Page 296 - New Trends in Eco efficient and Recycled Concrete

260 New Trends in Eco-efficient and Recycled Concrete

assessments using an attributional approach, the purpose is to estimate the absolute
impacts of a product system at a given point of time assuming a status-quo
situation.
The model is intended to present a ‘snap shot’ of the product system. A conse-
quential approach is intended to provide information on the environmental burdens
that occur, directly or indirectly, as a consequence of a certain decision, that is,
results are intended to represent the net environmental impacts of the change caused
by this decision, change in demand. Two major differences between these two
approaches which cause different results are:
Different system boundaries: An ALCA model does not include processes other than those
of the life cycle investigated, so cut-offs or allocation are usually applied when dealing
with multifunctional processes. A CLCA model includes unit processes that are signifi-
cantly affected irrespective of whether they are within, or outside, the life cycle, so system
expansion is the only way to resolve multifunctional processes. In practice, instead of sys-
tem expansion, system expansion with substitution method (the avoided burdens
approach) is usually applied as being conceptually equivalent at the process level
(Tillman et al., 1994).
Different data: An ALCA model should include average data on each unit process within
the product’s life cycle, while a CLCA model should include marginal data on bulk pro-
duction processes in the background system (Ekvall and Andrae, 2006). Besides, changes
in the production and alternative use of co-products in a CLCA should be based on an
analysis of how the relevant markets are affected and price elasticity of supply and
demand of each product or co-product (Ekvall and Andrae, 2006).
Although the consequential approach is the preferable option in comparative
LCAs (policy making instead of policy implementation), it is not easy to apply.
Many authors apply CLCA (with avoided burdens approach) based on the average
data and on the assumption that substitution is 1:1, and without the analysis of how
the relevant markets are affected. Simply subtracting the avoided burdens from the
product’s process does not make a consequential LCA, but is always favourable for
that product.
Combining an attributional LCI modelling with system expansion understood as
avoided burdens approach in resolving multifunctional problems cannot be sup-
ported (Vogtl¨ ander et al., 2001; Chen et al., 2010; Pelletier et al., 2015) since this
combination would cause various inconsistencies in the LCA, such as double count-
ing avoided burdens. The main problem is the mass and energy balance over the
various product systems: the outputs must equal inputs; mass and energy cannot
disappear. Therefore, allocation instead of system expansion in ALCA should be
applied. In that case, only benefits from recycling, for instance, are the benefits that
really exist at certain point of time: a certain amount of concrete waste that is
recycled is not going to be landfilled and a certain amount of NA that is replaced
with RCA is not going to be extracted.
In comparative LCA studies where a consequential approach with system expan-
sion (understood as avoided burdens approach) is applied, results are usually beneficial
for RAC. For instance, (Knoeri et al., 2013) showed that RAC environmental impacts
were reduced to 70% of the conventional concrete impacts. Turk et al. (2015) obtained