Page 28 - New Trends in Eco efficient and Recycled Concrete

Construction and demolition waste 5

Aside from the properties that are also typically assessed for natural aggregates
(NA), some innate to RA also have to be evaluated to ascertain their feasibility in
concrete production. The standard EN-933 11 (2009) presents a simple method for
the examination of coarse RA with the purpose of identifying and estimating the
relative proportions of constituent materials separated in the following six
categories:

Rc—concrete and products thereof;
Ru—unbound NA;
Rb—ceramic bricks and tiles, calcium silicate masonry units;
Ra—bituminous materials;
Rg—glass; and
X—other materials (includes gypsum plaster, plastic, metals, rubber, soil, wood).

1.3 Influence of recycled aggregates on the fresh-state
performance of concrete

The workability of RAC, being one of the fundamental properties of fresh concrete,
apart from being influenced by the concrete’s mix design, is also considerably
affected by the replacement ratio of NA with RA, physical properties of RA and
their moisture content. Most of the literature shows that a worse workability is
likely to be observed with increasing RA content (Etxeberria et al., 2007; Barra and
Va ´zquez, 1998; Khatib, 2005; Buyle-Bodin and Hadjieva-Zaharieva, 2002; Yang
et al., 2008). Assuming that the water absorption of the RA is not compensated, the
decrease in consistence is probably lower when using RCA than when using MRA
and RMA, which contain more porous particles capable of absorbing the mixing
water.
The quality of the original material can vary significantly, which also influences
the water absorption capacity of the resulting RA. The RA resulting from the bene-
ficiation of high-strength concrete or ceramic bricks tend to present lower water
absorption in comparison with those obtained from lower strength products and,
thus, do not have such a great influence on the workability loss of the resulting con-
crete (Khalaf and DeVenny, 2004; Pedro et al., 2014; Otsuki et al., 2003).
Conversely, finer RA fractions, which are generally looked upon as lower-quality
material, typically present higher water absorption levels when compared to corre-
sponding coarser fractions (Teranishi et al., 1998). This occurs as a result of the
fragmentation of particles during the CDW’s beneficiation process, wherein the
adhered mortar and other porous constituents tend to accumulate in the finer frac-
tion (Rodrigues et al., 2013).
To offset the workability loss due to the RA’s higher water absorption, the most
implemented approach and with the best results in terms of workability over time
has been through the addition of compensation water. This can be achieved by pre-
saturating the RA before the mixing process or by incorporating more water during
the process itself. The latter is likely to produce RAC with more stable consistence