From the smallest local agencies to state DOTs, there’s been a growing emphasis on making infrastructure more sustainable and resilient. Accelerating this is the November 2021 Infrastructure and Investment in Jobs Act which incentivizes the use of local materials, such as concrete, and funds investment in the decarbonization of such materials.
Concrete is one of the oldest building materials known to man, with a history dating back to ancient Rome and earlier. Today, concrete—comprised primarily of aggregate, cementitious materials, and water—is the most widely used building material on the planet. Without it, modern society could not exist. Concrete is often claimed to have a high carbon footprint, but this is due to its abundant use more than to its carbon footprint per unit produced. Most of its carbon emissions are associated with manufacturing portland cement, one of modern concrete’s major components. Furthermore, concrete is a long-lived material, which reduces the impact of the carbon produced during its manufacture over the extended service life.
What is Carbonation?
There is another, less-talked-about way in which concrete contributes to sustainability: carbonation. During the production of portland cement, limestone consisting of calcium carbonate is added to the kiln. When heated to a high temperature, the calcium carbonate (CaCO3) converts to lime (CaO)—the product required for portland cement production—and CO2, which is released into the atmosphere. But a partial reversal of the process begins once the portland cement is hydrated during and after construction. As the concrete surface is exposed to atmospheric carbon dioxide, the calcium hydroxide and calcium silicate hydrate in the hardened cement paste react chemically with the CO2, converting back to limestone and recapturing the equivalent of some of the CO2 that was emitted during production.
This carbon capture is known as carbonation, and over the life cycle of a concrete road, the potential for carbonation, although small, is significant.
How Much Carbonation Can Occur in a Typical U.S. Concrete Pavement?
Research by Anderson et. al quantified concrete’s uptake of CO2 by carbonation (ref. 1). One factor that determines the amount of carbonation taking place is exposure, i.e., whether the concrete is exposed to rain, sheltered from rain, indoors, or underground. The amount of exposed concrete surface area, the quality of the concrete, and the amount of cement also affects the amount of carbonation that takes place. As an example, a typical U.S. pavement can be assumed to be a 564 lbs./yard3 portland cement mix with a compressive strength of 4,000 psi and exposed to rain. Based on this, the amount of CO2 sequestered through carbonation is calculated to be between 0.2-0.3 lbs./ft2 of surface area in 50 years, which translates to roughly 14,000-20,000 lbs./lane mile (see figure 1 below).
Figure 1. Over a timespan of fifty years, concrete achieves carbonation of roughly 14,000-20,000 lbs./lane mile (pounds per lane-mile).NCE
An important observation is that concrete’s rate of sequestration decreases with time because the calcium carbonate begins to fill up void space in the concrete. This “pore blocking” effect slows the penetration of carbon dioxide into the concrete, so the rate of carbonation decreases at roughly the square root of time. The result is that about 45% of the carbonation that occurs over 50 years happens by year 10. In a typical pavement, approximately 2-3 mm of carbonation may occur in the first 2-3 years, whereas by year 20, the total depth of carbonation is only 5-8 mm, with little additional carbonation occurring beyond year 50.
The most practical way to counteract this decrease is to remove the carbonated surface and expose uncarbonated paste—an action most effectively accomplished through diamond grinding the pavement surface. Diamond grinding reopens the pore structure and exposes fresh calcium hydroxide and calcium silicate hydrate while also increasing surface area. Once a carbonated pavement surface is diamond-ground, the process of carbonation starts over. It’s worth noting that a typical pavement reconstruction strategy—overlaying the concrete with asphalt—seals the pavement against atmospheric CO2 and terminates sequestration.
Getting the Most Sustainability Benefit from Concrete Pavements
Carbonization is one of several ways in which concrete’s natural benefits can be optimized. Proper maintenance of a pavement is key to extending pavement life and lessens the amount of new construction that would otherwise be necessary. Less construction not only equates to less material acquisition, processing, and construction but also reduces congestion due to construction delays, thereby reducing vehicle greenhouse gas emissions. Keeping smooth pavements smooth is another way to reduce vehicle greenhouse gas emissions; it is well-documented that vehicle fuel efficiency is related to the smoothness of the pavement (by sources such as the World Bank HDM 4 model and recent research by NCHRP, Caltrans, and organizations in Europe).
Even considering there are some greenhouse gas emissions associated with concrete pavement preservation tasks (e.g., minor repair and diamond grinding), these are relatively small compared to those incurred due to rehabilitation or reconstruction. Yet the overall ecological benefits of concrete pavement preservation, especially diamond grinding, are such that they likely offset the emissions due to the preservation activity.
For roads with roughly 12,000 to 34,000 vehicles per day, 127 in./mile is the trigger value.
For roads seeing more than 34,000 vehicles per day, the recommended IRI trigger value is 101 in./mile.
Conventional triggers and time frames for concrete pavement preservation may not optimize carbon capture or other concrete sustainability benefits. Currently, concrete pavement preservation featuring diamond grinding is most often performed when a pavement is showing advanced stages of distress. Typically, agencies and other road owners assess road roughness using the International Roughness Index (IRI) and often do not trigger treatment until a high IRI threshold value (180 in./mile or more) has been reached.
According to studies performed by Wang et al., IRI trigger values should vary according to the amount of traffic on a given road to maximize sustainability benefits (ref. 2). For roads with daily traffic of roughly 2,500 vehicles or less, there is no optimal IRI trigger value based on greenhouse gas reduction whereas the recommended trigger value for pavement preservation of roads where daily traffic is between 2,500 and 12,000 is reduced to 152 in./mile. For roads with roughly 12,000 to 34,000 vehicles per day, 127 in./mile is the trigger value and for roads seeing more than 34,000 vehicles per day, the recommended IRI trigger value is 101 in./mile.
With a more deliberate timing of preservation projects, the sustainability benefits of concrete pavement could be even greater than they currently are. Diamond grinding can be successfully performed on the same pavement multiple times. If diamond grinding were to be routinely performed on a 10-to-15-year cycle, not only would vehicle-emitted carbon dioxide be significantly reduced through improved smoothness, but there would be a renewed rate of carbon dioxide sequestration. In fact, diamond grinding every 10 years would more than double the amount of sequestered CO2 over a 50-year period. (See figure 2 below.)
Figure 2. Diamond grinding pavement every 10 years would more than double the amount of CO2 sequestered over a 50-year pavement life span.NCE
Preserving concrete pavements to a high level of serviceability is inherently a sustainable solution. The above approach represents a rigorous quantification of the benefits of diamond grinding, making it easy to assess its advantages. Grinding concrete surfaces not only keeps them smoother and quieter, but also can more than double the amount of CO2 sequestered over a 50-year design life. By comparison, carbonation would happen more slowly on unground pavement—and not at all on concrete once covered with asphalt.
References
1 Anderson, et. al. 2019. “Carbonation as a method to improve climate performance for cement-based materials.” Cement and Concrete Research, Vol. 124, (October 2019) 105819.
2 Wang, et. al. 2013. “Network-Level Life-Cycle Energy Consumption and Greenhouse Gas from CAPM Treatments.” Research Report: UCPRC-RR-2014-05. University of California Pavement Research Center, UC Davis, UC Berkeley http://www.ucprc.ucdavis.edu/PDF/UCPRC-RR-2014-05.pdf
About the author
Thomas Van Dam, Ph.D., P.E., and Principal, NCE, has more than 35 years of civil engineering experience, specializing in pavement design evaluation, forensic investigations, materials assessment, sustainability and resiliency. He is a principal investigator on multiple projects looking to reduce the greenhouse gas emissions associated with concrete and concrete pavements. He is also principal author of the FHWA's "Toward Sustainable Pavements: A Reference Manual.”
