Tough floor for soft drinks

Concrete, Sep 2002 by Murgatroyd, Darren

Three technologies have been used in combination to construct the high-performance floor for a new 6,300m^sup 2^ factory for Macaw Soft Drinks, Nelson, Lancashire. To accommodate the high loads generated by bulk storage of soft drinks and the poor geotechnical conditions found on site, piling became necessary for this industrial flooring project. Altogether, 600 precast piles were driven to various depths across the site and pile caps were constructed to eliminate concerns of punching shear through the slab.

Steel and polypropylene fibres together with traditional reinforcement cages all played a key role in the design of this high-quality industrial floor. An engineering concern was that the high density of storage racking generated point loads in excess of 400kN on a 2700 x 3000mm grid on 400 x 200mm baseplates. There was an additional requirement for mezzanine point loads of 260kN on a 3750 x 3750mm grid on 250 x 200mm baseplates. The uniformly distributed load was 50kN/m^sup 2^.

Traditionally designed piled floor solutions would require a 500mm-- thick floor slab reinforced with orthogonal bars top and bottom. This would require pumping and jeopardise the achievement of high tolerances.

To overcome this, ABS Brymar Floors decided on a new approach, in conjunction with the main contractor DavRic Construction Ltd and consulting engineer Cooper Beal and Ross.

Solution

The solution consisted of a 325mmthick structural slab incorporating steel fibres, polypropylene fibres and steel reinforcement. The main concern was of structural integrity, accommodating the high loads from palletised drinks stored at heights up to 16m, with a specific need to achieve Category 1 flatness as specified in Concrete Society Technical Report 34 Concrete industrial ground floors: a guide to their design and construction).

Although concrete is strong in compression, it is brittle and weak under tensile stresses. However, adding steel fibres to the concrete creates three-dimensional reinforcement. With individual steel fibres providing anchorage in the concrete, steel wire fibres can provide some post-cracking load-carrying capacity. Steel fibres are randomly distributed in the mix and intercept early-age micro-cracks, stopping the formation of large cracks.

The residual load capacity is governed by the R^sub e,3^ value, as determined from a simple three-point beam test. This value increases with fibre dosage, indicating greater concrete toughness. By using a suitable fibre at high dosages, enhanced crack distribution characteristics can allow casting of jointless panels up to 2500m2, reducing the need for induced stress-- relieving joints, that could potentially cause future problems. For this project, it was decided to use a dosage of 45kg/m^sup 3^ RC65/60BN Dramix steel fibres, supplied by Bekaert Building Products. Concrete was directly discharged into position.

With thicker slabs, heavier particles in the mix settle towards the bottom of the slab and the lighter particles, including water, will migrate upwards. This can lead to excessive bleeding and plastic settlement cracking. Fibrin 23 polypropylene fibres from Adfil UK were incorporated, as they offer improved resistance to plastic shrinkage cracking and plastic settlement. They help with the achievement of high-quality surface flatness and the fibres reduce excessive mix bleeding.

Bleeding can lead to the surface of the concrete having weaker particles and a higher water/cement ratio than the underlying matrix. The surface may be of a lower strength than the underlying concrete. Furthermore, water may pond at the surface and make it more difficult to obtain good surface tolerances. To satisfy the abrasion resistance requirement AR1, the surface received a 5kg/m^sup 2^ Permaban Fibretop dryshake finish followed by a one-coat acrylic resin spray-applied curing, hardening and anti-dust agent.

Construction

Piled floors are essential when ground conditions are particularly poor, and a ground-bearing option is considered to be unsuitable. The inner piling grid was 3300 x 2800mm. The first line of piles begin 1m in from the perimeter of the building. All piles had a 900mm square pilehead at a depth of 450mm. The piling platform consisted of two well-graded, compacted layers of granular material with a total thickness of 300mm.

Adding steel fibres to a concrete mix improves the post-cracking tensile strength of plain concrete and plays a decisive role in load-bearing and deformation behaviour. Traditional strips of reinforcement were manufactured to exact specification by ROM Ltd and 12mm-diameter bars were prefabricated into cages to provide virtual beams within the slab depth. These cages were laid progressively as the floor was constructed. The concrete was a prescribed mix using 20mm-down aggregate with a Portland cement (CEM I) content of 330kg/m^sup 3^ and a water/cement ratio of 0.50. A superplasticiser helped achieve the target slump of 125mm.

Using mass-pour construction techniques with a Somero 240 laser screed and STS 130 dry-shake spreaders, this floor has many benefits over traditionally suspended floors. In addition, the combination of fibres and conventional reinforcement gives high crack-- control performance, with the floor designed in accordance with Eurocode 2 and to specified crack widths.