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Hydrodemolition Swede dreams

Concrete conservation - One of Europe's longest road bridges has been under repair for nearly 15 years. Dave Parker reports from the shores of the Baltic, where robots have been at work.

Since it opened in 1972 the 6,072m long Ölandbrucke has formed a vital link between Sweden's south eastern coast and the holiday island of Öland. Up to 30,000 vehicles cross it every day during the summer.

But the structure has been dogged by problems, and to keep the bridge open and safe the Swedish National Road Administration (SRNA) has had to spend more than £50M since 1990. The latest phase of repair work is nearing completion - but no one believes the saga has ended.

'It started with the insitu piers, back in the early 1980s, ' explains SRNA project manager Jan-Olof Bolin. 'No-one expected chloride attack because the Baltic is not very salty. But tests on the areas which were spalling and cracking showed chloride levels more than 3% by weight of cement - more than 10 times the allowable maximum.' Further investigation revealed a litany of errors; low concrete grade, low reinforcement cover, higher than specified water/cement ratio and porous sandstone aggregate amongst them. To top it all, the concrete had been mixed with Baltic water, which added around 0.1% chloride by weight of cement to the mix.

The rest of the chloride ions were soaked up from the brackish Baltic by the sandstone aggregate, via badly cracked surfaces - the result of a combination of high heat of hydration from a rapid hardening cement and early striking of formwork.

More than 100 of the crossing's 156 piers needed attention. In 1990 a £19M contract was let to jacket a total of 105 piers over five years with a reinforced shell of high quality air entrained concrete. But even as the work neared completion problems were emerging on the decks above, where concrete quality was much higher.

'In 2000 we carried out a full inspection of the parapets and the bridge's 19 movement joints, ' Bolin reports. 'As a result, it was decided that about 2.3km of the parapets would have to be replaced, as well as all the movement joints, and some of the roller bearings on the pier heads as well.' Freeze-thaw cycling and chloride ions from de-icing operations had taken their toll.

The cost was another £7.5M.

But contractor NCC's task was eased by the adoption of hydrodemolition, particularly the latest generation of Conjet robots (see box).

These could swing their jets into position on all faces of the parapets - even underneath.

Conjet AB president Carl Strömdahl says the Multi Purpose Arm (MPA) model range was a natural development for the company. 'For the first 10 years we concentrated on the classic Conjets, then saw there was a need for a machine that could work on vertical or sloping surfaces - or even below decks.' On the Öland Bridge specialist subcontractor Waterjet used both MPA models and the standard design, the latter for the joint repairs. Large open boxes were

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