Sweden´s supersonic waterjet

2002-05-30 — Removing damaged or poor quality concrete from sensitive structures such as bridges, dam walls, tunnels, docks, harbours and multi-storey car parking decks using pneumatic and hydraulic breakers or road milling machines is universally accepted as being very 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These once favoured techniques are now outdated and indiscriminate as they remove and leave both good and bad concrete and cause damage to the remaining healthy and sound concrete and steel reinforcement, as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 as 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. Operatives using hand held breakers have extreme difficulty chipping away concrete around and below reinforcement without causing additional damage to the sound concrete and rebar. Any exposed reinforcement has to be cleaned from rust by sandblasting before pouring in fresh concrete. Independent laboratory pull 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.

In the early 1980s the Swedish National Road Administration, Vägverket, were becoming increasingly concerned with the disadvantages of using the then conventional methods of cutting out concrete on bridge decks damaged by salt and frost and started to research safer and more efficient alternatives. The organisation was aware that water jetting had been used for many years for cleaning dirty machinery and considered the concept could possibly be adapted and adopted 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 jointly evaluate and develop the idea.
The joint venture focused its research into 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 made in 1983. The purpose built Conjet Robot equipment, together with the integral and the 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 Conjet Robot’s introduction 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. Also the Conjet Robot equipment did not cause any new damage to the roughened surface, after the concrete had been removed by the water jet, or create micro cracks in the remaining healthy concrete.

The joint venture’s research and development was supported by equally extensive and successful independent trails carried out by 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.

In 1990 the senior staff running Atlas Copco’s Conjet project purchased the organisation and formed the independent company Conjet AB. The new company totally focused on the design, development and manufacturer of Conjet Robot high pressure water jetting equipment and now exports 95% of turnover. The firm has expanded rapidly and in 1994 moved into new premises at Haninge, 20km south of Stockholm. Conjet AB, jointly owned and run by Carl Strömdahl, Lars-Goran Nilsson and Kent Fahlström, 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 Conjet Robots, including the very latest Conjet Robot 362 which has just been added to its extensive product range. The new Conjet Robot 362 together with all the other Conjet machines have 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 7m, can remove concrete to depths of up to 500mm and can blast away between 0.5m3/h to 1.5m3/h, depending on the machine and strength of concrete.

Conjet hydrodemolition equipment relies on a jet of high pressure water exiting from a special nozzle at supersonic speed and forcing its way into the damaged concrete’s porous and cracked surface. The cracks, caused by hardening, ageing, bending, frost, alkali silica reaction, carbonisation and ingress of de-icing salt, propagate from the surface and dramatically reduce the concrete’s tensile strength, which in normal undamaged concrete is about 7% of the compressive strength.

Water, at pressures of 900bar to 1200bar and flows ranging from 150litres/min to 300litres/min, is fed to the nozzle from 350kW to 550kW diesel driven high pressure pumps housed in a sound insulated standard 20ft ISO container. The water jet penetrates the damaged surface creating a hydraulic pressure in the concrete, which breaks and lifts away when the internal pressure rises above the tensile strength of the concrete.

The special nozzle is coupled to a remotely operated, computer controlled, fully adjustable Robot water jetting machine. The nozzle is set at a predetermined angle of attack to the concrete and mounted on an oscillating cassette, which is attached to a traversing cradle running back and forth along a feed beam. When the cradle reaches the end of its travel the nozzle swivels over to maintain the same angle, which enables the jet to operate with a sweeping action to cut away concrete behind reinforcement. At the same time the machine moves back a predetermined distance ready to make the next adjacent cut.
Safety is paramount and the entire nozzle assembly is attached to the end of the Conjet Robot’s multi-positioning arm and covered by a protective shroud. This articulating boom gives the operator considerable flexibility to use the machine for a wide variety of hydrodemolition tasks on horizontal, vertical, angled and curved surfaces, ceilings and soffits. The boom can also reach under a bridge deck soffit while the machine stays on the deck above.
Hydrodemolition, using Conjet Robots, has increasingly proved to be a considerably more cost effective concrete removal method on sensitive structures than traditional pneumatic breaker and milling techniques. Conjet water jetting machines, which normally only require a single operator, remove concrete between 25 and 50 times faster and far more efficiently than normal breaking and milling equipment. The remotely operated, computer controlled water jetting machines also have built in automatic quality control. Once a Conjet Robot has been pre-set by the operator the machine only removes weak and damaged areas of concrete to a pre-determined “quality depth” above or below any steel reinforcement, which, if exposed, is also cleaned of rust.

The remaining healthy and sound concrete’s rough and uneven textured surface provides a very efficient and much stronger bonding interface for the new replacement concrete than the surface left by milling machines and breakers. The preferred hydrodemolition method does not cause any new damage to the concrete surface or leave micro cracks.

Details of all the machine’s computer controlled operations and functions, such as nozzle angle, cradle and oscillating speed, cutting width and production rate in m2/hour, are displayed on a screen on the Conjet Robot’s control panel. A constant monitoring system provides the operator with a visual alarm display on the screen if the Conjet hydrodemolition equipment deviates from its pre-set sequence. The computer controls also incorporate a self diagnostic system and again provide the operator with a visual alarm and display of any machine faults, which, if serious, will trigger the computer to automatically shut the equipment down.
The operator selects one of several pre-loaded software programs, which have adjustable parameters to ensure that only concrete to a predetermined quality depth is selectively removed in a continuous, uniform and safe operation. Additional programs are available to suit customers’ individual requirements. The Robot operator can also easily adapt and program the computer on site to match a specific operation or set the computer to memorise and save specific working settings for future use.

All machine functions are controlled from a hand portable, remote control console ensuring safe operation for the operator. An additional safety feature requires the operator to respond to a flashing indicator light on the control box every two minutes and reactivate the Robot’s control system, otherwise the machine will automatically shut down.
Concrete on a damaged structure must first be analysed to determine the extent of the repair and the depth to be removed. Core samples need to be taken and concrete strengths checked. Chloride content and penetration, together with frost resistance also has to be verified to develop a profile of the overall damage and assess the depth of poor and weak concrete to be cut out. This quality depth is first tried on a small area of the structure, but may need to be deepened if cracks are still showing in the rough and uneven surface or spots of salt, especially where aggregate has been embedded, are showing. When chloride content of the concrete is lower than 0.2% the aggregate, if struck with a hammer, should just crack, but stay in place. When chloride content reaches between 0.2% to 0.5% the aggregate will bounce out leaving a matching impression in the concrete. And if chloride content is higher than 0.5% large pieces of concrete will break away, indicating that the bonding is insufficient and more concrete will have to be removed prior to finalising the specified quality depth.

After the selected removal of the poor concrete, the rough and uneven textured surface has to be thoroughly cleaned with a high pressure water jet immediately prior to overlaying with fresh concrete. The cleaning is essential to prevent risk of delamination at the interface. Efficient compaction of the fresh concrete is also vitally important to prevent formation of air pockets in any depressions created in the old concrete’s roughened surface.
Hydrodemolition Robot machines are now the only authorised and accepted method of selectively removing damaged concrete from bridge decks and other similar sensitive structures in Sweden and Norway. Hand held water jetting lances can occasionally complement hydrodemolition machines and are only used on very small areas inaccessible to the purpose built robotic equipment. The hand held water jetting lance is also not as safe, selective or productive as an automatic and remotely operated, computer controlled hydrodemolition Robot.

The proven and very successful hydrodemolition repair technique is also preferred and extensively used throughout North America, Europe, Russia, the Middle East and South East Asia over the past 15 years. Steel reinforced concrete, weakened and damaged by salt and frost, has been very efficiently removed from bridge decks, joints, soffits, pillars and columns, harbour and dock walls, dam faces, tunnel and hydro-electric turbine linings, car parking decks and airport runways with Conjet’s versatile hydrodemolition water jetting equipment.

Biography

Conjet AB, based in Stockholm, Sweden, is the world's leading specialist in the design, development and manufacture of remotely operated, computer controlled Conjet Robot high pressure water jetting machines, which selectively remove damaged or poor concrete from numerous structures using the hydrodemolition repair technique.

For further information please contact:

Lars-Göran Nilsson or Carl Strömdahl

Conjet AB
P.O. Box 507
S-136 25 Haninge
Sweden.

Tel: +46-8-556 522 40
Fax: +46-8- 556 522 60
E-mail: conjet@conjet.se
Internet: www.conjet.com

Company presentation (pdf 192 kb)
The Conjet story (pdf 1,3 mb)

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