Page 298 - New Trends in Eco efficient and Recycled Concrete

262 New Trends in Eco-efficient and Recycled Concrete

will be reabsorbed or uptaken by the concrete structure. The question is whether
this part is relevant for a proper environmental assessment of a concrete structure.

10.3.3.2 Calculation of CO 2 uptake during use and
end-of-life phase

Carbonation is a slow, mostly diffusion-controlled process which starts from the
concrete surface and slowly penetrates into the interior of concrete. The maximum
theoretical CO 2 uptake for totally carbonated concrete is correlated to the amount
of reactive CaO in the binder. It can be calculated according to prEN 16757 (CEN/
TC229/WG5-N012, 2016) as:

w% reactive CaO m CO 2

CO 2 -uptake tcc 5 Ubinder content (10.2)
100 m CaO
where CO 2 -uptake tcc : maximum theoretical uptake of CO 2 in totally carbonated
concrete (kg); w% of reactive CaO: part of reactive CaO (100 kg CaO/kg binder);
: molar weight of CO 2 5 44 g/mol; m CaO :
binder content: mass of cement (kg); m CO 2
molar weight of CaO 5 56 g/mol.
Portland cement includes at least 95% of clinker and a typical value for reactive
CaO is 65%. So, for one kilogram of Portland cement (CEM I) the maximum theo-
retical CO 2 -uptake tcc is:

65 44
CO 2 -uptake 5 0:95 5 0:49 kg CO 2 =kg cement (10.3)
tcc
100 56
During the use phase, part of the concrete will carbonate and that part, measured
from the surface, is called the carbonation depth. It can be calculated through
Eq. (10.4):
p ffiffi
x c 5 k t (10.4)

0.5
where x c : carbonation depth (mm); k: carbonation rate coefficient (mm/year ),
k-factor; t: time (years).
However, not all of the available CaO in hydration products will convert to
CaCO 3 during the use phase of the concrete structure. Because of that, a concept of
the degree of carbonation is introduced in the estimation of the CO 2 -uptake. In a
so-called carbonated concrete, the degree of carbonation can differ widely depend-
ing mainly on exposure conditions. So, prEN 16757 (CEN/TC229/WG5-N012,
2016), based on Andersson et al., (2013), recommends k-factors for different con-
crete strength classes, exposure conditions and degree of carbonation for different
exposure conditions given in Table 10.1. For other cements than CEM I consider-
ation of clinker content should be done by multiplying the uptake of a CEM I