Could Composite CLT be the Next Big Thing in Australia? 1

Wednesday, September 7th, 2016
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Composite flooring systems consisting of cross laminated timber with reinforced concrete have significant potential for use in multi-storey buildings such as offices, hospitals and schools throughout Australia, according to a leader within the engineered timber sector in Australia.

Following tests in the US in which a flooring system involving a cross layered timber deck topped with a thin layer of reinforced concrete was shown to support loads of around eight times what is required by code in that country, XLam technical manager Nick Hewson says there are opportunities to use similar types of systems in Australia.

Hewson says concrete and timber work well together as an efficient way in which to structure the form of floor spans, with one working in compression and the other in tension.

Because of this, whereas the maximum span length which pure CLT flooring systems on their own could reach in an economically feasible manner is around six metres, use of a composite could instead see this increase to around eight or nine metres.

Moreover, Hewson says, concrete screeds were being used in many CLT buildings in any rate in a non-structural function in order to achieve required levels of performance with regard to acoustics and vibration.

In such cases, he says there is a case to be made for using concrete to achieve gains in structural performance as well.

Whilst pure CLT models were generally working well in apartment buildings without the need for the composite concrete topping, Hewson says composite-based models could well be a feasible solution in more of a commercial and open plan environment such as offices and other public buildings such as schools or libraries.

“I think the composite concrete CLT shows a lot of promise for longer span applications in office buildings,” he said. “I see it in commercial buildings, public buildings and schools – maybe less in apartments.”

Hewson’s comments follow the results of testing which was performed in the US by Oregon State University along with architecture and engineering firm Skidmore, Owings & Merrill LLP of a 36-foot-by-eight-foot specimen consisting of a cross laminated timber deck which was topped with a thin layer of concrete. The two were joined together and made to form a composite by virtue of connectors.

During testing – throughout which the specimen was loaded with a hydraulic actuator and from which results were recorded by 48 different sensors over the course of two hours – the system was found to support an ultimate load of 82,000 pounds, which is roughly equivalent to eight times the required design load under the US building code.

Despite its advantages, however, Hewson acknowledges that the system has drawbacks.

Whilst timber is clean, fast and relatively easy to work with, he says the re-introduction of concrete brings with it additional complications.

These include having to deal with ‘wet’ trades on site; needing to manage concrete lorries, dust and water; having to manage the moisture associated with concrete and how that interfaces with timber; and needing to allow sufficient time for the concrete to cure.

Additional weight associated with the reintroduction of concrete, meanwhile, might necessitate thicker walls or stronger foundations, Hewson said.

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  1. Linda Lodetti

    Innovation is about pushing the boundaries. Making the results of testing more widely available will add to general body of knowledge and advise in what circumstances to apply hybrid solutions for optimum application.