Precast concrete is often chosen for its aesthetic architectural appeal. The vast array of available colours, textures and finishes and the flexibilities afforded by a controlled manufacturing process allow designers to achieve a wide variety of design objectives.
With the unlimited design possibilities available when designing with precast, it’s easy for designers to get carried away with ambitious designs that can result in budget blow-out. Here are some cost-saving measures that can help to reduce expenditure without detracting from the quality of the design.
Using Standard Products
Opting to use readily available product lines presents immediate savings due to the elimination of mould establishment costs. SVC’s range of standard products have a defined geometric form, but can be made in many different colours and finishes. With a bit of creativity, stock lines can be used in varying combinations and formats to create more intriguing designs.
Simplifying Product Shape
The key factor that can easily hike up the cost of a bespoke concrete product is its geometry. The cost of a precast concrete product is generally determined by materials, labour and mould fees. Objects with complicated shapes and multi-directional curves require similarly complex moulding, which can only be achieved with a fibreglass mould. Fibreglass moulds incur double the expense of a standard steel mould and can only be re-used about 20 times.
In contrast, steel moulds can be re-used over 100 times, and allow for adjustments to unit length without the need for extra moulds. Steel moulds can suitably accommodate products with straight edges and curvature in a single direction. By keeping product geometry within these guidelines, designers can gain significant savings in mould fees.
Chamfered vs Rounded Edges
Choosing to use a chamfered edge instead of a rounded one will allow the option to use a steel mould rather than the higher priced fibreglass alternative. Designers should consider whether a rounded edge is worth the additional expense of a fibreglass mould. A chamfered edge can achieve a similar effect and is much lighter on the budget.
An example of a project featuring concrete products with chamfered edges is the Box Hill Gardens redevelopment project.
Incorporating Modular Design
For a large-scale design, using one or two concrete modules in repetition is a good way to cut down on mould establishment fees. The alternative is to utilise many different moulds for separate items, which incurs additional fees per mould.
Despite being more cost-effective, modular designs do not necessarily look boring or bland. Depending on the specific design, concrete modules can be very simple or quite complex, and multiple modules can be positioned or combined in different ways to create complexity and variety within the design.
When it comes to precast concrete, a full spectrum of colour is available due to the wide range of coloured oxides that can be added to a concrete mix.
However, for the budget-conscious designer, oxides may be cost-inhibitive as they add a surcharge to the cost of the product. In terms of affordability, standard grey concrete is the lowest-priced option, followed closely by off-white or charcoal concrete. Natural, earthy colours are on the next pricing tier, followed by pigmented colours that can only be achieved by the addition of oxides, making them the most expensive option.
Despite costing the least, the visual appeal of plain grey concrete is not to be dismissed. Many love the authenticity and rawness of grey concrete, and its subtle beauty makes it the preferred colour choice for many designers. Significant projects that have utilised beautiful grey concrete are the impressive fire pit at Forge Apartments and Shadow Play’s luxurious rooftop bar.
What’s in the Mix
A precast concrete mix always contains fragments of aggregate material in addition to sand, water and cement. Aggregates are used to stabilise and add strength to the concrete mix – they also contribute significantly to the visual appearance of the concrete product. For instance, the type of aggregate materials used can affect the overall colour tone of the concrete. ‘Exposed aggregate’ concrete is also a popular aesthetic style, where the surface of the concrete is shot-blasted, ground or honed to reveal the aggregates within the mix.
Typical aggregate materials are versions of quarried stone that are available in varying sizes. Common aggregates used by SVC include river pebble, quartz and blue metal. It is possible to use alternative aggregate materials to create a unique look, which surprisingly may not add a lot to the material costs.
When sourcing aggregates, the most cost-effective and eco-friendly option is to select stones from local quarries, as this reduces the cost and delivery times associated with transport.
The surface of any precast concrete product can be treated in various ways to achieve different visual and textural styles. Finishing techniques include shot-blasting, grinding, honing and polishing, each of which incurs an additional cost.
The standard off-mould finish is a zero-cost option, where the concrete is very smooth to the touch and aggregate materials remain unseen. This is a common selection for products that are made in off-white or standard grey concrete.
Shot-blasting is a common finishing technique recommended for dark-coloured concrete products such as charcoal. Blasting removes the thin topmost layer of the concrete, taking away any visible signs of efflorescence (which is particularly evident on charcoal concrete) and revealing a porous, textured surface. The aggregate materials are usually exposed during the shot-blasting process, and the intensity of the shot-blasting correlates to how much of the aggregate can be seen.
Grinding and honing of concrete are additional surface treatments that are performed after an initial round of shot-blasting. The concrete product is passed over several times with sanding discs of various grades, resulting in a smoother surface texture and complete exposure of the aggregates in the mix. A ground concrete finish has a texture that is still a little rough to the touch, whereas a honed finish involves more passes of the sanding disc to a much finer grade, creating a surface that feels completely smooth to the touch. Honed surfaces remain porous and small air bubbles may be visible.
Polishing is the final step that occurs after a concrete product has been blasted, ground and honed. The product is passed over again with a sanding disc with an extremely fine grade and a densifier and sealant is applied to infill the concrete ‘pores’, making the surface impervious to liquids. Finally, the product is buffed to give the polished concrete a shiny, reflective look.
A visual comparison of the differing textures of blasted, honed and polished concrete surfaces.
Additional fees apply when opting for any of these surface finishing techniques. Shot-blasting adds about 10% to a product’s purchase price. Grinding adds a 40% increase to the price, and honing adds an 80% increase. Polished concrete is the most expensive treatment by far, typically adding 150% to the base product price.
By keeping these tips in mind, designers can confidently incorporate cost-saving measures into their precast concrete design, without compromising on their design intent or quality.