PSI - Issue 11

Gibson R. Meira et al. / Procedia Structural Integrity 11 (2018) 122–129 Author name / Structural Integrity Procedia 00 (2018) 000–000

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3

prepared with deionized water and had pHs ranging from 12.6 to 13.5. Calcium and potassium hydroxide were chosen because they are the main products responsible for the high alkalinity of the liquid phase of the concrete, according to Cascudo (1997) and Mehta and Monteiro (2008).

Table 1. Solutions used in test cells. Solution Reagents

pH

I

Saturated calcium hydroxide (Ca(OH)2

12,6 13,1 13,2 13,5

II

Saturated calcium hydroxide (Ca(OH)2) + 0,15M of potassium hydroxide (KOH) Saturated calcium hydroxide (Ca(OH)2) + 0,20M of potassium hydroxide (KOH) Saturated calcium hydroxide (Ca(OH)2) + 0,50M of potassium hydroxide (KOH)

III IV

The cells were kept hermetically sealed in laboratory environment (UR=65 ± 10% and T=25 ± 5ºC) and constantly monitored by electrochemical measurements of the reinforcement bars and pH measurements of the solutions. 2.3. Carbonatation of test cells Before starting the carbonation process, the reinforcements were kept in the solution for about 15 days, until the stabilization of the electrochemical parameters and the passivation of the reinforcement could be verified. Afterwards, the carbonation of the solutions was then performed. For this purpose, 5% carbon dioxide (CO2) gas was introduced into the cells. This action lasted until the pH reduction of the solutions reached values around 8. After that, the electrochemical parameters of the bars were monitored to identify changes in their behaviour or maintenance of their pre-carbonation conditions. It should be noted that the carbonation procedure was only adopted in the cases was identified a previous stabilization of the electrochemical parameters of the reinforcements and thus the passivation of these bars. 2.4. Electrochemical monitoring The monitoring of the bars was carried out using the electrochemical technique of linear polarization resistance with previous measurements of the open circuit corrosion potential. This allowed to obtain the instantaneous corrosion current density. The equipment used in the measurements was a bench potentiostat, model GILL AC of ACM Instruments. For the accomplishment of the measurements, a reference electrode of Cu-CuSO4 (CSE) and a graphite counter electrode were used, keeping the entire assembly in a Faraday cage to avoid external interference. Considering the researches of Angst and Vennesland (2008) and Meira et al. (2014), in this work, the linear polarization resistance technique was used to detect armor depassivation. This technique combines an efficiente reponse to the identification of the corrosion process, starting with the agility of the measurements. The readings were performed at constant intervals throughout the testing period. Corrosion potentials more electronegative than -350 mV (CSE) and corrosion current densities higher than 0.1 μA/cm² were used as criteria to identify the moment of reinforcement depassivation, as recommended by Cascudo (1997). 3. Results 3.1. Electrochemical monitoring The TC electrochemical monitoring results are presented in this section. The variables studied were the corrosion potential and the corrosion current density. The corrosion potential used classification bands according to ASTM C- 876 (2009) and the current density used the classification proposed by Cigna et al. (1997). Figures 1 to 4 show the results of the corrosion potential readings and pH as a function of the testing time (days) for the TC with the studied solutions I, II, III and IV, respectively. Considering that only the bars immersed in