Page 300 - New Trends in Eco efficient and Recycled Concrete

264 New Trends in Eco-efficient and Recycled Concrete

time perspective, earlier uptake during service life and on the dimensions of debris-
pieces. However, for a long-term time perspective it can be assumed that maximum
total potential CO 2 uptake (for all life cycle phases) is 75% of the maximum clinker
uptake according to Eq. (10.3) (CEN/TC229/WG5-N012, 2016). So, in the landfill-
ing case and for CEM I concrete, the total CO 2 uptake is:

CO 2 -uptake 5 0:75 0:49 5 0:37 kg CO 2 =kg cement (10.6)
total

10.3.3.3 Relevance of the post-use phase (secondary life of the
concrete structure)

Total concrete CO 2 footprint depends significantly on the postuse applications of
the demolished concrete structure, its so-called secondary life. If demolished con-
crete is crushed into RCA and stockpiled for a certain period of time or used in a
new concrete construction, the CO 2 uptake will be higher than CO 2 uptake during
service life only. The exposed surface area relative to volume of RCA is greatly
increased compared to the concrete structure itself or landfilled concrete waste,
which increases its capacity to reabsorb CO 2 . However, the amount of captured
CO 2 depends on the RCA application: whether it is used in below-ground applica-
tions (for sub-base and base of road structures, where it is commonly used) or as an
aggregate in new concrete. In the latter case, CO 2 uptake is significantly smaller
and comparable to CO 2 uptake during the primary service life of the structure.
Also, the uptake in crushed material depends on the earlier uptake in the original
structure being demolished and crushed.

10.3.3.4 CO 2 uptake results from research
Two extensive investigations on CO 2 uptake of concrete were carried out in Europe
for Nordic countries (Kjellsen et al., 2005) and in the United States (Gajda, 2001).
Approaches and results of these and two other studies (Dodoo et al., 2009; Collins,
2010) are shown in Table 10.2 (Marinkovi´ c et al., 2014).
The Kjellsen et al. (2005) and Gajda (2001) studies were based on a survey of
the volume of produced concrete, its applications, typical thickness, recycling prac-
tices, etc., in Nordic countries and the United States, respectively. The Dodoo et al.
(2009) and Collins (2010) studies were based on data for specific concrete struc-
tures. Only in Gajda (2001) the study of the post-use of structures was not taken
into account, while in other analysed studies the secondary life was considered,
although with different post-use RCA applications. There are also certain differ-
ences in the carbonation model, especially in the assumed percentage of CaO avail-
able to carbonation and the value of the carbonation rate coefficient.
Despite these differences, one conclusion is obvious: CO 2 uptake of concrete
during service life is much smaller than CO 2 uptake during secondary life, if demol-
ished concrete is crushed into RCA compared to the 8.6% from the Gajda (2001)
study to 33% 57% in the Kjellsen et al. (2005) study, or even 86% in the Collins