Detection of internal tendon breaks by fiber-optical measurements at concrete surfaces

Undetected internal tendon breaks pose a risk for prestressed concrete structures. If announcement by cracks on time cannot be proven in calculation, demolition and replacement are often the consequence. This paper presents an experimental method for detecting and localizing such damage using fiber-optical measurement technology. Due to a loss of prestress and the re-anchoring of the tendon in the surrounding concrete, characteristic changes occur in the strain field on the concrete surface. These are detected using a two-dimensional grid of fiber-optical sensors. In experimental tests on prestressed concrete beams, tendon breaks were generated and the resulting strain fields measured on the concrete surface. The re-anchoring in the concrete was evaluated using sensors applied to the tendons. Relevant influencing variables on the measurement signal were investigated by varying the concrete strength, the depth of the tendons and the prestressing force. In addition, the influence of bending cracks on the measurement signal was evaluated. It has been shown that tendon breaks are clearly detected and localized up to depths of 25 cm from the surface. The strain changes along the tendon axis are particularly suitable as a measurement parameter. The strain changes orthogonal to the tendon are weaker but also indicate the point of failure and supplement the measuring system.