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Remix it up to boost concrete's sustainability

How the changing the ratios of concrete components can improve its sustainability.

Concrete is one of the most consumed substances on earth, second only to fresh water.

Concrete is, of course, an incredibly useful construction material offering durability, fire resistance, thermal mass and end-of-life recyclability, but it does have a significant environmental impact.

The production of cement, one of the constituent materials in concrete, is estimated to account for 5 per cent of global carbon dioxide emissions according to the Cement Technology Roadmap 2009.

The other main materials, coarse and fine aggregates, while only making a small contribution to the embodied carbon of concrete, create other social and environmental impacts associated with extraction, affecting communities local to the quarries through movements of heavy goods vehicles, dust and noise. 

Minimising concrete’s impact

The single most significant opportunity to minimise the impact of concrete is to minimise its use through efficient and intelligent design.

It is worth investigating the suitability of voided biaxial slabs, which can reduce the amount of concrete required by 30 to 50 per cent, or contemplating the use of post-tensioned slabs, which can cut slab depth by about 25 per cent.

“To promote low-carbon concrete, it is crucial to ensure the cementitious content is kept to the required minimum”

The cementitious content of a concrete mix design for the same strength and exposure class can vary by more than 10 per cent. To promote low-carbon concrete, it is crucial to ensure the cementitious content is kept to the required minimum. 

Sometimes the inclusion of cement substitutes such as pulverised fuel ash (PFA) or ground granulated blast furnace slag (GGBS) can lead to an increase in the total cementitious content, thereby reducing the beneficial impact these substitutes have on the embodied carbon of concrete.

Admixtures play a valuable role in enabling the use of cement substitutes without an increase in the cementitious component.

Consider the ratios in the mix

It is important to stress that mixes incorporating up to 30 per cent PFA and 50 per cent GGBS do not experience any impact on 28-day strength profiles. 

Above these ratios, to a maximum of 40 per cent PFA and 70 per cent GGBS, design teams will need to explore options for specifying 56-day strength characteristics, based on a clear understanding of construction programme and risk.

“Early engagement with the supply chain including the design team, concrete contractor, formwork supplier and concrete supplier is paramount”

With regards to coarse and fine aggregates, the environmental impacts associated with quarrying can be minimised by specifying a recycled or secondary aggregate; stent – a by-product from the China clay industry, glass sands, recycled concrete aggregate (RCA) and granulated blast furnace slag (GBS) all offer opportunities for substituting primary aggregates.

A 100 per cent replacement of coarse aggregate with stent and GBS can be accommodated without significant impact on finish quality or performance.

Supplier engagement vital for sustainable concrete

When pushing the boundaries of sustainable concrete mix design, early engagement with the supply chain including the design team, concrete contractor, formwork supplier and concrete supplier is paramount, particularly where architectural finishes are required, concrete is precast or alternative design techniques such a post-tensioned slabs are proposed.

There are now many successful industry examples of sustainable concrete, but there is still considerable room for improvement. As with many aspects of sustainability, that improvement can be delivered through early collaboration and co-ordination throughout the whole value chain.

Chloe Souque is senior sustainability consultant and Kirsten Henson is a director at KLH Sustainability

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