Fast-speed hydrodemolition with Conjet robots has been key to speedy concrete removal within short nightime closures to repair the Liberty Tunnel highway bores in Pittsburgh and avoid overrun penalties.
Wall preparation is needed for Pennsylvania’s roads department to have the schedule freed up for shotcreting work as part of a US$18.8 million rehabilitation programme for the 90-year-old tunnels.
The twin Liberty tunnels are 1,795m (5,889ft) long, each with two lanes, and link the South Hills of Pittsburgh to the city through Mount Washington. The tunnels were constructed from 1922 to 1924 and were vital in the expansion of the South Hills suburbs. Heavy traffic volume of 48,000 vehicles/day, and extremely heavy during rush hours, makes the scheduling of maintenance difficult.
The rehabilitation project is designed to increase service life, reduce maintenance cost and improve visibility for drivers. Contractors on the project include local firms Swank Associates as general contractor, with HTNE Hydrodemolition Service and Hydro-Technologies of Jeffersonville, Indiana.
Chief Engineer for Hydro-Technologies, Ed Liberati, said hydrodemolition is only a fraction of the cost and more efficient than using jackhammers on the project. Work began in April 2013 and involves removing aged concrete over 19,785m2 (212,960ft2) of walls, working at night between the engineering hours of 10pm and 6am. “It is a heck of an undertaking,” said Liberati.
If the tunnel is not opened by 6am the contractor would be assessed for liquidated damages of US$7,800/hour until the tunnel is opened to traffic once more. With only a maximum of eight work hours per night, the possible working time is slashed even more during sporting events and concerts in Pittsburgh. After Pittsburgh Pirates’ games, for example, construction could not get going until after midnight.
The speed and effectiveness of hydrodemolition allows the work to be tackled even within the shortened hours, and penalties are avoided, not least because of the relative ease of cleanup and extracting the equipment from the tunnels. Project Manager Denis Mazzoni of Swank Associates said: “Given the allowable working hours, there would be no chance to complete the job in one construction season using different methods.”
Hydrodemolition only needs three to four people to operate the robots whereas jackhammer operations would have needed about 30 workers. The technique uses high-pressure water jets operated by a hydrodemolition robot.
On the Liberty Tunnel project, two Conjet 363 vertical robotic units performed partial depth, selective removal of concrete on the vertical walls of the tunnel and on horseshoe profiles that have 4.42m (14.5ft) posted vertical clearance. The robot water jets are controlled by a technician while the flow rate, pressure, dwell time and cutting head functions are computer-controlled. These combined controls are used to remove deteriorated concrete selectively and leave the sound concrete intact.
The Conjet robot is powered by a 11kW (15hp) electric motor. This can handle the rotors and the oscillation tools efficiently and is able to withstand reaction forces working with high-pressure pumps of up to 550kW (737hp). The removal area can be as wide as 2m (6ft 7in). After hydrodemolition removal, the surface is water-blasted resulting in a high quality surface that is ready for the shotcrete application to repair the tunnel walls.
There are three types of tunnel repairs where hydrodemolition is used. These are in areas where the concrete surface is removed to a depth of 25mm (1in) for replacement with shotcrete; to remove deteriorated concrete extends deeper but not as far as rebar reinforcement; and to remove concrete deterioration that goes deeper than the rebar layers and requires more of the reinforcement surface to be exposed.
Initially confined to the bridge rehabilitation market in the USA, hydrodemolition is also used now on tunnel projects and wherever deteriorated reinforced concrete has to be removed to rehabilitate and preserve a structure.