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."