Page 29 - New Trends in Eco efficient and Recycled Concrete

6 New Trends in Eco-efficient and Recycled Concrete

values in comparison with concrete containing pre-saturated RA (Ferreira et al.,
2011). The water compensation method consists of using additional water during
the mixing process corresponding to the amount absorbed by RA throughout the
mixing stage until the slump test (Fonseca et al., 2011; Pereira et al., 2012a;
Ferreira et al., 2011; Bravo et al., 2017b). Poon et al. (2004) studied the influence
of concrete containing 100% RCA at different moisture conditions and with similar
total w/c ratio. The results showed that RAC containing dry RCA and with greater
amount of mixing water presented higher initial slump values when compared to
natural aggregate concrete (NAC) whereas RCA at a saturated and surface-dry state
produced concrete with similar slump values to those of the control mix.
Nevertheless, after 15 30 min, all mixes, regardless of the RCA’s moisture state,
presented equivalent slump levels to those of the NAC indicating that most of the
RCA’s water absorption capacity had been reached.
Naturally, aside from the addition of extra water, it is possible to offset the
workability loss of RAC mixes with equivalent total w/c ratio to that of a corre-
sponding NAC with the use of water reducing admixtures. However, it is likely that
this only occurs at an initial stage as the uncompensated RA will absorb much of
the free water after the mixing process (Domingo-Cabo et al., 2009, 2010), resulting
in a progressively stiffer mix over time regardless of the water reducing admixture
content. Furthermore, even in circumstances where the RA’s water absorption is
partly compensated, the admixture’s effectiveness may decrease with increasing
replacement levels due to the RA’s rougher surface and thus higher surface area
(Cartuxo et al., 2015, 2016; Pereira et al., 2012a,b; Teranishi et al., 1998; Meddah
and Sato, 2010; Pedro et al., 2017a).
Stability properties, such as bleeding and segregation, are more likely to be
affected as cases where the mixes are more fluid and with higher w/c ratios.
Although this intuitively would not pose a problem in RA-containing mixes due to
their rough surface and absorption of the mixing water, as most of the literature
indicates (Kutegeza and Alexander, 2004; Koulouris et al., 2004; Yanagibashi
et al., 2002; Kou and Poon, 2009b; Corinaldesi and Moriconi, 2011; Dhir et al.,
1999; Van Der Wegen and Haverkort, 1998; Yang et al., 2008; Carro-Lopez et al.,
2017; Kumar et al., 2017), since complete water compensation approaches may be
implemented, the resulting mixes may present some stability-related issues. It was
ascertained that the incorporation of saturated and surface-dry RA, obtained through
pre-saturation, is more likely to cause some bleeding when compared to correspond-
ing NAC mixes (Dhir et al., 1999; Debieb and Kenai, 2008). Furthermore, this phe-
nomenon may also occur in cases where the RA’s absorption capacity is completely
compensated through the addition of extra water during the mixing process (Poon
et al., 2004).
There is a consensus in the literature that increasing the replacement ratio results
in a linear decrease of the fresh density of concrete. An average loss in density of
about 5% is expected with the incorporation of 100% coarse RA (Amorim et al.,
2012; Buyle-Bodin and Hadjieva-Zaharieva, 2002; Cachim, 2009; Ferreira et al.,
2011; Park, 1999; Vieira et al., 2011; Thomas et al., 2013; Casuccio et al., 2008;
Adams et al., 2016; Akca et al., 2015; Maleˇ sev et al., 2010; Pickel et al., 2017;