Page 31 - New Trends in Eco efficient and Recycled Concrete

8 New Trends in Eco-efficient and Recycled Concrete

chemical composition, fine RMA will likely present some pozzolanicity that can
result in improved mechanical performance (Alves et al., 2014; Vieira et al., 2016).
The quality of material from which the RA was obtained also participates in the
strength development of RAC. The use of RCA from end-of-life high-strength con-
crete is likely to produce RAC with negligible strength loss and even strength gain
when compared to control mixes (Nagataki et al., 2004; Otsuki et al., 2003; Dhir
and Paine, 2004; Wang et al., 2011; Pedro et al., 2014, 2015, 2017a). This can also
be seen in concrete made with RMA from high strength brick units (Khalaf and
DeVenny, 2004, 2005).
It is a well-known fact that the mechanical behaviour of concrete improves with
elapsing time, as a result of the continuous formation of hydration products. Long-
term studies on the use of 100% coarse RCA in concrete showed that, even though
there may be an initial difference in mechanical performance, this gap tends to
close over time (Poon and Kou, 2010). This phenomenon is probably due to the
continuous hydration of cement particles, both in the new and old cement mortar,
which results in increasingly stronger RCA as well as greater bond strength devel-
opment at the interfacial transition zone between the new cement paste and RCA.
The use of water reducing admixtures, by enabling the reduction of the con-
crete’s w/c ratio for equivalent workability levels, consequently leads to enhanced
mechanical performance (Fig. 1.4). However, these admixtures may be based on
different constituents and, thus, present various levels of effectiveness. It is possible

80
Without WRA WRA-1 WRA-2
70
Compressive strength (MPa) 50
60

40
30

20
10
0
0 10 30 50 100
Fine RCA replacement ratio (%)

Figure 1.4 28-day compressive strength of concrete with increasing fine RCA content and
two different water reducing admixtures (lignosulphate-based WRA-1; polycarboxylic
acid WRA-2).
Source: Adapted from Pereira, P., Evangelista, L., de Brito, J., 2012b. The effect of
superplasticizers on the mechanical performance of concrete made with fine recycled
concrete aggregates. Cem. Concr. Compos. 34 (9), 1044 1052.