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Under the surface

Aperio director Simon Brightwell reckons the most important development since the Concrete Society Guide was drafted is the ability to remove the surface pulse - that part of the radar signal that reflects back from the concrete surface without penetrating further. This usually masks returns from the first 40mm-50mm, and by eliminating it during the signal processing phase Brightwell says a new clarity of visualisation up to 70mm is now possible.

'This enables us to check on the bond between floors and screeds, and to look for areas of low reinforcement cover,' he says. 'Previously, radar was most accurate over depths between about 50mm and 500mm.

Standard covermeters work best up to 100mm depth, according to Brightwell, who adds: 'We normally take a covermeter with us on concrete investigations, but nothing gives as much detail as radar.

Aperio now has a new American high resolution antenna operating at 1.5GHz. Measuring only 100mm by 150mm, the new antenna is 'highly manoeuvrable and very useful in confined areas', Brightwell says. 'And it gives fantastic resolution of rebars and bond between layers, and is accurate down to 400mm.'

Continuing advances in affordable computing power is also an important spur to development, he adds. This could result in the solution of one of the last great problems in radar testing - the measure- ment of the velocity of the electro- magnetic pulse as it travels through the non-homo- geneous concrete matrix.

'What we actually measure at the moment is the time the pulse takes to return. To calculate the depth of any feature we must work out the speed the pulse travelled at inside the concrete, and this is affected by many factors, particularly moisture content.

'So we have to cut cores and test these to calibrate the system - but there is still a lot of expertise needed to get a reliable result.'

Brightwell believes developments such as multiple antenna arrays could soon be providing automatic velocity calculation. In the meantime, he points to Aperio's current ability to take readings at 250mm centres at 110km/h - which means surveys of busy motorway fast lanes can be carried out without disrupting traffic.

But Aperio's investigation of the 400m long Saltash Tunnel, in Cornwall, is perhaps its best illustration

of radar's current capabilities.

Brightwell explains: 'The tunnel has major cracking and leakage problems throughout, although three different constructional techniques were used. Short lengths of conventional insitu reinforced concrete lining at each portal were linked by a section of classic NATM construction and a second section consisting of steel arches encased in concrete.

'From inside they all look the same, but with the radar we were able to pick up not only the boundaries between each section, but the thickness of the concrete - which was very variable - and the exact location and cover to the steel arches. We also found areas at the back of the lining

where we suspect there is either very high moisture content, or micro- cracking, or both.'

Aperio never relies solely on radar when conducting an investigation. Thermographic measurements and falling weight deflectometers will be deployed as appropriate, Brightwell says. And he welcomes the Concrete Society report, adding: 'It will help foster the growing awareness among clients of radar's capabilities.'

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