Evaluating the effect of external and internal factors on carbonation of existing concrete building structures

Carbonation is one of the key factors that affect the durability of reinforced concrete structures. This research investigated the carbonation of existing concrete building structures located in a coastal city (Shenzhen, China) under subtropical maritime monsoon climate. The relationship between carbonation depth and external influencing factors (e.g. temperature, humidity, concentration of CO₂, and surface coating) as well as internal factor (reflecting concrete characteristics e.g. compressive strength) were analyzed. In addition, a prediction model was modified from the existing empirical models with an attempt to evaluate the carbonation risk in existing concrete structures. Test results showed that the carbonation level of indoor ends was higher than that of outdoor ends due to the higher average concentration of CO₂ at indoor ends as well as presence of surface coating at the outdoor ends. The natural carbonation depth was found to have a negative correlation with the compressive strength of concrete and thickness of mortar cover. When the thickness of mortar coating was over 8 mm, no carbonation was observed in existing concrete structures having an age of up to 25 years. Finally, the predicted results from the modified model, which took into account the influence of both internal and external factors, agreed well with tested data. Hence, the modified model can be used to predict the carbonation in existing concrete structures with/without surface coatings to benefit the durability design of existing building structures.