Page 36 - New Trends in Eco efficient and Recycled Concrete

Construction and demolition waste 13

which depends on the concrete’s age, constituents, relative humidity, surrounding
temperature, shape and size of the element. Since RA have greater porosity, which
allows greater movement of water and means a material with lower stiffness, these
have a lower restraining ability on the resulting RAC, thereby increasing its shrink-
age strain (Buyle-Bodin and Hadjieva-Zaharieva, 2002; Manzi et al., 2013; Silva
et al., 2015b; Pedro et al., 2015; Cartuxo et al., 2015; Khatib, 2005; Andal et al.,
2016; Ferreira et al., 2011; Evangelista and De Brito, 2004; Kou et al., 2004).
Therefore, as the replacement ratio increases, so does the concrete’s shrinkage; the
incorporation of 100% coarse RA is expected to result in an increase of 80% (Silva
et al., 2015b). However, shrinkage stains three times higher than that of the control
concrete have been reported for RAC containing 100% coarse and fine RA from a
French CDW recycling facility after a one-year curing in an outdoor environment
(Buyle-Bodin and Hadjieva-Zaharieva, 2002).
As in previous properties, the quality of RCA influences the extent of volume
variation due to shrinkage (Andal et al., 2016). Subjecting crushed concrete to more
crushing stages will result in RCA with a rounder shape and lower adhered mortar
content and, as a result, it becomes more capable of restraining the cement paste’s
shrinkage more effectively (Pedro et al., 2015).
The use of a water compensation method, wherein part of the RCA’s absorption
capacity is offset during the mixing process, allows producing RAC mixes with
lower shrinkage strains in comparison with those made with pre-saturated RCA
(Ferreira et al., 2011).
Owing to the relatively high-adhered mortar content in fine RCA, these are
likely to produce concrete exhibiting higher shrinkage strains than concrete made
with coarse RCA (Cartuxo et al., 2015; Evangelista and De Brito, 2004; Khatib,
2005). However, in the case of mixes made with fine RMA, studies have shown
that the resulting shrinkage strains may not be as significant as those of equivalent
mixes made with fine RCA (Khatib, 2005). The former type present some pozzola-
nicity that can further strengthen the cementitious microstructures and may also
provide and internal curing thereby preventing excessive evaporation during the
drying period (Khatib, 2005).
Like shrinkage, concrete deformation by creep is a complex phenomenon that is
influenced by the mix design, surrounding temperature, relative humidity, loading
conditions, shape and size of the element (Neville and Brooks, 2010). In the pres-
ence of a sustained stress, concrete may show sliding of nanoparticles, causing a
local increase of packing density towards the jammed state associated with limit
packing densities, beyond which no particle sliding is possible without dilation of
the granular media (Vandamme and Ulm, 2009). The extent of this deformation
largely depends on the type of aggregates used; as RA are less stiff than NA,
increasing replacement ratios lead to higher deformations due to creep (Silva et al.,
2015a; Manzi et al., 2013; Fathifazl et al., 2011; Gomez-Soberon, 2002; Wesche
and Schulz, 1982; De Pauw et al., 1998; Ravindrarajah and Tam, 1987). The incor-
poration of 100% coarse RA is expected to result in an average increase in creep
strain of about 50% (Domingo-Cabo et al., 2009; Silva et al., 2017), though
increases up to 110% were reported in one case (Bravo et al., 2017a).