Millions of miles of pavements are placed or rehabilitated across the globe every year. Nearly 2.8 million lane-miles of pavement are in the U.S. and are a focus in the effort to lower U.S. greenhouse gas emissions (GHG), according to a February 2023 study from the MIT Concrete Sustainability Hub (CSHub). This represents an opportunity for leaders to make pavement type and treatment selections that can improve sustainability and address climate change.
The American Concrete Pavement Association (ACPA) recently released a white paper, “Concrete Pavement’s Role in a Sustainable, Resilient Future,” which states achieving carbon neutral paving material by 2050 is possible by implementing alterations to improve the environmental impact of concrete pavement systems throughout their life cycle. These activities fall under two primary categories: improving designs and reducing use-phase impacts.
By taking advantage of the inherent properties that make concrete sustainable, infrastructure decision-makers can make an impact on the fight against climate change, lowering GHGs and leading to a healthier environment. Here are some of the ways concrete can pave the way to a healthier climate future.
Concrete has a long lifespan — 40 or more years — compared to alternative pavement materials. With proper maintenance, it can last even longer. This longevity is inherently more sustainable, requiring less material to be produced and hauled, and fewer construction projects to occur. Additionally, concrete’s long lifespan means cost savings to taxpayers.
In previous research, CSHub found stiff, smooth pavements like concrete decrease rolling resistance, which is the resistance a tire encounters as it travels over a road surface. Excess fuel consumption is one of the most significant use-phase impacts for pavements, especially on higher-volume roadways with a large amount of truck traffic. Concrete pavements can make a major contribution to sustainability by reducing the amount of fuel required to overcome rolling resistance.
Concrete pavements have higher albedo — which is the measure of the fraction of solar energy reflected by a surface — than darker pavements because they are lighter and more reflective. Alternatively, dark surfaces absorb light and have a low albedo. Pavements with high albedo help mitigate climate change and global warming potential in two ways:
Through radiative forcing, increasing albedo radiates more energy out from the Earth and has a cooling effect. Radiative forcing is a measure of Earth’s energy balance. It is the difference between the amount of energy that enters Earth’s atmosphere and the amount of energy that radiates out into space.
Higher albedo reduces the urban heat island effect. The lower amount of heat reflected back from a concrete pavement surface, as opposed to asphalt, allows ambient air temperatures to remain lower.
Studies show concrete’s naturally high albedo can improve sustainability by mitigating or delaying some of the consequences of warming from carbon dioxide (CO2) emissions.
Concrete absorbs CO2 from the atmosphere through a process called carbonation, which happens as the pavement ages. Carbonation occurs when hydrated portland cement is exposed to atmospheric CO2, which reacts with the water and the calcium compounds in concrete to produce calcium carbonate.
CSHub researchers have investigated the carbon uptake of all concrete pavements in the U.S., running thousands of simulations to calculate the potential carbon uptake in each state based on road conditions, maintenance actions, budgets and road lengths. After a 30-year analysis period, CSHub found that 5.8 million tons of CO2 could be sequestered thanks to concrete pavements, with 2.8 million tons of CO2 coming from the use phase and 3 million tons of CO2 coming from the end-of-life phase.
Concrete has unique strength, durability and load-distributing properties, making it one of the most resilient building materials available. Resilience is an often-overlooked aspect of sustainability, and is a material’s ability to withstand, respond to and recover rapidly after a disruptive event such as a flood, earthquake or other severe weather event. In a changing climate where extreme weather events are greater in frequency and intensity than in the past, it is impossible to have sustainable infrastructure without resilient materials like concrete.
Concrete is 100% recyclable and is often reused on the same project site. In fact, concrete is one of the most recycled materials in the U.S. — Approximately 140 million tons of concrete are recycled each year. Opportunities to recycle concrete on grade reduce truck hauling, further improving the carbon footprint of a concrete pavement project.
When recycled, concrete has a variety of uses including base material, aggregate in new concrete and granular fill. Additionally, when concrete is recycled it accelerates the carbonation process, absorbing more CO2 from the atmosphere.
Enhancing Concrete’s Sustainable Properties
In addition to taking advantage of the inherently sustainable qualities of concrete, researchers have determined ways to optimize the concrete itself.
Quality pavement designs can reduce the long-term environmental impacts of pavements. Mechanistic-empirical design tools can optimize a pavement to reduce over-design, which lowers the initial and life-cycle environmental impacts of concrete. These tools also can evaluate the long-term benefits of optimizations, determining how small changes to design thickness and other design parameters can extend the lifespan of the pavement with minimal economic and environmental impact.
It’s also important to evaluate the ingredients of the concrete mixture. One ingredient, cement, makes the biggest environmental impact, but can be mitigated by using portland-limestone cements (PLCs). PLCs have an increased amount of limestone in the cement blend that decreases carbon emissions with minimal effect on performance. This increase results in up to 10% less embodied CO2 for the cement.
Reducing the amount of clinker required to produce cement is another option to reduce concrete’s CO2 emissions across the value chain. This can be done by incorporating supplementary cementitious materials (SCMs) into a blended cement.
It’s also possible to reduce the carbon footprint of the concrete mixture by creating performance-engineered mixtures (PEMs). PEMs were developed to better design and evaluate concrete mixtures and to ensure achievement of long-term durability — enabling the material to meet the pavement’s design life.
A Path Toward Sustainability
The challenge presented to global and local communities is how to build more durable structures using concrete, while minimizing the carbon emissions generated during the manufacturing and supply of cement and concrete.
The concrete pavement industry and others across the concrete value chain are working together to implement the Portland Cement Association’s “Roadmap to Carbon Neutrality,” which has a goal of achieving net-zero carbon emissions by 2050.