Page 267 - New Trends in Eco efficient and Recycled Concrete

Life cycle assessment applied to recycled aggregate concrete 235

a sensitivity analysis to assess the range in which the recycled concrete showed a
better performance than the conventional concrete. Marinkovi´ c et al. (2010) con-
cluded that, when the same transport distances were considered, the recycled con-
crete (made with 100% gravel substitution and 3% additional cement) exhibited a
11.3% 36.6% increase for the different impact categories with respect to the tradi-
tional mix. Thus, the authors recommended a 20 km shorter routing trip of recycled
aggregates in order to ensure that the environmental impacts of the recycled con-
crete remained lower than those of the conventional mixture. Nevertheless, it should
be mentioned that different means of transport are considered, ships and trucks are
employed for natural and recycled aggregates, respectively. Besides, both the
assumptions made in the attributional modelling and the change in mix design had
a great influence on the results. Also under Serbian conditions and as result of a
multicriteria optimisation by the VIKOR method, Toˇ si´ c et al. (2015) affirmed that a
50% replacement of the natural aggregates with coarse RCA was the most environ-
mental friendly option; meanwhile conventional concrete resulted as the most eco-
nomical alternative. In order to make recycled concrete more competitive, the
authors suggested a 50% tax raise for river aggregate extraction and a 53% cost
increase for landfill disposal.
Knoeri et al. (2013), who included the environmental burdens (building disman-
tling) and gains (avoided CDW landfill and recuperated steel) in consequential
modelling of the recycled aggregates, found that recycled concretes presented gen-
eral environmental benefits (around 30%) compared to conventional concrete.
Moreover, regarding the global warming potential, the environmental gain only dis-
appeared when more than 10% of additional cement is used or when transport
distances exceed 15 km. The authors also stated that the CDW composition has little
effect on the results. However, a sensibility analysis performed by Wijayasundara
et al. (2017) showed that CDW composition greatly influenced the results due the
different amount of embodied energy in each type of component. For instance, lower
cement contents or the use of secondary materials had a positive effect on the overall
environmental performance of the recycled concrete. When the quality of the recycled
aggregates is taken into account, Fraj and Idir (2017) argued that the manufacturing of
recycled concrete was only beneficial for high-grade recycling aggregates up to a 20%
substitution. For 50% replacements of high grade recycled aggregates, only the impact
categories of water usage, water pollution, ozone layer depletion and eutrophication
were lower than for conventional concrete. The higher global warming potential pre-
sented by the recycled concrete resulted from the additional cement used, which is the
main responsible in this impact category. From the results of the sensibility analysis
performed on delivery distance of natural aggregates, long distances (250 km) favoured
the use of recycled aggregates whereas close availability of natural aggregates
(,22 km) yielded less beneficial recycled concretes. Nonetheless, if the quality of
recycled aggregate was considered, the minimum distance increased up to 50 km for
high grade recycled aggregates.
In addition, Wijayasundara et al. (2017) argued that some studies (e.g., Knoeri
et al., 2013; Marinkovi´ c et al., 2010), gave the impact of transport an overrepresen-
tation within the environmental burdens of concrete manufacturing and downplayed