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Equivalent functional unit in recycled aggregate concrete 299

11.3.2 Influence on the properties of fresh concrete

RAs have a direct influence on the fresh-state properties of concrete made with
them. First, this is due to the greater water absorption of RA, which will influence
the free mixing water (de Juan and Gutie ´rrez, 2009). Then, the rougher surface of
the RA will increase concrete viscosity, leading to a higher water demand (de Brito
and Robles, 2010). Moreover, RA tend to have ultra-fine particles adhered to the
surface, that will demand more free water in order to maintain the same workability
(Quiroga and Fowler, 2004). Consequently, the moisture condition of the RA, as
well as the mixing techniques, have a major role on the rheological performance of
fresh-state RA concrete.
Poon et al. (2004a,b) tested the influence of the moisture condition of coarse
RAs in the fresh-state properties of concrete and concluded that the oven-dry condi-
tions produced the biggest slump loss with time, due to the high water demand of
RA. On the other hand, RA in saturated surface-dried conditions produced con-
cretes that exhibited bleeding, as it was later confirmed (Poon et al., 2006).
As for self-compacting concrete, Kou and Poon (2009) state that using RAs in
air dry conditions leads to an increase of workability at initial stages, but also to a
faster slump decline due to the RA’s water absorption.
The effectiveness of water reducing agents is also influenced by the use of RA.
Pereira et al. (2012) and Matias et al. (2013) concluded that both plasticisers and
super plasticisers loose efficiency when fine and coarse RAs are used, respectively.
In both cases, the authors state that the larger specific surface of the RA is the key
factor for the loss of performance.

11.3.3 Influence on the mechanical properties of concrete

Mechanical performance of concrete is affected by the presence of RA. Overall, the
vast majority of the mechanical properties present lower results as the replacement
ratio of NAs with RA increases (Xiao et al., 2012). The results are intrinsically
related with the existing interfacial transition zone (ITZ) within the RA, as well as
the new ITZ between the RA and the new cement paste (Poon et al., 2004a,b; Lee
and Choi, 2013; Xiao et al., 2013), as well as the weaker mechanical performance
of the RA themselves, due to the presence of adhered mortar (Casuccio et al., 2008;
Gonza ´lez-Fonteboa et al., 2011; Pedro et al., 2017). Based upon 119 different publi-
cations, Silva et al. (2015b) established that compressive strength of RAC (RAs
concrete) made with 100% replacement of NA ranges from 0.56 to 1.17 times that
of NAC, with an average value 5 0.89.
Similarly, the tensile strength of RAC is also weaker than that of NAC, for a
comparable composition. The decrease of performance is related to the same factors
presented for compressive strength, even though the magnitude of the effect of
these factors seems to differ for both properties. In a thorough literature review con-
ducted by Silva et al. (2015a) the relative tensile strength ranges from 0.40 to 1.14
for 100% replacement, with an average value of 0.88. The authors also concluded
that the relationship between the compressive and tensile strengths of RAC is not