Contractors are likely to have heard of low carbon concrete, used in mega projects like Amazon’s second headquarters in Arlington, Virginia and Microsoft’s Quincy, Washington, data center. Although not yet the norm on job sites, the use of greener cement mixes is growing rapidly.
Some of that growth has been spurred by the federal government, which is developing potential global warming requirements for concrete as part of its Buy Clean initiative that will be used to award funding for its projects. The federal government is creating these requirements to allow the environmental impacts of different materials to be compared.
Concrete is the literal foundation of the modern building, the second most used substance on Earth after water, but it also contributes at least 8% of all human-caused carbon emissions, according to a study from the University of Rochester. There are now a wide variety of low-carbon mixes and carbon sequestration methods on the market that have a lower environmental impact than standard Portland cement.
There are many considerations for contractors regarding these new formulations. Green mixes tend to behave differently than Portland cement alone, and the availability of a given mix in a given area varies as concrete is produced regionally.
Using one of these materials for the first time can be intimidating, but there are simple ways to get started, said panelists at the Greenbuild 2023 International Conference and Expo in Washington, DC, last month. Here are some of his tips on how to successfully experiment with green concrete.
start small
The first time you try a low-carbon material, don’t use it for the whole project – start with a low-level component that requires only minimal load, such as rat slabs, temporary access mats, curbs or walls of gardening, Lindsey said. Maclise, licensed structural engineer and director of Forell | Elsesser Structural Engineers.
Use these test projects to understand how the material behaves and to win over the more skeptical members of the project team.
Take time for the test
Low-carbon concrete typically uses less cement by replacing some percentage with supplementary cementitious materials such as fly ash, slag and silica fume, all industrial by-products that can play a similar binding role. SCMs can be used individually with Portland or mixed cement and in different combinations, according to the Portland Cement Association.
Do more test cylinders when using a new cement for the first time, Maclise said. Try a few different types of low-carb mixes and see what works best for your needs.
Find the right customer
According to Jessie Buckmaster, director of sustainability at San Francisco– based at Hathaway Dinwiddie Construction. Some homeowners value reducing their project’s carbon footprint more than others and are more willing to experiment and pay the cost of experimentation.
As the market goes through the process of adjusting to this flood of new options, low-carbon concrete continues to cost more than Portland cement. Fifty years ago, fly ash blends were similarly novel and at a premium, but now they have become commonplace, Maclise said. Prices vary, but are coming down, and green mixes sometimes qualify for carbon offsets, Buckmaster said. Work with the owner to establish bid leveling criteria to account for low-carbon concrete.
Talk to your provider
There are now at least dozens of types of low-carbon cements on the market, but not all are mass-produced or available in all regions. Talk to your supplier about which green mixes can serve a given project, said Juan Gonzalez, manager of strategic development and sustainability at Central Concrete Supply in San Jose, California. Some helpful questions to ask your provider include:
- What low carbon mixes are available in your concrete catalog?
- What are your low carbon concrete strategies?
- What SCMs are available for mixing with Portland cement?
- What are ways to take carbon reduction to the next level in your project?
There’s a significant amount of overdesign with low-carbon mixes because they’re still new, Gonzalez said. It is important to share concrete field data with suppliers so they can use the feedback to requalify low-carbon mixes for their true design strength.
Collaborate with team members
The GC, concrete supplier and structural engineer should meet in advance to ensure mixes are optimized to minimize global warming potential and to discuss how to manage the project schedule to accommodate the new formulations, Maclise said. The structural engineer should be involved throughout the process for the experiment to be successful, Maclise said, and it’s also important to collaborate with the design and preconstruction teams.
Be ready for the push
Sometimes engineers who aren’t already familiar with low-carbon mixes are resistant to change, but contractors can sometimes push engineers out of their comfort zone, Buckmaster said. Let them know that reducing your carbon footprint is a priority, and connect them with other engineers who have used the material to answer questions and offer advice.
“As a GC, don’t be afraid to ask questions, don’t be afraid to suggest new things,” Buckmaster said. “I think sometimes we feel like, ‘Well, we just build what we’re told,’ but I think we’re in a very unique position today where we can also help drive the conversation.”
Know the standards
Note that new technologies and low-carbon mixes may need approval from government authorities. The General Services Administration, which leads the federal government’s procurement efforts, released standards for low carbon concrete in 2022 which are being modified based on industry feedback.
In the meantime, contractors could use these federal benchmarks to guide their maximum embodied carbon limit, or carbon budget, for their project, said Prateek Jain, senior environmental designer at Atelier Ten USA in San Francisco.
Adjust the project schedule
You’d expect low-carbon mixes to behave differently than Portland cement, Maclise said. Green concrete mixes can take longer to set and have a slower strength gain, so contractors should adjust project schedules accordingly. Coordinate work hours and formwork rotation, taking into account longer curing times, as well as the season and expected temperature extremes.
Request mix design submissions in advance to allow for testing, as well as sample environmental product statements, Buckmaster advised.
Have a backup plan
Get used to the excess force assumed in the design, or the force higher than the seismic code requirements, Maclise said, and don’t get nervous about not hitting the excess force numbers. Know your local building codes and their true requirements, he said, and know your “outs,” or alternative ways to add strength and meet requirements for a particular part of a project if the concrete doesn’t fit as was expected
More test cylinders will likely be needed to give contractors a more accurate idea of how the material behaves, Maclise said, and tools like maturity sensors can help provide reassurance. Have alternative mixes ready that are still within the project’s carbon budget in case one doesn’t perform as intended, but don’t give up after a bad pour.
“Cement manufacturing is quite carbon intensive and cement drives the high carbon content of concrete,” Jain said. “By encouraging these new technologies, even on a small scale, what we really want to do is send market signals to manufacturers to decarbonize.”
The Greenbuild Conference is owned by Informa, the parent company of Industry Dive.
