South Africa is 30 to 40 years behind the developed and much of the developing world in the construction of multi-storey buildings. The substantial economic advantages which European, American and Asian construction companies are realising through the extensive use of hollow-core concrete slabs and other precast concrete elements, has to date, largely been lost to the local construction industry.
Precast concrete is the preferred building medium for multi-storey superstructures in much of the world. In fact in Europe today, 90% of building superstructures comprise precast concrete with only 10% being cast in-situ. In South Africa the reverse applies. Precast concrete is regarded as an alternative system while 90% of the concrete used on local building construction is still cast in-situ. This results in buildings which are far more costly and time consuming to erect, to the detriment of property developers and the consumer.
In Europe building with precast elements is the default system and is incorporated into the design at the outset. The way it works here is that when hollow-core slab flooring and other precast elements such as beams and columns are actually used, they are specified by the construction engineer after the architects’ plans have been drawn. Only very rarely is precast built into architect’s designs at the outset, which means that when hollow-core slabs are specified by the engineer, the plans must be returned to the architect and amended, and this of course adds to the overall cost.
Notwithstanding South Africa’s seeming reluctance to take advantage of hollow-core and precast technology, this situation looks set to change, and quite rapidly too. Not as one would suspect due to the substantial cost/time and other advantages of the technology, but for the more mundane reason that the skills pool in sin-situ building practice has shown a marked decline over the past five years. Small to medium companies have lost much of their in-situ expertise during the downturn and now it is only the major construction companies which retain sufficient skills to pursue in-situ construction. Hollow-core and precast concrete is ideally placed to take up the slack.
Fortunately for South Africa the precast concrete industry is a healthy one and companies such as the Echo Group, Africa’s largest precast concrete supplier, are already geared to assist the local building industry make the switch. It seems evident that as soon as some of the major architectural practices start to design with precast, it won’t be long before the rest follow.
In 2009 Echo Prestress commissioned the Concrete Manufactures Association (CMA) to manage cost case-studies on two medium-rise buildings, Bridgeview, a five-storey load-bearing masonry residential structure in Johannesburg and Capital Park, a seven-storey residential building in Pretoria. Both buildings were built with hollow-core slabs and the case study was conducted to highlight hollow-core’s superior cost attributes over in-situ construction.
Both projects realised substantial savings, 30.76% in the case of Capital Park and 23.79% on the Bridgeview project. These results were authenticated by quantity surveyors, Bathuleng, Wallace, and Raubenheimer and design verification was handled by DG Consulting Engineers (Capital Park) and Knutton Consulting (Bridgeview).
Hollow-core slabs were first introduced in the 1950s to fast-track flooring in multi-storey apartments during the reconstruction of a bomb-ravaged Europe in the wake of the Second World War. And over the years other precast concrete elements such as beams, columns, cladding, walling, double T slabs and staircases, were adopted such that, as we have seen, 90% of building superstructure concrete in today’s Europe is precast. Hollow-core slabs can be used for most applications requiring a floor and roofing system. Flats, office buildings, schools, and hotels all lend themselves to the technology.
They are produced in well-equipped, up-to-date plants using advanced technologies where it is far easier to ensure consistently high quality standards than it is in an in-situ building environment. Slabs are manufactured on steel casting beds, and in the case of the Echo Group, using slipform technology. Echo manufactures both reinforced and prestressed hollow-core slabs. Reinforced slabs run to a maximum length of five metres whereas the additional strength afforded by prestressing facilitates spans of up to 12 metres. Longer spans and the capacity to carry heavy loads means better space utilisation, lower floor/floor heights and more profitable buildings.
As its name implies hollow-core slabs are cast with continuous voids which reduce weight and cost and offer excellent sound insulation. The voids or hollow channels are used to conceal wiring and/or mechanical ducts. For example, they can be sued for lighting circuits and can allow fixtures in an exposed slab ceiling without unsightly surface-mounted conduit. Concrete batching plants with automatic control of weights, cement/water ratios and the hot curing of concrete in controlled conditions, mean that very high and consistent quality standards are achieved.
The great versatility of hollow-core slabs allows their use not only for floors, but also as walls for reservoirs, as retaining walls and as external bearing walls for various types of buildings. The numerous examples of such buildings in other parts of the world demonstrate that the various possibilities of hollow-core slabs have not been fully exploited. In fact its development world-wide should be considered as in its infancy.
Building with precast in Europe
The Emirates Arena and the Sir Chris Hoy Velodrome in Glasgow, Scotland
The Scottish buildings include the Emirates Arena and the Sir Chris Hoy Velodrome, both situated on 10.5hecatre site in the East End of Glasgow, and a multi-storey car park adjacent to the City’s exhibition and conference site.By way of example, four recently completed structures, three in in Glasgow, Scotland, and the other in Manchester, UK, all contain a high percentage of precast concrete elements and are typical of the type of precast concrete construction which dominates the European construction landscape.
In addition, precast concrete cladding was used on much of the external facades of the Arena and Velodrome as well as an interconnecting hub between them.
Robust, durable and providing inherent acoustic and thermal qualities, the architecturally striking selection of an almost black exposed granite aggregate finish to the precast concrete cladding on both buildings created a rugged impression, mirroring the heritage of the region’s coal mining past and providing a lasting legacy for its future. The ‘shiplap’ pattern represents the different seams evident in an exposed coalface.
Also comprising a high percentage of precast concrete elements, the car park was constructed using building contractor, SCC’S newly developed inverted IPCs (Integrate Precast Component) and PFV (Precast Finished Voided) hollow-core flooring system, manufactured under ISO 9001 Accreditation Scheme. Specifically designed for car park construction, the system offers rapid erection times, off-site fabrication and just-in-time delivery. Manufactured at SCC’s Reddish site, the system offers a high standard of specification and quality finish within a realistic price tag.
Built off pile foundations, the garage’s seven suspended decks are accessed by two semi-circular ramps on its north elevation. These too, form part of SCC’s precast car-park methodology, as they are assembled entirely from precast concrete and are thought to be the first of their type to be constructed in this way.ontractor, SCC’s newly developed inverted IPCs (Integrate Precast Component) and PFV (Precast Finished Voided) hollow-core flooring system, manufactured under ISO 9001 Accreditation Scheme. Specifically designed for car park construction, the system offers rapid erection times, off-site fabrication and just-in-time delivery. Manufactured at SCC’s Reddish site, the system offers a high standard of specification and quality finish within a realistic price tag.
[The multi-storey car park built extensively with precast concrete elements.]
At the peak of construction, the main frame was erected at a rate of 925m2/day, the equivalent of enough space to park 48 cars, along with their respective driving aisles. One of the benefits of the system is the addition of the pedestrian guarding, which is pre-assembled and fixed to the structural members before delivery to site. This avoids the need for temporary guard rails and protects the workforce as soon as the unit is erected.
Pedestrian access to the parking decks is via a main stair tower on the south elevation. This connects to a podium deck and link bridge at level two, for access over the carriageway to the arena site. The stair tower has four machine-roomless lifts, with a high standard of finish throughout. There are two additional escape cores, one each on the northwest and northeast corners of the building.
The building in Manchester comprisesa £60 million 24,000m2 health, psychology and social-care building on the City’s Metropolitan University’s new campus.
It is the largest construction project in a ten-year plan consolidation plan. The scheme has utilised the benefits of precast concrete extensively and will be the greenest university campus in the UK, with environmental sustainability integral to all aspects of the design.
[The 24,000m2 health, psychology and social-care building under construction at Manchester Metropolitan University’s new campus] The multi-storey car park in Scotland, built extensively with precast concrete elements.
Design and build precast specialist, Creagh Concrete Products Ltd, provided a superstructure including structural cores and lift shafts, stairs and landings, columns (including 17 metre circular single formed columns), beams, steel beams, plate flooring and hollow-core slabs including structural toppings. Added to this was the requirement to create a TABS building using cast-in embedded pipework. Overall there were 2,500 precast concrete components on the project involving 500 deliveries.
The overall on-site duration was only 21 weeks with lower level floors handed over five weeks early due to the back-propping design and integration of J&P Building Systems’ BSF Connectors.
European standards adopted locally
In order to promote the use of precast construction and advanced best-practice building standards, local precast companies such as Echo Prestress, are switching to European standards, as are several construction-related engineering practices. The multi-storey car park in Scotland, built extensively with precast concrete elements
Euro codes for precast concrete are far more detailed than local codes and provide extensive and very detailed guidance, especially on hollow-core slabs. By contrast, the local standard for the suspended precast slab is very basic. For example, it doesn’t cover durability properties and thermal values are not well described. Nor are U values and sound transmission values. For these and other reasons local construction professionals are turning to European standards and specifications for precast concrete in their quest to build better building more efficiently.