# Risk-Based Selection of Inspection Method for External Post-Tensioning System of Bridges
## 2 Literature Review
### Defining and Classifying Applicable Inspection Methods
#### 2.2.2 Mechanical Waves and Vibration Methods  
Inspection methods based on mechanical waves and vibration have the affiliate mechanical source and taking advantage of the disturbance and the fact that the wave propagation velocity varies in different perimeters [42]. Sounding, acoustic emission, impulse response, impact echo, vibration response, and ultrasonic are NDE methods that can be classified under mechanical waves and vibration methods.  
Sounding inspections are carried out on PT bridges on a regular basis by tapping an impactor (Figure 4). Although sounding inspections need professional inspection experts, they are simple to do in the field and are a quick way to find voids in ducts. The sounding inspection method has trouble detecting microscopic voids, but it could detect relatively moderate and big voids. Voids indicate the possibility of corrosion. As a result, the sounding method can be an efficient tool for locating voids in the field [43]. Acoustic emission is based on recording (in situ) damaging events from a structure emanating mechanical waves through the medium (called acoustic emissions). Data acquisition systems record events picked up by sensors connected or installed in the structure when damage happens, whether it's concrete cracking or strands/wire anchored in tendons. The acoustic emission method is a passive NDE methodology which allows a structure to be tested continuously rather than at regular intervals while it is in use [44]. The impulse response approach includes creating flexural vibrations in a structural component by delivering a low-frequency impact and using load cells and attached transducers to measure both the force of impact and the structural reaction (accelerometers, geophones, etc.). In contrast, vibration response is a technique in which a dead-blow hammer is used to strike an external tendon. Accelerometers at fixed places record the vibrational patterns. The tension forces in the tendon then would be estimated, modeling the response as a dynamic system [45]. The impact echo method generates stress pulses on post-tensioning elements by impacting it mechanically. The impact echo approach is particularly promising for discovering defects in post-tensioned concrete structures since the impact generates a high energy pulse and can penetrate the structure [46].  
Pulse-Echo, through-transmission, linear array, and ultrasonic guided waves have been classified under ultrasonics methods. Pulse echo uses a single device to send and receive sound waves to detect cracks, voids, and other defects [47]; however, through-transmission uses a pair of ultrasonic probes instead of a single sensor. One probe sends the sound waves, and another probe receives the waves on an opposite surface [47]. The linear array method uses several sensors in the shape of a matrix to monitor the condition of the structure [16]. Guided Wave Ultrasonics is a nondestructive testing technology that uses sound waves to detect corrosion or other damage along pipe walls. A ring of transducers is placed around a pipe to conduct this procedure. These transducers produce sound waves that flow in both directions down the tendon. If they come into touch with rust or damage, they will reflect back to the transducers, which will immediately capture the data [48; 49].