Above: Matthew Adams in Hastings-on-Hudson, N.Y., where walls, sidewalks and a school use low-carbon materials.

On a cold morning in early 2019, I stood at a city transportation facility along the Gowanus Canal in Brooklyn watching a concrete crusher convert mammoth chunks of former sidewalks, manhole casings and building slabs into walnut-sized bits. Towering several stories above me, the rubble pile would soon float by barge down the East River, a small fraction of it delivered to regional roadway projects, as fill underlying new pavement, but most of it to landfills. As a civil engineer focused on sustainable materials, I asked the question: Can we recycle this razed urban infrastructure at a net benefit to the environment?

Here’s the problem: Concrete, the world’s most pervasive man-made material, has a colossal impact on energy and natural resources consumption. In 2020, 3.66 billion tons were manufactured worldwide, the equivalent of over 4,000 Hoover Dams. The production of its core component — cement — is estimated to be responsible for about 8% of global CO2 emissions.

Nearly all of it incorporates newly mined rock. Yet in the U.S. alone, we send more than 134 million tons of concrete waste from demolition each year to landfills. Among other benefits, reusing these materials would reduce the pollution, energy use, habitat destruction and massive costs associated with mining and the construction of new landfills. 

In my lab at NJIT, which looks more like a miniature construction site than a laboratory, researchers make and test new types of concrete that include novel materials such as recycled concrete aggregate, waste from the coal and steel industries and pulverized recycled glass. We squeeze, pull and bend the concrete until it breaks to assess how strong it is; we freeze, heat and dry it, and spray it with de-icing salts and other corrosive chemicals to see how resistant it is to environmental degradation. Our goal is to reduce the embodied carbon content of concrete without compromising its ability to withstand heavy loading, natural disasters and long-term wear.

Despite its promise, however, building codes at the local, state and national level are slow to permit the mixture of new engineered compounds in building materials. What’s more, local agencies lack the resources to develop regulations to support the use of more sustainable products that already have regulatory approval. It’s clear we need to do a better job addressing concerns over durability, while also publicizing
the benefits.

As a public policy fellow with the Rockefeller Institute of Government since 2020, I investigate the barriers that hinder the adoption of sustainable concrete. One of the main stumbling blocks is the lack of accessible, easily digestible information about how these new technologies perform. The state agencies that write and control materials specifications need to know what happens when we mix them with local rock. An official in New Jersey familiar with granite is rightly skeptical, for example, about research performed on limestone in Texas.

We need to develop education campaigns that inform these officials and construction engineers about up-to-date research, testing and real-world case studies, while emphasizing that there is no one-size-fits-all solution to the greening of concrete — no single mixture that works in every scenario.

Recycled concrete mixtures that are sturdy enough for a house foundation or a sidewalk, for example, might not be suitable for a bridge deck. Therefore, we need to develop design tools that help engineers decide which sustainable materials are best suited to their construction projects.

Finally, many public agencies and engineering companies are afraid to embrace new methods without strong proof of their long-term durability. While laboratory testing can show that novel sustainable materials are strong enough to withstand structural loading, our methods for predicting their — or any concrete’s — ability to last for 50 to 100 years are less accurate. There is risk in using novel materials that have not been subjected to real-world conditions for decades. Fortunately, there are opportunities to ease these concerns, and they start in towns and cities. While policies on sustainability at the national level are mired in political division, a significant amount of concrete is purchased with state and local tax dollars. As such, municipalities and states can write rules that encourage the use of approved sustainable construction materials.

One great example is a resolution adopted in 2020 by Hastings-on-Hudson, N.Y. that directs the village to provide education and support to community stakeholders about low-embodied carbon concrete, while encouraging them to use it and other novel, sustainable materials. By agreeing to accept the risk of durability failures if the concrete does not last as long as predicted, the village is able to mandate low-carbon materials in project specifications. It has since been used in walls, sidewalks and even the new Hillside Elementary School. 

When it comes to sustainable construction, the adage “change starts at home” has never been more accurate.

When enough municipalities decide to hold their contractors to higher standards, states are more likely to adopt them, and then we’ll begin to see significant improvements in the sustainability of our concrete infrastructure. In the meantime, my team of engineers will be busy in the lab designing the next generation of materials that are both durable and green. We invite sustainable communities to help us spread the message.