Faced with cutting out damaged concrete as part of repair and strengthening of eight bridges on the UK's main A1 trunk road, main contractor John Mowlem opted for hydrodemolition.
Specialist Laser Civil Engineering was brought in to use its high pressure water jetting Conjet Robot 362 on the bridge decks, joint areas and parapets, prior to Mowlem replacing with new reinforced concrete.
Mowlem is working on all eight bridges out simultaneously and round the clock to minimise disruption to motorists on its ú2M ($3.1M) contract for client the Highways Agency, which manages the motorway and trunk road network on behalf of the Government.
But despite the continuous 24 hour working, the A1 Catterick bypass in North Yorkshire, has been reduced to a single lane in each direction, with a 50mph speed limit and contra-flow, for the duration of the three month project.
'At this time of year we expect about 45,000 vehicles a day along this stretch of the A1, so closing two lanes will cause delays to traffic, ' said Highways Agency project manager Alan Ellison. 'But when the work is complete it should be a very long time before the agency has to revisit.'
Refurbishment of the bridges involves a range of work, which has been designed and supervised by consulting engineer Halcrow on behalf of the HA.
The main contractor had to carry out rewaterproofing and drainage improvements, install new parapets and barriers, replace bearings and bearing shelves, paint structural steelwork in addition to the hydrodemolition.
Laser Civil Engineering's work involved removing 240m 3of concrete with the remotely operated, computer controlled Conjet Robot 362. This was supported by a Conjet 545 Power Pack incorporating a 750hp, diesel driven Hammelmann pump, delivering 240 litres/ minute at a pressure of 1,000bar.
Concrete was cut out at the rate of 0.5m 3/hour to 1m 3/hour.
The Swedish-built unit made cuts ranging from 90mm deep on the decks up to 720mm in the parapets in a single pass. This method leaves the contractor with a rough clean surface which provides a good bonding interface for the new concrete. Reinforcement is left intact and cleaned and without micro craking in the healthy concrete left behind.
In the deck slab, the majority of cutting was at 300mm depth. More than 40% of the concrete removal was on the Agricola Bridge over the River Swale with 50m 3cut out on each carriageway.
This involved cutting a slot about 1.75m wide through the 300mm thick deck slab either side of the joints directly above each of the bridge's two piers and across the carriageway into the central reserve.
In addition, the Robot was used to cut out a 500mm wide, 300mm deep channel along the bridge's central reserve, followed by an abutting step 1.7m wide and 90mm deep.