University project delivers lessons in low-carbon concrete
When Belfast Valley Contractors topped out the new Georgetown University residence hall in June, the day marked the completion of both an ambitious contract and a new foray into green building for Belfast Valley.
Striving to reach LEED Platinum certification, the project team for the 12-story, concrete frame, post-tension building on H Street had decided to forego using traditional concrete and instead use ECOPact products. ECOPact integrates upcycled construction and demolition materials as well as other supplementary cementitious materials to produce mixes that generate 30 percent to 100 percent less carbon emissions than standard concrete mixes.
“This was a learning curve for everyone involved,” said Kevin Riley, Project Manager at Belfast Valley Contractors.
Working with ready mix supplier, Aggregate Industries, as well as the project’s structural engineer and a third-party inspector, Belfast Valley tested numerous, custom mixes to determine which concrete mixes would meet the project’s specifications. The ECOPact products easily met or exceeded 28-day strength requirements.
“But getting early strength was a little tougher to achieve,” Riley said. “The biggest challenge was the mix for the post-tension decks because with post-tension decks, you only have a certain window of time to stress the cables. We were also tasked with accelerating the original bid schedule by two weeks, so we had to hit our cable stressing windows of two or three days after we poured each deck.”
Meticulous testing and mix adjustments ultimately delivered a mix suitable for the post-tension decks.
“It was almost like it was a specialized product just for us, just for that job,” Riley said.
Furthermore, the ECOPact products “pumped beautifully” onsite without experiencing any clogs or mix segregation. By using ECOPact, the project team also lowered the carbon footprint of the concrete to 56 percent of the footprint of traditional concrete.
Impressed by the unconventional project, Riley suggests that more projects should consider using low-carbon concrete.
“Cement is a finite resource. Eventually, we are going to run out of limestone, we are going to run out of cement, so I think as an industry, we need to start looking at these types of solutions to further the longevity of this industry,” Riley said.
Build With Strength – a coalition of industry professionals brought together by the National Ready Mixed Concrete Association (NRMCA) – is striving to move the design and construction industries to lower carbon concrete.
“Through education of the industry and increased demand for lower carbon products, our industry has become more efficient at mix designs and that has resulted in improved benchmarks for carbon impact,” said Lionel Lemay, Executive Vice President for Structures and Sustainability at NRMCA.
Benchmarking completed by NRMCA in 2019 showed a 13 percent reduction in carbon footprint industry-wide in the previous five years. The association has signed onto the Architecture 2030 challenge to reduce the carbon footprint of construction materials 50 percent by 2030.
The carbon reductions realized to date are due mostly to more efficient use of Portland cement, the main contributor to carbon emissions from cement mixes.
The concrete industry, Lemay said, is capable of making significant, additional gains in cutting carbon impact by “becoming more performance-oriented and eliminating some prescriptive requirements” in construction design documents that often unnecessarily increase the Portland cement content.
“If we can eliminate some of the artificial limitations on materials, then concrete producers who have qualified people and laboratories to do testing and experimentation and quality control, can really develop cutting edge mix designs that optimize the cementitious material content and meet all the requirements as a structural material,” Lemay said.
By educating owners, engineers and builders, NRMCA is also hoping to create another opportunity for greater use of low-carbon concrete. Post-tension slabs, Lemay said, are probably the best example of how construction schedules, not structural requirements, limit the use of low-carbon concrete. If a builder needs to strip forms and tension a slab two days after a concrete pour, the concrete contractor will have to use higher levels of cement in the slab mix to achieve high strength levels just two days after the pour.
“But if an owner says I am willing to compromise a day or two or a week in between each floor slab, then that concrete slab doesn’t have to be overdesigned,” Lemay said. “You are going to have a lower carbon footprint because you didn’t have to meet such a rigorous schedule.”