Abstract
During the research study focused on 100 years, old concrete bridges in Slovakia an unexpected observation was made. Two bridges were found with a very low carbonation depth under an ordinary cement-based render coat used at the time of its construction for aesthetic reasons. The average measured value by phenolphthalein test was less than 2 mm after more than 100 years of service life in exposure class XC3 of EN 206. The remaining investigated bridges, from this period of construction, exposed to the same environment and made of concrete of comparable quality, showed significantly higher carbonation depths. Low carbonation depth, can be explained by the presence of a thin (2-4 mm) layer of the protective render coat (PRC) applied to concrete surface around 100 years ago. The place, where the PRC was of good quality was almost impermeable and the carbonation of the concrete underneath was even 0 mm. A narrow free space filled with the carbonates can cause increased non-permeability of the thin PRC creating thus the best condition for the built limestone-based (anti-carbonation) barrier with the ability to dramatically reduce CO2 penetration into the beneath concrete over time. This article is focused on the probable explanation of this phenomenon.
Highlights
The carbonation reactions of cement-based materials a) occur in natural environment at a very slow rate due to the low CO2 concentration (400 ppm or 0.04%) in the atmosphere [1]; b) are the highest within the R
The unprotected concrete SL-NON - demonstrates an evident increase in CaCO3 content (10.32 % wt.) in the surface 0-20 mm layer (NON-1) compared to the internal 150-180 mm (NON-2), whereas this difference is negligible in the surface-protected concrete COV-1 vs. COV-2 that takes only 0.72 % wt
3) The results show the close correlation between the surface permeability of protective render coat (PRC) and the depth of carbonation of the underlying concrete
Summary
The carbonation reactions of cement-based materials a) occur in natural environment at a very slow rate due to the low CO2 concentration (400 ppm or 0.04%) in the atmosphere [1]; b) are the highest within the R. When the matured concrete is attacked, the CO2 penetration depth is mostly limited to the outer layer because of the slow diffusion rate through the dense microstructure. In such a condition, the increase in strength of OPC-based systems due to the carbonation is not prominent [12,13]. Because of the reduction of total porosity, the carbonation reaction firstly reduces the permeability [7,16] and increases the microhardness of the cementitious matrix [17]. Due to the increase in capillary porosity, further progressing carbonation increases the diffusivity of ions through the cementitious matrix and the permeability [19]. The relation between mixture composition and carbonation resistance is in a detail assessed in the papers [20,21,22,23] and the major conclusions from the study are listed below
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