Precast hollow-core flooring – all set for substantial growth
The local demand for precast hollow-core flooring and load-bearing structures looks set for substantial growth. This message was brought home in October when over 200 delegates comprising mainly of consulting engineers, architects, and other professional classes, attended three hollow-core flooring seminars in Johannesburg, Port Elizabeth and Cape Town.
Organised jointly by the Concrete Manufacturers Association (CMA) and the Concrete Society, the seminars were presented by world-renowned precast concrete expert, Dr Kim S. Elliott, in collaboration with Echo Prestress technical director, Daniel Petrov.
Vice president of the CMA Floor Slab Division, Monique Eggebeen, comments that although precast hollow-core flooring has been locally available for the past 30 years, the potential for further growth is considerable.
“The aim of these seminars was to promote the use of modern precast concrete technologies which the South African construction sector must embrace if it is to provide the efficient, attractive and dynamic building systems that the market requires,” says Eggebeen.
CMA director, Hamish Laing, says professional interest shown at the seminars certainly bodes well for precast concrete technologies in an increasingly competitive market.
“The main focus was how prestressed concrete hollow-core floor slabs can be used in a wide range of building applications, including medium and high-rise residential and office buildings, parking garages, hotels and stadiums, among others,” said Laing
Interviewed before his return to the UK, Dr Elliott said that hollow-core-flooring market penetration is extensive in the Developed World, much more so than in South Africa. In Europe alone 1 billion sq.m. of hollow-core flooring has been made since 1970, enough to fill the land enclosed by the Johannesburg ring road.
“One of the main reasons for this is that hollow-core flooring provides for large open spans and flexibility of application. It also allows for structures which are safe and quicker to erect. ”
Dr Elliott observed that the technology was particularly suited to residential and apartment blocks, as well as to medium and high-rise buildings, and during the seminars he showed examples of 40-storey office blocks in northern Europe where hollow-core flooring was used even though the floor plans were in some cases oval, and in others, triangular.
“Speed of construction and a reduced formwork requirement were of great benefit and the advantage in terms of overall cost savings was a major consideration. Moreover, by the time these structures reached the fifth floor, services and other trades were already working on the first and second floors. However, it should be noted that at these heights hollow-core flooring requires a mesh-reinforced structural topping to prevent progressive collapse in the event of accidental loads due to explosions or impact.
Dr Elliott said that in Britain and Europe the use of hollow-core flooring for longer spans in depths of around 400 – 500mm had been greater than the use of double-T flooring over the past 20 years.
“Double-T slabs have a ribbed soffit with deep down-stands, making the lighting of parking garages problematic, unlike hollow-core flooring which has a very clean and reflective finish which is not only easier to light, but is also much friendlier and safer. Moreover, 400 mm deep hollow-core units, cast together with a 75 mm topping, are capable of spanning the full width of parking garages – up to 16m – covering both the parking and access areas.
“In residential buildings, slab depth usually varies between 150 to 200mm and spans of up to eight or nine metres are achievable. In addition, hollow-core flooring facilitates the alteration of internal layouts by simply moving partition walls, which in Europe are generally lightweight. Hollow-core floor spans in northern hemisphere office buildings tend to be 8m to 10m, and the latter includes space for a corridor.”
Elliott says a further benefit of hollow-core flooring is that the cavities also provide for air-conditioning ducting.
“Cool air can be pumped through the ducts during the day and warm air at night. This application is particularly widespread in the Middle East.”
Dr Elliott is a member of the FIB (Federation International du Beton) Commission on Prefabrication, and will soon be publishing Recommendations for Prestressed Hollow-Core Floors, a document of some 120 pages on the design, manufacture, construction and application of hollow-core in a wide range of buildings. See the FIB web site for this and other work on precast concrete at: fib.epfl.ch.
Laing cites another advantage in that CMA member companies manufacture precast hollow-core slabs in a factory environment where high-quality standards are more easily achievable than in in-situ construction.
“Besides the applications covered in the seminars, precast hollow-core flooring can make a significant contribution to subsidised housing initiatives and there are several examples where this has already been achieved. One of the latest was in the Legacy Housing project in Cape Town where the technology was successfully used in the construction of a semi-detached double-storey house,” concluded Laing.