Page 303 - New Trends in Eco efficient and Recycled Concrete

266 New Trends in Eco-efficient and Recycled Concrete

study. Dodoo et al. (2009) reported that at the end of concrete building service life,
CO 2 uptake was 23% of the calcination emission, while after the concrete was
crushed after demolition and exposed to air for four months, the absorption of CO 2
was almost twice as large 43% of the calcination emission. The absorption of
CO 2 during a concrete bridge service life in the Collins study was only about 3% of
the calcination emission. But if the concrete is crushed into RCA after demolition,
and used in the construction of the new bridge for another 30 years, CO 2 absorption
was as high as 55% and 86% of the calcination emission, depending on RCA
application.
Therefore, if demolished concrete is crushed into RCA and used in any unbound
application, CO 2 uptake can be significantly increased compared to that from ser-
vice life only, depending on the exposure time. The main reason is, many times
larger surface area of RCA than that of concrete structure. Based on analysed
research, CO 2 absorption during the service life of a concrete structure is about
20% of the calcination emission at maximum, or about 10% of the total CO 2 emis-
sion from the cement production. If the concrete structure has a secondary life,
unbound RCA application in new construction for the next 30 years, CO 2 uptake
can reach 80% of the calcination emission, or 40% of the total CO 2 emission from
the cement production. This is certainly important for an overall judgement of the
total concrete CO 2 footprint and shouldn’t be neglected in a LCA model of a spe-
cific concrete structure. The other question is how to do it. When applying ALCA,
care must be taken on the allocation of CO 2 uptake in the post-use phase. As with
impacts from recycling, the amount of reabsorbed CO 2 in this phase should be allo-
cated between the product that generates waste and the product which receives it,
following allocation rules (cut-off, mass or economic allocation). In CLCA, the
amount of reabsorbed CO 2 should be considered as an avoided impact of the recy-
cling process depending on the postuse of recycling products.

10.4 Case study: reinforced concrete floor slab in
residential building made of conventional and
recycled aggregate concrete

Ten case studies were performed with a goal to compare the environmental impact
of the RC floor slab life cycle, made of two different concrete mixes: NAC and
RAC. Varied parameters were: quality of RCA (two different qualities), exposure
class in the carbonation case (XC) (two classes, XC1 and XC3), LCI modelling
approach and CO 2 uptake (taking it, or not taking it, into account). Since the state-
of-the-knowledge has reached a level which enables the design of structural RAC, a
comparison was based on the design of the slab, according to Eurocodes, instead of
on the experimental results.
The investigated slab was a typical floor structure in the residential building
shown in Fig. 10.1 (examined slabs are marked). The building (four storeys, a
ground floor 1 and one underground parking level) is located in the capital of
Serbia, Belgrade. All floors are for dwelling and the roof is not open to the public.