Researchers Underestimate CO2 Sequestration Rate in Dry-Cast Concrete

A new ASTM International paper presents results of a National Concrete Masonry Association (NCMA) Education and Research Foundation investigation quantifying the amount and rate that machine-molded concrete products absorb and permanently lock up atmospheric carbon dioxide.

“Conceptual Test Protocols for Measuring Carbon Sequestration of Manufactured Dry-Cast Concrete Products” covers lab work undertaken to develop and refine procedures that deliver repeatable, accurate measurement of CO2 uptake in building or hardscape block or other similarly formulated and fabricated concrete elements.

While they continue to gauge long-term sequestration characteristics, investigators observed block specimens averaging 1.3 lb./ft.3 (21 kg/m3) uptake at 28 days, the most common interval for reporting cured concrete compressive strength. That CO2 volume, “Conceptual Test Protocols” authors note, is “significantly higher than would have been predicted using wet-cast concrete carbonation data.”

The thrust of the investigation, they add, centered on “the relatively porous nature of dry-cast concrete compared to wet-cast concrete, making it a potentially attractive candidate for carbon sequestration. While considerable research has been conducted on carbonation of wet-cast concrete, there has been limited research on quantifying carbon uptake in manufactured, dry-cast concrete products, including concrete masonry units.”

“Some investigations have focused on quantifying the potential shrinkage of concrete as it carbonated to mitigate cracking, whereas other studies have measured changes in pH levels within the concrete and the resulting effects on the corrosion of embedded ferrous metals,” the authors explained.

Current market drivers, however, have shifted beyond the functional performance aspects of carbon sequestration of cement-based materials and are now placing much more emphasis on the embodied carbon, and potential carbon sequestration, of construction materials.”

Authoring “Conceptual Test Protocols” are Craig Walloch of ACM Chemistries, Norcross, Ga.; Donald Broton and Laura Powers of CTLGroup, Skokie, Ill.; and Jason Thompson of ICPI-NCMA.

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