SQUID system for magnetic inspection of prestressed tendons on concrete bridges

Superconducting quantum interference devices (SQUIDs) are magnetic field sensors with the highest sensitivity and largest dynamic range known to date. In a joint German R&D project, the authors are developing a multi-channel SQUID system for non-destructive detection of tendon ruptures in prestressed members of bridges. The system uses the magnetic stray field measuring method. The tendons are magnetised by means of a yoke magnet scanning along the members. The magnetic stray field during magnetisation or the remanent field subsequent to magnetisation is measured using four SQUID sensors mounted in the yoke. Signals from stirrups of the mild steel reinforcement are suppressed with two types of techniques: either a best fit of typical stirrup signals to the stray field signal and their substraction, or the comparison of remanent field signals after changing the magnetisation direction of the stirrups. Subsequent correlation analysis with the dipolar signal of a typical void yields a rupture probability distribution and rupture signal amplitudes. Results of measurements on a prestressed concrete highway bridge are presented. At three locations, signal amplitudes above the threshold values were found. Two of them were verified by opening the bridge deck. In one case, one out of eight wires in the strand was broken, in the other case two. Thus, it was shown that the unsurpassed sensitivity and dynamic range of SQUID sensors can be applied to localise ruptures in prestressed members of bridges.