Carbon Impact of Structural Materials

By Johnson Jones, PE – Structural Engineer | Seattle Office

Structural Engineering’s Role in Sustainable Development

The first core tenet of the ASCE Code of Ethics is to create safe, resilient, and sustainable infrastructure for the public. The fact that sustainable development is a primary value of civil engineering may seem counterintuitive given how the construction industry accounts for the largest percentage of global greenhouse gas emissions of any sector. While that fact can be disconcerting, it also presents an opportunity for professionals within the industry to revolutionize our approach to development and forge a path to sustainability. This article aims to shed some light on the causes of embodied carbon of structural materials and discuss innovations that could provide a path to net-zero within the field of structural engineering.

The Environmental Impact of Construction Materials

Concrete and steel combine to represent about 15% [1][2] of global greenhouse gas emissions. To put that into perspective, passenger vehicle emissions only account for approximately 12%-15% of global emissions’ so while we typically picture large trucks and gridlock traffic as the driver of climate change, steel and concrete use is just as much to blame.

Concrete

Cement production accounts for 7-8% of total greenhouse gas emissions worldwide. Cement is typically manufactured by taking crushed limestone and a few other additives and heating them in a rotary kiln to thousands of degrees, then grinding the contents into a fine powder once cooled. This process is highly energy-intensive due to the high heat required. In addition, the kilns are usually heated with coal or natural gas, resulting in a highly carbon-intensive process.

Steel

Steel production accounts for 8% of total greenhouse gas emissions worldwide. The largest crude steel producer in the world by far is China, which produces roughly half of the total amount of crude steel worldwide. China, and many other steel-producing nations, still primarily use basic oxygen furnaces for smelting iron ore into steel, which is a highly polluting and energy-intensive process.

Path to Carbon Neutrality in Structural Engineering

Modern society depends on steel and concrete for its infrastructure, which will not change anytime soon. So how do we develop sustainably, knowing that we must use these currently carbon-intensive materials? A white paper titled “Achieving Net Zero Embodied Carbon in Structural Materials (2020)” [3] outlined an analysis of how this could achieve carbon neutrality. Two design strategies that could make the most significant impact in achieving net zero in structural design were shown to be Material Specification and Structural System Substitution.

Material Specification

As discussed previously, concrete and steel materials are highly carbon-intensive, but there are emerging developments within the production of these materials and design strategies that can significantly decrease the Global Warming Potential (GWP) of the materials. For concrete, simply specifying a lower compressive strength when high strength concrete is not required will drastically reduce the GWP, as the compressive strength is proportional to the amount of cement required in the mix. Beyond that, new developments from companies like CarbonCure are capturing and storing the CO₂ produced from the cement production process and introducing it to the concrete mix to permanently store the emissions within the concrete without reducing performance.

Steel is unique because it can be recycled using scrap steel and electric arc furnaces. Because of this, as the grid becomes more renewable from sustainable electricity production like solar and wind, steel production with recycled materials becomes increasingly renewable as well. In addition, well over 50% of steel production within the USA uses electric arc furnaces, so specifying locally produced steel not only decreases the energy required for transportation of the materials but will also likely be the least carbon-intensive steel.

Structural System Substitution

An emerging and increasingly popular sustainable building approach is mass timber construction. While this technology has been used for decades in Europe, it is still relatively new in the US. Mass timber uses small pieces of wood lamina which are glued or fastened together to create a larger composite section. Mass timber products such as glulam members, cross-laminated timber (CLT), and other engineered wood offer higher strength and dimensional stability than lumber or timbers while also increasing the fire resilience of the structure. Because of this, recent code updates in the 2021 IBC allow mass timber structures to be constructed up to 18 stories or 270 feet and allow wood buildings to compete in the mid-rise and high-rise development space, which has been dominated by steel and concrete. Mass timber structures can offer many benefits over traditional steel and concrete, such as panelized/modular construction and simple connections that can allow for an accelerated construction schedule. Additionally, wood has a high strength-to-weight ratio, allowing for timber structures to be lighter than comparable concrete and steel structures; this can significantly impact the size of the foundations required. Since wood is 50% CO₂ by mass and stores rather than produces carbon, it is a much more sustainable alternative to concrete or steel.

SE2050 Commitment

In 2018 the SE2050 Commitment was formed in response to the International Panel on Climate Change (IPCC) report detailing the ramifications of not achieving net-zero by 2050. This commitment acknowledges the role that structural engineers have in the development of sustainable infrastructure and is a commitment to achieving net-zero through utilizing emerging technologies and sustainable design techniques within structural engineering. Read more about Coffman’s commitment to SE2050 and how we are working as a company to apply sustainable development principles and be a leader in this challenge to create a greener future.

[1] Global Efficiency Intelligence

[2] Fast Company

[3] Carbon Leadership Forum