September 18, 2006

Environment

Recycled materials improve concrete mix

Industry steps up efforts to cut emissions

SAUL CHERNOS

correspondent

TORONTO

With increased recognition that greenhouse gases contribute to climate change, the concrete industry is stepping up efforts to reduce carbon-dioxide emissions by boosting recycled content.

While using wastes from other industrial processes may predate the blue box, additives such as fly ash, slag, silica fume and demolition and leftover concrete are gaining popularity thanks to the availability of new recycling and integration technologies, coupled with rising environmental concern and the advent of green construction standards.

Ingredients such as fly ash, a by-product from coal-fired electric plants, and slag, a leftover from steel production, may seem antithetical to an environmental effort. However, John Hull, president of the Ready Mixed Concrete Association of Ontario, says waste additives can improve the mix.

“These additives actually improve many of the qualities of concrete. The whole premise of concrete is the denser you make it, the better it is.”

Each additive is unique. Fly ash is finely-powdered, similar to cement but rounder and smoother, while slag needs to be ground up.

“These additives actually improve many of the qualities of concrete.”

John Hull

President, Ready Mixed Concrete Association of Ontario

Hull says silica fume, a waste from the silicon chip industry, is so fine that adding five to eight per cent of it by weight of cement into concrete fills all the voids and produces an extremely dense, durable product.

Another popular additive is concrete itself.

Hull says delivery leftovers often end up used in retaining walls, precast curbs and other such objects, while concrete from demolitions is crushed with pulverizers, and large magnets are used to remove rebar (which has its own waste stream). It can then be effectively used to replace gravel in a new concrete mix.

“If we weren’t using that, we would be using raw aggregates, which is a non-renewable resource,” Hull says.

Availability varies by product and region. Fly ash and slag are inexpensive, but if a project is nowhere near a coal or steel plant, transportation costs can boost the price tag significantly. Silica fume also has availability issues and tends to be costly.

Use of additives can present construction challenges. Rico Fung, director of engineering with the Cement Association of Canada’s Ontario region, says this can prolong setting. While this sometimes causes minor delays, it can give work crews extra time for finishing, particularly during the summer when cement tends to dry quickly.

“In warmer weather, when you want to slow the process, you might use a higher percentage of materials, perhaps (as high as) 30 to 50 per cent,” Fung says.

One factor driving recycling in the concrete industry is increased desire for environmental protection. Green construction standards, such as LEED Canada, reward builders using recycled content.

“We recognize the use of supplementary cementitious materials in our Materials and Resources Credit 4,” explains Ian Theaker, LEED Canada program manager with the Canada Green Building Council.

“With most materials we base the percentage on the total cost, but that doesn’t work for materials that substitute for cement and concrete, so we modified that particular indicator so it refers to the amount of carbon dioxide that would otherwise be used to create the cement that has been displaced.”

Some details still need to be ironed out. While it’s easier to set comparatives for poured-in-place concrete, concrete block presents technical complexities.

Theaker says a technical advisory group task force is currently looking at how to address materials within a concrete block.

Another effort under way to facilitate recycling is the development of web-based software to help optimize use of recycled materials in individual projects. Ready-mix providers generally perform quality-control so that proportions of all ingredients are known and tested.

However, Michel de Spot, president of EcoSmart Foundation, a nonprofit organization that serves as a resource centre for the cement and concrete industries, says builders generally use conservative estimates when deciding how much recycled content to use.

The SOS Project, as the software development effort has been dubbed, is looking to automate the process so it’s easier to optimize to the right amount.

“We want to have a repository of knowledge and help users access it so they can figure out for themselves how much fly ash and other materials they can use in a particular project,” says de Spot, who is LEED-accredited.

The software, slated for availability in 2009, will allow multiple parties in a project to work in an interactive fashion, in keeping with principles of integrated design.

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