Marin broke ground last month with the passage of a new ordinance that regulates the production of concrete, a material whose manufacture results in an estimated 6 to 10 percent of global anthropogenic carbon emissions.
The ordinance, the first of its kind nationwide, specifically sets new limits on the use of cement, the primary ingredient in concrete and the main cause of concrete emissions. Anyone pouring new concrete in Marin now must either reduce the amount of cement used in their mixes according to new standards or demonstrate that they have lowered overall emissions through other means. That can mean employing alternative materials that act like cement, such as fly ash, a byproduct of coal combustion.
“Embodied carbon emissions are a big deal, and cement in particular comes at a very high cost to the climate,” said Bruce King, a San Rafael structural engineer who was instrumental in drafting the ordinance.
County planner Alice Zanmiller explained the focus on so-called embodied emissions in a letter to the Board of Supervisors, which passed the ordinance on Nov. 19.
“Because the emissions from material extraction, transportation, manufacturing, and building construction are already emitted by the time the building is occupied, there is little potential to mitigate those impacts later in the building’s life, as is possible with energy efficiency retrofits for operational emissions,” she wrote. “The importance of addressing embodied carbon emissions is heightened by the pressing need to reduce emissions in the near term to avoid the most catastrophic impacts of climate change.”
Cement is formed by subjecting limestone in a kiln to high heat and then grinding the result and combining it with gypsum, a soft sulfate mineral. Its creation causes emissions in two primary ways: bringing a kiln to high temperatures involves burning fossil fuels, and the heating of limestone, which is composed mainly of calcium carbonate, directly releases carbon dioxide.
Last year, the Bay Area Air Quality Management District awarded the county and several partners—including Mr. King’s group, the Ecological Builders Network—$200,000 to develop Marin’s new standards. A group of stakeholders, which included other representatives from academia, the building trades, the concrete industry and other local governments in the Bay Area, convened over the past year to create the new policy.
On Dec. 3, Berkeley passed a similar ordinance, and other cities are also considering it.
The ordinance, which added a subchapter to the county’s building code that will go into effect in January, establishes a new sliding-scale maximum amount of cement allowed for concrete mixes of varying strengths.
Should builders choose the second pathway the regulation offers, they could innovate new ways of reducing the overall emissions in the production of concrete in a number of ways. For example, Mr. King said builders could select brands and types of cement that cause less emissions, leave more time for the concrete to cure, or use other materials that act like cement.
The new rules should not result in the weakening of concrete, he said, in part because current practice overuses cement. “I’ve been a structural engineer for 40 years, and I’ve been aware for a long time that we always make concrete a bit stronger than we need to,” he explained. “All my career, engineers everywhere have done that as an insurance, since no one wants a callback.”
Terry Nordbye, a contractor in Inverness who has completed several projects for the Community Land Trust Association of West Marin, said he has been using alternatives to cement for years, including fly ash. It is not more expensive, he said, and there is no reason its use should not be more prevalent.
In her report to the board, Ms. Zanmiller highlighted the enforcement challenges. The new standards apply to all projects that include newly poured concrete; but if someone is pouring concrete for a patio or driveway or another type of project that does not require a building permit, the county will be unable to regulate the use.
“Ongoing education of the public, building industry, and ready-mix suppliers will be important to promote the use of low-carbon concretes regardless of local permit requirements,” she emphasized.