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.