Civil Engineering Association

Carbon dioxide uptake in demolished and crushed concrete

Recycled Concrete Aggregates (RCA) produced in the Nordic countries was summarized and its
scenario applications with regard to the grain size were described. The CO2 uptake to different crushed
concrete types was then measured by conducting extensive accelerated laboratory tests, in order to
document any differences in the uptake rate between the different crushed concrete samples.
Furthermore, the maximum uptake of CO2 within reasonable testing time in laboratory was also
determined.
The annually volumes of concrete rubble generated in the Nordic countries, except for Island, was
estimated to be in the range of 0.6 to 1.2 million ton. From these concrete rubble volumes, the
production of RCA were calculated to be in the range of 0.2 to 1.0 million ton corresponding to a
recycling level of 30-90 %. In Norway, Finland and Sweden the current recycling level is at 30, 50 and
60 % respectively. However, these countries reported that the target recycling level is 70 % by the end
of 2010. The current recycling level in Denmark was reported to be 90 % and is expected to be the
same by the end of 2010. In Finland, however, a major increase in the concrete rubble generation
(from 45 % to 60 %) as well as the expected increase in the recycling level results in a major increase
in the RCA production by the end of 2010. The annual concrete rubble generation in Island is
approximately 50 000 ton which is landfilled.
In the laboratory different concrete mixes were tested for CO2 uptake. After hardening the mixes were
crushed into different grain sizes. It was found that 60-80 % of the CO2 released released during
calcination is reabsorbed to the concrete mixtures with w/c of 0.6 or higher for the grain size of 1-8
mm within 20-35 days of exposure. Furthermore, the calculation showed that 60-90 % of the total CaO
in the same samples was carbonated. Determination of the total carbon in the carbonated samples by
total combustion and CO2 detection showed reasonable agreement with the measured CO2 uptake.
The w/c ratio was found to be crucial as expected. Comparison of the mixes with the w/c ratios from
0.4 up to 0.75 showed large differences as the highest w/c ratio gave the highest carbonation rate.
Thus, it was found that more than 90 % and less than 10 % of the CO2 was absorbed within the first 50
hours of exposure for the mixes with w/c of 0.75 and 0.4 respectively testing samples with grain size
of 1-8 mm. Coarser aggregate samples carbonated significantly slower.
Although the reaction kinetics varied due to the changing CO2 partial pressure in the exposure
desiccators, this test setup provided a fairly rapid quantification of the carbonation rate in between
different concrete mixes. The tests also gave a realistic measurement of the total CO2 uptake for the
different samples which can support the documentation of concrete carbonation during service life and
secondary use.