Page 30 - New Trends in Eco efficient and Recycled Concrete

Construction and demolition waste 7

Garcia-Gonzalez et al., 2017; Lotfi et al., 2015; Wardeh et al., 2015). This is due to
the RA’s higher content of adhered mortar or ceramic particles, which present
greater porosity and are less dense when compared to NA (Hansen, 1992).
Naturally, the magnitude of this variation varies according to the type of RA; RMA
generally have lower density than RCA and thus cause greater losses in fresh den-
sity. Decreases of about 10% have been reported (De Brito et al., 2005; Ridzuan
et al., 2005).

1.4 Influence of recycled aggregates on the mechanical
performance of concrete

When producing RAC, the different properties of RA in comparison to those of
NA, must be taken into consideration. Owing to RA exhibiting generally lower
quality, their increasing incorporation in concrete typically leads to a decline in
mechanical performance, the scale of which depends on the recycled materials’
type, size, original quality and moisture content. This is true for all mechanical
properties of concrete (compressive and tensile strength, modulus of elasticity and
abrasion resistance), even though with varying intensities (more intense for the
modulus of elasticity and less intense or even inconclusive for abrasion resistance).
Therefore, the description provided below, illustrating the concepts for compressive
strength, is also generally valid for those other mechanical properties. RCA present
the closest basic physical properties to those of NA and are, thus, more likely to
produce RAC with equivalent mechanical behaviour when compared with concrete
containing the same amount of MRA or RMA. Conversely, as the content of RMA
increases, the mechanical performance is expected to decline at an even greater
rate, thereby demonstrating the necessity for stricter quality control during construc-
tion and demolition activities, as well as during the recycling process, in order to
separate the materials by type as much as possible.
It was reported that the incorporation of 100% coarse RA may result in compres-
sive strength losses of around 50% when compared to corresponding NAC mixes
(Silva et al., 2014a). However, strength gains of up to 40% were also reported
(Ridzuan et al., 2005), indicating that there is a number of different factors affecting
this property at the same time. The results from the literature suggest that the incor-
poration of fine RA usually result in equivalent (Evangelista and de Brito, 2007;
Sarhat, 2007; Khatib, 2005) or lower (Kenai et al., 2002; Kikuchi et al., 1998; Silva
et al., 2014a) compressive strength values in comparison with control mixes.
Therefore, it can be said that concrete made with coarse RA is more likely to pres-
ent better mechanical behaviour than those made with fine RA (Kikuchi et al.,
1998). Nevertheless, although this may be true for RCA and MRA-containing
mixes, it is probably not the case when using RMA. Finer fractions of this type of
RA often have equivalent density and water absorption when compared to corre-
sponding coarser fractions, thereby indicating that there is no added frailty as that
observed with fine RCA. Furthermore, due to their greater surface area and specific