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Wasted potential

CONCRETE ENGINEERING - Using lightweight aggregate made from refuse material could slash the cost of concrete bridges. So why is it so rare in the UK? Colin Cleverly reports.

Lightweight aggregate concretes (LWACs) are extensively used in the US, Canada, Europe, and Japan for the construction of highway bridges.

But their take up in the UK has been slow.

To help deal with this, the Concrete Bridge Development Group (CBDG) has published a new technical document called 'Guide to the use of LWAC in Bridges'. The paper is aimed at helping UK engineers understand the pros and cons of using the material.

The CBDG is a body of 100 major companies from the concrete and bridge industries, and includes bridge owners, consultants, contractors, suppliers and academics.

Using lightweight aggregates primarily means that the weight of the concrete is reduced, so cost savings can be made from using less reinforcement and prestressing steel. The reduced weight also leads to lower transportation costs for precast elements, and less formwork and propping for in-situ construction.

The size and number of foundations used to support the structure can also be reduced.

The biggest benets of using LWAC are realised on long-span bridges, where the weight of the concrete and bridge finishes is relatively high compared to the amount of traffic across them.

But there are benefits beyond reduced weight: a lower thermal expansion coefcient;

higher strain capacity; and less cracking from drying shrinkage and thermal effects due to a 30% lower elastic modulus compared to normal-weight concrete.

LWAC is also more resilient to frost and fire damage; and its lower thermal conductivity means it can be placed at lower temperatures.

So where are the drawbacks?

Well, LWAC will cost more initially, although studies in Norway and the UK have found that there are overall savings due to reductions in concrete volume, reinforcement and prestressing.

In fact, the biggest sticking point for LWAC is that if it is used as a substitute for normal concrete, modifications to bridge design and construction may be necessary.

The CBDG guide says that the design needs to accommodate a lower tensile strength and a reduced resistance to locally concentrated loads.

It also says that during construction, more rigorous control is needed to accommodate differences in the characteristics of the aggregate and the resulting concrete.

But this does not seem too much to take on board if the cost savings stack up.

Colin Cleverly is director of the Concrete Bridge Development Group.

What is lightweight aggregate?

Lightweight aggregate can be manufactured from waste such as fly ash, a by-product of coalred power stations. Its use is encouraged by the government to reduce dependency on primary aggregates. Other potential sources of lightweight aggregate under investigation include fly ash from the incineration of refuse, paper, bio-mass conversion and silt from sewage works.

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