The history of hydrodemolition

High-pressure water jetting equipment is extremely efficient at removing damaged concrete.

Carl Strömdahl, President of Conjet AB, outlines the development of this increasingly popular concrete repair technique.

Removing damaged or poor quality concrete from sensitive structures such as bridges, dam walls, tunnels, docks, harbours, multi-storey car parking decks and runways, using pneumatic and hydraulic breakers or road milling machines, is universally accepted as being inefficient and even damaging to the structure being repaired. These once favoured techniques are now outdated and indiscriminate as they remove and leave both good and bad concrete and cause damage to the remaining healthy and sound concrete and steel reinforcement, as well injuring jackhammer operators.

Road milling machines are severely restricted when working on reinforced concrete and cut out damaged and good material only down to any reinforcement.

Contractors operators who use hand held breakers experience extreme difficulty in chipping away concrete around and below reinforcement without causing additional damage to the sound concrete and rebar. Any exposed reinforcement has to be cleaned of rust by sandblasting before pouring in fresh concrete.

Independent laboratory pulls off tests of the old and new concrete have also shown that concrete breaks at the bond.

Breaker chisels and milling machine cutters also hit and vibrate the steel reinforcing causing a 'zip fastener effect' leading to micro cracking in the surrounding and previously undamaged concrete and a break down of the rigid bounding to the rebar. This can contribute to the speeding up of any corrosion and cause lamination in the concrete. Exceptionally noisy and vibrating hand held breakers are also very stressful on operators, and hearing difficulties and vibration injuries to fingers and arms are extremely commonplace.

Water jetting
In the early 1980s, the Swedish National Road Administration, Vägverket, became increasingly concerned with the disadvantages of using what were at the time conventional methods of cutting out concrete on bridge decks damaged by salt and frost and started to research into safer and more efficient alternatives. The organisation was aware that water jetting had been used for removing paint and hard cement deposits and considered that it might be possible to adapt the concept to cut away or demolish weak concrete. Vägverket, together with Swedish construction and mining equipment manufacturer Atlas Copco and one of Sweden's major civil engineering contractors, formed a group to evaluate and develop the idea.

The joint venture focused its research at the time on extremely high pressure water jetting technology and producing equipment, capable of selectively removing only the damaged or poor quality concrete from bridge decks while leaving healthy and sound concrete in place. Suitable high-pressure water jetting equipment was successfully developed by a special Conjet project group formed within Atlas Copco and a prototype machine was made in 1983. The purpose built Conjet Robot equipment, together with the integral and vitally important bonding of new concrete to the old and sound material left in place, was thoroughly tested and proven by the joint venture.

Extensive trials were conducted on specially constructed test slabs, made up of stepped layers of different strengths of concrete, and on a variety of damaged structures in Sweden prior to the introduction of the Conjet Robot to the world market in 1984. The tests clearly demonstrated that the high pressure water jet equipment could selectively remove the different layers of concrete and that the equipment did not cause any new damage to the roughened surface or create micro cracks in the remaining healthy concrete.

The joint venture's research and development was supported by equally extensive and successful independent trials carried out by Professor Johan Silfwerbrand at Stockholm's Royal Institute of Technology during the 1980s. The Royal Institute of Technology performed pull off tests to compare the bonding of new concrete to old, which had been prepared by robotic high pressure water jetting equipment and hand held pneumatic breakers. Base slabs were cast and left to cure for several months prior to their surfaces being broken away by breakers and high pressure water jets.

The prepared surfaces were cleaned by compressed air jets and vacuum cleaners and the slabs kept moist before casting the new overlay. The composite slabs were carefully cured to minimise risk of shrinkage and left for a year before carrying out pull off tests.

Laboratory tests conclusively proved the bonding of the water-jetted interfaces to be twice as strong as those prepared by mechanical chipping. The selective removal of concrete using high-pressure water jets was christened "hydrodemolition", and several Conjet Robots were soon in use around the world.

Conjet
In 1990, the senior staff running Atlas Copco's Conjet project purchased the organisation and formed the independent company Conjet AB. The new company focused all its attention on the design, development and manufacture of Robot high-pressure water jetting equipment and now exports 95% of its turnover. The firm has expanded rapidly and in 1994 moved into new premises at Haninge, 20 km south of Stockholm. Conjet AB, jointly owned and run by Lars-Göran Nilsson, Kent Fahlström and Carl Strömdahl, is now the world's leading specialist in the design, development and manufacture of remotely operated, computer controlled high-pressure water jetting hydrodemolition machines.

The firm makes an extensive range of Robots, including the very latest Conjet Robot 322, which has just been added to the product range. The new Robot 322, together with all the other Conjet machines, has been designed to comply with the stringent European regulations and carry the appropriate CE marking.
Conjet Robots are now available in cutting widths up to 7 m, can remove concrete to depths of up to 500 mm and can blast away between 0.5 m3/h to 1.5 m3/h, depending on the machine and strength of concrete.