John Duffy, chairman of the Precast Flooring Federation Marketing Committee, talks about the benefits of hollowcore flooring.

OVER THE past four decades, concrete flooring has become the material of choice for housing and low-rise commercial premises, largely replacing timber. Of the various types of precast concrete floor systems available, hollowcore has the largest share of the market. Hollowcore floor planks, as the name indicates, are precast prestressed concrete elements with continuous longitudinal voids providing an efficient lightweight section. Units are generally available 1200mm wide, although some manufacturers produce up to 1500mm or narrower units of 600mm or 750mm. Depths are 100mm to 400mm, depending on span and loading conditions, providing efficient flexible solutions across all markets for most building types.

When grouted, the effective shear key between adjacent planks ensures that individual planks behave similarly to a monolithic slab. They may be used to produce a diaphragm to resist horizontal forces, either with or without a structural topping.

Precast concrete floors have always been perfect for improved thermal performance in all types of building. The increased mass of a concrete floor enables heat to be stored in the fabric of the construction – core heating – and precast concrete ground floors are ideal for providing the higher levels of insulation required under the new legislation.

Out of vogue
Perhaps the most familiar way of providing this improved performance is by sheet insulation simply laid over hollowcore slabs finished either with at least 65mm concrete screed or with a floating timber floor.

However, the thickness of applied insulation has gradually increased as the regulations have become ever more stringent. For example, achieving 0.22W/mÇK calls for at least 120mm expanded polystyrene or 80mm polyurethane, depending on the PA value being considered. Of course, as insulation works all year around, helping to keep dwellings warmer in the winter and cooler in the summer, comfort as well as energy savings are increased.

Raw power
An often-overlooked benefit of concrete floors is the improved acoustic performance. With the world getting noisier, such improvements are most welcome. Concrete floors are also more cost-effective than timber and enable thinner floor zones as well as longer spans. In addition, they provide a safe working platform for ongoing construction.

Add to that their intrinsic one-hour minimum fire resistance and compatibility with other precast concrete components such as stairs and balconies and it is easy to see why the technique continues to gain ground.

When used in domestic upper floors,hollowcore planks eliminate the problems associated with traditional timber floors: e.g. maintenance costs resulting from shrinkage of the timber and subsequent cracking of finishes. The benefits to the homeowner are flexibility of design, elimination of squeaks, improved sound insulation between floors and the opportunity of incorporating block partition walls to further improve sound insulation between rooms.

Speed is also a factor. Precast hollowcore flooring is, by definition, manufactured offsite and delivered just in time; typically, installations are undertaken within one day. Many floor specifications can be available within days rather than weeks and simple installation techniques enable up to 350m2 of hollowcore to be installed in one day. To aid erection, some hollowcore slabs incorporate lifting hooks, enabling them to be craned quickly into final position.

Achieving such strengths can be attributed to the combination of the high compressive strength of the machine-compacted concrete (C40/50 to C50/60) and the use of hightensile low-relaxation prestressing wire or strand ( f pu from 1570 to 1860N/mmÇ). What is more, the strict quality control obtainable under factory conditions means that performance and behaviour can be guaranteed.

At the same time, PFF member companies continue to invest in larger factories and improved quality control, with developments in extrusion/slipform machine technology maximising product efficiency.

Also, at a time when the world is looking more and more at the efficient use of materials and avoidance of waste, concrete scores well. One of its main features as a material is its sustainability.

Its shelf life is measured in decades rather than years and, what’s more, it can be broken up when no longer required and reused in a variety of ways, including as aggregate for new concrete. No other construction material can make the same claim.

Mass Delivered Concrete
The following projects illustrate just a few of the growing number of applications of hollowcore concrete floors.

Innova Park, Enfield
A £3.5m project to incorporate hollowcore units for the upper floor was constructed by ROK at Innova Park, Enfield, for SDG Enfield. The four separate two-storey office buildings comprise ten individual units.

The overall floor area of the development is 28,636ft2 with car parking and associated access roads, paved areas, landscaping, incoming services and ancillary works. The design, manufacture and installation of approximately 1270m2 of 1200mm-wide precast prestressed hollowcore flooring units150mm and 200mm deep were carried out by Coltman Precast Concrete.

British Gas Offices, Solihull
This typical commercial application consists of a steel frame with 7.5m by 8.7m bays and 200mm precast hollowcore units spanning 7.5m. A total of 9100m2 of precast units were used and 40m3 of precast stair.

Multi-storey car park, Nuneaton
Here, 10,000m3 of 150mm deep hollowcore floor units and 67m3 of precast stair units was used to provide excellent durability and  performance in a busy, robust car park environment.

IKEA outlet, Manchester
Constructed using a steel frame with precast flooring units, the project called for more than 40,000m2 of 200mm-deep hollowcore precast floor units. The grid reflected the use of car park, retail and offices on different floors.

Sport City, Manchester
For this innovative project Countryside Properties specified 8500mÇ of hollowcore floors. The development of two circular buildings will accommodate 111 one and twobed apartments.

Arora Hotel, Heathrow
Bison supplied more than 30,000mÇ of bespoke precast concrete products for the huge construction programme of six hotel blocks at the new Sofitel London Heathrow site, which will service the airport’s Terminal 5 development.

Set to be one of the largest luxury airport hotels in Europe, the £180m, 605-bedroom hotel is a striking design that includes five towering atriums and a linking bridge that will connect directly to the airport. Bison delivered both prestressed hollowcore and precast solid flooring, 32,000mÇ of wall panels and 300m3 of stairs.

The punch line
Members of the Precast Flooring Federation can provide a complete service from conceptual design stage to manufacture and installation of an approved structural solution on site, a package approach that is already commonly used in the commercial sector with great success. Between them, the member companies of the PFF produce around 90% of the precast concrete flooring systems manufactured in the UK. Using a PFF member as a supplier is a guarantee that the products:

• have been manufactured under strict quality-controlled conditions
• will be of a high standard and comply fully with specific design requirements of specifiers and builders
• will be delivered to site to suit the contractor’s programme for rapid fixing by the supplier’s or the contractor’s operatives.

In regard to this last point, the PFF operates a training programme for installers, leading to individual accreditation cards.

However, perhaps the PFF’s greatest claim to fame in construction, an industry where serious injuries once were commonplace, is its preoccupation with health and safety. Indeed, the federation is the acknowledged ‘market leader’ in this respect. Very early in its existence, it recognised the dangers of working from height, for
example, and has for years produced a regularly updated and revised Code of practice for the safe erection of precast concrete flooring and associated components. Indeed, the latest edition – which takes into account developments such as Work at Heights Regulations 2005, CDM regulations 2007, Health and Safety in Construction (2006 version) and Noise at Work Regulations 2005 – has just been launched and may be downloaded free from www.precastfloors.info.