As in many other areas also the construction industry requests more and more methods of online-monitoring of corrosion related values. Monitoring the reinforcement corrosion, which is the main cause for damages or early rehabilitation measures, promises a subjectively higher safety feeling to unexpected lack of usefulness or sudden need for rehabilitation. In the paper a survey of methods and sensors for corrosion monitoring in reinforced concrete structures are presented.
Basing on extensive tests as well as in the laboratory as on-site performed in many research projects during the last 10 years different devices and methodologies are described regarding their special features and application areas. Hereby techniques using portable equipments and permanently installed systems are shown. The permanent systems are divided into systems installed during construction of new structures and post-mounted devices for installation in existing structures.
Within a majority of structures the static-construction safety is provided by steel reinforcement. The alkaline pore water usually protects the steel from corrosion. In case of unfavorable environmental conditions (carbonation, chloride ingress) the passive layer on the steel surface might be damaged. First the resulting corrosion products are being consumed by the pore structure of the concrete without causing visible changes on the concrete surface. So these damages cannot be discovered by the usual visual structure inspections. They will be noticed only if corrosion caused secondary damages on the concrete surface occur, like discolorations, cracks, delaminations.
Early and nearly non-destructively determined information regarding the current corrosion behavior is very important in order to initiate the rehabilitation measures required in terms of safety aspects as well as economic considerations. Therefore methods and procedures for permanent and regular corrosion surveillance of reinforced steel structures continuously gain greater attention to research and development as well as to on-site use. Electrochemical measurement procedures like measurements of corrosion potentials, determination of short-circuit currents on corrosion cells and electrochemical investigations with external control offer the right tools for that. So the measured corrosion potentials can be interpreted in that way that more negative potentials point to a higher probability of corrosion processes. In passive stage the corrosion potential of reinforcement can fluctuate in a very large area, depending on different parameters. Therefore a clear classification of active (corroding) or passive conditions is not always possible. In such cases investigations using a galvanostatic external power might be useful. Here a short DC impulse is applied to the reinforcing steel using a counter electrode. Beside measurements of parameters for evaluation of the corrosion behavior the determination of essential influencing factors like moisture content, chloride concentration and pH-value are very important in order to evaluate the real corrosion risk. Also the concrete resistance plays an important role, because it combines a lot of influencing values like temperature, moisture, salt content and concrete composition. For the influencing parameters mentioned above sensor systems are already available or are being evaluated. In order to achieve a quite systematic of described measures and methods it is distinguished between monitoring of structures with and without permanently installed sensors.
