SQUID system for magnetic inspection of prestressed tendons on concrete bridges

Superconducting quantum interference devices (SQUIDs) are magnetic field se nsors 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 prestresse d members of bridges. The system uses the magnetic stray field measuring me thod. The tendons are magnetised by means of a yoke magnet scanning along t he members. The magnetic stray field during magnetisation or the remanent f ield subsequent to magnetisation is measured using four SQUID sensors mount ed in the yoke. Signals from stirrups of the mild steel reinforcement are s uppressed with two types of techniques: either a best fit of typical stirru p signals to the stray field signal and their substraction, or the comparis on 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 a mplitudes. Results of measurements on a prestressed concrete highway bridge are presented. At three locations, signal amplitudes above the threshold v alues 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 cas e two. Thus, it was shown that the unsurpassed sensitivity and dynamic rang e of SQUID sensors can be applied to localise ruptures in prestressed membe rs of bridges.