Page 37 - New Trends in Eco efficient and Recycled Concrete

14 New Trends in Eco-efficient and Recycled Concrete

As observed previously, the strength of the source concrete affects the quality of
the resulting RCA. If sourced from a concrete with higher w/c ratio, RCA are likely
to present greater porosity and lower stiffness; increases in specific creep of around
60%, 75% and 125% were reported for RAC mixes with the coarse NA fraction
completely replaced with RCA from concrete with effective w/c ratios of 0.30, 0.45
and 0.65, respectively (Geng et al., 2016).
Due to the fine RCA’s higher adhered mortar content, considerable increases in
creep strain can be observed in specimens containing them. Owing to their lower
stiffness, in one case, the use of 100% fine RCA led to an increase in creep defor-
mation close to 150% when compared to that of the control specimens (Cartuxo
et al., 2015). However, in other cases wherein 100% fine and coarse RCA were
used, an increase in creep strain of about 75% was observed (Pedro et al., 2017b)
and just 40% in another case (Gomez-Soberon, 2002). Thus, it is clear that there
are several factors related to the characteristics of RA apart those of the mix design
influencing this property and that considerable amount of research still needs to be
carried out on this matter.

1.7 Concluding remarks

Over the past five decades, several research entities have been investigating a way
to incorporate increasing quantities of CDW-based RA in concrete so that its per-
formance remains the same as that of conventional concrete or, at least, becomes
predictable. Indeed, the vast majority of investigations points towards the technical
feasibility of this material in building structures. However, as durability-related
properties seem to be much more affected by RA incorporation, little acceptance
has been observed in actual practice, especially in the light of the considerably
changing composition and properties of RA coming out of CDW recycling plants.
Although concrete is a versatile material, concerning the constituents it can con-
tain, not all materials are adequate for the manufacture of structural concrete.
Among all types of RA from beneficiated CDW, RCA are the most compatible RA
for concrete production, which is natural considering that they are obtained from a
product with similar characteristics to those of their future application.
Nevertheless, RCA can vary considerably depending on the source concrete’s mix
design, the quality control implemented during construction and demolition activi-
ties and CDW recycling plants’ techniques. Because of this variation, apart from
the necessity of proper quality control during all stages, more comprehensive classi-
fication systems need to be implemented in order to properly ascertain the RA’s
influence on the properties of concrete. Furthermore, although it has been demon-
strated that the use of RMA or MRA are feasible for the production of concrete
with good-enough mechanical behaviour, the durability-related performance will
probably be more affected in comparison with a corresponding concrete made with
RCA. Therefore, future specifications for the production of RAC should impose
limits to their incorporation and physical properties.