# Experimental Methods to Evaluate the Carbonation Degree in Concrete--State of the Art Review
## 2 Qualitative Methods
### Image Analysis  
The degree of carbonation of the cementitious materials is normally estimated by the measurement of carbonation depth [50]. Depth of carbonation is a significant parameter related to durability performance and microstructure stability of the composite. The distribution or repartition of carbonation depth in concrete is generally variable accompanied by the distinctive minimum and maximum depth along the border of carbonated areas under both weathering and accelerated carbonation conditions due to the effects of coarse aggregates [51]. Characterization of carbonation depth is carried out by means of image analysis after detecting carbonation areas by using pH indicators. [PERSON] et al. [25] investigated the correlation between depth and degree of carbonation in cementitious composite over periods of six- and eight-week accelerated carbonation. They pinpointed that at the depth of colorless phenolphthalein, the carbonation degree is always more than 75%, while this value is almost zero when phenolphthalein turns red. The biggest advantage of the image analysis technique is that it is uncomplicated to determine the carbonation front width, and it is relatively economical and time-saving. The use of this technique is, hence, highly recommended [52; 53]. [PERSON] et al. [15] employed a color threshold principle to measure carbonation depth distribution in mortar without and with reinforcement in different exposure periods. The observation from the carbonation detection images of samples tested, as shown in Figure 1, illustrated that corrosion has only occurred once the partially carbonated zone appears in the reinforcement-cementitious matrix interface. In addition, the increase in corrosion of the reinforcement has been observed due to the deeper carbonation depth in the sample. This is in accordance with the conclusions of [PERSON] et al. [14], although the corrosion propagation process differs due to the difference in the CO\({}_{2}\) penetration inside the cementitious matrix. Another method has been applied in [54]. In this method, the carbonation area was determined by means of ImageJ software, corresponding to the average width of progressive carbonation in the middle of the specimen. After the carbonated area is confirmed in the cylindrical specimen, carbonation depth is calculated with respect to the radius of the specimen and carbonated area. This simple method is also affected by the difficulty of differentiating between partial and non-carbonated zones. Otherwise, [PERSON] et al. [55] compared the change in microstructure of sample using the observation of SEM photomicrographs before and after carbonation curing. It was observed that the microstructure becomes denser, as shown in Figure 2, due to the creation of high crystalline CaCO\({}_{3}\) and silica gel that are capable of filling the gaps of the spatial structure made by the hydrated cement products when increasing the solid volume fraction [56; 57; 58; 59], even if the calcium carbonate product is not easy to be detected in the images. More details can be found in [35; 60]. It was indicated that denser structures of hydrates and high porosity of C-S-H have been observed when CO\({}_{2}\) diffuses and converts into calcium carbonate in the position of the carbonation front. To compare the change in microstructure due to accelerated carbonation curing, a SEM image has been shown in Figure 3, indicating a border of the carbonated specimen. Due to the high pore rate in calcium silicate hydrate C-S-H structure, carbon dioxide could easily diffuse deeply into the microstructure, leading to the formation of calcium carbonate, which induces the formation of two different areas in the specimen: the outer layer is noted as a carbonated zone and the inner layer is noted as non-carbonated. Based on the edge of the two zones, carbonation depth is estimated quantitatively.