Concrete

Concrete is a material with very high embodied carbon.  Its manufacture and use accounts for 7% - 8% of carbon emissions worldwide, more than sectors like aviation, shipping, or landfills.¹   

Concrete is manufactured by mixing aggregates, water and a cementitious compound. Throughout the life cycle of a concrete project there are carbon emissions - from mining to implementation. The mining of aggregates impacts our natural resources, and its processing and transportation uses huge amounts of energy, currently usually provided by fossil fuels.

The largest source of emission in the life-cycle of concrete occurs during the manufacte of portland cement, the traditional cementitious compound used in making concrete. It is manufactured by heating  limestone and clay minerals to extremely high temperatures, typically through burning of fossil fuels.  Also the minerals themselves releases CO2 during processing.¹ Concrete is heavy and transporting concrete has a high carbon footprint. 

The concrete industry is very aware of the high carbon footprint of their product, and is actively researching ways to reduce the carbon footprint of concrete. Worldwide there many cutting-edge pilots and experiments seeking to reduce the carbon footprint in various ways.  At the current writing, there are reports of new ways to reduce  carbon in concrete almost every week. 

This document only lists technologies and methods that can be readily implemented today in Oregon.  New ways to reduce or eliminate the carbon footprint of concrete will be developed, and supply will reflect demand.

References

¹ Cracking the problem of cement, one of climate’s hardest problems | MIT Technology Review

Carbon drawdown: Sustainable Use of Concrete in landscape applications

In many landscape architecture projects, concrete is by far the largest source of carbon emissions. We encourage landscape architects to specify low-carbon and carbon-neutral concrete options when possible.

Reduce, Reuse, Recycle

  • Many landscape architecture projects have existing concrete structures on site.  Some of these structures can be re-used in various ways. The structure itself can be incorporated into the new design. An example is Duisburg Nord in Germany designed by Latz + Partners. This was an old industrial site that was ingeniously transformed into a wonderful park.

  • Another way to re-use old concrete is using broken pieces of old concrete pavement and using these as pavement similar to flagstones or as stacked pieces in a retaining wall. These re-used broken pieces of concrete are often called “urbanite.” The beauty of this type of re-use is that existing site concrete can be broken up on-site, reducing both the need for new materials and the need for transportation of heavy materials. In addition, concrete sequesters CO2 out of the atmosphere when it ages, in a process called carbonation. The more surface area is exposed to the atmosphere, the more carbonation occurs. Broken pieces of concrete (urbanite) expose more surface area to the atmosphere. Oregon company “The Wall” created a business around recycling concrete and currently recycles over 600 tons of concrete every year.

  • The demolished concrete structures can also be crushed into aggregates of various sizes. These aggregates can be used  as a base layer for new pavement, as a crushed aggregate surface material, or as fill. Using aggregates in the a new concrete mixture is theoretically possible but because of the uncertain quality of the aggregate most manufacturers and contractors are weary of such an application. There are, however, pre-cast concrete products available that have been created from recycled concrete aggregates. 

    On the other hand, using recycled concrete aggregates as base layer is very feasible and is and has been done on many highway projects. Robert Metcalfe, Principal Geotechnical Engineer at GeoEngineers, Inc. in Redmond, Washington, says that  nowadays contractors are actively wanting to use recycled aggregates. Recycled aggregates are also used as fill to form a base pad for buildings. It  can be readily used as a base for walkways and parking facilities in our landscape architectural projects. For using  crushed concrete from the site on the site you need a large size crusher on-site which is only economically feasible on a large project. Otherwise recycled concrete aggregates are now readily available in the Portland, Eugene and Bend area in the gradations that landscape architects are used to (you can order ¾-minus). The advantage of buying from a vendor is that you can be assured that the product adheres to environmental regulations, and you have more quality control.

    However, the aforementioned geotech Robert Metcalfe experienced plants dying off because of recycled concrete aggregates that were present in the plant beds.  The lime in recycled aggregates increased the pH in soils significantly, which can make the soil too acidic for many plants and trees. He now specifies the use of recycled aggregates only in areas that are at least five feet from planting areas. For this same reason, it should not be used as a base layer under permeable concrete. Despite some of these drawbacks, the material can be used and provides significant environmental benefits such as reduced landfill waste and less need for virgin gravel material that impact our natural resources. 

    The Oregon Department of Transportation (ODOT) has specifications for the use of Recycled Coarse Aggregate (RCA) in road construction projects. These specifications ensure that the material meets certain quality standards for strength and durability. They can be found in the ODOT Standard Specifications Manual. 

    Local successful examples include:

    1. Portland International Airport: The airport has used RCA in various construction projects, including runway and taxiway repairs. This has helped reduce costs and environmental impact by reusing materials on-site.

    2. City of Portland: The city has incorporated RCA in several street and road construction projects. By using RCA, they have been able to reduce the need for new raw materials and minimize waste.

    3. Oregon State University: The university has used RCA in the construction of new buildings and infrastructure on campus. This has supported their sustainability goals and reduced the carbon footprint of their construction activities.

  • This point is related to our lean design carbon drawdown strategy which will be discussed below.  Steel reinforcement comes with an embodied carbon footprint; in addition, the  steel reinforcement makes recycling the concrete more difficult. New technologies are under development that will make steel reinforcement unnecessary. For example, the TU Dresden in Germany completed a building in which steel rebar was replaced with carbon fiber. Introducing fiber-reinforced concrete (FRC) is an emerging alternative to standard concrete practices that reduces a structure’s carbon footprint , strengthens its durability, and is more cost-effective than conventional processes.

    Consider using recycled content such as reinforcement fibers, for example from Electrek, that turn old wind turbine blades into fibers that can be used as reinforcement. 

  • As landscape architects we are already aware that paving reduces site permeability, which interrupts the hydrologic cycle. Porous paving is one way in which this effect is reduced.  As mentioned before, concrete will keep uptaking carbon from the environment throughout its life cycle. The more porous the and the more surface area the more contact there is between the concrete and the air, the more uptake will happen.  

Lean Design

The next strategy to decrease carbon use on your site concrete is employing lean design strategies by using less concrete. A simple example is using a 15-inch wide concrete seat wall instead of a 18- or 24-inch wide seat wall. 

  • Engineers and landscape architects often overdesign footing dimensions because they don’t take into account the shape of the concrete structure. For example, an arched wall will not tip over as easily as a straight wall; nonetheless, frequently the same footing is used no matter the shape of the wall.  In addition, the National Ready Mixed Concrete Association (NRMCA) has noted that pavements are often unnecessary thick and the psi strength requirement is often unnecessary high (see point 5). Geotechnical engineers are not always consulted on projects, which results in over-dimensioning of footings, specifying pavements that are unnecessarily thick or specifying too much rebar for the specific soil strength of the project. 

    Consultations with a Geotech results in more upfront design cost; however, it can lead to material and construction savings that are a multiple of the additional design costs. Therefore, both for environmental and budgetary reasons, it is good practice to include a geotechnical investigation in the design process. Our landscape architecture projects are often part of larger projects that include buildings for which there is already a need for geotechnical investigation; it will not cost much extra to extend to scope of the engineer to include the soil strength considerations for our structures.

    Similarly , in many cases wall footing designs are over-dimensioned.  As mentioned earlier, curved walls  are inherently stronger because of the curved configuration as compared with a straight wall.  If an engineer would be  allowed to actually calculate the footing size (versus simply using the footing recommended for a straight wall) the potential concrete savings for such a shape could be substantial. 

  • In many cases, concrete landscaping elements are over-designed because the structural engineer rarely performs analysis on items outside of the primary structural frame, unless it is coordinated upfront in the design fee or covered through additional services. This often leads to landscape architects or engineers utilizing rules of thumb derived from dimensions that have stood the test of time on previous projects. It’s also important to point out that oftentimes the concrete mix requirements for these elements are taken from the structural engineer’s specifications even if they weren’t directly involved in the design, which often means much higher strengths are used than needed. Then when the concrete mix submittals come in, the structural engineer typically won’t pay close attention to non-structural mixtures and will defer to the rest of the design team for review. Cement is the most impactful ingredient in concrete, and it is important to understand that the higher the strength of the concrete, the more cement typically needs to be used. This tells us we need to reduce strength whenever possible and coordinate this alongside the impacts of reinforcement to ensure we’re capturing overall reductions.

    Looking ahead, project teams will need to start placing more importance on reducing concrete and embodied carbon in general outside of the building’s core & shell through more proactive coordination between landscape architects and engineers. This is evident as we look towards the release of LEEDv5 where LCA is required for all projects and hardscape is now required in the scope.

  • The best way to reduce the embodied carbon associated with concrete after implementing lean design is putting together a performance-based specification. Historically, structural engineers have specified conservative criteria like test age and water-to-cement ratios in an effort to ensure the performance of concrete. However, it’s important to note that the structural engineer doesn’t craft the concrete mixture itself and it’s up to concrete supplier to create a mixture that meets the criteria set forth in the drawings. Over time, structural engineers have maintained this practice and have failed to update their specifications even as design codes compile more research and relax their recommendations. This is most evident in determining an exposure class for the concrete. It’s incredibly important to coordinate the exposure class for concrete because it affects the compressive strength, water-to-cement ratio, and air content required, which will in turn affect the amount of cement needed in the mixture.  Looking ahead, our codes are working on developing even more performance-based approaches, so it’s critical to stay up to date as research continues to progress.

  • Another highly influential area of the spec is the concrete test age. Engineers have generally set test ages at 28 days, but over time, concrete continues to cure and increase in strength. So, if test ages can extend to 56 or even 90 days, the supplier can utilize less cement because they’re accounting for that longer curing and strength gain. However, it’s incredibly important to make sure your contractor is aware of extended test ages when determining schedule and sequencing. For example, oftentimes contractors want to backfill and pull formwork for retaining walls as quickly as possible, and the concrete must be cured to a certain extent before this happens; so they’ll need to think about this while coordinating the construction schedule.

Include sustainability goals in the concrete specification section

Landscape architects should coordinate with the project team to make sure sustainability goals are included in the concrete specification section. It is best to include these goals in the concrete section and not in a general section because when contractors bid on projects they often only read the section applicable to their trade.   Below is an example of a written specification sustainability goal:

SECTION 03300 – CAST-IN-PLACE CONCRETE

PART 1 – GENERAL | 1.1 SUSTAINABILITY GOALS

This project has a goal of reducing the embodied carbon footprint over a typical project by 20%*. To accomplish this goal, we are targeting a carbon footprint reduction for concrete of 35%* under benchmark established in the NRMCA’s Cradle-to-Gate Life Cycle Assessment Version 3.2*. Specific targets for Global Warming Potential (GWP) are provided in Section 2, CONCRETE MIXTURES. To accomplish this goal, we are encouraging the use of innovative products and processes for concrete and will consider proposals for mix designs that can demonstrate they meet all performance criteria for strength, durability, constructability, and cost, in addition to reducing carbon footprint.

* These values are for demonstration purposes only.” (Source THE TOP 10 WAYS TO REDUCE CONCRETE’S CARBON FOOTPRINT, Build with Strength, pdf)


This specification language allows  the concrete supplier freedom in creating the most efficient batch of mixtures to minimize the project’s total carbon footprint as a whole. However, note that the combinations of psi and maximum GWP do not always work well and can be difficult to achieve in certain regions.

Reducing portland cement

The most important way in which the carbon footprint of new concrete can be reduced is reducing the portland cement content by using so called Supplementary Cementitious Materials (SCMs) to the largest extent possible. Currently there are several SCMs readily available, including fly-ash,  ground glass, natural pozzolans, and limestone calcined clay cement (L3). 

Traditional portland cement is expensive therefore many mixing plants already have  substituted some of the portland cement content with other supplemental cementitious materials (SCM).  In most situations where landscape architects specify a concrete product (pre-cast or cast in place), there are many additional options available to further substitute portland cement with such other cementitious materials.  These options are NOT more expensive, and in most cases do not compromise the structural integrity of the concrete work. In fact, some supplemental cementitious materials make the concrete stronger. The biggest construction challenge is that mixes with greater SCM content typically gain strength slower, potentially requiring longer cure times.  However, studies indicate that the strength required for typical  landscape architecture projects is easily achieved within standard cure time periods.  A study by the Carbon Leadership Forum (CLF) indicates that concrete contractors will need to gain familiarity in finishing lower carbon mixes.² Several municipalities in Oregon are now requiring low-carbon concrete mixes for  city-owned projects. These requirements are  increasing the number of contractors with experience in using low-carbon concrete, which in turn will make it possible for design professionals to specify using experienced contractors. 

Lower carbon concrete mixes are readily and widely available in Oregon. Especially Type L1 concrete which reduces the global warming potential by about 5 to 15% , the industry is moving rapidly to using Type L1 as the standard mix (ODOT does this, for example). As with any concrete mix, the reduction in global warming potential varies regionally because aggregates vary regionally, and some aggregates require more cement.  

The most typical SCM used is slag and fly ash.  These come as by-products from coal burning and steel production. Oregon does not have these industries so it needs to be imported. Using ground glass is currently also not readily available locally. The industry and environmental researchers are investigating new cementitious products. For now, best practice is to specify strength performance and not specify specific SCM. This recommendation might change in the future if/when local SCM become readily available. 

References

² ConcreteCaseStudy-ResBirdsmouth.pdf (oregon.gov)

Include early communication with concrete manufacturer and contractor

After you have taken the first step of including sustainability goals in your specifications, the next and most important step is coordinating those specifications with the contractor. If coordinated early enough, they could potentially help with a holistic bidding exercise where cost is weighed alongside carbon in concrete bids. Then after a concrete supplier is chosen, it’s a good idea to hold a meeting with them as well to go over the specifications to see if there’s any areas prohibiting innovation or opportunities for further reductions. This meeting should also include the contractor to make sure they are aware of any considerations associated with included supplementary cementitious materials, admixtures or other ingredients that could affect the wet and hardened properties of the concrete.

Use performance specifications instead of prescriptive specifications.

Current specifications are often prescriptive and cement ratios are included. These cement ratios, in many cases, lead to the use of more portland cement than required for the needed strength of the structure.  A better way is to use a performance specification. “The best example of a performance criterion is strength. By specifying compressive strength, a concrete producer can design a mixture to meet the strength criteria through experience and testing. The mixture proportions are not specified, just the target strength, leaving the product formulation entirely in the hands of the manufacturer. It permits the producer to develop a mixture that not only meets the strength requirement but does it economically, so they can minimize cement content, use supplementary cementitious materials (SCMs) such as fly ash and slag cement or other innovative technologies to reduce cost, improve performance such as workability and durability, and reduce environmental impact.” (Concrete Academy, last accessed 4/19/24: CE Center - Concrete Innovations (bnpmedia.com) 

And as mentioned before, specify strength requirement further out, for example 56 days instead of 28.

Carbon-negative concrete aka Biochar: the carbon sequestrating concrete

A relatively new development in concrete mix design is the sequestration of biogenic carbon in the form of adding biochar to the mix. Biochar is already known in the industry as a great soil amendment product (learn more about Biochar). Photosynthesis by plants is the beginning point for biochar. Through this process, mother nature captures and compresses carbon dioxide over 1,000x, creating biomass in the form of plant material. Humans run this biomass through a thermal process known as pyrolysis, an anaerobic heating environment. Biochar can be added as an amendment to a concrete mix. It will make the mix darker colored, and the resulting concrete will have a different strength performance depending on the amount of biochar that has been added (typically, the addition of biochar is performance-neutral or increases the concrete’s strength). 

While concrete only has a few ingredients (Sand, Rock, Cement and Water), the hydration reaction is a very complex process both chemically and thermodynamically, requiring rigorous scientific analysis to understand how new materials will work in concrete mixtures. In addition to the primary hydration reaction which takes place in weeks and days, there is the potential for a secondary reaction in concrete to take place which occurs over weeks and months. This reaction is called the Pozzolanic reaction. Supplementary Cementitious Materials (SCMs) rely on this Pozzolanic reaction to develop strength.  

The Carbon Dioxide Removal (CDR) industry looks for sequestering carbon in ways that are durable, scalable and cost effective. Concrete provides durability by the nature of this long-lasting material, so the sequestration has the potential to endure. There are concrete structures that are 2,000 years old, making this approach one of the most durable CDR technologies around. 

Concrete is also the most used human-made material on the planet. In fact, there is more concrete used in construction than wood, steel, plastic and glass combined. By the sheer volume of concrete, this sequestration solution is massively scalable. Solid Carbon is an Oregon company that produces a biochar product that can be added to concrete, which then sequesters that carbon for hundreds of years.  Solid Carbon’s approach is able to sequester at a rate of 100 times the gas phase mineralization approaches used by other technologies in concrete. 

Biochar in concrete is cost effective because biochar can be made from waste materials at a very low price. The capital and operational expenses of creating biochar are very low. Sequestering carbon in concrete is likely the only way to achieve a carbon neutral landscape design in most urban settings. 

Solid Carbon products are tested for outdoor use including freeze / thaw durability.  They have been deployed recently at Remy Wines in McMinnville, Oregon and the Burnside Skate Park addition in Portland. Currently BioLockTM is available for use in concrete applications on a limited supply basis. Solid Carbon’s next product, BioPozz, is planned to be released in 2025. This product will be a partial cement replacement.

Read more about Biochar in our Resource Guide.

Additional Information on Concrete

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