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Development

Back in the early 1980s, Professor Johan Silfwerbrand was one of the first academics to quantify the advantages of hydrodemolition.

Working at the industryfunded Swedish Cement and Concrete Research Institute Silfwerbrand was investigating the performance of concrete overlays on damaged bridge decks, a hot topic in Sweden at that time.

Harsh winters demand heavy and constant use of de-icing salts. As traffic levels increased the Swedish National Road Administration (SNRA) became increasingly concerned at the effect on its highway structures, especially those dating from the 1960s. The main challenge was to find a cost effective and structurally efficient technique for removing and replacing chloride-soaked concrete and corroding reinforcement from the top of bridge decks and from parapets.

'At the time the usual answer was to use jackhammers to remove the concrete, fix studs into the lower sections then pour a topping, ' Silfwerbrand explains. 'The problem was getting a decent bond between the old and the new so the repaired section would act monolithically and be as structurally strong as the original deck.

'This can be achieved with jackhammers, but the method is very reliant on the skills of the operatives and there is always the danger of damage to reinforcement and microcracking in the good concrete.' To meet the challenge the SNRA joined forces with equipment manufacturer Atlas Copco and major civils contractor NCC. The technique investigated was high pressure water jetting, already in use in Italy and the US. There, however, the business end was always a hand held lance, which again needed skill to operate and extensive safety precautions and enclosures.

The Swedish solution was to build a 'robot'; a shrouded, remotely controlled water jet mounted on a frame that allowed the jet to track backwards and forwards across the area under treatment. Water pressure, flow rate and rate of tracking could be adjusted to produce consistent results. Only one operator was needed and, most importantly, the sensitivity of the robot made 'selective' removal of substandard concrete possible.

Silfwerbrand explains: 'The high pressure jet penetrates into the pores of the concrete and bursts it apart. How much concrete is removed is a function of its tensile strength. With the robot it is possible to programme it to break out concrete only with a tensile strength below a predetermined limit. This leaves good quality concrete intact.' Convinced by these benefits, the joint venture dubbed the technique 'hydrodemolition' and went ahead to develop a practical machine. In 1984 the first Conjet Robot entered service - and is still in service more than 20 years later.

In the meantime, however, a management buyout in 1990 created Conjet AB.

Over the last 20 years some 300 Conjet Robots have seen service all over the world.

Overlays on bridges where hydrodemolition was used are still being monitored, Silfwerbrand reports. 'Some of these are 20 years old.

Our results showed that bond strength had actually increased slightly - there was no deterioration due to traffic loads or continued use of de-icing salts.'

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