## 3 Reinforcement Corrosion Monitoring Techniques
### Future and Research Challenges of Sensors for Corrosion Monitoring of RCS  
The development of monitoring systems for new and existing RCS allows a rational approach to the assessment of repair options and schedule of inspection and maintenance programs. In the last few decades, different types of electrochemical and optical systems for corrosion monitoring have been developed. The high interest in optical sensors has led to the publication of several review articles in recent years [61; 224; 225; 226; 227; 228; 229]. Fiber-optical sensors (FOS) are small, lightweight, robust, corrosion resistant, immune to electro-magnetic interference, highly sensitive and can be tuned to detect different signals. They are suitable for use in severe environments such as RCS. They allow a continuous and permanent observation of the corrosion conditions of RCS and can be used as early warning systems in critical situations. The massive development of electronics, will undoubtedly, lead to the development of powerful and low-cost data acquisition systems, data transfer, storage and assessment [96].  
The development of integrated monitoring systems (optical and electrochemical systems) for new and existing RCS is an area that has expanded in the last few years. Several sensors based on electrical and optical fiber technology [49; 224; 225; 230] have been reported and have demonstrated that they can monitor several physical and chemical properties of concrete directly related to RCS durability. Each system shows advantages and disadvantages. Therefore, the combination of both methods will ensure a proper maintenance strategy with decreased repair and rehabilitation costs of new and existing RCS.  
Effective systems to monitor the durability of RCS require sensors that can be embedded in the concrete. Desirable properties are (i) robustness; (ii) good design and location (cover zone of concrete), (iii) high reproducibility and sensitivity; (iv) low cost, (v) environmentally friendly and (vi) easy to store data. Currently, most research focuses on the development of advanced sensing technology, and sensor and durability monitoring. Numerous smart sensors, such as FOS [231; 232; 2334], cement-based strain sensors [235; 236] and nanomaterial-based sensors [237] have been implemented in several structures. Multifunctional sensors have also been developed such as investigation of the pH, moisture or relative humidity, chloride ion concentration and the corrosion rate of the steel rebar [238; 239]. However, most of them show poor and inconsistent responses [240; 241], drifting of the signal due to dye leaching [238], reduced lifetime [241; 242] or unsuitability to operate at a pH > 12 [242; 241; 242]. Additionally, embedding several conducting wires for corrosion monitoring of RCS is very laborious and expensive, therefore, in the future it should be minimized or avoided.  
Cutting-edge nanotechnology/microelectromechanical systems (MEMS) represent another innovative solution for monitoring RCS. In 2008, [PERSON] et al. explored, for the first time, the feasibility of using MEMS devices as embedded sensors to monitor concrete temperature and internal humidity [108]. MEMS devices are manufactured using identical materials and processes as microelectronic devices such as memory chips and microprocessors. These methods allow the obtaining of small and low-cost devices that can be distributed, in number (from hundreds to millions) throughout a RCS. The common manufacturing process also allows the integration of mechanical actuators with electronics for control, processing and communication. MEMS technology has the potential to measure the properties of interest directly ensuring a good coverage of a structure. The MEMS size and encapsulated nature avoids damage while wireless technologies make them easy to interrogate. MEMS-based systems have integrated technologies that allow structural health monitoring and condition assessment of RCS [108; 243].