Tesco wasn't prepared to queue for long to get its new Essex distribution centre, so Taywood proposed an unusual steel-fibre reinforced concrete floor that could be installed in a mere 25 days. Thomas Lane finds out how it worked.
Tesco needed to get its distribution centre in Thurrock, Essex, up and running in double-quick time. A conventional reinforced concrete floor would take three months to install, so that was out. But main contractor Taylor Woodrow had a better idea: it said the warehouse's 46,000 m2 floor – the size of six football pitches – could be laid in just 25 days if steel-fibre reinforced concrete supported on piles was used.

This type of floor gets its strength from thousands of tiny steel fibres – each the size of a small nail – that are added to the concrete before it is laid (see box overleaf). This makes it quicker and cheaper to lay.

The main benefit for Tesco was getting the building completed more quickly. However, Les Howarth, senior development construction manager for the supermarket, points out that a fibre-reinforced floor has other advantages.

For one thing, the joints can be up to 50 m apart, instead of the 5 m for a concrete slab. The laying technique makes it easier to achieve a flatter surface overall, which means better support for the high storage bays full of groceries and less damage to the 100 or so forklift trucks that will work in the distribution centre 24 hours a day. This in turn minimises maintenance costs.

For Taylor Woodrow, another recommendation of the system was that Luxembourg-based flooring contractor Silidur would take control of the process from the beginning. Dave Cullen, concrete consultant for Taylor Woodrow, sums it up: "The fact that there are no supply-chain problems with fibre, materials or plant make it one secure package from our point of view." Before the floor could be laid, however, the ground had to be prepared. The site is in an old chalk pit and had to be filled to level the ground. Up to 12 m of backfill was added. Next, piles were sunk to the level of firm bedrock.

Some 5500 piles with enlarged heads were required at 3 m centres to support the 230 mm thick slab.

Finally, a levelling layer was added. Before the slab could be laid, the ground had to be at or above the level of the pile heads to ensure that the slab would be fully floating. The ground also had to be level to ensure that the slab was of uniform thickness. This would prevent stress concentrations building up. Specially graded earth was brought in and levelled by a laser-guided JCB, then rolled to the correct level. Polythene was laid on top to act as a dampproofing and movement-slip membrane.

Work started on installing the floor's 18 40 × 53 m bays on 5 March. Ready-mixed concrete of grade C35 was brought to site by lorry. Silidur added two litres of superplasticiser to the mix for every cubic metre of concrete. The metal fibres were emptied into a drum rather like a washing machine, and spun to prevent groups of fibre clumping. The machine then blew the fibres into the lorry and the mix was turned for six minutes to ensure evenness.

Next, the lorry backed up a ramp and pumped 4.5 m3 of the mix into a specially adapted dumper with large, low-pressure tyres to stop it rutting the prepared sub-floor. The dumper moved up to the working area and evenly distributed the mix. A laser-controlled screeding machine levelled and compacted the mix, after which a topping spreader shook a dry mixture of quartz and cement evenly over the new surface.

Once the concrete had cured sufficiently, power floating machines rubbed the surface to "case harden" the slab, making it more resistant to wear. This meant that the floor's top layer E E ended up with less water than the main body, and so was stronger. The dry-shake topping also reduced water content at the surface and provided a strong, fibre-free covering layer. The process continued overnight until a hard and glossy surface was achieved. Finally, the surface was sealed to keep residual moisture in the mix. Work stopped on 9 April.

The warehouse's perimeter floor is divided into a series of much smaller bays. These use fibre-reinforced concrete and conventional reinforcement fixed to the piles. Taywood opted for a traditional reinforcement in this area to minimise shrinkage: a critical factor as the perimeter runs between the docking bays and the exterior of the building.

Silidur's Delta Joint was used to separate all the bays. This presents a metal edge level with the floor to prevent subsequent damage. The joint allows movement in two directions and, in the case of the larger bays, shrinkage will cause joints to open up to about 10 mm after one year and will eventually be filled with polysulphide sealant.

Tesco's Howarth is delighted with the result: "I wish I knew about this earlier. Silidur's professionalism and how well they have integrated into the partnership is very impressive. The dumpers are very good and there are positive benefits on the health and safety side too, as it creates a better working area."

Three times faster and much, much stronger – but you’ll need an onsite lab

The concrete gets its strength from steel fibres that act as tiny reinforcements. The fibres used on the Tesco floor are 1 mm thick and 54 mm long and have conical heads at each end to anchor the fibre to the concrete. About 130,000 were used in each cubic metre of concrete: this equals 45 kg of steel – about half the weight of the steel used with conventional reinforcement. However, only one-third of these fibres are calculated to be actually doing the work of reinforcement, as they are oriented randomly in all directions in the concrete. Although the same number of workers is required to lay this floor as a conventional concrete floor, they are only there for one-third of the time, and there is no need for steel fixers. No steel mesh means that large quantities of concrete can be taken by dumper straight to the working edge and laid continually, rather than having to pump concrete into the reinforcement area, then lay more reinforcement and so on. One disadvantage of the system is that if anything goes wrong during the pouring of a bay, the whole area has to be ripped out and redone. A meticulously planned production-line approach is adopted to ensure everything runs smoothly. Mix suppliers are lined up well in advance and a second supplier is held in reserve in case of supply problems. Spare parts for the laying machines are kept on site in case of breakdown. Quality control is essential to minimise variability in the mix and get an even distribution of fibres, as this could compromise the structural integrity of the finished product. This is why Silidur prefers to take control of the whole supply chain. An onsite lab constantly tested concrete samples to ensure that the mix met the required standard. Shrinkage is controlled by invisible micro-cracking of the concrete, but as these cracks are bridged by the fibres, the performance of the floor is unaffected. The incorporation of steel fibres allows a much bigger bay than could be achieved with a normally reinforced floor. In fact, there is no limit to the size of bay that can be laid – Silidur has laid one measuring 12,000 m2. The disadvantage of larger bay sizes is that shrinkage at the joints becomes greater. The optimum size is 2000-3000 m2. The bay size at Tesco was determined by a combination of the final positioning of the racking and the amount of concrete that could be delivered and laid during the working day.