From Fig. 1, it can be observed that the concrete carbonation
depth increased with increased CO2 concentration but the significance
is reduced when the CO2 concentration is higher than 20%.
The increase rates in different zones are also different. In Fig. 1,
the arrow lines display the slopes of 56d carbonation depth lines
for different CO2 concentration zones. The slope of zone r (2–
10%) is the biggest. It indicates that when CO2 concentration
increased from 2% to 10%, increase of concrete carbonation depth
is the most obvious. The slope of zone s (2–10%) is smaller than
that of zone r. The slope of zone t reduces greatly, and the slope