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Life cycle assessment applied to recycled aggregate concrete 227

9.3.1.1 Functional unit

Of great importance in LCA of concrete, is the definition of the functional unit
(FU). This unit is seen as the reference unit of the product system for which the
environmental impact will be calculated (ISO 14040, 2006; Desmyter and Martin,
2001). Literature review done by Van den Heede and De Belie (2012) shows that
the scale of this FU for LCA can vary significantly from (1) the material level on to
(2) the structure level:

1. When comparing the environmental impact of different concrete mix designs, a small-
scale FU on the material level can be appropriate. An efficiency indicator similar to the
one proposed by Damineli et al. (2010) is seen as a first example, although it should not
only be expressed in terms of a compressive strength unit, but also relate to a unit of ser-
vice life. Another possible functional unit choice is the amount of concrete needed in a
simple structural element (column, beam, slab, etc.) with a given mechanical load and a
predefined service life in a given environment. This way, additional concrete manufactur-
ing due to replacement or repair over time, is included in the LCA. The same goes for dif-
ferences in strength. The use of a high strength concrete implicates that structure
dimensions and thus the overall concrete amount needed can often be reduced consider-
ably. The resulting environmental benefit will be visible in the LCA output.
2. When LCA is used to evaluate the environmental impact of a specific structure, the FU
usually corresponds with the structure itself. For a LCA study on pavements, Sayagh
et al. (2010) used a 1 km pavement with a lane width of 3.5 m as FU. The service life
was set at 30 years with a traffic load of 9.4 million trucks per lane within this period.
Both durability and mechanical load aspects are included in the study that way. Park et al.
(2003) calculated environmental impacts for 1 km of a four lane highway with a prede-
fined service life of 20 years and repair once every 7 years. However, construction activi-
ties and maintenance are not always included. For instance, Chowdhury et al. (2010)
chose a road section with a thickness, width and length of 600 mm, 2.5 m and 1000 m,
respectively, as FU. The focus of the study was simply on the material level with no spec-
ification of a service life. Besides road structures, entire buildings can be the object of an
environmental evaluation. Xing et al. (2008) studied the difference in impact between steel
and concrete construction office buildings with a use life of 50 years. Both the construction
2
and use phase were taken into account. 1 m of building area was adopted as FU. For the
assessment of wood and steel reinforced housing construction, Gerilla et al. (2007)
2
expressed the FU in kilogram emission per year and per m . It was assumed that the
2
detached houses had a 150 m floor space and a design life of 35 years. The whole life
2
cycle of the houses was considered. Specifying a 1 m area is not always correct (Lo ´pez-
Mesa et al., 2009). When comparing the construction related impact of in situ cast floors
with precast floors, the smallest precast hollow core slabs on the market performed much
better than a normally dimensioned in situ cast floor regarding structural strength. As a con-
sequence, the spans achievable with precast concrete slabs are higher and this reduces the
number of columns and spread footings. Therefore, the FU comprised the whole building,
although the main goal of the study was only an evaluation of the floor type used.

Thus, regardless the scale of the FU for LCA that compares the environmental
impact of a novel, potentially ‘green’ concrete, the unit should be able to deal with
the workability, strength and durability/service life differences between the two
concrete types.