Climate Action: 10 Ways to Reduce Concrete’s Carbon Footprint

Lionel Lemay | Executive VP, Structures and Sustainability, National Ready Mixed Concrete Association

The following is a list of strategies for reducing concrete’s carbon footprint through collaboration, design optimization and performance-based specification:

1. Communicate carbon reduction goals
2. Ensure good quality control and assurance
3. Optimize concrete design
4. Specify innovative cements
5. Specify supplementary cementitious materials
6. Specify admixtures
7. Set targets for carbon footprint
8. Don’t limit ingredients
9. Sequester carbon dioxide in concrete
10. Encourage innovation

Why is this important?

According to the UN Environment’s Global Status Report 2017, the world is projected to add 2.5 trillion sq ft (230 billion sq meters) of buildings by 2060 driven by soaring needs for housing and infrastructure development. The UN report urges building designers and owners to reduce operational carbon by designing disaster-resilient buildings for the future with zero-energy consumption. Concrete has long been the material of choice for energy efficient and disaster resilient buildings and infrastructure. The UN report also urges the building industry to reduce the embodied carbon of building materials. Given the amount of concrete used in buildings, and the carbon footprint of making cement, the challenge is to offer lifetime benefits of concrete at a lower carbon footprint.

To meet this challenge, the National Ready Mixed Concrete Association (NRMCA) adopted in 2012 the Architecture 2030 Challenge, which has goals to reduce operational carbon and embodied carbon from the built environment to net zero by 2050. However, concrete is unique among building materials. Design professionals and contractors have a greater influence on concrete formulation than they do with other building products. Therefore, it is critical that architects, engineers, contractors and concrete producers collaborate to reduce concrete’s carbon footprint. And, given that structural engineers often specify concrete on projects, architects need to know more about how to talk with their consultants to reduce the embodied carbon in concrete.

Things you can do right now

1. Communicate carbon reduction goals
   Sustainability goals, including carbon footprint targets, should be communicated to the design team and to product manufacturers. (This not only applies to concrete but to the majority of building products specified.) We suggest collaborating with your structural engineer, local concrete producers and contractors. Invite them in for a meeting or charrette with your design team. Understand what technologies and concrete ingredients are available locally. Emphasize the carbon reduction goals for the project in pre-bid meetings. Carbon reduction of 50% and higher is readily achievable.

Recommendation: State a carbon reduction goal (carbon budget) for all the concrete on the project. See Section 1.2 of NRMCA Guide Specification.

2. Ensure good quality control and assurance
   Concrete is made from local materials and its performance can be affected by weather conditions, variability of materials, delivery, placing, handling, and testing. Although the materials used to make concrete meet rigorous standards, the variability can be quite high. Concrete rarely tests well when proper manufacturing, installation and testing protocols are not followed. If test results constantly show lower strength, then the only way to overcome that is to increase overdesign which generally raises cementitious material content. For example, if poor testing increases the necessary overdesign from 600 to 1000 psi the cementitious materials content would increase by roughly 40 lbs for 4,000 psi concrete, increasing the embodied carbon footprint by as much as 6%.

Recommendation: Specify minimum qualifications for the concrete producer, installer and testing facility. See Section 1.7 of NRMCA Guide Specification.

3. Optimize concrete design
   If a structural element such as a column or beam is designed larger than required, then excessive concrete is being used which increases embodied carbon. Alternatively, for a high-rise building, reducing the size of the columns might be critical to keeping the rentable space to a maximum. That means using high strength concrete which generally means higher carbon footprint, but at lower volume with a net benefit to the project. Additionally, higher quality materials can allow for innovative design strategies such as increased deck spans with less deflection, or narrower columns to withstand equivalent seismic and wind drift.

Recommendation: Discuss sizing of structural members with your engineering consultants. Use life cycle analysis software to quickly calculate the embodied carbon of concrete elements throughout design iterations (structural and architectural). Also, consider exposing concrete wherever possible. Finish materials have a considerable carbon footprint and exposed concrete can be attractive and is fire resistant without the need for additional protection. Additionally, exposed concrete has a higher rate of carbon uptake through a process called carbonation, which permanently sequesters CO₂ directly from the atmosphere (see Strategy 9).

4. Specify innovative cements
   There are several innovative cements on the market. The most common are called blended cements. These combine ordinary portland cement (OPC) with other materials. The most common type of blended cement is portland-limestone cement (PLC) or, technically, ASTM C595 Type IL (pronounce “one el”) cement. This blended cement combines up to 15% limestone interground with OPC to make a cement with a carbon footprint that is up to 10% lower than OPC with performance that is identical to— and in some cases better than—OPC. As a 1-to-1 replacement, incorporating PLC into a project allows for direct reduction in carbon across all classes of concrete. There is also another standard, ASTM C1157, for performance based blended cements with no limits on cement composition which allows considerably more flexibility.

Recommendation: Coordinate with your structural engineer to permit the use of all hydraulic cements including ASTM C150, ASTM C595, or ASTM C1157. See Section 2.2 of NRMCA Guide Specification.

5. Specify supplementary cementitious materials
   Nearly all concrete uses supplementary cementitious materials (SCMs). SCMs offer the greatest opportunity for the reduction of carbon footprint today. The most common are fly ash, slag cement, and silica fume in that order. However, there are others, such as metakaolin, volcanic ash, rice husk ash, and ground glass, just to name a few. All of these also enhance the performance of concrete when combined with portland cement, including increased strength, increased durability and enhanced workability. Given that they are often recycled or naturally occurring products, SCMs are a carbon reduction strategy that frequently yields negligible impact to cost.To give an idea of how effective the use of SCMs are in reducing carbon footprint, going from a 100% portland cement mix to a 50% fly ash/slag cement mix can reduce carbon footprint by roughly 40%. Additionally, present day projects can easily realize over 70% cement replacement with SCMs for foundations that won’t experience ultimate loading until the project nears completion.With SCMs other than silica fume, the rate of strength gain might be lower initially, but strength gain continues for a longer period compared to mixtures with only portland cement or blended cement, frequently resulting in higher ultimate strengths. This slower strength gain can sometimes be overcome with admixtures. However, the most common way to overcome slower strength gain is to specify a later test age. For example, foundations are often specified to reach compressive strength tested at 56 or 90 days to allow for slower strength gain.

Recommendation: Permit all types of SCMs in your specification. See Section 2.2 of NRMCA Guide Specification. Coordinate with the engineer and contractor to maximize the use of SCMs while balancing schedule demands including using compressive test age of 56 or 90 days depending on the application.

6. Specify admixtures
   Nearly all concrete produced today uses admixtures. Most enhance the plastic properties in order to make concrete more workable, economical, shorten or lengthen set time, and so on. There are water reducing admixtures that in effect reduce cement demand, accelerators that improve strength gain, and viscosity modifiers that permit concrete to flow into very tight spaces. As an example of how effective admixtures can be, using a water reducing admixture that reduces water content in a mixture by 12% will result in a reduction of cement content by 70 lbs for equivalent slump and strength with a carbon reduction of roughly 10% for 4,000 psi concrete. High-range water reducing admixtures can reduce water content by as much as 40%.

Recommendation: Permit all types of admixtures in your specification. See Section 2.2 of NRMCA’s Guide Specification.

7. Set targets for carbon footprint
   The best approach to achieve carbon footprint reductions is to use whole building life cycle assessment to set a carbon budget for all the concrete on a building. Use industry average benchmarks to establish a baseline and use industry wide environmental product declarations to establish a carbon budget (see recommendations). It is still necessary to have a general idea of what the carbon footprint of each mix will be to set a carbon budget for the building, but by setting a carbon budget for all the concrete, it provides enough flexibility to the contractor and concrete producer to meet the all the performance criteria such as strength and durability while also meeting carbon reduction goals.For some elements, such as post-tensioned slabs, the ability to reduce carbon footprint from industry average benchmarks will be difficult because of early strength requirements. But for other elements such as foundations and shear walls, the carbon footprint can be significantly lower than benchmark. The carbon budget approach allows for the contractor and concrete producer to combine many of the strategies presented such as implementing good quality control and using innovative cements, SCMs and admixtures to help meet the performance requirements and carbon reduction goals.

Recommendation: Specify a target global warming potential (GWP) for all the concrete on the project by using NRMCA Industry Wide Environmental Product Declarations and Benchmarks to estimate how to compare your proposed designs to a baseline (benchmarks). See Section 2.11 of NRMCA’s Guide Specification.

8. Don’t limit ingredients
   Do not list a maximum or minimum cement content, maximum or minimum SCM content, or quantity of admixtures. These random limits on material ingredients in project specifications limit the concrete producer’s ability to meet performance criteria, including carbon footprint reductions. As an example, unnecessarily limiting the water-cementitious ratio to 0.45 versus an alternative of 0.50 can result in an increase in GWP of 7-12%. Instead, use ACI 318’s exposure classification table when conditions would require limits on the water-cementitious ratio.

Recommendations: Include a table in the specification that lists performance criteria such as strength and exposure class without limitations on material quantities. See Section 2.11 of NRMCA Guide Specification.

9. Sequester carbon dioxide in concrete
   Carbon dioxide (CO₂) can be captured or permanently sequestered in concrete through natural processes or carbon mineralization technologies. The rate of carbon uptake depends on exposure to air, surface orientation, surface-to-volume ratio, binder constituents, surface treatment, porosity, strength, humidity, temperature, and ambient CO₂ concentration. Research conducted by Possan, et al., indicates that during its lifetime, concrete can uptake anywhere from 40 to 90% of CO₂ emitted in its manufacturing process. Other commercially viable technologies accelerate carbonation. This is accomplished either by injecting CO₂ into concrete, curing concrete in CO₂, or creating artificial limestone aggregates using CO₂.

Recommendation: Expose concrete as much as possible on projects. Permit the use of recycled and artificial aggregates in concrete. Permit the use of carbon mineralization technologies. See Section 2.2 of NRMCA Guide Specification.

10. Encourage innovation
    For an innovative product or process to be successful, demand must be created, but the current design-bid-build process discourages innovation.

Recommendations: Communicating the carbon reduction goals to contractors and producers during the design process is critical. Let them know that you are looking for innovative solutions. Most sophisticated producers are experimenting on new formulations all the time. Ask them to discuss some of their low-carbon concretes. Will they meet all the performance criteria set by the design team and the contracting team?

Tools and Resources

Guide to Improving Specifications for Ready Mixed Concrete with Notes on Reducing Embodied Carbon Footprint, NRMCA Publication 2PE004‐21c, 2021. www.nrmca.org/sustainability

NRMCA Member Industry-Average EPD for Ready Mixed Concrete NRMCA 2021. https://www.nrmca.org/wp-content/uploads/2022/03/NRMCA_EPDV3-2_20220301.pdf

A Cradle-to-Gate Life Cycle Assessment of Ready-Mixed Concrete Manufactured by NRMCA Members – Version 3.2. NRMCA 2021. https://www.nrmca.org/wp-content/uploads/2022/02/NRMCA_LCAReportV3-2_20220224.pdf.

2016. Possan, E. F. Felix, W. A. Thomaz, CO₂ uptake by carbonation of concrete during life cycle of building structures, Springer International Publishing, Switzerland, October 2016.

Engage the Concrete Design Center for help with sustainable concrete design and specifications, www.buildwithstrength.com/design-center.

Learn about the latest concrete innovations, www.concreteinnovations.com.

Ample daylighting and window views can help make your next building net-zero, healthier, and more sustainable.

By Lisa Heschong, Fellow Illuminating Engineering Society

“Daylighting” is the process of designing and operating interior spaces to utilize natural daylight as their primary source of illumination. Optimizing the distribution of daylight illumination requires climatically appropriate architectural design, cognizant of locale, climate and orientation. Daylight optimized interior design and furniture layouts enable occupants to take maximum advantage of the available daylight; electric lighting design responds to the dynamics of daylight, providing supplementary illumination only where and when necessary. Occupants in successfully daylit buildings regain control of their environment, with the ability to make local adjustments to furniture, window shading systems, and electric lighting to best meet their changing needs over the course of the day and year.

There are many fenestration design strategies, including windows, skylights, atria, and sunlight redirection devices, that can contribute to successful daylighting. Multiple approaches often make for a more robust system. Ultimately, daylighting both benefits from and encourages a holistic design approach; one which integrates input across design professions and establishes performance goals that are pursued continuously throughout building design and operation. Thus, daylighting is a fundamental benefit of good architectural design and coordination.

How are window views different from daylighting?

Windows designed for views provide occupants with a visual connection to the outdoor environment. While view windows may also deliver daylight illumination into a space, they do not necessarily need to be optimized for that purpose. Via thoughtful design, architects become curators of good views, which can add enormous financial value to buildings.

Occupants may get more circadian health benefits[i] from window views than from interior ambient illumination. This is because window views are generally brighter and more interesting, thereby attracting occupants’ attention, both consciously and unconsciously. Window views have also been associated with many other health benefits, including eye health, cognitive performance, and emotional well-being.

How are daylighting and window views related to climate change?

Efficiency: Using daylight as the primary illumination sources inside of buildings during the daytime is inherently energy efficient. No electricity is needed for most of the daylight hours. Fully daylit buildings can be extremely efficient, by using passive solar design principles. During cold weather, sunlight can be utilized for both light and heat (as long as it is ‘gentled’ first, to avoid overheating or glare.) During warm weather, daylight from the sky and broadly diffused sunlight can be more efficient than equivalent electric light sources, contributing less net heat per unit of light and so further reducing building cooling loads. Views can also add to building energy efficiency, by providing the bright vistas that occupants crave, without a need to increase electric lighting power densities.

Sustainability: Daylighting may be the ultimate sustainable design practice. Well daylit buildings have endured for hundreds of years. Buildings with ample daylight and views are easily repurposed, from offices and factories to schools and homes. Windows and glazing materials are long lasting. They can easily last for 50 (or more)      years, compared to a 15-to-20     -year life for many electric lighting systems and the current 5-10 year change-over cycle for new electric lighting technologies     . While glass industries do utilize a lot of energy, the materials are simple, widely available, and non-toxic.  Likewise, maintenance is a piece of cake: water with a little soap or vinegar

Resilience     : Daylight is the most reliable and resilient form of interior illumination. It works regardless of power availability and grid reliability. You want to be in a daylight building when your power supply is uncertain, such as after an earthquake, a flood, or a fire storm.

Simplicity     : Daylight from windows is easily managed using common materials and common sense, such as curtains, blinds, awnings. Repairs and re-configuration do not require an electrical engineer or computer programmer (as with advanced electric lighting controls.) Thus, in case of failure or reconfiguration, any problems can be easily and quickly solved.

Emergency response: Window views provide essential information about our environmental context, enabling occupants to assess sudden weather changes and other threats. They can also enable essential emergency communication and egress options.

Health: This is the big one. What’s the point of climate adaptation if it leaves people sick or uncomfortable? We now know that we need the circadian cues from daylight (and dark nights) to stay healthy. We have specialized photoreceptors which seek signals from the dynamic colors of daylight to keep our fundamental body and cellular rhythms in sync. Views are likely the most potent form of circadian stimulus inside of buildings, because they are bright; perfectly in sync with the exterior illumination; and most of all, interesting, drawing our attention. As a result, ample daylighting and views are an essential component of IEQ—indoor environmental quality.

Happiness: Daylight and views make people happy. They have been shown to lower depression and anxiety. They have also been shown to motivate healthy behaviors, like more physical activity and better sleep patterns.  We could all use a little more happy.

Things you can do right now:

Design buildings that use daylight as the primary form of interior illumination.

• Create spaces that have daylight entering from more than one orientation, such as view windows on two or three sides, entering from high windows like clerestories, and/or entering from above, such as from skylights, roof monitors or atria.
• Design the electric lighting to supplement the daylight only when and where needed, and to provide appropriate nighttime illumination.

Use clear, highly efficient glazing materials that provide maximum light transmission per window area (VLT of 60% or greater).

• Both people and plants benefit from the full spectrum of sunlight, so avoid glazing tints or coatings that change the appearance of the glass.
• Instead, minimize heating and cooling impacts via proper shading, orientation, use of triple or quadruple glazing layers, and by keeping total glazing area to the minimum needed for well-balanced daylight and views.
• Operable shading systems can also be designed to improve both heating and cooling profiles, thus reducing whole building energy use. When closed during nighttime operations, such operable shading systems also improve privacy and reduce light pollution.

Avoid allowing too much direct sunlight into the space that may cause visual or thermal discomfort for the occupants.

• Pay careful attention to solar geometry, and wherever possible, provide exterior shading for windows during the hottest times of the year.
• View windows should also include occupant-controlled blinds or shades to provide intermittent privacy and glare control.
• Skylights should be diffusing to ‘gentle’ the sunlight and spread daylight broadly
• Clerestories and roof monitors should be carefully sized and oriented so that any direct sunlight entering the space will reflect off of another surface at least once before it reaches any eyeballs.

Use design analysis tools that will help you understand the dynamics of daylight in the space. This can include physical models combined with photography and physical measurements, or computerized simulation models that are capable of hourly daylighting analysis.

• If you don’t have the expertise to do daylighting analysis in-house, hire a daylighting consultant.
• Go visit well-daylit buildings and analyze the strategies used. Learn from the masters.
• Read up on daylighting design and analysis in the resources listed below.

Advocate for daylighting design to your clients, to other building owners and occupants, and to public officials.

• Access to daylight and window views during the day should be a basic right for all workers. It is a fundamental equity issue for healthy indoor environments.
• Help your mechanical and electrical subcontractors better understand how to design in response to the dynamics of daylight, rather than ignoring it or fighting it.
• Advocate for minimum daylighting and view requirements in building codes, especially for vulnerable populations such as children, the elderly and low-wage workers.

Resources:

Seven very different books that cover a wide range of the art and science of daylighting:

• Visual Delight in Architecture: Daylight, Vision and View by Lisa Heschong, Routledge 2021.

An exploration of the many ways that daylighting and view are important for our health, well-being, social structures and culture, derived from both recent research and personal experience.

• The Architecture of Natural Light by Henry Plummer, The Monacelli Press 2009

A lush photographic tour of beautiful daylit spaces designed by great architects

• Approved Method: IES Spatial Daylight Autonomy(sDA) and Annual Sunlight Exposure (ASE), published by the Illuminating Engineering Society, 2013

A manual explaining a methodology used to analyze daylight illuminance adequacy in commercial spaces. Soon to be updated. preview_IES+LM-83-12.pdf (ansi.org)

• Effective Daylight for High Performance Facades by Kyle Konis and Stephen Selkowitz. Springer 2017

The latest synthesis of work by Lawrence Berkeley National Labs on advanced daylighting strategies and technologies, packed with technical data and examples.

• Daylighting: Architecture and Lighting Design by Peter Tregenza and Michael Wilson, Routledge, 2011

A seasoned educator’s guide to architectural daylighting design, including tools, graphics, and case studies.

• Windows on the World by Matteo Pericoli, Penguin, 2014

A delightful collection of hand drawings of window views from around the world, accompanied by loving descriptions by the writers who live and work with those views.

• Sunlighting as a Form Giver for Architecture by William Lam, Van Nostrand Rienhold, 1986

Old school daylighting advice and case studies by one of the early masters, using physical models for analysis. Out of print, but available used:

More extensive daylighting bibliographies with short reviews:

BOOKS – The Daylight Site | Daylighting research, architecture, practice and education The Daylight Site

77 Best Lighting Design Books | ArchDaily

Online resources:

Daylighting Pattern Guide – New Buildings Institute

A free, interactive tool that helps design teams incorporate proven daylighting strategies into commercial building projects for reductions in lighting power consumption and overall building energy use, from 2015.

“Visual Delight – The Importance of Views in the Workplace” by Lisa Heschong – YouTube

A preview of some topics covered in Visual Delight in Architecture, from a17 minute talk by Lisa Heschong in 2019.

[i]The term  ‘circadian’ refers to our body’s daily rhythms, synchronized to the 24-hour day-night cycles of the planet. Biologists have found that every cell in our body follows these rhythms, and just like a complex orchestra, they need a steady signal from a conductor to keep them all in sync. That signal is provided via light receptors in the retina. Disruption of circadian rhythms is associated with impaired sleep, cognitive performance, immune response, and metabolic function; it is also associated with weight gain and obesity, diabetes, hypertension, heart disease, stroke and depression. (For further detail see Chapters 1-3 of Visual Delight in Architecture (Routledge 2021) in references)