# Experimental Methods to Evaluate the Carbonation Degree in Concrete--State of the Art Review
## 2 Qualitative Methods
### Estimation of the Carbonation Degree Based on pH Indicator  
Determination of the carbonation degree based on pH indicator is considered as a simple and effective method. The main effect of carbonation is to diminish in the pore solution's pH values, which causes the destruction of the thin passive film of iron oxides or hydroxide on the facade of the steel bars in concrete. The evaluation the change in pH level of concrete is, hence, a simple approach to determine the degree of carbonation. Among qualitative methods, the suitable indicator was used as the conventional and most popular method in the assessment of pH value. The utilization of portlandite during the carbonation reaction leads to a decrease in pH level, resulting in the formation of different zones in concrete, such as completed, partial and non-carbonation areas. The most popularly used indicator to observe the depth of carbonation, for many years, has been phenolphthalein [20; 21], followed by thymolphthalein, alizarin yellow R, aqueous ethanol and tropaeolin O. The clean freshly broken surface specimen is sprayed by indicator solution. A solution consisting of 1% phenolphthalein in 70% ethyl alcohol is highly recommended, according to RILEM recommendation [22]. Previous studies have given different opinions on the specific correlation between the degree of carbonation and pH value, depending on the production of the specimen and existing indicator. In fresh concrete, the pH value is normally more than 12; in carbonated concrete, however, this value could decrease to 7, depending on the carbonation degree. The change of pH value reflects the carbonation degree according to indicators, as shown in Table 1.  
As can be seen in Table 1, most studies used phenolphthalein as the main indicator to control the boundary causing the carbonation front in which concrete is carbonated, resulting in the evaluation of carbonation rate and degree [29; 30; 31]. Phenolphthalein is chosen as the most popular indicator because it can change color in a pH range between 8.3-10 and is more accessible in comparison with other indicators. It will appear pink in non-carbonated concrete and clear in carbonated. Additionally, the intensity of the color on the surface of the sample, which can be seen with the naked eyes, also reflects the different levels of carbonation. The increase in the intensity of the hue of reddish-purple illustrates the decrease in the carbonation degree [27]. Besides, based on the color reaction, [PERSON] et al. [32; 33] took advantage of the value of boundary length of the non-carbonated zone in the middle sample to numerically model the irregularity of carbonation depth. This potential method could help in the numerically calibrated results of several statistical parameters required by a numerical carbonation model through comparison with that predicted by experimental data results. On the other hand, the main obstacle in using phenolphthalein as an indicator is the difficulty in detecting the partial carbonated area where the pH is more than 10, according to [PERSON] et al. [34]. Therefore, they proposed two new reactants, which show color change at the higher pH scale, to prevent the disadvantages \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Indicator** & **pH Value Range** & **Carbonation Degree** & **Ref.** \\ \hline Phenolphthalein & 8.3–9.5 & & \\ \hline Thymolphthalein & 9.3–10.5 & & \\ \hline Alizarin yellow R & 10.2–12.2 & & \\ \hline Tropeaolin O & 11.1–12.7 & & \\ \hline \multirow{2}{*}{Phenolphthalein} & \textless{}10.5 & & \\ \cline{2-4}  & 10.5–11.5 & Partial carbonation & \\ \hline \multirow{4}{*}{Phenolphthalein} & \textless{}7.5 & 100\% & \\ \cline{2-4}  & 7.5 \textless{} pH \textless{} 9 & 50–100\% & \\ \cline{2-4}  & 9 \textless{} pH \textless{} 11.5 & 0–50\% & \\ \cline{2-4}  & \textgreater{}11.5 & 0\% & \\ \hline Thymolphthalein & 9–10.5 & & \\ \hline Phenolphthalein & 8.2–9.8 & & \\ \hline Phenolphthalein & \textless{}7 & & \\ \hline Phenolphthalein & \textless{}8.2 & & \\ \hline \hline \end{tabular}
\end{table}