By Colin Foreman
Precast concrete is no longer limited to just a few aspects of design and construction; nowadays, it is used in projects ranging from columns and beams to wall systems and roof slabs. And techniques such as sand blasting and acid etching can transform the product from a functional material into an eye-catching effect. CW reports.
Precast forms a solid success for variety of building projects |~|DOH003200.gif|~|Intricate precast concrete elements, like those used on the West Bay Complex in Doha, can now be achieved using innovative design techniques.|~|The benefits of using precast concrete are no longer limited to just a few aspects of design and construction.
Technological advances and product innovations have pushed the material to the forefront of modern building construction, and precast construction techniques can now be found on almost every project, big or small.
The product offers clear advantages which include increased strength, load bearing capacity, environmental performance, quality of finish and faster installation — providing the transport system that delivers the product to the site functions effectively. “Precast definitely gives a more structured approach on site,” says John Glackin, Middle East business manager, BSI.
Precast can also be used for a wide variety of functions including columns, beams, wall systems, roof slabs, tunnel segments, decking and piping.
Many may think that precast is an unattractive functional product, but this is not the case as it is often used by architects to create intricately designed architectural panels.
These are made using a number of techniques including acid etching, sandblasting, waterblasting and grooving.
Custom-made forms can be used to create panels in the exact sizes and shapes specified by the designer, as well as to introduce reveals, joints, patterns and other detailing to the panel surface. Specific colour effects can be achieved through varying sands, cements and pigments. Coarse aggregates add further dimension to the range of possible architectural effects. With hundreds of aggregate types, shapes, sizes and colours to choose from, precast manufacturers can provide architectural panels with a wealth of distinctive and unusual colours and textures that could not be achieved with any other material.
Textures can also be customised using chemical and mechanical treatments ranging from acid washes to sandblasting, polishing, tooling and hammering. Stone, tile or brick veneers can also be attached or cast into the panels at the plant, allowing designers to achieve prestigious visual effects at a reasonable cost.
Branding can also be included, as it has been on several high profile jobs recently, such as the new bridge leading into Ibn Battuta mall from Sheikh Zayed Road, which has the developer’s logo cast into the abutments.
It also allows architects to overcome parameters that would otherwise constrain the design of a building. For example, precast beams allow for much larger column spans than those used normally. This means more open car parking areas, which increases the number of spaces available, and greater line-of-sight improves both safety and security within the building.
All of this gives limitless architectural effects offering designers more flexibility than almost any other material.
Although aesthetics are important, precast’s greatest asset is the fact that it allows contractors to build faster and more efficiently, while offering significant opportunities to save on costs. Evidence from North America has shown that precast systems speed up the construction process by up to 30% and reduce time spent on follow on work by as much as 55%, without any sacrifice of quality.
Experience in this region is similar. “As far as time is concerned, the precast blocks reduce the time that it takes to build 100 villas out of conventional concrete by half. This is because the block is finished in the factory, so no preparation work is needed before tiling or other wall coverings are installed,” says Harry Buitelaar, deputy general manager, E-Acico.
The savings in time come from the fact that precast wall systems can be cast at a factory as soon as the designs are complete, even if they are not yet needed on site. This means that the structure is ready for assembly as soon as the site is ready, rather than having to wait for concrete pours that may be delayed. All that is needed is a good transport system to deliver the panels, and a crane ready on site for placement duties.
Another important advantage of using precast concrete is quality control. Unlike other methods of construction that are at the mercy of the conditions on site, precast manufacturers can rely on a stable and controlled factory environment that helps achieve the best quality possible.
Casting in a factory environment avoids cutting many of the corners that are normally cut on site, and the design specifications are strictly adhered to.
As with all other factory-produced products, precast concrete can be standardised. Many precast products, such as blocks,
are of a regular shape and size and can be easily standardised.
Even irregular shapes and sizes can be standardised as often it is only the configuration of the moulds that is changed which only makes a slight impact on the actual performance of the product. Agencies such as the British Standards Institution (BSI) conduct external audits and visit factories for testing to see if certain performance criteria like durability, weathering, water resistance and strength are met. If successful, the product is then awarded a kitemark or another standard such as European Norms (EN).
Precast concrete also offers environmental benefits, as it requires less energy to produce than alternatives like structural steel frames and system curtain walling. When used on site it creates less air pollution, noise and debris. The high quality of finish that is achieved means that unlike other finishes, it can be left untreated and exposed.
Thermal insulation is also improved. Precast products like E-Acico’s Hebel blocks offer excellent thermal insulation, which has proved to be very popular elsewhere in the Gulf as it reduces electricity bills by slashing cooling loads.||**||