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.

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 in a special project, called Conjet, 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.