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Code cracking

New European standards are set to improve the way that concrete structures are repaired. Matthew Jones reports.

Everyone knows that those Eurocrats in Brussels have long since troubled the Brits with their petty rules and regulations. First they tried to ban the crisp flavourings we all know and love. Then - shock horror - they tried to make our cherished pints of bitter into demi-litres.

But not every dictate passed down from Europe is inherently bad. A small team of experts from the UK, Sweden, Germany and France is currently beavering away to produce new Europe-wide standards for concrete repairs and waterproofing concrete structures. The new code, in draft form at the moment but expected to be complete and ratified by the end of 1999, should dramatically improve the quality to which concrete structures are renovated or altered in the future.

One of the key sections of the code is devoted to the removal of concrete and reinforcement bars from structures. The draft version, currently being circulated throughout the European Union, states that the method used to remove concrete should not cause micro-cracking in the remaining concrete, and that any reinforcement should be removed in a way which does not damage reinforcement bars left behind.

Code working group member Pereric Westergren of Swedish National Road Administration believes that the tighter controls will lead to safer, more durable repairs, and put specialist contractors tendering for work throughout Europe on a level playing field.

The new code is also likely to change the way that bridges and other sensitive structures are repaired in the future, since traditional percussion methods of breaking out concrete will become more difficult to do in an acceptable way.

'Using a jack hammer or milling machine causes vibrations throughout the concrete which lead to cracks, and also leaves exposed reinforcement bars in a tangled mess,' explains Westergren.

Mechanical methods of concrete removal will still be allowed under the new code, but for the last 50mm removed only light, hand operated hammers will be acceptable to avoid cracking. 'This will be very expensive for large areas, because it is labour intensive,' he says.

Westergren believes that the best way to meet the new stipulations will be to use more sensitive techniques such as high pressure water jetting, or hydro-demolition.

The method has been used extensively in Sweden since the mid-1980s and is now virtually the only way that damaged concrete is removed from bridges.

'We prefer mechanised water jet equipment because we can get very high production from it and it also helps to avoid health problems such as white finger which are caused by percussion methods,' he says.

Over the past 10 years, Westergren has been involved in a number of research projects to prove that the technique can effectively remove concrete without causing vibrations and cracking in the material left behind. The Roads Authority has also funded work which demonstrates that using modern, remotely controlled water jet machines the surface of the concrete can be scoured in a uniform way, removing only damaged material and leaving full strength concrete untouched.

The advantage of this selectivity, apart from avoiding unnecessary work, is that it leaves behind a very rough surface. The Swedes have shown that this produces an excellent bond between the new concrete used for the repair and the old left behind, without having to drill dowel bars into the structure.

'The roughness also effectively helps to divide the new concrete into smaller 'blocks', reducing the area over which shrinkage takes place and so minimising the amount of surface cracking,' adds Westergren.

Yet despite the success of the technique in Sweden, and also to an extent in Norway, Italy and Switzerland, it has still not gained much acceptance in the UK. Where water jetting has been used it has tended to have been with hand lances, which cannot be controlled to the same accuracy as a remotely controlled machine.

This can mean that either too much or not enough concrete is removed, and also has safety implications. At pressures of around 1,000bar, the water jet can be difficult to control and is easily powerful enough to cut an operative in two.

Head of Swedish contractor NCC's water jet department Thomas Aberg believes that using remotely controlled machines is much safer and achieves as much as three times the production rate as the hand lances.

He has carried out a couple of contracts in the UK using Conjet Robot machines, but says that despite good results the British still treat the technique with suspicion.

'The British engineers did not like the fact that it produced a rough surface because they could not get to grips with the idea of selective concrete removal. They were so afraid of the new technology that in the drawings they specified an exact profile.'

The new code should go some way to allaying this suspicion and will oblige British engineers to take hydro-demolition more seriously.

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