# Non-Destructive Testing Applications for Steel Bridges
## 3 NDT Methods with Potential Application to Steel Bridges
### Test Methods Applicable to Structural Steel Cables, Tendons, and Ropes
#### 3.3.3 Chemical and Electrical Testing (CET)  
CET is typically used for the detection of corrosion, voids, and inclusions of considerable sizes in the structural element. This NDT method can be categorized into:  
Electrochemical Impedance Spectroscopy (EIS)--This is an impedance method in which a low-amplitude voltage (alternating current) is implemented to the steel under inspection over a wide range of frequencies. The changes in signal amplitude and phase shift can be measured to calculate the impedance of the concrete-steel interface. This method requires physical access to the element being inspected. The application of this method has been mostly for the external tendons. Special training and sophisticated approaches are needed for the interpretation of the data and extracting meaningful results [60]. This method is generally slow, as low frequencies are required to determine the corrosion rate. Its application has been mainly in laboratory conditions. Figure 25: Application of MsS for George Washington Bridge, New York City [42]. Electrical Capacitance Tomography (ECT)--This method was introduced in the late 1980s. ECT obtains capacitance data with the use of multi-electrode sensors and generates permittivity images of sections by several iterations (Figure 26). It is considered a fast, safe, and inexpensive NDT method. However, it has not been used widely, except in special investigations on small parts of structural elements. Its inherent problems with the image resolution are the major drawback for its application. Chemical Testing (CT)--This method is mostly used to detect corrosion in the reinforced concrete elements but can be implemented for other structural elements. In the marine environment, the measurement of chloride in the element can be a means for detecting corrosion. This method for measuring chloride employs chemical testing or the use of sensors/indicators [64]. Another method of chemical testing has been investigated in the research conducted by [PERSON] and [PERSON] in 2014. The compositions of structural members were analyzed by wavelength through an optical emission spectrometer (Figure 27). In their work, the content of the chemical composition was analyzed by the strength on-site. Accordingly, weld crack sensitivity (Pcm) and carbon equivalent (Ceq) composition were evaluated by this method. These two components can be used for the detection of corrosion in the steel element [65]. The literature cited can be referenced for more details on procedures and applications. Half-cell Potential Measurement, Galvanostatic Pulse Technique, and Potential Mapping--These methods are electrochemical techniques and have been used widely for corrosion detection in reinforced concrete elements, and, with some modifications, they can be used for stay cables and external post-tensioning tendons. The method requires a direct electrical connection to the steel element embedded in the grout/concrete and access to the face of the grout/concrete. The difference in potential between electrodes and electrolyte is utilized in a half-cell potential measurement technique. A schematic example is shown in Figure 28 [66]. Application of the half-cell potential method is inexpensive and simple to perform. Limitations exist when the grout/concrete is contaminated, leading to difficulties when obtaining quantitative measure for corrosion [67]. Figure 27: Optical emission spectrometer [65]. With a concept similar to half-cell potential measurement, the galvanostatic pulse method applies a current pulse to the steel embedded in grout/concrete, and polarizing potential is recorded after the pulse is turned off. In this way, the method attempts to eliminate the interference by concrete resistance, [68]. Using the above corrosion potential measurement methods, Corrosion Potential Mapping can be performed to obtain the distribution of corrosion in an area or length.