In a series of seminars organised jointly by the Concrete Manufacturers Association (CMA) and the Concrete Society towards the end of last year, world-renowned precast concrete expert, Dr Kim S Elliott, and Echo Prestress technical director, Daniel Petrov, addressed the use of modern precast concrete technologies in construction.
Vice president of the Floor Slab Division of the CMA, Monique Eggebeen, commented that although precast hollow-core flooring has been available locally for the past 30 years, the potential for further growth is considerable.
CMA director Hamish Laing said, “The main focus was on how prestressed hollow-core concrete floor slabs can be used in a wide range of buildings, including medium and high-rise residential and office buildings, parking garages, hotels and stadiums, among others.”
Interviewed before his return to the UK, Dr Elliott said that the market penetration of hollow-core flooring is much more extensive in the developed world than in South Africa. In Europe, one billion square metres 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,” he said.
Elliott pointed out that the technology is particularly suited to residential and apartment buildings as well as medium- and high-rise office buildings. In the seminars he showed examples of 40-storey office buildings in Europe where hollow-core flooring has been used, even with floor plans in some cases being oval or triangular.
“Speed of construction and a reduced formwork requirement are great benefits and the advantage in terms of overall cost savings is a major consideration. Another benefit is that by the time such structures reach the fifth floor, services and other trades are already working on the first and second floors.
“However, at these heights hollow-core flooring does require a mesh-reinforced structural topping to prevent progressive collapse in the event of accidental loads due to explosions or impact,” Elliott said.
He also highlighted that in Britain and Europe, over the past 20 years, the use of hollow-core flooring for longer spans in depths of around 400 to 500mm has been greater than the use of double-T flooring.
“Double-T slabs have a ribbed soffit with deep down-stands. In parking garages this makes lighting problematic. By contrast, hollow-core flooring has a very clean and reflective finish which is not only easier to light but is also much friendlier and safer. Furthermore, 400mm deep hollow-core slabs, cast together with a 75mm topping, can span the full width of parking garages – up to 16 metres – covering both the parking and access areas.
“In residential buildings, slab depth usually varies between 150 and 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. In the northern hemisphere, hollow-core floor spans in office buildings tend to be eight to ten metres, the latter including space for a corridor.”
Elliott says a further benefit of hollow-core flooring is that the cavities provide for air-conditioning ducting. “Cool air can be pumped through the ducts during the day and warm air at night. This application is especially widespread in the Middle East.”
Dr Elliott is a member of the FIB (Federation International du Beton) Commission on Prefabrication and is soon to publish Recommendations for Prestressed Hollow-Core Floors, a document on the design, manufacture, construction and application of hollow-core floors in a wide range of buildings.
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