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Life cycle assessment applied to recycled aggregate concrete 231

A LCA is considered either retrospective or prospective depending on whether
the analysis is looking backward or forward in time. According to McManus and
Taylor (2015) this differentiation is strongly related to the differentiation between
attributional and consequential LCA. While attributional LCA usually has a retro-
spective viewpoint by relying on present and past information for rather contained
system boundaries, consequential LCA is more prospective and relates to future
policy decision making. Logically, the latter approach implies a much broader
scope.
Earles and Halog (2011) distinguish only two general LCA types, that is, the ear-
lier mentioned attributional and consequential LCA. Attributional LCA deals with
the immediate physical flows (such as use of resources, materials and energy, emis-
sions, etc.) within the life cycle of a product. Average data are used to assign a
quantitative value to each of these flows. This is completely different for conse-
quential LCA. There, the aim is to describe how physical flows can/will evolve in
response of an increase/decrease in demand for the product system that is being
investigated. To do so, the mere use of average data on the physical flows involved
is insufficient. Additional economic data to measure physical flows of indirectly
affected processes as well as marginal data on which technologies will be affected
and how much, are needed as well. In short, one can say that attributional LCA
assesses the status of the flows in a system, while consequential LCA assesses the
changes in the status of the system caused by a certain decision (Santos Vieira and
Horvath, 2008).
In attributional LCA, inputs and outputs are attributed to the functional unit of a
system by linking and/or partitioning the unit processes of the system under a speci-
fied normative rule (Sonnemann and Vigon, 2011). This approach uses allocation
procedures to divide the impacts arising from the same process between both the
products and by-products (Ekvall et al., 2016) in a way that the underlying causal
relationships between them are realistically represented (Gursel et al., 2014).
Attributional modelling is the most common approach when including secondary
cementitious materials, that is, blast-furnace slag, fly ash, etc., in the concrete mix
design. Usually, allocation is made in terms of mass or economic value (i.e., ratio
between market value of the by-product and the primary material), the former being
more constant with time and the latter being more unstable due to market changes.
Nonetheless, economic allocation could be used to avoid overburden of the by-
products, for example, fly ash (Chen et al., 2010a,b; Marinkovi´ c et al., 2016; Van
den Heede and De Belie, 2012) (see Section 9.2.1.2).
In consequential LCA, activities are included in the system to the extent that
they are expected to change as a consequence of a change in demand for the func-
tional unit (Sonnemann and Vigon, 2011). For instance, recycling causes a reduc-
tion in the upstream processes (they appear as negative inputs), which represents a
burden avoidance and implies a decrease in the demand of the parallel positive
input. This practice is known as system expansion or substitution (Tillman et al.,
1994). Note that allocation is preferably avoided in consequential LCA. System
expansion has the preference (Weidema, 2001). Different substitution ratios should
be applied depending on the functional equivalence between the primary product