Over the next two years more than 1M. m 3of concrete will be needed on the massive Heathrow Terminal 5 project. Dave Parker reports on a construction revolution that will make sure it is poured on time.
Everyone knows how reinforcement for concrete is procured. First, the structural engineer designs the bridge, or tunnel, or structural frame, then detailed reinforcement drawings are produced and bar bending schedules taken off.
The schedules are sent to a specialist rebar producer, which takes steel bars in a wide range of diameters from its stock bay, cuts them to length, and transforms them into the hundreds of items of reinforcement needed for the project.
As these come off the production line they go to a storage yard until transport is available, then off they go to site, where the contractor stores them until the steelfixers are ready to go.
This is not how it happens on the ú3.7bn ($5.9bn) Heathrow Terminal 5 project. Here reinforcement is designed only when the Laing O'Rourke team on site signals it has a pour planned and four days later the steel for the pour, and only the steel for that pour, turns up on time at the right place.
Fixing it is easy, because almost all of it is preassembled.
Pours that would normally take four days for steel fixing take only one.
Achieving this productivity demanded a massive early investment - for which the T5 team had very good reasons.
'It's a 250ha site with 36 work areas, four batching plants and 28 tower cranes, ' says T5 demand fulfilment team leader Nigel Harper. 'Currently we're pouring up 2,500m 3of concrete a day, say 13,000m 3a week, mainly into heavily reinforced slabs and walls. Yet in all those 250ha there is virtually no storage area.
'We can store about one day's supplies on the actual site. Worst of all, there's only one entrance to the site - and to get to it you have to pass through the major roadworks we're doing just outside.'
To keep the concrete flowing and the 3,000-plus workers on site busy, more than 100 vehicles an hour have to negotiate the cones and get into and out of the site during the restricted delivery hours.
Each lorry has to turn up at the right work area at the right time with the right load. And when it gets there the site team has to be ready to roll.
'Construction has always been known for its low productivity compared to factory-based activities like car manufacture, ' Harper explains. 'The main reason for that is unreliability - materials arrive late, pours aren't ready when the concrete arrives, the weather may be a problem. We calculated that if we could increase reliability by 20% we would boost productivity by 40%.'
One obvious option, and one already pioneered by client BAA on other projects, is to take as much construction as possible away from the unpredictability of site and carry it out under factory conditions. The current target is for a staggering 70% of the whole T5 project to be prefabricated or preassembled off site.
All these components still have to get on to site however, so unless the logistics problem is tackled effectively, pre-assembly is only a partial solution.
The T5 team's response began with the setting up of two 'logistic centres' - one just to the south of the site and one in Colnbrook - both within 5km of the entrance.
Bulk materials - aggregates, cement, steel, shuttering and so on - are delivered to these by rail. Significant pre-assembly takes place there, with packages of materials and components delivered direct to the specified tower crane, as needed, by road.
But much more was needed to ensure that reliability increased significantly. 'Just in time' delivery, a concept pioneered by the motor industry, can only be successfully applied if there is a cultural revolution within the project team.
'When we say this is a fully integrated team, we mean it, ' says structural engineer Connell Mott MacDonald divisional director Tim Dawson.
'Detail design starts as late as possible to avoid delays caused by last minute changes.
And the first thing we do is talk to the team which is actually going to build the section we're designing.
'This means the steelfixers have an input into the detail design, so buildability is the starting point, ' he adds.
To speed formwork assembly and minimise working at height, wall and slab geometries are simplified. Bar sizes are rationalised and kept to a minimum range. The sequence of reinforcement assembly is agreed in advance. And, increasingly, fixing information is supplied as 3D graphics rather than the traditional 2D drawing.
'Steelfixers find this much easier to use, especially in the pre-assembly production lines, ' says Dawson.
At the moment site work is focused on underground structures such as station boxes and cut and cover tunnels.
Waterproof concrete is needed, primarily a C40 mix with limestone aggregate and 30% of the cementitious content made up of pulverised fuel ash.
Demanding work At Colnbrook, BAA has invested $33M in a state of the art facility unlike any other in the UK. This is where the individual components of the 1M. m 3of reinforced concrete that T5 will need first come into proximity.
Cement, PFA, aggregates and rebar arrive by train, to be marshalled, processed and delivered to site by road - but only when the site needs them.
'This is not a storage depot, ' emphasises T5 Laing O'Rourke operations manager Stuart Barr. 'Everything is driven by the 'pull' from the work area on site. Material enters Colnbrook only when we know the site will need it within three days. The same applies to reinforcement production. Nothing starts until we have a call from the site for a specific pour.'
Barr says the Colnbrook reinforcement factory is unique in the UK. 'We have short production lines but lots of them, to give maximum flexibility. Each line is dedicated to a specific area of the site and there are three production teams each dedicated to two lines.'
A dedicated pile cage line and a line producing rollmats from eight specialised machines supplement six fully flexible production lines.
These latter can turn out rollmats up to 12m wide made up from bars up to 40mm diameter spot welded to mild steel straps.
'Once the bars get to 32mm diameter they are very difficult to unroll on site, ' Barr adds. 'So we roll an air hose in with the mat. On site this is pumped full of air, and air pressure alone neatly unrolls the mat.'
Processed bar passes into six pre-assembly areas, where modular, reconfigurable jigs are served by four-strong cross-functional teams. Preassembled cages weighing up to 8t are welded together in this area and loaded straight on to trailers - held at Colnbrook for only a few hours until called for delivery.
Barr says this procurement philosophy makes two major contributions to improved reliability. 'First, if any bar has been detailed incorrectly we've only made one set before we find out. And, second, any problem on any particular work area has little effect on the rest of the factory and therefore on the site as a whole.'