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Wired sound


Sometime in 1997 visitors to the Transport Research Laboratory's Crowthorne facility might have encountered a bizarre sight. Grown men and women were apparently assaulting a large concrete element in the grounds - solemnly flogging it with steel chains, dropping sizeable lumps of metal onto it, even pinging it with steel rulers. If the understandably bewildered observers had asked for elucidation, the answer might have confused them even further.

TRL research fellow Dr David Cullington was one of those carrying out the assaults. 'We were actually trying to fool engineers in Canada, who were listening to the experiment over the Internet,' he explains.

'The idea was to see if they could reliably distinguish between the sorts of noises we were producing and the much quieter but much more significant sound of a pre-stressing strand wire snapping.'

TRL first encountered the Canadian SoundPrint system when engineers from its developer, Calgary-based Pure Technologies Inc, visited the British Cement Association's Crowthorne HQ in 1995.

At the time the TRL was embarked on a major Highways Agency-funded programme of research into non-destructive investigative techniques which could be used to monitor the condition of ageing post-tensioned concrete highway structures.

Concerns over the corrosion of post-tensioning strands had been raised after a few well-publicised failures, but most investigations relied on invasive techniques like core cutting.

SoundPrint was originally developed to monitor tendons in the much quieter environment of post-tensioned floors of high-rise buildings and multistorey carparks, Cullington says. TRL was happy to add the system to the programme, he adds.

'We had been trying things like X-rays and impulse radar, and already had a large box girder beam from a demolished motorway footbridge in the yard to try them out on. We had even developed a system for producing accelerated tendon corrosion insitu, using what is effectively the reverse of impressed current cathodic protection.'

Adding the 'passive' SoundPrint system to the 30m span test beam was straightforward. An array of piezo-electric microphones were simply glued to the concrete soffit at 3m intervals and cables led to the first stage hardware - a data acquisition unit with primary filters.

'The wires that make up the strands are comparatively small, so the noise they produce when they break is tiny - but distinctive,' says Cullington.

'Extraneous noises have to be filtered out first before the distinctive signal of a wire failure can be identified.'

Signals from this preliminary stage are then transmitted over the Internet to Pure Technology's Calgary labs. There, armed with engineering drawings of the structure concerned, engineers reprocess the signals to identify the time and location of the failure.

Which is why TRL investigators began their assault on the test beam. Anyone who has ever been inside a heavily-trafficked box girder bridge knows how many 'clicks, bangs and rumbles' the concrete transmits, but Cullington says that TRL did more than attempt to swamp the SoundPrint system with simulated traffic and impact noise.

'First of all, facsimile wire breaks were generated by attaching a test frame to the beam and overstressing external wires which had either been pre-ground to reduce their cross-section or which were subjected to accelerated corrosion. This allowed Calgary to tune the system for this particular element - an exercise that has to be carried out for each structure.'

Once Pure Technologies reckoned the system was ready, the assaults began. Mixed with the spurious sounds were random facsimile wire breaks.

The challenge for those listening via the Internet was to pick up these facsimiles, identify them as wire breaks - and pinpoint their location.

Such was their success that TRL had no hesitation in supporting a full- scale trial installation on the Huntingdon Viaduct. But Cullington says the system can be used on more than just post-tensioned structures.

'Cable-stayed and suspension bridges can also have problems with wire breaks. So do some types of ground anchor. And Pure Technologies is already monitoring the cable-supported roof of the Calgary Saddledome, the home of the ice-skating competitions in the 1988 Winter Olympics.'

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