## 2 Background
### Non-Destructive Corrosion Monitoring  
To monitor corrosion in RC structures, there are various NDT techniques, which may be classified into six categories as indicated by [PERSON] [22] and they are as follows: (1) visual inspection; (2) electrochemical methods (i.e., open circuit potential method, resistivity method, polarization resistance, galvanostatic pulse method (GPM), electrochemical noise (EN)); (3) elastic wave methods (i.e., ultrasonic pulse velocity, (UPV), acoustic emission, (AE), and impact echo (IE)); (4) electromagnetic methods (i.e., ground penetrating radar (GPR)); (5) optical sensing methods (i.e., fiber Bragg grating (FBG)); and (6) infrared thermography (IRT).  
Visual inspection is a commonly used inspection technique that can be used to monitor the corrosion damage on the surface of RC structures. Nevertheless, this method requires highly experienced staff, and it is not effective in detecting unseen corrosion [22]. Thus, other methods, such as electrochemical methods (i.e., HCP) and elastic wave methods (i.e., ultrasonic testing and AE), are more suitable in detecting corrosion at the rebar level below the surface.  
Electrochemical methods, such as HCP, are known to be the most suitable technique for corrosion monitoring. HCP is capable of detecting corrosion regardless of the depth of concrete cover, the rebar size and detailing, and, in addition to indicating the corrosion activity at the most external layers of reinforcement, it can indicate the corrosion in greater depth [23]. However, this method has some limitations, which will be discussed later.  
Ultrasonic testing (UT) is an evaluation method consisting of a transmitter and receiver circuit, a transducer, and a display device [24]. The UT measurements can be used to locate the cracks and determine the flaw size. This evaluation system is known for its speed of scanning and good resolution capabilities.  
AE is another NDT method that has been extensively used for corrosion monitoring in R structures. [PERSON] and [PERSON] [25] compared AE with HCP for non-destructive testing of corrosion in RC slabs and analyzed the concentration of chloride ions around the rebar. It was demonstrated that AE monitoring can detect the onset of corrosion activity earlier than HCP. With investigation of the ingress of chloride ions, a correlation between AE activity and chloride concentration was observed. Once the chloride contents surpass 0.3 kg/m\({}^{3}\), a high AE activity period is observed, which marks the first stage of the deterioration process caused by NaCl attack. Another stage of high AE activity can be observed once the chloride content becomes higher than 1.2 kg/m\({}^{3}\), which corresponds to the second stage of the deterioration process. [PERSON] et al. [26] used AE to detect corrosion activity in RC structures at an early stage, where a significant increase in cumulative AE hits can be an indication of the onset of corrosion. Thus, AE can be used as a real-time corrosion monitoring technique that can detect corrosion activity at an early stage.  
In this research, UGWL and HCP have been used as two NDT methods for continuous monitoring of corrosion in RC structures. These two methods are further explained in the next section.