The hydrodemolition technique, which uses high pressure water jets to remove damaged concrete from various structures, is playing a crucial role in the major restoration of the vital and adjoining Central Bridge and Söderled Tunnel in the centre of Sweden’s capital Stockholm.
Six Conjet Robot hydrodemolition machines have been working on the current phase of Stockholm’s largest concrete repair project, which is estimated will cost over SEK1billion (€106M) and five years to complete. The Stockholm City Traffic Office initiated the extensive repairs, which includes the removal, by the Conjet Robots, of the concrete surface from the ceiling and lower parts of the salt damaged tunnel walls, and removal of the bridge’s salt damaged concrete deck, followed by replacement with fresh concrete.
The 1,550m dual two lane rectangular box Söderled Tunnel and connecting dual two and three lane Central Bridge are a vital 2.6km long link in the north south Stockholm traffic artery. Around 85,000 vehicles every 24 hours travel through the two adjacent north and southbound box tunnels and about 128,000 use the bridge. A comprehensive inspection in 2000 of the two adjoining structures revealed water leakage in the tunnel roof and serious corrosion of the concrete reinforcing in the low part of the tunnel walls and on the bridge’s concrete deck, primarily due to de-icing salt sprayed by passing traffic.
Stockholm City Traffic Office embarked in 2005 on a comprehensive programme of repairs and modernisation, which included a new fire protecting sprayed concrete lining in the tunnel soffit, sacrificial precast concrete panels along the base of the tunnel walls and upgrading the lighting, ventilation and sign boards. In addition the Central Bridge’s concrete deck is being repaired and strengthened and central crash barrier replaced.
To minimise traffic disruption as much as possible Stockholm City Traffic Office opted to phase the repairs over a five-year period, but only during the 13 summer weeks, between June and September, when traffic flows are down by about 15% to 20% from normal. One of the Nordic region’s largest contractors NCC Construction is the main contractor while Stockholm based E-Schakt , one of
Sweden’s leading hydrodemolition specialists, is using the technique to perform all the concrete removal on the bridge deck. The company is also working as the specialist hydrodemolition subcontractor for NCC removing the damaged concrete lining from the tunnel walls and ceiling. In addition E-schakt is also repairing the concrete deck on the Central Bridge.
Stockholm City Traffic Office this summer focused on renovating the 850m northern stretch of the north bound Söderled Tunnel, following completion of the southern section of the same tunnel last summer. The northbound tunnel was closed to traffic, which was temporarily diverted through the southbound tunnel, during the round the clock restoration. E-Schakt, working two 12 hour shifts used its smallest Conjet Robot 322 and larger Conjet 362MPA and very latest Conjet 364MPA, both with Multi Purpose Arms for high reach and fitted with the optional hydraulically driven rotor head, which is ideal for scarifying and preparation of concrete surfaces. The 322 was fitted with a single nozzle, which was modified to reach below road level and into the channel at the base of the tunnel walls.
High pressure water was supplied from three separate Conjet Powerpacks to each of E-Schakt’s three Robots at a pressure of around 1000bar and flow of 200litres/min. The larger 362MPA and 364MPA Robots were reaching up and working directly above on the tunnel ceiling with the rotor heads removing between 5mm and 10mm of the concrete, leaving a rough and textured micro crack free surface for bonding on the new fire repellent overlay. At the same time E-Schakt used its smaller Robot 322, with single water jetting nozzle, to selectively remove just the salt damaged areas of concrete along the base of the tunnel walls. Depending on the intensity of the chlorides from the de-icing salts the 322 removed concrete in an approximate 1m high swath to a variable of depth of between 30mm and 70mm and in some cases exposing and cleaning the rust from the reinforcing steel.
During this summer’s phase E-Schakt had to prepare about 10,000m2 of the tunnel’s damaged concrete surfaces with its Conjet hydrodemolition Robots. “Hydrodemolition was specified by Stockholm City Traffic Office for this project,” says NCC project manager Hans Qvarnström. “E-Schakt, with its Conjet Robots, has done a very good job and finished all the hydrodemolition by the middle of July. We worked round the clock to a very tight programme, the tightest schedule of any project that I’ve been on, and finished on time by the middle of September.”
On completion of E-Schakt’s hydrodemolition NCC followed on spraying a 35mm thick porcelain impregnated fire retardant concrete lining to complete the soffit restoration. On the prepared tunnel walls NCC sprayed an approximate 60mm thick layer of stainless steel fibre reinforced concrete, followed by a covering of abutting 2m high, 4m long precast concrete sacrificial panels, which can be individually and easily replaced in case of any future salt corrosion.
At the same time E-Schakt’s team used the same hydrodemolition technique on the adjacent Central Bridge removing damaged concrete from the deck, which had also been corroded by de-icing salts. Here E-Schakt used a further three of its Conjet 322 Robots, initially fitted with rotor heads removing and scarifying the concrete surface, which also enabled E-Schakt to identify the worst areas to go deeper. The rotor heads were then replaced with single water-jetting nozzles for the deeper concrete removal. Working at similar 1000bar pressure and 200litres/min flow, each Robot, supplied by its own Conjet Powerpack, then selectively removed only the areas of damaged concrete to a depth of about 70mm to 80mm and exposing the reinforcement.
After E-Schakt’s hydrodemolition, which leaves a very uneven and textured surface, the deck was replaced with 45 MPa bridge concrete, using the rough micro crack free surface as a key to bond on the new concrete overlay. The new concrete deck was then covered with a waterproofing membrane prior to a final asphalt base and 50 mm wearing course topping for a return to traffic. E-Schakt and NCC completed this summer’s restoration to part of the Central Bridge and Söderled Tunnel by the middle of September. They will return again next summer for the next phase and finally complete the restoration and upgrading of the two vital structures by the early autumn of 2010.
