Fig. 4.38. Permeability versus weight loss of concrete cubes, for drying and wetting [4.57].
carbonation in parallel to the changes of capillary porosity. Kropp [4.71] reported a reduced rate of water vapour diffusion in
carbonated mortar. For an OPC mortar with w/c=0.5, results are given in Fig. 4.51.
Bier [4.73] investigated the effect of carbonation on the pore structure of cementitious systems that were manufactured with
different types of cement. It was found that for cements with a high clinker content, carbonation led to a considerable
reduction of the capillary pore volume, whereas for cements with a slag content exceeding approximately 50% by mass, a
coarser capillary pore system was formed during carbonation. This was attributed to a considerable decomposition of the CSH
gel in these cement pastes. The changes in the pore structure were reflected in the permeability of the different materials for water.
Whereas systems containing OPC showed a decrease in permeability upon carbonation, PBFSC with slag contents exhibited a
drastic increase in permeability after carbonation (cf. Fig. 4.52).
These results demonstrate that permeation of chemically reactive media through concrete may alter its permeability
significantly, thus also controlling the resistance of concrete against other types of external attack. If the durability of concrete
is expressed in terms of permeability these effects have to be taken into account.
Patel et al. [4.72] found that measured reductions of capillary porosity due to carbonation were associated with reduced
rates of counter-diffusion, as pore water was exchanged with methanol. Martin [4.75] reported that nitrogen permeability
measurements were repeatable whereas carbon dioxide permeability measurements were not; the observed reduction was
attributed to the carbonation reaction. The changes of pore structure with carbonation are of practical significance for in-situ
permeability testing of concrete and interpretation of results: measured values of permeability could exhibit a decrease over a
long period of time, and such a decrease could be more pronounced in concretes made with a high water/cement ratio. Thus it
is necessary to make some allowance for carbonation if the condition or quality of the concrete at some earlier age is to be
assessed from in situ permeability data.