Investigation of Carbonation Profiles of a 60 Years Old Carbonated Concrete Specimens Subjected to Re-Alkalization

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Investigation of carbonation profiles of a 60 years old carbonated concrete specimens subjected to re-alkalization M.O. Mohsen1,*
1 College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar Re-alkalization method is an electrochemical treatment which is commonly used to repair carbonated concrete elements. This method prevents further reinforcement corrosion by increasing the PH of the element which results in maintaining the reinforcement passivation. In this paper, the re-alkalization sacrificial anodes method is applied to 60 years old carbonated concrete specimens extracted from a demolished building in the state of Qatar which. The carbonation profiles in the specimens are studied by Thermogravimetric (TGA), phenolphthalein and Gammadensimetry analysis before and after re-alkalization. The thermogravimetric analysis (TGA) was supplemented with chemical analysis (CA) to proportion the sample cement content, the sand content and to correct the TGA results. These measurement methods provided a qualitative as well as a quantitative view of the carbonated portions. The results illustrated a significant change in the structure of the carbonation profiles after the re-alkalization process. However, repassivation of the steel rebars could not be demonstrated. Keywords: Re-alkalization, Carbonation, pH

INTRODUCTION

Steel bars in reinforced concrete are protected from corrosion by the highly alkaline environment inside the concrete (pH≈13). If the pH decreases in the environment of the reinforcement, this protection ceases and the steel bars can corrode. Consequently, the rusting of the reinforcement usually leads to a severe reduction of the durability of the structure. One of the main processes facilitating the destruction of the protection by alkalinity is carbonation, i.e. the reaction of alkaline constituents of the concrete with atmospheric carbon dioxide to produce carbonates. The main carbonation reaction may be described by CO2 + Ca(OH)2 → CaCO3 + H2O (1)

CO2 participates in concrete carbonation basically through two processes: 1) the diffusion of CO2 in the concrete’s pores containing air; and 2) the

* Corresponding author. Tel.: 0097455829769. E-mail: 200202128@qu.edu.qa. reaction of CO2 with the alkalis that are dissolved in the pores’ water. Consequently, the carbonation of concrete does not take place, under natural conditions, to a noteworthy degree, at: 1) conditions of very low relative humidity values in the concrete pores, because the availability of pore water is necessary for the dissolution of alkalis hydroxides; and 2) at conditions of extremely high relative humidity values in the concrete pores, because the availability of air in concrete pores is essential for the diffusion of CO2 in concrete. Past studies have reported that significant carbonation reactions occur under a relative humidity between 40 and 70% which is more likely the case in the gulf region. Several studies for protection and rehabilitation of carbonated reinforced concrete elements are available nowadays. The most common one is the patch technique. However, this method has a disadvantage which occurs when a patch of a very high pH, is placed side by side with a carbonated area. This leads to an anode effect that can induce Fig. 1 Re-alkalization treatment

the corrosion of the rebars covered with the carbonated concrete [1]. Another technique that is used to repair carbonated concrete is the electrochemical re-alkalization treatment. These techniques consist in using the concrete rebars as a cathode, to temporarily apply an anode on the concrete surface (Fig. 1), and to generate a current between both electrodes. Electrical conductivity is assessed by an alkaline electrolyte contained in a cellulose pad. Current can be supplied by an external electrical power source (impressed current (IC) process) or by using specific anodes composed of a metal less noble than the rebars...
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