Detection of internal tendon breaks by fiber-optical measurements – influence of the re-anchoring behavior

• Published in: Procedia Structural Integrity 2024, Vol.64, p.1287-1294
• Main Authors: Paul, Aeneas, Sanio, David, Mark, Peter
• Format: Article
• Language: English
• Subjects: bond stress; DFOS; Fiber-optics; monitoring; prestressed concrete; re-anchoring; tendon break
• ISSN: 2452-3216; 2452-3216
• DOI: 10.1016/j.prostr.2024.09.199

Unannounced tendon breaks belong to the most dreaded failure mechanisms in prestressed concrete bridges. Failure of single wires or tendons does not necessarily cause concrete cracking. Hence, such damage cannot always be detected by visual inspection. In the case of tendons bonded in concrete, re-anchoring after breaking induces characteristic local strain fields: tensile strains occur between the separated extremities, while compressive strains develop around the tendons, until the initial prestressing strain is reached again. On the more distant concrete surface, these strains are rather small and depend on multiple factors, e.g., the depth of the tendon, the transfer length or the concrete stiffness. Nevertheless, such strain fields can be detected by fiber-optical sensors attached to the surface in two-dimensional grids. By evaluating the backscatter of emitted light, minimal strain changes (uncertainty: ±1 µm/m) can be recorded in quasi-continuous resolution (0.65 mm pitch). An experimental investigation of artificial tendon failure in two prestressed concrete beams is presented. The tendons were mechanically cut to simulate failure, while strains were measured on the concrete faces. The failure can be localized within a few centimeters by monitoring. The bond to the tendon mainly depends on the concrete strength. To investigate its effect on the surface strain a normal strength concrete is compared to a high-performance concrete. Better bond conditions result in lower expansion of the strain field and smaller strains in general. The results show potential to detect the position and quantity of broken tendons, solely from the strain signal on the concrete surface.