Q and A: Low-carbon concrete

My project requires low-carbon concrete. What is that, anyway?

There’s no universally accepted definition of low-carbon concrete. Typically, though, low-carbon concrete has a lower global warming potential than traditional concrete with the same strength.

How can I reduce the global warming potential of my concrete?

St. Croix Crossing bridge aerial view | Beton Consulting Engineers
Beton proportioned sustainable concrete mixtures incorporating supplementary cementitious materials for the St. Croix Crossing between Minnesota and Wisconsin. Photo: MnDOT.

The first strategy is to reduce its portland cement clinker content. Making portland cement clinker burns a lot of fuel to heat the kiln. It also releases CO2 from the calcination of limestone. In all, producing a pound of portland cement clinker releases about 0.9 pound of CO2. You can reduce the clinker content by using a blended cement, a Type IL (limestone) cement, or a mixture of portland cement and supplementary cementitious material(s).

Keep in mind, though, that even if the strength is comparable, other performance characteristics may not be. Be sure to specify any other performance criteria you need. Some things that could change are setting times, finishing characteristics, and freeze-thaw durability.

Optimizing the concrete mixture proportions can help you further reduce the clinker content. The aggregate suspension method and judicious use of admixtures are ways to reduce the overall cement paste content.

What is overdesign? How does it relate to low-carbon concrete?

Ready-mixed concrete producers often target a higher average strength than the specification requires. Essentially it’s a form of “insurance” against things they can’t control: just add some extra cement to avoid rejection of the concrete. We’ve discussed previously how improper testing yields artificially low results. But for every pound of extra cement, you’re increasing the carbon footprint by 0.9 lb. Prescriptive specifications can also result in overdesign, as they don’t allow the concrete producer to optimize the mixture proportions.

Are there other ways to reduce the carbon footprint?

Yes. We’ve talked about how to reduce the carbon footprint of a cubic yard of concrete. But it’s worth looking at how the design of the structure affects its carbon footprint. For example, if you prestress or posttension a girder, you could use less concrete to carry the same load over the same span. Or you could lengthen the span and have fewer columns. If you use high-strength concrete for the columns, you increase the cement per cubic yard but use less concrete per column.

Good concreting practice reduces the carbon footprint by doing the work right the first time. That way there’s less need to remove and replace substandard work. And the concrete lasts longer, too.