In Situ Monitoring Of Chloride Ion Concentration In Concrete

Chloride induced corrosion of reinforced steel is one of the most important factors influencing durability deterioration of concrete structures. The timely and accurate measuring choride ion distribution in concrete structure can supply an important sicientific evidence to correct assessment and prediction of concrete durability and rehabilitation of damaged structures. In this paper, one type of electrochemistry-based all-solid-state chloride sensor was developed using a Ag/AgCl electrode as the working electrode, a MnO2 electrode as reference electrode through one special preparation and surface protection treatment. And it can be used for in situ monitoring of chloride ion concentration in concrete.The paper presented the potential response, stability and reproducibility of the chloride sensor in different chloride concentraiotn and chemical composition solutions, and influences of other ions, solution temperature and external disturbance current on the working stability of the sensor were also tested. The results show that there is a good linear relationship between the potential output value of the sensor and the logarithm of chloride ion concentration (0.05mol/L~ 5.0mol/L) in high-alkaline synthetic concrete pore solutions, and the potential reading maintains very stable with the increasing time. The influence of K+, Ca2 +, Na+ and SO42- on the potential reading of the sensor is less than 2.4 mV. Meanwhile, no visible change was detected in the potential of the sensor when the temperature changed from 15 to 40℃. The sensor has a high resistance to polarization under the interference current of 10 ?A. The maximum potential drift due to galvanostatic polarization is among±4.5 mV in 0.1mol/L chloride concentration solution and among±1.7 mV in 1.0 mol/L solution. During potentiodynamic polarization measurement, a good exponential relationship exists between the current value and the induced potential. CV curve shows a good cycling symmetry, reversibility performance.Designing the concrets with different water-cement ratios and mineral admix- tures and the cement with the same conditions. Test the potential value of sensor embedded in concrete specimen and concentration of free chloride ions in cement paste. The relationship between the potential reading of the sensor and the detected chloride ion concentration was analyzed. The results showed that: for three different water-cement ratio concretes of 0.32, 0.4 and 0.5, the sensor showed the minimum reading at 8, 6 and 4 days respectively; the development tendency of potential reading with age is similar for all samples; and the potential reading of the sensor embedded in concrete increases with the higher water-cement ratio. With the same water-cement ratio, the higher addition of fly ash in concrete leads to a lower potential reading of the sensor. The higher addition of chloride into concrete leads to a lower potential reading of the sensor when the concrete mixing proportion keeps the same. After 60 days immersion of single surface concrete to chloride solution, the sensors show similar potential readings at the depth of 1cm and 2cm. Except for concrete specimens with the addition of 6% NaCl ( the chloride concentration in pore solution is higher than 10 mol/L), a very good linear relationship exists between the potential reading of the sensor embedded in concrete and the free chloride ion concentration in cement paste under the same curing condition at all the test ages. Therefore, the developed chloride sensor is basically proved to be applicable for in situ monitoring in concrete structures with different mixing proportions.