A close friend of mine from my days at the University of Waterloo’s Civil Engineering program is now completing his Master’s degree, with a focus on concrete. Jeffrey Ianni, P.Eng, describes a way to reduce carbon emissions in concrete production by up to 15%:

Concrete is the most used building material in the worldIn the face of rising CO2 emissions due to human development and increasing global populations, any effort to find material efficiency can contribute to the solution for attaining global sustainability as a species.

In 2007, CO2 emissions from cement production represented 4.5% (377 million metric tons) of the global CO2 releases. Current concrete supply practice typically uses only two grades of aggregate: fine and coarse, causing “gap graded” or “poorly graded” concrete pours.

“Well graded” aggregates can save up to 15% of cement paste required. Therefore, aggregate selection can potentially reduce 15%*4.5% = 0.675% of global CO2 emissions.

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Poor or gap graded aggregate requires more cement to fill in the gaps, and therefore requires more carbon in production. Credit Civil Engineer’s Forum

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Gap Graded Aggregate

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Well Graded Aggregate, credit Portland Cement Association

The above image represents what “well graded” aggregate looks like: a perfect amount of every size of stone from sand to pebble. Well graded aggregate can reduce porosity, permeability, and shrinkage, which improves performance and durability. It also makes for a more consistent finish, which I hear architects loveFurthermore, A reduction in cement content can lower crack vulnerability, making concrete less susceptible to corrosive damage and future repairs, which reduces the life-cycle CO2 costs of concrete  and litigation costs due to failed concrete.

If you are an Architect on a project and you can’t get around using concrete, you can require your contractor to provide this kind of aggregate to reduce on emissions. Concrete with exposed aggregate finishes illustrate whether or not the pour was “well graded”; I would love to have included a photo of what “well graded” concrete looks like, but its use in the field is exceedingly rare due to the aggregate industry primarily supplying mostly two sizes of stone to contractors. This could conceivably be addressed by changing our energy codes.