of zone u is nearly zero, which means that, when CO2 concentration
exceeds 20%, a great increase of CO2 concentration cannot lead
to a great increase of concrete concentration depth. Concrete carbonation
depths under 50% and 100% CO2 concentration levels
are almost the same. Therefore, when CO2 concentration exceeds
50%, the increase of CO2 concentration does not lead to the increase
of concrete carbonation depth. For 7d, 14d and 28d carbonation
ages, the relationships between CO2 concentration and carbonation
depth are similar. The relationship between CO2 concentration and
carbonation depth of Grade 40 MPa concrete (see Fig. 2) and Grade
50 MPa concrete (see Fig. 3) are also similar to that of Grade
30 MPa shown in Fig. 1.