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The dash for ash

Lightweight concrete is usually restricted to high-end buildings where engineers need to keep the weight down. Declan Lynch explores how the product could also benefit smaller structures.

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Savings: Lytag can achieve weight reductions of up to 31% in concrete frame buildings

London’s skyline is rapidly changing, with eccentrically named high rise buildings like the Gherkin, the Shard and the Cheesegrater springing up in recent years. To build such large structures, engineers have used lightweight concrete, which uses lightweight aggregate products for the floors, to help reduce dead load.

Relatively expensive lightweight concrete is mainly used in tall buildings where its can offset its high cost. But could these benefits also be applied more widely?

Richard Baldry, business director at Lytag Ltd, which produces lightweight aggregate Lytag, certainly thinks so.

“Engineers often only looked at Lytag concrete when they had a problem,” he says.

Lytag can be used to replace coarse or fine aggregates in concrete. Its low density and high strength means it is about 25% lighter than concretes using traditional aggregates.

But despite the inherent benefits of reducing the concrete’s dead load it has never been universally adopted, according to Baldry.

“It’s most commonly used for screed [concrete facing] or structural concrete,” he says.

Lytag is produced from pelletising pulverised fly ash (PFA), a by-product of burning coal at power stations. It is technically known as “sintered pulverised fuel ash lightweight aggregate” and is made in specially designed pelletising pans by adding water to the material, which is then heated to temperatures of around 1,100oC.

The finished product is a hard, honeycombed structure ranging from 14mm diameter pellets to fines.

In support of its efforts to get Lytag more widely used, Lytag Ltd recently commissioned independent civil and structural engineering consultant Peter Kelly to conduct a study to see if using the product in lower rise structures could generate cost savings.

He found that savings could be made in the cost of foundations, structural steel and structural concrete in building. “It might eliminate the need for piles, for example,” says Baldry.

Kelly calculated loadings in a six and 30 storey high building using steel frames and concrete frames.

The biggest weight reductions came in a 30 storey concrete frame building, which was 31% lighter, while a similarly sized steel frame structure with Lytag floor slabs could be 21% lighter. Weight reductions in a six storey concrete frame were 25%, while a six storey steel frame was 16% lighter.

“Engineers often only looked at Lytag concrete when they had a problem”

Richard Baldry, Lytag Ltd

Baldry believes savings in other parts of a structure can offset the fact that Lytag is between 30% and 100% more expensive than standard aggregates.

It also has the benefit of increasing a building’s insulation properties compared to standard aggregates.

The only other area where the use of lightweight aggregate is less prevalent is in columns. Baldry says there is no reason why it cannot be used in these, but says that because the Eurocodes make no specific reference to its use, designers and contractors are more cautious about using it.

He adds that even though coal fired power stations are slated for closure over the next few years, there will be no shortages of Lytag, as there are huge stockpiles of PFA.

Readers' comments (1)

  • This isn't new. I attended a lecture on concrete skyscrapers by a Chicago Engineer in 1967 in which he assumed low density concrete concrete 30% stronger than we normally used. This meant substantial dead load savings throughout a structure which would accumulate down to the foundations as lower loads meant smaller sections saving even more.

    Uffa Fox said that weight was only an advantage in a steam roller.

    Archie

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