Forecasts of a bleak winter in the UK will bring little joy to Swale Crossing design and construct contractor Carillion, which is programmed to be concreting the bridge's deck right through from now until March. If Siberian winds blast in from the east as anticipated, conditions on the pours some 35m above the water will be bone-chilling - and a real test for the mix designers, readymix concrete producers and concrete gang alike.
Carillion design manager Kevin Reid says the crossing's distinctive profile has already posed some problems for the concrete technologists.
'Maximum gradient on the deck is 6%. We wanted to place by pump, but this implies a high workability mix. So there were worries that when we started vibration, the concrete would slump downhill, ruining the pour.' The steep gradients were chosen to minimise approach embankments across the marshy foreshores on each side of the Swale, which separates the Isle of Sheppey from the UK mainland (see box). A twin concrete box girder form of construction featured in the original illustrative design produced for the Highways Agency by consultant Mott MacDonald, but the option selected by Carillion lead designer Capita Symonds and viaduct designer Cass Hayward was composite construction, with a 21.1m wide, nominally 250mm thick concrete deck sitting on four fabricated steel girders up to 3.2m deep. Maximum span was 93m.
Grade C40/50 concrete was specified for the deck. To ensure it could be pumped, placed and finished on the steepest sections, trials were carried out on 2m by 1m specimens of the deck, complete with reinforcement.
Carillion materials manager Richard Burkinshaw says the trials revealed two key factors.
'The transverse reinforcement is very dense, and this was a major restraint on the fresh concrete. And we found that by using twin beam vibrators rather than pokers we could get full compaction without any downhill displacement.
'To date we've completed pours totalling more than 1,300m 3, and it's all gone very well.' Burkinshaw adds. 'If really cold weather hits we'll be ready with hot mixing water for the concrete and heated enclosures on top of the pours.' Before then Carillion should have inserted the last section of supporting steelwork into the last gap in the deck. Most of the 19 span plate girder structure was push launched, the only realistic option given the access problems on each side of the Swale.
But dealing with the curves of the two main transitions between tight horizontal and vertical alignments represented too great a challenge for push launching technology. So superstructure contractor Fairfield Mabey decided to divide the deck into three push launched sections linked by transition pieces which would be craned into position once the launches were complete.
'In fact we start to smooth out the vertical transition between the southern straight gradient and the central hogged section early. The outer end of the last span and a half of the straight gradient was allowed to droop down under its own weight by about 500mm, ' says Fairfield Mabey temporary works manager Keith Dibben.
'This gave us a longer launch and simplified the transition, although it built in stresses in the steelwork.'
Even so, slotting the first 43m long fabricated transition section into place was a nail biting operation. Over the normal ambient temperature range the gap could vary by 110mm as the two long stretches of steelwork each side expanded and contracted. Fairfield Mabey site manager Justin Marriot says the maximum temperature difference which the fabrication could cope with was 8-10¦C.
'We had to erect it at night because the gap was over a road. So we had in effect to predict the temperature that particular night months in advance. As it turned out, we were within 1¦C, ' Marriot adds.
Precast concrete Omnia planks act as permanent formwork to the centre section of the deck, between the outer pair of girders. Getting these and the deck reinforcement into position was another logistical challenge that had to be solved.
'The object of the exercise is to be able to lift eight Omnia planks at the same time, and then place them precisely [one at a time] on the deck steelwork, ' explains Tony Gee & Partners project engineer Tim Burgess. 'Any conventional lifting cradle would become seriously unbalanced by the time five or six planks had been placed.' A precisely balanced, angled frame with eight pairs of hooks has provided the answer.
As it lifts off, the planks are stepped up, one above the other. As the lower planks are placed and detached the frame tilts downwards, so that the last plank is within reach of the men on the deck.
The cradle itself was suspended from a mobile gantry, consisting of a 40m long 'Toblerone truss', supported by two vertical 7m high portal frames running along the top of the outer girders. 'Typically such gantries run on railway lines, but this would have clashed with the reinforcement, ' Burgess explains.
'Here we use wheels - with 'skis' to guide them between the shear studs on the girders.' Special thermally blanketed cantilever shutters will be used to complete the deck pours.
On top will go an acrylic based waterproof membrane and low noise surfacing. Parapets will be precast concrete.