The role of concrete pavements. By Steve Friess
The almost 2.8 million lane-miles (4.6 million lane-kilometers) of pavement in the United States are a significant consideration in the effort to lower greenhouse gas emissions, according to a February 2023 study from the MIT Concrete Sustainability Hub (CSHub). The good news is researchers believe achieving carbon neutral paving materials by 2050 is possible if industry and governmental actors apply a range of solutions. Such solutions include taking use-phase impacts into account and making strategic pavement construction and repair choices – with concrete pavement being one of the suggested materials to contribute to carbon neutrality.
A major benefit in choosing concrete over other paving materials is the long life span it can achieve. Concrete can last 30 years or more before requiring a maintenance cycle. Departments of transportation and other agencies have long understood that this long lifespan translates into economic sustainability and offers the greatest value for taxpayers. It is now being realized that a long service life also equates to environmental sustainability, as it reduces the need for raw materials.
Another significant way in which concrete pavement contributes to sustainability is by improving fuel efficiency. It does this by reducing rolling resistance (that is, the resistance a tire encounters as it travels over a road surface). CSHub has shown in previous research that stiff, smooth pavements decrease rolling resistance – and concrete pavement is not only stiffer than other paving materials, but it stays smoother longer. Excess fuel consumption (EFC) is one of the most significant use phase impacts for pavements, especially on higher volume roadways with a large amount of truck traffic, so concrete pavements stand to make a major contribution to sustainability by reducing the amount of fuel required to overcome rolling resistance.
Mitigating mechanisms
Concrete pavements are more reflective than darker pavements, which is to say they have higher albedo. Albedo is the measure of the fraction of solar energy reflected by a surface. Lighter color surfaces reflect light and have a high albedo, while dark surfaces absorb light and have a low albedo. Choosing pavements with high albedo helps mitigate climate change and global warming potential by two major mechanisms. One is radiative forcing (RF), a measure of the Earth’s energy balance. It is the difference between the amount of energy that enters the Earth’s atmosphere and the amount of energy that radiates out into space. Increasing albedo radiates more energy out from the Earth and has a cooling effect. A second mechanism is the reduction of urban heat island effect (UHI). The lower amount of heat reflected back from a concrete pavement surface, as opposed to asphalt, allows ambient air temperatures to remain lower.
A phenomenon receiving increasing attention is concrete’s ability to absorb CO2. This 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. The process is known as carbonation and it occurs as the pavement ages. CSHub researchers have investigated the carbon uptake of all concrete pavements in the United States, running thousands of simulations to calculate the potential carbon uptake in each state based on road conditions, maintenance actions, budgets, and road lengths. They found that after a 30-year analysis period, 5.8 million tons of CO2 could be sequestered, with 2.8 million tons of CO2 coming from the use phase and three million tons of CO2 coming from the end-of-life phase.
Performance-engineered concrete
One often-overlooked aspect of sustainability is resilience. Resilience is the 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 resilience. With its unique strength, durability, and load distributing properties, concrete is one of the most resilient building materials available.
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 supplying of cement and concrete. Achieving sustainability during this phase begins with addressing the embodied environmental impact of concrete and its mix ingredients. Fortunately, 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, with a goal of achieving net zero carbon emissions by 2050. Current efforts focus on reducing the carbon footprint of cement (the ingredient with the most significant environmental impact) by using Portland-limestone cement or supplementary cementitious materials (SCMs) to replace all or part of the ordinary Portland cement in a mix. Performance-engineered concrete mixtures take the process a step further by not only optimizing the use of SCMs and Portland-limestone cement content in the mix, but optimizing aggregate gradation and sourcing, as well.
Considering its resilience and its sustainability benefits, concrete is well-positioned to address the planet’s climate change concerns. The American Concrete Pavement Association (ACPA) has more information in its white paper, ‘Concrete Pavement’s Role in a Sustainable, Resilient Future.’
For a list of the sources used in this article, please contact the editor.
Steve Friess is Chairman of the Board at the American Concrete Pavement Association. The American Concrete Pavement Association (ACPA) is the national trade association for the concrete pavement industry. ACPA’s mission is to develop and protect concrete pavement markets through education, advocacy, marketing, and industry technical leadership. ACPA’s vision is for concrete to be the pavement material of choice, benefitting communities and society within US state and local economies. Founded in 1963, ACPA is the world’s largest trade association that exclusively represents the interests of those involved with the design, construction, and preservation of concrete pavements.