Development and assessment of Nile blue-immobilized pH sensor to monitor the early stage of concrete carbonation

Concrete carbonation causes the performance degradation of structures. Thus, timely maintenance by monitoring the carbonation is required to guarantee the safety and serviceability of structures. Destructive tests are common methods to observe carbonation. However, they are cumbersome and time-consuming, and also introduce cracks accelerating deterioration. To overcome such limitations, research on non-destructive methods using optical sensors or electrodes has been conducted. In this study, an optical sensor that can monitor the early stage of concrete carbonation was proposed. The sensor was made by covalent immobilization between Nile blue and cellulose, and its reaction process was relatively simple and short. The sensor showed color changes in the pH range from 10 to 13.4, which covers the pH range of uncarbonated and semi-carbonated concrete. In addition, it exhibited fast responsiveness within 10 s and reversible responses. An accelerated carbonation test was performed to verify the applicability of the sensor system, and the system was able to monitor the early stage of concrete carbonation.