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Detecting progress

CONCRETE - Testing: Advanced non-destructive testing technology is now available on site. Aperio sales and marketing director Simon Brightwell reports on the latest developments.

With the growing acceptance of nondestructive investigative techniques, has come a new generation of test equipment and processing software. Most new instruments are digital, and are consequently more compact and able to store and process more data than the analogue systems they replace, allowing insitu investigation in challenging environments such as confined spaces and at height using rope access methods.

Just as important is the ability for initial data processing on site to give a quick result and to target intrusive investigation to verify results. Computer-based processing of large datasets allows major civil assets such as the road network of a county or even a country to be assessed.

Multi-channel ground penetrating radar (GPR) systems are now sufficiently compact to be operated from a hand-pushed cart with one person achieving progress rates four to six times those of a typical two strong team five years ago. Spatial accuracy on large structures such as car park decks or roads has been improved by use of encoder wheels that control the radar sampling and by the increasing influence of Differential GPS.

Depth measurements are improving too, usually accurate to within -8% in bound materials without calibration and as good as -4% with calibration (see box) Despite advances in GPR technology, materials such as fresh concrete, very heavily reinforced concrete and steel fibre concrete still cannot be penetrated. For these materials, a new ultrasonic system has been used over the last year with impressive results. Although already established for investigation of fine grained and predictable materials such as iron and steel, until recently the use of ultrasonics for the more coarse grained and therefore attenuative concrete was severely limited.

The emergence of new low frequency ultrasonic flaw detection systems has brought exciting possibilities. A sophisticated array of 24 cushion mounted transmit/ receive units in a handheld measuring head can take a reading on rough unprepared surfaces every 10 seconds.

Working in this way on a heavily reinforced concrete basement floor, an Aperio team built continuous profiles though 600mm of concrete containing three layers of reinforcement where a GPR survey had penetrated no more than mid-depth.

Although virtually blind to reinforcement, the stress waves transmitted by the ultrasonic system cannot cross an air gap - they are therefore highly effective in mapping voids in concrete and delamination between layers, even in steel fibre concrete where other methods will fail.

Many within the construction testing industry look enviously at the resources committed to developing imaging systems in areas such as medicine and aerospace. A number of recent spinoffs, however, have benefited civil engineering. Examples include highly sensitive infrared thermal imaging cameras capable of detecting structural features such as wall ties and fixings and building defects such as moisture and delamination in rapid surveys, often without expensive access equipment.

Trials of thermal imaging as a tool to check the bond of carbon fibre strengthening materials to concrete structures have shown promising results - an example of new materials and new test methods working together.

Survey success

Two very different Aperio investigations illustrate the potential of modern GPR technology.

In the first, a client suspected the presence of sandball inclusions within a 10,000m 2distribution centre floor. Originating as frozen lumps of sand incorporated in the concrete mix, hundreds of these fist-sized lumps were causing small punch-outs under forklift loading.

A high frequency cart-based GPR system was used to map the sand ball inclusions with sufficient accuracy to enable targeted repairs. Only by filtering out the effects of the concrete surface and much of the reinforcement could the sandball response be distinguished from the surrounding concrete.

The project also highlighted the benefits of modern processing software by trpansferring data from more than 500 GPR profiles to a single plan view overlaid on to CAD drawings of the building.

A project which demonstrated the accuracy of GPR was the recent strengthening of the Medway crossing on the M2 motorway in Kent. Main contractor Edmund Nuttall needed to cut more than 1,800 holes through the post- tensioned concrete box girders - without weakening the structure by damaging the post-tensioning tendons or secondary reinforcement.

Aperio was commissioned to survey a total of 320 girder elements and to locate all tendons and reinforcement before drilling began. The results were not 100% conclusive - much of the bottom steel in the 500mm thick lower flanges could not be mapped - but it was clear that without the GPR survey it would not have been possible to drill the holes without damaging the structure.

Concrete conquest

A try out of a Canadian-developed GPR system on a 10 year old light industrial unit in Hertfordshire gave a rare picture of the effects of flood damage on subfloor condition.

The lightweight portable 3D imaging Conquest GPR system developed by Toronto-based Sensors & Software (S&S) was used to survey the 150mm thick reinforced concrete floor, and the advanced digital imaging quickly showed up complicated zones of break-ups, enlargements and overlaps in the rebar mesh and a gas pipe under the concrete.

'This was a unique test because the site flooded after our initial survey. The buildings were in a valley which has been dry for over 100 years, ' explains president of Sensor & Software, Peter Annan.

'Increasing groundwater pressure caused water flow under the test slab. After the flood we carried out another two hour survey.

'When we compared pre- and post-flooding data, we found a pocket of silty mud surrounded by coarse gravel, and concluded that the base has not been properly prepared during original construction, ' Annan reports.

The testing was run jointly with Bedfordshire-based Geomatrix Earth Science which has just launched the system in the UK.

Geomatrix director Chris Leech was impressed by the results and says the real beauty of Conquest is that it does not need expert operators. 'Someone who operates a core drill, say, can use Conquest on site to get almost instant results with the minimum of training.'

Leech adds that Conquest is light enough to be run over the surface of concrete structures whether vertical or horizontal. It also has a menu system by which maps of the concrete or other material can be created at various depths.

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