Italian civil engineering contractor CO.I.PI. S.p.A. has successfully used its two Swedish Conjet Robot 362 hydrodemolition Robots and its own purpose built “Cammello” hydrodemolition machine, with Conjet computer control system, to assist with high speed concrete repairs to a section of one of the two road tunnels on the main A22 Verona Brennero autostrada at Bolzano, in the north east of Italy, about 80 km from the Austrian border. Damaged parts of concrete tunnel lining, in a section of the southern 6 m radius horseshoe shaped tunnel, were removed in just 18 days, following corrosion due to a combination of carbonisation, chloride attack and freezing and thawing cycles.
CO.I.PI. S.p.A., based in Colle Umberto north of Venice and working for client Brennero Motorway, used the two high-pressure water jetting Conjet Robot 362s and the “Cammello” to remove the weakened concrete and clean the reinforcement in the tunnel lining. A 400 kW Conjet PP345 and two 350 kW Conjet PP340 pump units provided high-pressure water for the hydrodemolition machines.
The Conjet Robot 362s, with multi-purpose extension arms, and “Cammello” selectively removed the damaged concrete to a depth of 10 mm to 40 mm both above and below the reinforcement, which was also cleaned of rust. Approximately 360 m 3 of 27 N/mm 2 concrete had to be removed from a 488 m long stretch of the multi section 12.4 km long southern tunnel. The three hydrodemolition machines each worked approximately 15 high-pressure hours/day on three shifts for the first 10 days, followed by 10 high-pressure hours/day on two shifts for the remaining 8 days. This resulted in an approximate combined average production from the three machines of around 20 m 3 /day of concrete, equivalent to approximately 0.52 m 3 /high pressure hour/machine.
The concrete surface of the tunnel lining had suffered mainly from carbonisation and chloride attack, combined with freeze and thaw cycles and traffic vibration. Carbonisation occurs because concrete is a naturally basic material but carbon dioxide in the air causes a chemical reaction in the concrete, which destroys the ability of the cement to protect the steel reinforcement from corrosion. The pH in the concrete protects the steel from corroding, but carbonisation lowers the pH value and when this drops below a level of pH9 the corrosion of the reinforcement accelerates, leading to a breakdown in the protection.
Chloride, which is spread as salt on the road surface for de-icing, splashes up and penetrates the concrete through surface cracks and capillary attraction. The ingress of salt speeds up corrosion of the interior steel reinforcement, which expands as it corrodes causing cracking and bursting of the concrete structure.
“Water-jet technology using automated selective hydrodemolition with high-pressure jets of water offers advantages and technical solutions that are safer than traditional mechanical demolition methods,” says CO.I.PI president Dario Piccin. “Hydrodemolition does not cause any further cracking or breaking to the concrete. It selectively removes the decayed concrete, leaving an uneven, textured, undamaged surface for an excellent bonding of new concrete. Also the reinforcing bars are perfectly cleaned and the chloride in the porosity is washed away. This project is an example of how a job that is completed very rapidly, using hydro-demolition technology, is extremely advantageous for a road or rail authority, as the time that the service is interrupted is reduced to a minimum.”
After CO.I.PI. S.p.A. removed the old and damaged concrete from the tunnel lining and exposed and cleaned the reinforcement, another contractor followed on to complete the restoration with a new cast in-situ surfacing layer of high strength concrete, prior to the tunnel's return to traffic.
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