## 5 Prestress Loss Tests
### CWI Analysis Results
#### 5.3.2 Wave Velocity Variations  
Figure 20 compares wave velocity variations related to bridge temperature changes between tests #1 to #6 and tests #7 to #11. A reduction in velocities in both vibration and ultrasonic wave fields was observed as the post-tensioning force decreased. The velocity changes varied more significantly among different ultrasonic sensor pairs than across different vibration Station pairs in both the south and north spans (comparing the same marker but different colors in Figure 20a,b). Notably, during tests #11-#7, the difference between the south and north spans (\(-2\%\) and \(-4.5\%\)) at a post-tensioning force of 250 kN in the ultrasonic wave field in Figure 20b was as large as 2.5%, while in the vibration wave field in Figure 20a, the difference between the south and north spans (\(-7.5\%\) and \(-8\%\)) at a post-tensioning force of 250 kN was less than 1%. Similarly, during tests #1-#6, the difference between the south and north spans (\(-2.7\%\) and \(-4\%\)) at a post-tensioning force of 200 kN in the ultrasonic wave field in Figure 20b was up to 1.3%, while in the vibration wave field in Figure 20a, it was less than 0.5%. Nevertheless, this indicates that while the vibration recordings reveal a global vibration behavior, they can also highlight local phenomena on different spans, similar to ultrasonic recordings through the CWI analysis. Figure 19: Max. _CC_ values in the vibration recordings (Station pairs 1–2, 3–4, and 1–4) and ultrasonic wave fields (A–B and D–E). In addition, the influence of bridge temperature is observed when comparing different tests conducted at the same post-tensioning force (comparing the same color but different markers in Figure 20a,b). The temperature effect on ultrasonic wave velocity variations is larger at the north span than at the south span, based on the difference between two velocity variations in two tests with the same post-tensioning force. The temperature effect on vibration wave velocity variations is also larger at the north span than at the south span, based on the difference between two velocity variations in two tests with the same post-tensioning force. These consistent results indicate that both scales of wave-screening methods are reliable and can be applied in the same cases. However, a significant decline in vibration velocity is noted in Figure 20a when the bridge is prestressed below 300 kN. This phenomenon indicates damage states such as prestress loss and subsequent crack opening, consistent with previous studies [43,44]. Additionally, this observation correlates with the drops in the second and third modal frequencies in the modal analysis, aligning with previous findings (e.g., [45]). Therefore, further research is encouraged to focus on how higher modes are affected by changes in prestress. In contrast, the gradients of the ultrasonic velocity changes observed in Figure 20b between different sensor pairs remain relatively consistent at each step of prestress loss. Furthermore, the negative velocity variation in the north mid-span was markedly higher than in the south mid-span under identical bridge temperature conditions. Based on formulations that establish the relationship between the modulus of elasticity (E) of concrete and ultrasonic pulse velocity [46], where a lower E modulus corresponds to a lower wave velocity, it can be concluded that the north mid-span has sustained greater damage than the south mid-span due to its considerable velocity reduction. Figure 20: Velocity variations during prestress loss monitoring: (**a**) vibration wave fields; (**b**) ultrasonic wave fields. In the short-term measurement, changes in wave velocity were found to be an effective method for detecting overall structural integrity changes resulting from the post-tensioning force. The temperature effects were revealed by local properties, and a direct relationship between the decreased velocities and the reduced prestress was evident.