Description
Unannounced tendon rupture is a dreaded failure mechanism in pre-stressed concrete bridges. Failure of single wires or tendons does not necessarily lead to concrete cracking. Then, damage cannot be detected by visual inspection. In the case of tendons bonded in concrete, re-anchoring after cracking causes characteristic local strain fields: tensile strains occur between the crack edges, while compressive strains develop around the tendons, until the initial prestressing strain is again reached. On the surface, these strains are small and depend on multiple factors, e.g., the depth of the tendon, the new anchorage length and the concrete stiffness. Such strain fields can be detected by fibre-optic sensors attached to the surface in two-dimensional grids. By evaluating the backscatter of emitted light beams, minimal strain changes (uncertainty: ±2 με) can be detected in quasi-continuous resolution (2.6 mm pitch). An experimental investigation of pre-tensioned concrete beams with three bars of 10.5 mm diameter is presented. These bars were mechanically cut to simulate failure, while longitudinal and transverse strains were measured on two opposing concrete faces. The location of rupture can be narrowed down towards a few centimetres. The shape and peak of the measured strain fields indicate the depth of the crack, as well as the anchoring length and the remaining pre-stressing force.
The effect of the bonding between tendon and concrete on the detected strain is investigated by varying the type of concrete as well as the surface conditions of the tendon (indented or smooth). Increased concrete stiffness (C40/50 to C 60/75) results in lower expansion of the strain field (less than 10 cm on the concrete surface), but higher strain peaks. The surface condition yields comparable results. For smooth tendon surfaces the anchoring length is increased and the strain field on the concrete surface is dispersed over a length of around 20 cm with smaller strain peaks. Both influences interact with the tendons’ depth. The results are used to develop a monitoring system to detect the position and quantity of ruptured tendons, just from the strain signal on the concrete surface.
