Page 261 - New Trends in Eco efficient and Recycled Concrete

Life cycle assessment applied to recycled aggregate concrete 229

perceived low significance in terms of environmental loads within the concrete
manufacturing process, admixtures are rarely covered in the literature. However,
during the synthesis of a superplasticiser, emissions from substances with a high
atmospheric acidification potential such as NO x ,SO x , acids, etc. are released
(Serres et al., 2016), which could alter the results of impact categories dealing with
ecotoxicity (Gursel et al., 2014). For instance, Jime ´nez et al. (2015) also reported
that chemical admixture related emissions mainly related to ozone layer depletion
and terrestrial ecotoxicity. Moreover, the use of superplasticiser can reduce concrete
CO 2 emissions by 26% since a lower cement content is required to maintain the
same water-to-cement ratio (Purnell and Black, 2012) which could also alter the
final results of a comparative LCA when different amounts of superplasticisers are
used. Some exceptions to that statement can be found (Braga et al., 2017; Flower
and Sanjayan, 2007; Jime ´nez et al., 2015; Panesar et al., 2017; Serres et al., 2016).
Key aspects that are not accounted for in pure cradle-to-gate LCA studies are
differences in workability, strength and service life between concrete types. To
overcome this problem Van den Heede and De Belie (2012) proposed a modified
cradle-to-gate approach. This methodology intends to quantify the environmental
impact of the total amount of concrete that the manufacturer will need to produce
and maintain a specific structure bearing a given mechanical load during its prede-
fined service life. Although such an approach still represents cradle-to-gate produc-
tion of an amount of concrete, additional concrete manufacturing needed for
expected repair and rehabilitation works during the use phase is included in this
amount. As such, the main functional properties are considered within the system
boundaries.
Although cradle-to-grave LCA holds the preference, several researches have
excused the inclusion of the construction, service and demolition phases from the
comparative LCA under the assumption that the exclusion should not detrimentally
affect either mixture in the comparison for an equivalent mix design (i.e., same tar-
get compressive strength, workability and service life) between the conventional
and the recycled concrete (Colangelo et al., 2018; Ding et al., 2016; Knoeri et al.,
2013; Marinkovi´ c et al., 2010; Toˇ si´ c et al., 2015). Nevertheless, an extension of the
system boundaries could significantly affect the results. For instance, if concrete
placement is considered the different densities of natural and recycled concrete
would affect the need for transport, which would result in lower emissions for the
recycled concrete.
The cradle-to-cradle concept promotes the design of products with a beforehand
recyclability scenario in mind. As an example, the environmental benefits and bur-
dens of completely recyclable concrete were analysed using this concept. In brief
on the technology: the mix design of this concrete type is to be adjusted to favour
the reutilisation of concrete waste as secondary raw material for the production of
cement (De Schepper et al., 2014, see corresponding eco-profile shown in Fig. 9.3).
Although less common, there are also studies applying the cradle-to-cradle theory
and closed-loop approach for sustainable construction when dealing with CDW as
raw material for a second life cycle in construction (Colangelo et al., 2018; Ding
et al., 2016).