Offsite Structural Options: The Case for Precast Concrete | C-Link

Offsite Structural Options: The Case for Precast Concrete

by James Finnie

This is the first of three articles focusing in detail on the three main offsite structural options: Precast, Steel and Timber.

I must start this article with an apology to concrete, being a biased member of the timber camp, and profess I don’t know everything about it. So, I was pleasantly surprised to discover that concrete hasn’t been “twiddling its fingers”.

Not having paid too much attention to this field, I was impressed by the quality of design, level of detail and building efficiency being achieved these days. The last time I really looked into the use of precast concrete was probably twenty years ago.

I was pleasantly surprised to come across a residential complex which had been thoughtfully designed and detailed in Aberdeen Scotland, near where I was brought up and studied architecture. Being a minimalist at heart, another project that really grabbed me was a 22-storey residential tower in the city centre of Birmingham. Big in stature, clean lines and minimal, it really caught my eye. The large panels appear very smooth and look like stone cladding and small, more sophisticated, details give the building its zing.

I have been dismissing concrete as an offsite option, I see now that it has something steel and timber will struggle to mimic: robustness. It also has thermal mass.

Not everyone’s cup of tea, but my head was turned and interest tweaked.

Cost and programme

I get the feeling that timber, steel and concrete are all very similar options when it comes down to cost and programme, though the precast concrete elements may be slightly quicker to procure. All these materials come with their specific advantages and disadvantages. So it really comes down to best fit for the project type, complexity, size, shape – and possibly gut feel.

When undertaking background reading for this article, I shouldn’t have been (my bias again) but was surprised to see a house being almost totally procured using offsite precast concrete. This highlighted the advantage of adopting a “manufacturing” approach to building where the factory set- up can allow for all shapes and sizes.

Some factories have become incredibly sophisticated with a high degree of automation, which has required a large investment only possible by some of the bigger players, but it’s wonderful to see buildings being procured in such a manner.

Buildability

With offsite construction, buildability is key. The fact that offsite relies on having practically everything designed up front is wonderful. However, as a form of fast track, it’s worth remembering that fast track has a degree of risk in that the upfront design elements are identified and tackled first. So, it’s crucial to ensure all the critical path items are addressed to meet the programme dates, which means constant pressure.

While working for a large commercial American practice, I learnt that the emphasis was to find all the troublesome details and work them out before you got to site. So I like how this (for me anyway) fit into the offsite ethos.

With the emphasis on buildability and the need for it to be established early on, BIM has become crucial to this process. The increasing influence of technology and the opportunities it allows to streamline and smooth the information flow is wonderful.

With precast, everything can be made of concrete and it’s an acceptable finished material. The current specialists in the field are clearly perfecting their systems, as every interface seems to have been considered and fine-tuned.

All elements of the building structure from the frame, floors, lift and stair cores, compartment walls and the external panels are produced and brought to the site, then lifted and slid or dropped into place just as required, having been designed months ago.

The façade panels can come as an insulated sandwich layer where the outer panel is thinner and has the external finish. It is tied through the insulated middle layer to the inner, thicker panel which acts as the structural element. Alternatively, the inner structural panel can be installed first and the outer cladding fixed later. However, with the visual quality that can be achieved with the “sandwich”, why add an extra step later which will require time and money to install?

The thermal mass also acts as a good acoustic barrier which eliminates the need for sophisticated acoustic layers on top of and under the floor.

The panels can incorporate finishes which can be applied in the factory, and electrics and conduiting can also be integrated. All the services cut-outs required have already been predetermined and co-ordinated to allow for the seamless erection of the structure on site. Balcony elements which often form a key design feature in residential developments have seamlessly become part of the system.

With houses being erected in 3 days, a group of 4 apartments being constructed in just over a week, and larger residential complexes being built in half the time, the buildability and speed are impressive. The smooth progression from factory to site to final position in less than a day is also impressive.

I like the fact that the detailing has become a little more sophisticated than the “lego” style sequence of construction. Not all wall and floor panels are simple rectangles, but can sometimes be odd shapes with cut-outs and openings for doors and windows. All parts have been carefully considered for fit efficiently.

They are still the same fundamental elements of a precast building from thirty or forty years ago but just so much more sophisticated and refined.

There are many time-lapse videos available of the various Specialist systems which really help to visualise and clearly explain how such buildings are brought together on site. The speed and efficiency are very impressive.

Sustainability

Concrete leaves a bigger footprint than timber. However, offsite does make for less waste, which means the product is being used more efficiently. Offsite makes for better quality and better detailing, so the building life should increase, which can definitely enhance the sustainability.

Precast concrete buildings are better insulated now, partly due to regulation requirements which address cold bridge interfaces. The thermal mass of concrete allows for a more stable and controllable internal environment.

In well insulated concrete buildings, heating and cooling costs can be reduced. The building performance could be increased, though this would come at an increased cost. With all the usual benefits that offsite construction brings, the precast concrete building is proving to be tough competitor.

Although they may not have some of the benefits of steel and timber which allow for lighter buildings, precast concrete buildings are robust and, depending on the type of external cladding used, façade maintenance could be reduced – another potential benefit.

The ingredients for concrete are all plentiful and typically widely available, which helps in sourcing and local availability.

Impact on Design

Residential design typically involves a degree of repetition that precast can use to its advantage. Using the architectural design principal that buildings should have a base, middle and top, this can easily be extrapolated to suit precast. Take for example a five-storey residential development: the ground floor could be set back using precast columns to transfer the loads from above.

The first, second and third floors could be a more standard module. If the external precast comprises two main panel types, they can be used to break up the rhythm of the façade and introduce a randomness and a more interesting façade.

On the top floor, the introduction of a different finish or opening size also helps break up the face and make it more appealing. Often for lower buildings, a top is not necessary and the design on the middle floors can simply be extend up to the last floor.

Balconies can help the design and create variation and interest on the façade. More importantly a balcony is often a key selling feature to any apartment if they can be accommodated. Alternatively, the “‘Juliet” balcony aims to give the feeling of a balcony, but it never really does.

If circulation stairs and lifts are brought to the external face of the building, they can be used to break up the rhythm of the façade and make it more interesting – and indeed turn the building into a quality design which becomes desirable to the potential buyer.

Some of these principals are reflected in the Forbes Place development at Stoneywood in Aberdeen.

These guidelines are very generic and are dependant on site and orientation and budget, but hopefully help to show how a façade can be broken up and articulated to create interest. There are obviously exceptions where these rules are thrown out the window. There are several (such) examples of social housing in Europe where the designers have pushed the envelope successfully:

Forbes Place, Aberdeen
Aston Place, Birmingham
The Hoxxton, London
Vale de Cambra, Portugal
House in Red Concrete, Norway

In the UK, residential is synonymous with brick. Although often poo-pooed by architects, the “imitation-brick” façades that I have seen have been cleverly integrated and at first don’t give the impression of a veneer. The size of the façade panels and the careful location of the joints and rainwater pipework can help to create this effect.

With the seeming flexibility of precast to enable a high-quality design, this flexibility can maximise passive benefits as well – for example, if the key living spaces have been orientated south to maximise the benefits of the sun.

Again, the robust concrete finish is ideal for common areas and escape stairs where off-shutter concrete can look stunning, particularly if used as a feature element in the space. The main entrance areas and receptions tend to be well but efficiently designed, and the need for double height spaces doesn’t pose challenges for precast structurally.

Apartments can come with internal walls already finished. A screed finish is then required to “iron out” any unevenness in the floor and ceilings installed to the underside. With all services integrated there is no difference from your typical bricks and mortar and you get a nice solid wall around your apartment.

Fire and acoustic design obviously benefit from the mass of concrete. After the recent Grenfell fire tragedy, close attention has to be paid to the external sandwich insulation, which typically uses a glass or stone based insulation to meet regulation.

In Summary, how suitable is precast?

Writing this article has educated this writer. I would say precast is definitely a suitable structural material for offsite construction and that it is up there with timber and steel. On a cost comparison basis it’s difficult to get specific numbers but it would seem that precast is a more expensive material initially, though over the life of the building the costs should balance out and potentially be cheaper, as timber and steel require more maintenance.

Concrete brings a robustness and a feeling of solidity to a building and I think subconsciously most people prefer that. Although we have tended to associate precast buildings with large areas of old grey boring concrete. that certainly isn’t the case any longer. Precast facade finishes can create slick and modern buildings.

I would say that precast is right up there and maybe shows itself to be slightly more versatile than steel and timber after having studied quite a few residential examples.

As architects and builders, we should be open to new and more efficient systems – or at least consider their qualities in relation to our practices.

Image credit: Photo by Ricardo Gomez Angel on Unsplash

About James Finnie

James has been based in Cape Town for the last 14 years, trained in Scotland, and has worked with some prodigious practices over the years. He recently started his own architectural practice where the philosophy of Kind Design puts kindness at the centre of the creative process.

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