Figure 1. Excerpt from Marin County, CA, Low-Carbon Concrete Code (2019)
Resources:
WB Strategy 5: Carbon Bank
A broader strategy for limiting GWP is the ‘Carbon Bank’, or ‘Carbon Budget’ approach. In this method, an embodied carbon reduction target is set for a portion of the project. The contractor then chooses a cost-efficient approach to meet the stated target. For example, a carbon bank could be established for a single sub-contractor such as the concrete supplier. The concrete supplier may then decide to only reduce GWP in the concrete for the building’s foundations, and keep the rest of the concrete as a standard mix, as long as the overall target is met. This allows more freedom for the contractor to control costs and think creatively to meet the target. This also creates a creative ‘buy in’ mindset of the contractor and shared ownership in decision making. These interactions facilitate the potential for a positive adoption of sustainability goals.
It is recommended that the specifications ‘bank’ or ‘budget’ is divided between trades to avoid confusion. Setting a carbon ‘bank’ or ‘budget’ for the entire project is likely too confusing for general contractors and leaves significant room for error.
Note – at the time of publishing this guide, setting a Carbon Budget for a new project is largely conceptual due to limitations of project data and benchmarks. A significant goal of the SE 2050 Program and Database is to collect data for setting statistically informed benchmarks and to guide implementation of such strategies.
WB Strategy 6: Sustainability Goals
Specify the project’s sustainability goals in Part 1 – General – Summary. This can be applied to any material specification section, and encourages contractors and manufacturers to work closely with the A/E towards embodied carbon reduction goals. Thorough communication is encouraged among the project delivery team to make sure goals are met.
Example: 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% over the benchmarks established in the concrete industry’s Cradle-to-Gate Life Cycle Assessment Version 3.1.
Material-Specific Strategies
The following strategies apply to specific project materials.
Structural Steel
90% of the embodied carbon of fabricated structural steel occurs between the cradle-to-mill-gate Life Cycle Assessment (LCA) stages. Therefore, an engineer should take steps to consider the mill processes that affect the embodied carbon outcomes. Electric Arc Furnace (EAF) produced steel has significantly lower embodied carbon because that process uses a high (~93%) amount of recycled content as a steelmaking input, rather than the virgin iron products used in basic oxygen furnace (BOF) produced steel. Steps taken to choose EAF structural products can have a large impact on embodied carbon reduction goals.
Steel Strategy 1: Consider Domestically-Produced Structural Steel
At the time of this publication, specifying domestic steel can reduce the embodied carbon associated with steel production and transportation, depending on the project and source locations.
Domestic hot-rolled sections are entirely produced in EAFs. Structural plate and hot-rolled coils used to form HSS may originate from either a BOF or EAF furnace. However, imported structural products are more likely to be produced in BOFs. The 2020 worldwide average ratio of BOF:EAF steel production was 72:28, and notably, China’s ratio was 90:10. The United States’ ratio was 30:70.
Additionally, not all EAF steel is created equal. In some instances, United States BOF-produced steel may have lower embodied carbon than foreign EAF steel (this variation depends on a mill’s input mix of scrap, direct reduced iron (DRI), or pig iron, and the energy grid source mix). Check producer-specific EPDs when available.
Steel Strategy 2: Maximize Recycled Content
In general, the greater the recycled content of steel, the lesser its embodied carbon. Specify minimum recycled content percentages to be required for all structural steel used on a firm’s projects. Consider whether these minimum percentages will apply to each piece of steel, or on average for the entire project.
Note – average percentages are more commonly used and often easier to achieve. Above 75% recycled content is typical for hot rolled sections. For HSS and plate, it may be more useful to specify EAF production rather than recycled content.
Recycled content requirements may also be applied to metals in other specification sections, including steel decking, steel joist framing, and cold-formed steel. Cold-formed steel and decking are often formed from steel produced in BOFs.
Steel Strategy 3: Lower Embodied Carbon Steel Products
Many structural steel, steel deck, and cold-formed steel producers have published product-specific EPDs, so it is becoming increasingly viable to directly specify a maximum GWP for these components. The EC3 tool provides access to a wide variety of producer EPDs, and can help project teams set appropriate thresholds depending on their region.
Note – engineers should exercise caution when interpreting EC3’s Boxplot Diagram results for steel. The diagram aggregates many EPDs together that have differing and non-comparable features that affect embodied carbon, and therefore it is not an adequate representation of the steel market at this time. For example, EPDs are lumped together regardless of whether or not they include the effects of fabrication or galvanizing, whether or not the LCA accounting was developed according to a consistent Product Category Rule (PCR), whether or not the LCA modules included are consistent, and whether or not the product fits the category as defined in EC3. Engineers should confirm the EPD assumptions and be specific about parameters when specifying maximum GWP.
Steel Resources:
Cast-in-Place Concrete (CIP)
CIP Strategy 1: Blended Cements and Supplementary Cementitious Materials
Most of the carbon footprint in concrete comes from cement. Reducing cement can be accomplished by using supplementary cementitious materials (SCMs) as a portion of the cementitious materials. SCMs can be separately batched when producing concrete or included when using a blended cement. Permit all types of SCMs in concrete and do not place prescriptive limits on their use, unless required by code (note that prescriptive limits are only required in certain circumstances, for example concrete subject to deicing salt application). Permit the use of ASTM C595 blended cements in the specification. The different types include Type IP (with pozzolans), Type IS (with slag) and Type IL (Portland-limestone cement). Type IL is generally available in all regions. The same quantity of SCMs can be used in concrete made with Type IL cement as with mixtures with Portland cement.
Considering that it may not be feasible to reduce the Portland-cement content in all concrete mixtures used on a project (due to lesser strengths, greater setting times, or other factors), an alternative strategy involves specifying a cap on embodied carbon considering the totality of concrete used for the project. This allows the contractor flexibility tailoring the SCM quantities in concrete mixtures to address sustainability requirements without sacrificing performance.
CIP Strategy 2: Concrete Strength
Over-specifying concrete strength results in increasing embodied carbon. Specify only the strength that is actually required in the design of members. If durability provisions in the code require a higher strength, consider taking advantage of that in design.
Requiring concrete to achieve its design strength at an early age also increases the embodied carbon. Specify 56-day (or longer) strength, unless high early strength is needed, such as possibly for post-tensioning. If specifying high early strength, also take full advantage of the resulting 28-day or 56-day strength for design. Concrete continues to gain strength well beyond 28 days, especially when including fly ash. Specifying longer cure times can allow less over-design of the concrete mix by giving it more time to come up to strength. Specifying the use of concrete maturity sensors is also recommended.
Reducing the cement content as much as possible will have a significant effect on lowering embodied carbon. Mixes with less water will meet higher strengths with less cement. Choose an appropriate prescription for the total amount of water in the mix, with provisions for use of plasticizers or water reducers for meeting slump/workability requirements. Additionally, designers may specify a higher allowable water / cement ratio to reduce unnecessary cement in the mix.
Air content also impacts the ability to achieve design strength – in general, higher cementitious materials content will be required for air-entrained concrete.
CIP Strategy 3: Specialty Services & Technologies
There are a growing number of technologies and services that can reduce the embodied carbon of concrete. Many of these technologies require changes to material specifications before they can be included in a project. It is recommended that structural firms work with these technology producers to draft specifications and insert language so that these can be more readily specified on a project.
In carbon mineralization, carbon dioxide is injected into concrete during production, usually by specifying proprietary methods. When introduced, the CO2 becomes chemically converted into a solid mineral – meaning that the CO2 is permanently trapped within the concrete. This technology requires a concrete specification without any prescriptive water / cement ratios.
Examples of specialty services and technologies include Green Sense, CarbonCure, Solidia, and Blue Planet.
Note – many of these technologies and services are specific to certain regions and are not available nationwide.
CIP Strategy 4: Lower Embodied Carbon Steel Reinforcement (Rebar)
The embodied carbon of metals depends upon the electricity source used in production. Rebar is typically produced in electric arc furnaces (EAFs). EAF steel generally has a higher recycled content and lower carbon footprint than blast oxygen furnace (BOF) steel.
Many rebar producers have published EPDs, so it is becoming increasingly viable to directly specify a maximum GWP for rebar. The EC3 tool can help project teams set appropriate thresholds depending on their region.
CIP Strategy 5: Recycled Content
Recycled water can be permitted by including a reference to ASTM C1602 (instead of specifying potable water). Also for some members like footings and foundations, recycled aggregates could be permitted. However, avoid specifying a minimum recycled aggregate content for concrete as it can have an adverse impact on performance.
Note – the embodied carbon benefits of these strategies are hard to quantify.
CIP Strategy 6: Performance Specifications
The NRMCA has launched the P2P Initiative, an acronym for ‘Prescriptive to Performance’ specifications. The initiative’s goal is to shift the concrete industry to performance-based specifications, to more easily allow for concrete mix design optimization to meet sustainability goals. See the ‘WBS Strategy 3 – Performance Specifications’ section for further information.
Concrete Resources:
Concrete Baselines:
Wood and Mass Timber
Engineers should research the availability and sustainable attributes of wood products in their region and specify product requirements that are consistent with the project’s carbon reduction goals. In many cases, the carbon benefits of timber products that must be shipped from outside the project region justify their use, particularly if the timber products are transported via lower carbon emission modes of transport, such as cargo ship, coastal barge, or freight rail.
Wood Strategy 1: Specify Sustainable Forest Management Practices
When preparing specifications, engineers should be aware of the impact of various forest management practices on the sustainability of timber products. Practices and standards that address sustainable harvest activities include forest certification, responsible fiber sourcing standards, and state-promulgated best management practices [1].
While research regarding the embodied carbon benefits of certification is underway, it is now common practice among green building designers to specify certified wood as a means of assuring a baseline of sustainable forest-management practices. The expectation is that these enhanced practices lower environmental impacts, including climate change impacts. Expert guidance from the Carbon Leadership Forum supports this recommendation, along with other strategies [see FAQ 10 in Reference 2].
The following gives a brief introduction to forest certification and other forest management controls:
a.) Forest certification is the highest level of quality assurance of sustainable forest management. Certification assesses a landowner’s forest management practices against a series of agreed-upon standards such as sustainable harvest levels, chemical use, water quality, old growth protection, land use conservation, biodiversity, wildlife, natural areas protection, indigenous rights, GMOs, community and public engagement, and forests with high conservation value (HCVF). The four primary systems in North America are Forest Stewardship Council (FSC), Sustainable Forestry Initiative (SFI), Canadian Standards Association Sustainable Forest Management System (CSA SFM) and American Tree Farm System (ATFS). SFI and ATFS are covered under the umbrella of the Programme for the Endorsement of Forest Certification (PEFC) in some cases. Different programs have varying levels of stringency, performance criteria, prerequisites, transparency, and internal governance. See “Information on Differences Between Certification Systems” below for links to further information.
While the U.S. is the largest producer of industrial roundwood, not all wood products consumed in the U.S. are harvested domestically. Specifiers should still be aware of the origin of their specified wood products and take appropriate precautions, such as specifying third-party forest certification, if sourcing from areas with higher risk of controversially harvested wood. More information on trends in global deforestation and locations of high-risk forests can be found here.
b.) Fiber sourcing standards are another type of certification that applies to mills and others in the supply chain, but not to organizations that own or manage forests. These standards limit the risk of fiber coming from undesirable sources such as high-conservation forests or illegally harvested forests. The three major responsible fiber sourcing standards are: PEFC Controlled Sources, FSC Controlled Wood, and SFI Fiber Sourcing. Wood products with these labels may not come from certified forests, but the originating forests must satisfy certain defined good management practices.
c.) Every U.S. state has developed best management practices (BMPs) for forest management. These are guidelines for protecting water quality and other environmental concerns such as soil erosion and regeneration. Some of these are codified into state forest practice regulation and others are voluntary. Many states do not monitor or enforce forestry BMP practices. BMPs are typically not as stringent as the requirements of a forest certification program and function alongside the requirements of those programs. Three strong examples of state codified BMPs can be found within Oregon’s Forest Production Laws: An illustrated manual, Forest Practices Illustrated: A simplified guide to forest practices rules in Washington State and California’s Forest Practices Act.
Wood Strategy 2: Sheathing
Consider specifying plywood sheathing in place of OSB sheathing. The 2020 AWC/CWC industry-average Environmental Product Declarations (EPD) indicates that OSB has over 10% more embodied carbon than plywood [3].
Resources:
Information on Best Management Practices:
Information on Differences Between Certification Systems:
References:
[1] 10 Questions with Sustainable Forestry Expert, Dr. Edie Sonne Hall
[2] Carbon Leadership Forum, “Top 10 Questions and Answers from the Wood Carbon Seminars,” Question 10 (https://carbonleadershipforum.org/download/11317/).
[3] American Wood Council, Industry-Average wood product EPDs.
Concrete Masonry Units (CMU)
CMU is a composite system of four materials, each of which can be addressed in specifications: concrete block, steel reinforcement, mortar, and grout. Clay masonry (hollow brick) can also be used in lieu of concrete masonry, but is not as common as a structural material.
CMU Strategy 1: Specified Compressive Strength Method for Grout
Use the specified compressive stress method instead of the volume method to proportion grout mix. ASTM C 476, Standard Specification for Grout in Masonry, permits the grout to be proportioned by volume (prescriptive) or using specified compressive strength in accordance with ASTM C 1019 (performance). Using the compressive strength method generally results in less cement, thereby reducing embodied carbon.
CMU Strategy 2: Supplementary Cementitious Materials
Use supplementary cementitious materials (SCMs) to limit the quantity of Portland cement in the mortar, grout mix, and/or concrete block to reduce CMU’s carbon footprint. See ‘CIP Strategy 1’ section for more information.
CMU Strategy 3: Carbon Mineralization in Concrete Masonry Units
Carbon mineralization by CO2 injection during manufacture applies to CMU as well as cast-in-place concrete. See ‘CIP Strategy 3’ section for more information.
CMU Strategy 4: Lower Embodied Carbon Steel Reinforcement (Rebar)
The embodied carbon of metals depends upon the electricity source used in production. Rebar is typically produced in electric arc furnaces (EAFs). EAF steel generally has a higher recycled content and lower carbon footprint than blast oxygen furnace (BOF) steel.
Many rebar producers have published EPDs, so it is becoming increasingly viable to directly specify a maximum GWP for rebar. See ‘WBS Strategy 1’ for more information on EPDs.
Resources:
Acknowledgements
Thank you to the following for your contributions to this document.
- Michael Cropper, Thornton Tomasetti
- David Shook, Skidmore, Owings & Merrill
- Carl Elefante, Architecture 2030
- Lindsay Rasmussen, RMI
- Brian Trimble, International Masonry Institute
- Max Puchtel, American Institute of Steel Construction
- Scott Campbell, National Ready Mixed Concrete Association
