CONJET ASSISTS IN RESKINNING CONCRETE ARCH BRIDGE.

2007-07-05 — Strong winds picking up salt from the sea have caused unexpected calcium chloride damage and led to complex and challenging repairs to Croatia’s spectacular reinforced concrete double arch Krk Bridge, which links the northwest mainland at Kralievica, just south of the major port of Rijeka, with the islands of Sveti Marko and Krk. Bridge owner Croatia Highways Authority, Hrvatske autoceste has instigated the €3 M of repairs, which includes the tricky and demanding reskinning of about two thirds of the bridge’s smallest arch with a new sprayed concrete cover by specialist contractor Spegra Konstrucktor Inzcnjering, based in Split.

Hrvatske autoceste specified that the high pressure water jetting technique of hydrodemolition be used to remove the concrete contaminated with calcium chloride(salt) that stretches about one third up from the base of each side of the arch. A Conjet Jetframe 122 high pressure water jetting hydrodemolition machine, adapted to work with a special frame and access scaffold designed and built by Spegra, is playing a vital role in the repairs. Spegra is removing contaminated concrete to a depth of about 30 mm over an area of around 2,000 m2 from all four faces of the rectangular section 8.3 m wide, 4m deep reinforced concrete arch. Any exposed reinforcement corroded to below 80% of its original area is also replaced.

“This is a very challenging and complex bridge repair project and Hrvatske autoceste specified hydrodemolition had to be used to remove the damaged concrete,” says Spegra project engineer Boris Pavic. “We did consider using hand lances, but this would have been far too difficult and dangerous working in the windy conditions at height over water. A Conjet Jetframe had been used successfully during repairs about 3 years ago to some of the Krk Bridge’s pillars and as this equipment is far easier and safer to use we decided to go for a Conjet 122 Jetframe. The standard frame was modified to enable the jetting nozzle to travel 9 m and was combined with our own special access scaffold to enable concrete to be safely removed from all four faces of the rectangular arch. I was initially concerned about working on the approximate 45° steep sloping arch, but the whole combined system has worked really well and better than expected.”

The single carriageway road bridge was built by Mostogradnja and when opened in 1980 was believed to be the world’s largest span concrete arch bridge. It has two main spans of 390 m and 244 m with a height of 67 m about water level and was designed to with stand wind speeds up to 142 km/h. The crossing bridges the 462 m wide Tihi Canal, between the mainland and the Island of Sveti Marko and the narrower 250 m Burni Canal between the islands of Sveti Marko and Krk. The frequent high winds have, over the years, picked up salt from the sea which has impregnated the lower sections of the arch and damaged the concrete skin.

Spegra’s approximate €1.5 M contract involves removing about 60 m3 of the salt infected concrete from both ends of the arch. For this process the contractor is using its Conjet Jetframe 122 with Conjet CCU 197 computer control unit connected to a 110 kW Hammelmann HDP 114 pump delivering clean fresh water at a maximum pressure of 2500 bar at a flow of 20 litres/min. Spegra operates the pump at a lower 2100-2200 bar pressure. “At this slightly lower setting we are able to remove an area of about 100 m2 in 4-5 hours with the Jetframe, compared with only about 20 m2 for a whole day if we had to do it with a hand lance,” says Boris Pavic. “Using our special wheel mounted cross frame, which runs on the top of the arch, we can use the Jetframe to cut an area 8.3 m by 1.5 m long on the top and bottom faces, which only takes about 45 minutes. For the underside we suspend and hang the Jetframe on a frame and working platform supported by Dywidag bars from the top frame. For the sides of the arch we have another special frame suspended from our top frame and capable of taking out a 4 m by 1.6 m area in one movement. The hydrodemolition part of the repairs is not critical and three days with the Conjet 122 Jetframe gives us about two weeks work on the remaining concrete repairing operations to that section.”

After the hydrodemolition phase Spegra first follows on with washing the exposed rough concrete with hand lances operating at 800 bar. The roughened concrete surface cannot be left exposed as it could get recontaminated and the washing is followed within 24 hours by a brushed coating of a gel inhibitor, followed by a second washing with clean water from a hose. Mortar containing high strength micro silica, to give added strength, is then sprayed onto the prepared surface to a depth of 40 mm to 50 mm, depending on the scarified surface irregularity and levelled by hand trowelling. “Hydrodemolition has produced a very rough scarified surface to give a good bond for the new sprayed concrete, which after levelling is covered with a 1 mm thick polymer paint applied with a roller,” adds Boris Pavic.

The faces on the remaining top section of the arch are also pressure washed at 800 bar and the structure impregnated with a silan sealant to provide surface protection against further salt contamination to complete the repairs. Spegra is also expected to tender to Hrvatske autoceste for similar renovation work to the bridge’s second wider arch which is anticipated will start during the first half of 2007.

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

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