Detection of Chlorides in Pore Water of Cement Based Materials by Potentiometric Sensors

摘要

Free chloride ions dissolved in pore water of cement based materials initiate the corrosion of steel rebars used for reinforcement of concrete and thus pose serious constant danger to the safety of reinforced concrete structures. The in-situ monitoring of the content of chlorides in pore water with second order Ag/AgCl electrodes is an elegant approach which, however, suffers from poor stability of Ag/AgCl electrodes in highly alkaline environment like pore water. In this work the electrochemical stability of plain and stabilized electrodeposited Ag/AgCl sensors in extreme alkaline conditions are presented. The electrochemical processes were elucidated which govern the potentiometric response of Ag/AgCl sensors and determine their performance. The results have shown suitable stability and reproducibility of stabilized Ag/AgCl sensors over five months, which was sufficient for laboratory testing of chloride ingress into cement based materials. The embedded calibrated sensor elements were used to monitor the chloride uptake into cylindrical mortar specimen dipped into simulated pore water containing about 3 wt% of chloride ions. The velocity of diffusing front and the diffusion coefficient of chloride ions in pore water of cylindrical mortar sample were estimated from concentration-time profiles determined by chloride sensors positioned at defined positions in the sample wall.