced by Ca ,K ,OH and Na ions in the highly
alkaline environment of concrete (Silva et al., 2005). On the other hand, most types
of recycled plastics (e.g., PP, PVC and HDPE) are non-biodegradable because they
have a high chemical stability (Puertas et al., 2003; Webb et al., 2012). Therefore,