Degradation of concrete structures due to corrosion of embedded steel reinforcement is an important problem with regards to durability and safety, with great economical consequences. One method for corrosion prevention is the application of corrosion inhibitors. Corrosion inhibitors can be applied as additives to new concrete during the batching process, or surface applied to existing concrete structures. This paper gives an overview of existing sensor technologies, previews new technologies, and proposes suitable methods for corrosion monitoring and inhibitor efficiency investigation on real structures. Many different sensor technologies have been developed, and new ones are under research. Advantages and disadvantages of different corrosion determination approaches are also discussed. Half-cell reference electrodes, linear polarization sensors, localized electrochemical impedance spectroscopy sensors and macrocell current sensors are based on electrochemical principles. Non-electrochemical sensors work on physical principles such as the Hall effect, magnetic flux leakage, magnetostrictive effect, eddy currents and on light modulation (Fiber optic sensors). Fiber optic sensors have many advantages such as immunity to chemical environments, long-term stability, and the ability to make distributed measurements of several parameters with a single sensor. However, they are underdeveloped in the field of concrete reinforcement corrosion. Fiber optic sensors can measure corrosion directly, or indirectly by measuring factors that influence the corrosion process (pH value, CI concentration, water content in concrete, CO2), and can also detect cracks due to corrosion. INTRODUCTION
Corrosion of steel reinforcement is the main cause of deterioration in reinforced concrete, adding significant costs to the repair of structures worldwide. Application of corrosion inhibitors may be the best solution when concrete is exposed to chlorides from the environment, or when concrete is prepared with chloride contaminated water or aggregates. The purpose of this paper is to give an overview of existing as well as potential new sensor technologies, suitable for monitoring corrosion and corrosion inhibitor efficiency. Based on working principles, corrosion monitoring techniques can be classified as electrochemical or non-electrochemical, and having direct or indirect methods. Direct methods measure corrosion processes (corrosion potential, macrocell current, magnetic induction), while indirect methods monitor consequences or parameters that stimulate corrosion (cracking, delamination, C1- ion content, pH value of pore water). Sensor technologies for corrosion measurement are presented in Table 1.
ELECTROCHEMICAL SENSOR TECHNIQUES
Embedded Reference Electrodes
The principle involved in this method is appearance of an electrical potential between reinforcing steel and a reference electrode. The half-cell consists of a metal rod immersed in a solution of its own ions. Different reference electrodes are commercially available. Pseudoreference mixed metal oxide electrodes developed by Cescor consists of mixed metal activated titanium rod cast in porous cement backfill (Figure 1). A new probe for taking true potential measurements when stray currents are present was patented (Figure 2) [1]. The ERE 20-Embbedable reference electrode developed by the FORCE institute uses a manganese dioxide electrode in a steel housing surrounded by an alkaline gel [2] (Figure 3). The advantages of these sensors are long-term stability and greater sensitivity compared with classical potential mapping methods. Austrian Engineer Biro Wietek manufactures a sensor in the form of wire wrapped around a steel bar to be monitored (Figure 4).
