Three technologies for one solution For the first time, three materials have been incorporated whilst constructing the floor for a new 7000 m² distribution warehouse in Burnley, UK. ABS Brymar Floors Ltd, UK, describes the flooring solution used in an industrial flooring project for Macaw Soft Drinks Ltd, UK. Piling of the site became necessary for this industrial flooring project in order to cater for the high loads generated by bulk storage of soft drinks and the poor geotechnical conditions found on site. As a result, pre-cast piles were driven to various depths across the site and pile caps were constructed to eliminate concerns of punching shear through the slab. A combination of steel and polypropylene fibres together with traditional reinforcement cages all play a key part in the design of a high quality industrial floor. However, engineering concern was that the high density of storage racking generated point loads in excess of 400 kN and traditionally designed piled floor solutions would require 500mm thick floor slabs reinforced with bars top and bottom in both directions. The result of this was that, in conjunction with main contractor Davric Ltd, UK, and consulting engineer Cooper Beal and Ross, UK, the company decided to adopt a new approach. Figure 1. Typical racking configuration in distribution centres. Solution The solution that was arrived at consisted of a 325mm thick lab reinforced with three main constituents. Whilst the main concern was one of structural integrity and accommodating the high loads, greater attention needed to be paid to achieving the level of flatness (CAT 1) required due to palletised drinks being stored at 15m heights. Whilst concrete is strong in compression, it is a brittle material and is also particularly weak when subject to the tensile stresses created by external loads. However, the addition of steel fibres to the concrete creates a 3-D reinforcement and, by individually providing anchorage in the concrete, steel wire fibres can enhance this brittle matrix, giving a ductile material with a high load carrying capacity. Steel fibres are randomly distributed in the mix and so intercept early age micro-cracks, inhibiting their tendency to form into larger cracks. The fibres bridging these cracks will provide considerable residual load carrying capacity and create improved concrete crack control properties. This enhanced ductility is governed by the 'Re,3' values of steel fibre performance in flexure, as determined from a simple three point beam test. These values increase as fibre dosages increase, indicating greater concrete toughness. By using a suitable fibre at high dosages, enhanced crack distribution characteristics can be utilised to allow large area 'jointless' panels of up to 2500 m² to be cast, reducing the need for potentially troublesome induced stress relieving joints. Polypropylene fibres were used to assist in the construction process, as they offered improved resistance to plastic shrinkage cracking and plastic settlement. In addition to aiding in the provision of a high quality surface flatness, another benefit is the reduction of risk of segregation and bleeding of a concrete mix as, when the concrete is poured, the heavier particles within the mix will gravitate towards the bottom of the slab and the lighter particles, including water, will migrate towards the surface. This is the mechanism that leads to the surface of the concrete having weaker particles and a higher water cement ratio than the underlying matrix and can result in the surface being of a lower strength than the underlying concrete. Adding to this, water may pond at the surface and lead it to being more difficult to obtain good surface tolerances. Construction Figure 2. Somero laser screed construction. Piled floors are essential when ground conditions are particularly poor and a ground bearing option is deemed to be an unsuitable form of construction. The addition of steel fibres into a concrete mix results in a post cracking tensile strength which is a function of the tensile strength of plain concrete, as well as the type and amount of steel fibres. In particular, impact and toughness are increased. For the use of steel fibres in piled supported floors, this toughness plays a decisive role for the load bearing and deformation behaviour of these slabs. Traditional strips of reinforcement are pre-fabricated into cages in order to provide virtual beams within the depth of the slab. These cages are laid progressively as the floor is constructed. Utilising flood construction techniques and incorporating the Somero 240 laser screed and Somero STS130 dry shake spreaders, this floor has many benefits over traditionally suspended floors. In addition, the combination of fibres and traditional rebar gives high crack control performance, with the floor designed in accordance with Eurocode 2 and to specified crack widths at the request of the client. An additional problem was introduced to the project by the presence of gas and ACS Linings Ltd, UK, were employed to design and supply a suitable barrier that would counter this problem and could be laid in advance of the floor without hindering floor slab construction. In order to reduce the number of trades involved in the construction process for this project, the gas membrane venting system was buried beneath 150mm of type 1 stone. The gas membrane was then laid as one continuous operation with the floor slab. Figure 3. The head office of ABS Brymar Floors Ltd. The six continuous pours were separated by a Armour Joint developed by ABS Brymar Floors. This provided the Macaw Soft Drinks Ltd with 8mm armour plates to protect the arises from fork lift truck damage and also provide load transfer between adjacent bays. This fully suspended floor can accommodate the extreme case in which the ground beneath settles so that the slab is entirely supported by its edge support strip and the piles, therefore taking no influence from the ground beneath. Conclusion Over the last 20 years, the use of steel fibres in ground supported industrial floors has been accepted and, with advantages in terms of price and performance, the use of steel fibres for this application continues to grow.