Page 35 - New Trends in Eco efficient and Recycled Concrete

12 New Trends in Eco-efficient and Recycled Concrete

Teranishi et al., 1998; Van Acker, 1998; Dhir et al., 1999; Hosokawa, 1999), most
have suggested that there is a decreased resistance to freezing and thawing with
increasing RA content (Nagataki and Iida, 2001; Zaharieva et al., 2004; Gokce
et al., 2004; Moon et al., 2002; Salem et al., 2003; Kimura et al., 2004;
Yanagibashi et al., 2002). The disagreements between the two trends can be mostly
explained by the RCA’s quality and whether the original concrete was air-
entrained. Using RCA with lower adhered mortar content, controlled by means of
effective contamination removal techniques, can improve the freeze-thaw resistance
when compared to mixes with RCA with higher adhered mortar content (Moon
et al., 2002; Gokce et al., 2004; Nagataki and Iida, 2001). However, the degradation
in these cases is still likely to be quite significant in comparison with control NAC
mixes. The onset of this degradation phenomenon takes place in the porous micro-
structure of the RCA’s adhered mortar, wherein there are disruptive stresses due to
the freezing water’s expansion, the effect of which is more noticeable due to the
existence of previously absorbed water (Salem et al., 2003). This leads to progres-
sively greater propagation of cracking with subsequent freezing and thawing cycles
causing the material’s disintegration. Nevertheless, if the original concrete was air-
entrained, the RCA obtained from it will probably result in a concrete with consid-
erably greater resistance to freezing and thawing than non-air-entrained RCA and
comparable to that of a corresponding conventional concrete (Gokce et al., 2004;
Nagataki and Iida, 2001).
Sulphate attack is one of the fastest and most detrimental chemical attacks that
can occur in concrete. Exposure to external sources of sulphate include, for exam-
ple, the contact of concrete with industrial wastewater, whereas internal sources
may be from sulphate-rich aggregates and excessive gypsum in the cement. As a
result of the potentially high contamination of gypsum in some CDW, the resulting
RA may produce concrete presenting disruptive expansions (Tovar-Rodriguez et al.,
2013; Aguiar and Selmo, 2004; Al-Attar et al., 2006). Although it has been estab-
lished that the incorporation of increasing RCA content may lead to lower resis-
tance to sulphate attack (Lee et al., 2008; Dhir et al., 1999; Lee et al., 2005), the
expansion of RAC when exposed to sulphate solutions is not significantly greater
than that of the control mix and can be easily managed by altering the mix design.
Furthermore, the extent of the degradation increases with increasing water absorp-
tion capacities of the RCA (Lee et al., 2005; Gokce et al., 2011). In such cases, it
has been observed that less mass loss due to sulphate attack can occur with decreasing
adhered mortar content as a result of a higher number of processing stages (Gokce
et al., 2011).

1.6 Influence of recycled aggregates on the
time-dependent deformation of concrete

As concrete cures, it experiences volume variations due to water loss by evapora-
tion, and hydration and carbonation related reactions (Neville, 2011), the extent of