Customer case

Boxberg coal-fired power station
Germany

Boxberg coal-fired power station repair with automated concrete removal

When German hydrodemolition specialist contractor, Krüger Wasserhochdrucktechnik, was called on to complete a very unusual and tricky concrete repair job at Boxberg’s coal-fired power station, they knew a Conjet ACR™ robot could get the job done. The repair was to a steep, new cooling water feed canal at the power station.

Krüger Wasserhochdrucktechnik, working for the power station joint venture main contractor Hochtief and Bilfinger & Berger, was able to quickly adapt the Conjet ACR™ robot and suspend it to operate on the steep canal.

The flat-bottomed canal, designed to take water from the base of the power station’s cooling water tower back down into the pump house for recycling, is just 21 meters long.

However, it is 25 meters wide at the inlet and gradually tapers down in a steep, shallow curve to 18 meters at the discharge. The 45% sloping canal has a heavily reinforced 1.5 meter thick base, and Hochtief and Bilfinger & Berger intended to cast the complete canal base in one continuous pour. But as the contractor was nearing the end of the pour the air temperature suddenly and unexpectedly dropped to -8C. Hochtief and Bilfinger & Berger had no option but to continue and complete the casting. The combination of the rapid drop in temperature and very strong biting winds caused extensive frost damage to a depth of 40mm over the canal’s entire concrete surface.

The solution

The damaged concrete was too weak to cater to the strong water flow, so Hochtief and Bilfinger & Berger, working for client and power station owner Arge Rohbau Boxberg, had to remove the spoiled layer prior to placing a completely new overlay. The client insisted that concrete had to be removed to a depth of 25 mm below the upper layer of reinforcement to achieve a good bond with the existing healthy concrete. To ensure the minimum required gap below the reinforcement was achieved, the depth of concrete to be cut out was set at 120 mm.

Hydrodemolition was the only good alternative for removing the damaged concrete on this repair contract, as it removes only the damaged concrete either above or below the rebar and provides a rough, clean surface to give a good bonding with the new concrete. The hydrodemolition technology doesn’t cause any micro-cracks in the sound concrete left behind and leaves all the rebar intact and cleaned. This is unlike pneumatic breakers, which can hit and vibrate the rebar and do a lot of extra damage by breaking the bond between the reinforcement and good concrete.

How it works

Before Krüger Wasserhochdrucktechnik could put the Conjet ACR™ robot to work, the company had to arrange to secure and support the machine on the damaged inclined surface during concrete removal. For this, the company anchored a winch at the top and in the center of the canal with a steel rope running down and fixed to the robot. This simple suspension system enabled the self-propelled Robot, pivoting on the anchored rope, to make traversing radial cuts across the concrete canal. After each pass the robot was lowered on the winch to make the next adjacent cut with the cycle repeated to the canal bottom.

The remotely operated computer-controlled robot was pre-set to selectively remove concrete to below the reinforcement using water at a pressure of 850 bar and flow of 156 liter/min. This was fed through a flexible hose to the Conjet ACR™ robot nozzle from a diesel-driven, high-pressure pump housed in a silenced 20ft long ISO container.

The nozzle, set at a predetermined angle of attack to the concrete, was mounted on an oscillating cassette. This was attached to a traversing cradle running back and forth along a feed beam which was mounted on the robot’s standard arm. When the cradle reached the end of its travel the nozzle swiveled over to maintain the same angle, which enabled the jet to operate with a sweeping action that cut away concrete behind reinforcement. With this system Krüger Wasserhochdrucktechnik was able to set the ACR™ robot to selectively remove concrete at the rate of 0.35m3/hour and expose the top layer of reinforcement.

Result

The Conjet ACR™ robot, suspended from a winch anchored at the top of the 45% inclined curved canal, systematically removed a layer of reinforced, water resistant concrete from the entire surface which had been badly damaged by an unexpected and very sharp frost during casting.

The Conjet ACR™ robot completed the job in only two weeks, allowing the joint venture contractor to return and complete the repair with a fresh concrete overlay, and complete the water feed canal.

Customer case

CUSTOMER: Krüger Wasserhochdrucktechnik

STRUCTURE: Canals

MISSION: Remove reinforced
damaged concrete

ROBOTS USED: 320-series

Robot 557 MPA

Conjet ACR™ robot 557 MPA features the latest technology in hydrodemolition. Combining the reach of Robot 367, the flexibility of Robot 327 and the diesel engine of Robot 437, this robot has it all.

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