# Experimental Methods to Evaluate the Carbonation Degree in Concrete--State of the Art Review
## 1 Introduction  
Carbonation in concrete is a well-known reaction of CO\({}_{2}\) in the natural environment with hydrated calcium compounds, such as calcium hydroxide Ca(OH)\({}_{2}\) and calcium silicate hydrate C-S-H, leading to the creation of calcium carbonate CaCO\({}_{3}\), as the consequence of CO\({}_{2}\) attack, resulting in change in the alkalinity level of cementitious composites [1; 2]. The carbonation reaction process in concrete is performed as a series of chemical equations as follows [3]:  
\[\mathrm{CO_{2}+H_{2}O\to H_{2}CO_{3}} \tag{1}\]  
\[\mathrm{Ca(OH)_{2}+H_{2}CO_{3}\to CaCO_{3}+2H_{2}O} \tag{2}\]  
\[\mathrm{C_{3}S_{2}H_{3}+3H_{2}CO_{3}\to 3 CaCO_{3}+2 SiO_{2}+6H_{2}O} \tag{3}\]  
Nowadays, the carbonation process in the cementitious matrix has been accelerating because the amount of CO\({}_{2}\) from the atmosphere has increased significantly (from 0.278 to 0.43% during the industrial period [4]). It is an indisputable fact that the long-term durability of the cementitious matrix structure is in grave danger from this increase in the CO\({}_{2}\) concentration [5]. With the aim of evaluating the effects of different possible carbonation reactions on the characteristics of reinforced concrete and preventing the potential harmful of the carbonation process, the carbonation degree must be quantified and predicted. So, what is the carbonation degree and how is this parameter measured? In general, the carbonation degree is ruled by the level of inward diffusion of CO\({}_{2}\) molecules from the surface to the inner parts of specimen, influencing the bonding between matrix and reinforcement. In cement-based concrete, the amount of calcium hydroxide Ca(OH)\({}_{2}\) (portlandite) mainly controls pH value. The portlandite dissolution and CO\({}_{2}\) outside diffusion result in the accumulation of calcium carbonate and pH reduction. The reduction of the pH value of concrete is a result of a wide range of processes, including penetration of CO\({}_{2}\), alkalis leaching, sulfate attacks and carbonation of portlandite in the concrete pore solution. In this context, the influence of carbonation on the performance of concrete has been extensively investigated in an attempt to limit the detrimental effects of weathering carbonation on outer constructions in concrete engineering [3; 6; 7; 8; 9]. It should be noted that the level of alkalinity of cementitious matrix relates strongly to the deterioration behavior of reinforced composite structures, which is one of the most challenging issues in the field of construction material. Steel reinforced concrete often faces severe durability problems in which its structure deteriorates heavily with increase in the age. The corrosion process was initiated by carbonating the concrete cover or by admixing chloride. From the perspective of reinforcement corrosion, this process could be divided in two separate stages, corrosion initiation and corrosion propagation stages [10]. The first step is CO\({}_{2}\) penetration towards reinforcement while the corrosion process starts in the second stage. Generally, the impact of environmental conditions on the carbonated area is multifaceted, depending on different factors, such as content of CO\({}_{2}\), temperature, relative humidity, and so on. Therefore, various methods could be used simultaneously to limit their weaknesses. Additionally, the decrease of hydroxide concentration in the intervals between cement paste and embedded reinforcing steel during the carbonation process has negative effects on the protection of the coating surrounding the reinforcement. At the reduced alkalinity level, the decomposition of the passive oxide layer protecting the rebar appears, leading to high sensitivity towards corrosion. Carbonation-induced reinforcing bar corrosion of concrete has been dominated mainly by the carbonation rate [11; 12]. Embedded reinforcement could be corroded in the low pH environment of carbonated concrete.