PSI - Issue 84

Gianni Moor et al. / Procedia Structural Integrity 84 (2026) 1079–1086

1081

This study focuses on inspection techniques for bonded post-tensioning by detecting these grout voids, addressing the challenge highlighted in CS 465 that critical tendon defects are often hidden and difficult to detect without special measures. That guidance suggests that the most vulnerable bridges require monitoring and periodic re-inspection, since uncertainty about tendon condition increases over time. Visual inspections alone are unlikely to reveal internal tendon problems before failure, whereas fully intrusive investigations are costly and potentially harmful to the structure unless clearly justified. The advanced NDT approach outlined here aims to bridge this gap by revealing internal tendon conditions quickly and non-destructively. 2. Bonded and unbonded post-tensioning systems Post-tensioning systems are generally categorized as either unbonded or bonded, referring to whether the prestressing tendons are bonded to the surrounding structure. In an unbonded post-tensioning system, each tendon is coated with grease and enclosed in a plastic sheathing to prevent direct contact with the concrete; this allows the tendon to move independently of the concrete. In a bonded post-tensioning system, by contrast, the tendons are placed inside metal or plastic ducts embedded in the concrete, and after the concrete has hardened and the tendons are stressed, the ducts are filled with grout. The hardened grout locks (bonds) the tendons to the surrounding structure along their length. Bonding the tendons in this manner ensures the prestressing force is distributed evenly into the structure and also provides better long-term corrosion protection for the steel. Although bonded systems involve an extra grouting step and make future tendon replacement more difficult, they are generally preferred in large-span bridges and critical structures where durability and structural integrity are paramount. Bonded tendons offer enhanced load-carrying capacity, lower long-term maintenance needs, and greater redundancy in case one or more tendons fail, thereby improving the overall safety margin of the structure. 3. Corrosion risk from grout voids in bonded post-tensioning Grouting defects or voids within a bonded post-tensioning duct can create a path for water to spread along the tendon, leading to corrosion of the steel strands – a serious risk addressed by the United States Federal Highways Administration, FHWA (2013, 2022). Figures 4 to 6 illustrate such scenarios. Identifying the locations of any grout voids is crucial so that they can be remedied before significant deterioration of the tendons occurs. In practice, voids in the grout are often found at high points along the duct’s profile, but they can occur anywhere, partly influenced by the locations of grout injection vents and drains. Therefore, it is advisable to inspect the full length of each tendon duct, regardless of the inspection technique used. The approach presented in the following sections employs advanced NDT tools to meet this goal: first by examining the span of the tendon between its anchorages, and then by inspecting the anchorage zones at the ends of the duct.

Fig. 4. Cross-sectional illustration of a post-tensioning duct at a high point: fully grouted (left) versus ungrouted with an air void (right).

Fig. 5. Schematic of a tendon duct’s longitudinal profile, indicating grout voids (defects) at various locations along the duct.