Hydro-demolition helps restore Stockholm's Central Bridge and Söderled Tunnel

Six Conjet Robot hydro-demolition machines have been used during the latest phase of Stockholm's largest concrete repair project to restore Stockholm's Central Bridge and Söderled Tunnel.

The SEK 1 billion (€ 106 million) five-year project, for the Stockholm City Traffic Office (SCTO), involves removal of the concrete surface from the ceiling and the lower parts of the tunnel walls and the bridge deck, which have both been damaged by salt, followed by replacement with fresh concrete.

The 1.55 km-long, dual two lane rectangular box Söderled Tunnel and connecting dual two and three lane Central Bridge are a vital 2.6 km-long link in the north south Stockholm traffic artery. Around 85000 vehicles travel through the two adjacent north and southbound box tunnels and about 128000 use the bridge every 24 hours.

An inspection of the two structures in 2000 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.

In 2005 the SCTO started a programme of repairs and modernisation. This included a new fire resistant 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 SCTO decided to phase the repairs over a five-year period, but with work confined to the 13 weeks of summer between June and September, when traffic flows are reduced by about -20%.

NCC Construction is the main contractor, while Stockholm based E-Schakt , one of Sweden's leading hydro-demolition specialists, is performing all the concrete removal on the bridge deck. E-Schakt is also removing the damaged concrete lining from the tunnel walls and ceiling and repairing the Central Bridge's concrete deck.

Last summer SCTO focused on renovating the 850 m-long northern stretch of the north bound Söderled Tunnel, following completion of the southern section of the same tunnel last summer. With the northbound tunnel closed to traffic E-Schakt's team worked two 12 hour shifts used its smallest Conjet Robot 322 and larger Conjet 362MPA, alongside the latest Conjet 364MPA.

All the machines were fitted with Multi Purpose Arms for high reach work and with optional hydraulically driven rotor heads, which are 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 Robot at around 1000 bar and with a flow rate of 200 litres per minute. The larger 362MPA and 364MPA Robots worked on the tunnel ceiling with the rotor heads removing between 5 and 10 mm of the concrete. This left a rough and textured micro crack free surface for bonding on the new fire repellent overlay.

At the same time the smaller, single nozzle 322 selectively removed 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 to a height of 1 m to a maximum depth of 70 mm.

Once the hydro-demolition was complete NCC sprayed a 35 mm thick porcelain impregnated fire retardant concrete lining to complete the soffit restoration. This was followed by a 60 mm-thick layer of stainless steel fibre reinforced concrete and a covering of the abutting 2 m-high, 4 m-long precast concrete sacrificial panels.

For the bridge E-Schakt's team used the same hydro-demolition technique to remove damaged concrete from the deck using another three Conjet 322 Robots, which were initially fitted with rotor heads, which removed and scarified the concrete surface.

The rotor heads were then replaced with single water-jetting nozzles for the deeper concrete removal. Working at similar 1000 bar pressure and 200 litres per minute flows each Robot selectively removed the areas of damaged concrete to a depth of about 80 mm exposing the reinforcement.

The exposed uneven and textured surface of the deck was then 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.

The next phase of the work will styart again this summer with completion of the whol;e project expected in the latter half of 2010.