No matter what other materials are used to construct a building, concrete provides the solid foundation needed to stand the test of time. It is humanity’s most widely used construction material dating back to before 6500 B.C.
As popular as it is, concrete has an environmental downside — it’s one of the largest sources of embodied carbon in construction materials. At PCL, we’re working to find ways to reduce or offset this embodied carbon so we can deliver outstanding results to our clients while meeting their sustainability goals.
Embodied carbon refers to the carbon dioxide created through a construction material’s manufacturing, transportation and usage on a job site. Clients and local governments are setting ambitious carbon reduction goals — the City of Vancouver, for example, recently targeted a 40% reduction in embodied carbon emissions associated with new builds by 2030.
Concrete is a composite material, and almost all its ingredients contribute to its embodied carbon through their extraction and manufacturing processes. The main ingredient in concrete is Portland cement, which is produced by burning limestone in kilns at temperatures between 2,300- and 3,000-degrees Fahrenheit (1,260-1,649 degrees Celsius). Getting those kilns up to temperature requires a massive amount of energy, and the combustion — along with the chemical reaction involved in making cement — releases significant quantities of carbon dioxide. All told, producing one ton of Portland cement results in roughly one ton of carbon dioxide emissions.
As much as 79% of the carbon emitted in making concrete comes from cement production, but cement only makes up 13% of most concrete mixes. The rest is sand, aggregate and water. According to the Zero Energy Project, concrete is the single largest source of embodied carbon in buildings, accounting for up to 55% in some cases. Additionally, concrete production is responsible for 8% to 11% of global carbon dioxide emissions.
As part of our broader focus on sustainable building practices at PCL, we’ve identified embodied carbon as an area where we can have a significant impact. With concrete being a large factor of a building’s embodied carbon emissions, our experienced construction and design experts have researched several options for lessening concrete’s effect on a project’s overall carbon footprint.
The first option is to use concrete mixes that contain less Portland cement, replacing it with other materials. For example, Portland-limestone cement — which incorporates between 5% and 15% limestone — can be used in place of Portland cement in concrete mixes, reducing carbon emissions in manufacturing by about 10%. Conversely, fly ash —particulates collected from flue gases in coal-fired power plants — can replace between 40% and 70% of Portland cement in concrete mixes, depending on the scale of the project.
Other replacement ingredients include slag from iron smelting blast furnaces; silica particulates collected from metal production; and natural materials like rice husk ash that, when finely divided and exposed to moisture, react chemically to form strong compounds that work well to strengthen a concrete mixture.
Another option involves sequestering carbon permanently into concrete, increasing its strength while offsetting manufacturing emissions. Some companies inject carbon dioxide directly into concrete mixes, while others combine it with steel slag to help concrete cure in climate-controlled chambers.
While some of these reformulated concrete mixes are only available in certain areas, they can dramatically reduce carbon emissions by 10% to 100%, or even be carbon negative in the right circumstances and are becoming more widely available as demand for low-carbon alternate mixes grows. Alternate mixes generally have little effect on placing, finishing or setting times. Curing can take longer, but that may be beneficial from a thermal control perspective for high-volume pours.
While there may be a premium cost for low-carbon mixes over more than standard concrete, the environmental benefits are huge and can aid an owner’s sustainability goals. Reducing a building’s concrete carbon intensity by 25% to 50% can lower the embodied carbon of the entire project by 12% to 25% while typically only costing 0.1% to 0.5% of the overall budget.
Other carbon-reduction methods include the following:
- Incorporating recycled aggregate and aggregate substitutes.
- Reducing rebar usage through alternate designs.
- Purchasing carbon credits from projects that deliver immediate emission reductions through sustainable development and renewable energy.
Research is a great first step, but it’s all for naught if it isn’t implemented. At PCL, we’re taking the next step, using low-carbon concrete mixes in some of our latest large-scale projects.
In building the new east deicing apron at the Calgary International Airport, our teams selected a concrete mix from CarbonCure Technologies with injected carbon dioxide. This was the first time this mix had been used in Canada, and the reviews were positive. On average, every cubic meter of CarbonCure concrete poured offsets 16 kilograms of carbon emissions; on this project, which was the size of six football fields, the offset totaled 160 tons.
Meanwhile, at the Capital Power Genesee Repowering project in Warburg, Alberta (southwest of Edmonton), our industrial team used EcoPact concrete from Lafarge, which replaces a high percentage of Portland cement with other materials. This resulted in a 20% reduction in emissions compared to standard mixes, equating to more than 340,000 kilograms, or 340 tons of carbon saved for our scope of work.
At PCL, we’re always looking for new ways to build sustainably for our owners and we’re always thinking about the best ways to put these new methods into practice for our clients. Our experts are ready to put their knowledge to work on innovative new projects that move the construction industry one step closer to a net-zero-carbon future.