Page 33 - New Trends in Eco efficient and Recycled Concrete

10 New Trends in Eco-efficient and Recycled Concrete

1.5 Influence of recycled aggregates on the durability
performance of concrete

Usually, with increasing replacement ratio of NA with RA, the durability perfor-
mance of concrete declines, the extent of which depends on the type, size and qual-
ity of RA. There is a consensus in the literature that, as the properties of RA
become closer to those of NA, the performances of the resulting concrete mixes
tend to be more similar (Silva et al., 2015c,d). This is typically seen in RCA-
containing mixes, whereas the increasing incorporation of RMA generally results in
materials with a worse durability (Gomes and de Brito, 2009; Dhir et al., 2005;
Poon and Chan, 2006). This occurs as a result of the latter RA type’s greater poros-
ity, which also increases that of the cementitious microstructure.
Coarse RCA have lower water absorption than finer fractions of the same mate-
rial due to the former’s lower adhered mortar content (Evangelista and de Brito,
2014; Silva et al., 2014b). Consequently, concrete made with fine RCA presents
greater porosity and thus exhibits a greater decline in durability-related properties
with increasing replacement ratios. On the other hand, a reverse trend is likely to be
observed in the case of RMA-containing specimens (Fig. 1.5), since fine enough
fractions of this type may present some pozzolanicity, thereby, resulting in a
decrease in porosity when compared to specimens with coarse RMA (Debieb and
Kenai, 2008; Sadek, 2012).
The properties of RCA depend on the mix design of the original concrete; by
decreasing the w/c ratio, concrete contains a less porous and denser cementitious
microstructure, which can also be seen in the resulting RCA (Silva et al., 2014b).
Studies carried out on the effect of incorporating RCA from concrete with different
strength classes showed that higher strength mixes generally result in RCA with
properties closer to those of NA and, consequently, in mixes with more comparable

160 Debieb and Kenai (2008) Sadek (2011)
Water absorption increase in relation to corresponding NAC (%) 120
140

100
80
60
40
20
0
100% fine RMA 100% coarse 100% coarse
RMA and fine RMA
Figure 1.5 Water absorption of concrete with different sized RMA fractions. RMA,
Recycled masonry aggregate.