# 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

Table 1: Summary of the experimental investigations on different pH scale range and carbonation degree depending on type of indicator. of phenolphthalein: (i) _alizarine yellow R_, which consists of 0.5% of this solution in acetone and ethanol mixture and shifts from yellow to deep reddish-violet at pH interval between 10.1 and 12.0, and (ii) _indigo carmine_, which consists of 0.5 % of this solution in acetone and water mixture and transfers from blue to yellow at a pH interval of 10.0 to 11.4. It was believed that these new indicators could give higher accuracy and dependability in alkalinity measurement of the partial carbonated zone, since they could detect carbonated hazards which the traditional indicator could not distinguish, thanks to a higher degree of reliability. Generally, using pH indicators leads to several drawbacks. The boundary of fully carbonated zones could be easy to detect while that of partial and non-carbonated zones could hardly be observed by the visual observation of the color change [16; 24]. Furthermore, this technique addresses the change in pH value instead of that in the amount of real CO\({}_{2}\) uptake during the process of carbonation. In fact, attention only to the alterations in the pore solution pH of the carbonated concrete to detect carbonation is also considered as a limitation of this conventional method [30]. Therefore, using pH indicators, associated with other approaches, to reduce restriction is necessary. In order to observe the change in the surface of the specimen, it is necessary to divide the surface into four concentric zones identified from the outer to inner zones of the sample [35; 36], following the decrease in carbonation degree: (1) the carbonated zone in which calcium carbonate is dominant, (2) the carbonation front could be a low-porosity tenuous layer due to the complete precipitation of CaCO\({}_{3}\), (3) the dissolution reaction front is a region of relatively high porosity owing to the fact that the cement hydration products constantly dissolve to create calcium carbonate, and (4) the non-carbonated zone is unmodified cement and not totally affected by carbonation curing, resulting in its retaining of initial hydration products. It is clear that the use of pH indicators could not detect these different zones. Therefore, the result from reagent solutions is always less credible than the result obtained from other methods [20; 37]. For example, [PERSON] et al. [3] pinpointed that carbonation depth observed from the phenolphthalein indicator is three time less than that acquired from the TGA technique in the reinforced mortar specimens. Nevertheless, it should be noted that this conventional method is still effectively used to determine the changes in the alkalinity of the sample. Another method of pH measurement for concrete is based on the ASTM D4972 standard. In this technique, a solution with a ratio of 1:1 of air-dried powder sample and distilled water is employed to determine pH value using a pH meter. However, this technique is rarely applied in the evaluation of the carbonation degree due to the negligible mass of sample and the time-consuming process needed for mixing. It is more suitable for investigating the alkalinity level of samples under different aggressive environments [38; 39; 40]. To limit the inconvenience of the weak accuracy of the pH indicator method, fiber optic systems were introduced, developed and installed for pH monitoring of reinforced concrete due to the carbonation of on-site concrete. Most recently, to control the corrosion of the internal chemistry of reinforcement structures under different conditions, a wide range of new fiber optic sensors has been proposed. These minimized the disadvantages of previous methods and demonstrated many advantages with extremely high accuracy monitoring, repeatability, stability, recovery test, and non-destructive samples. Several kinds of sensors were investigated, including temperature, chemical, pressure and strain sensors.