SPECIFICATION GUIDANCE

BF-BOF vs EAF

Primary steelmaking, also known as integrated steelmaking, utilizes a Blast Furnace and a Basic Oxygen Furnace (BF-BOF) to produce steel. With this process, virgin iron ore-based inputs are high, recycled content inputs are low, and the primary energy source utilized is derived from coal. Variation in embodied carbon of BF-BOF steelmaking depends on a mill’s input mix of scrap, direct reduced iron (DRI), or pig iron, and the energy grid source mix.

Secondary steelmaking utilizes an Electric Arc Furnace (EAF) to produce steel. With this process, virgin iron ore-based inputs are low, recycled content inputs are high, and the primary energy source utilized is electricity.

On a worldwide average, EAF steelmaking is 75% less carbon intensive than BF-BOF steelmaking, so choosing EAF products can have a large impact on embodied carbon reduction goals.

In the U.S., structural steel products come from both EAF and BF-BOF production, with availability varying by product type – so early coordination with the contractor is critical to confirm the feasibility of procuring EAF steel. Hot-rolled structural steel sections are exclusively EAF, while plate and manufactured products (e.g., HSS, joists, metal deck) may come from either method and require project-specific verification.

Domestic vs Non-Domestic

Overall, the U.S. steel industry is the least carbon-intensive among the world’s leading steel-producing countries. This is due to our high share of EAF steelmaking relative to BF-BOF steelmaking and due to our relatively clean electrical grid. Imported structural products are more likely to be produced in BF-BOFs, as the 2024 worldwide average ratio of BF-BOF:EAF steel production was 71:29, and notably, China’s ratio was 90:10. For comparison, the United States’ ratio was 28:72.

Additionally, not all EAF steel is created equal. In some instances, United States BF-BOF-produced steel may have lower embodied carbon than foreign EAF steel. Material test report (MTR) submittals identify the mill of origin and will confirm whether steel was sourced domestically or non-domestically. AISC maintains a database of shapes and sizes produced domestically for engineers to reference during design. As a practical rule of thumb, sourcing steel produced in the United States is often an effective strategy for achieving lower embodied carbon.

Consider Recycled Content 

There is a correlation between higher recycled content levels and lower embodied carbon intensity in steel, but using one as a proxy for the other may not be fully accurate for a specific product. Mill reporting of recycled content reflects annual averages, not the characteristics of a specific steel product from a specific production heat. In reality, the recycled content of steel products fluctuates from melt to melt, so recycled content letters from mills, which reflect annual averages, may not sufficiently represent the recycled content of an individual product.

Specifying a minimum recycled content percentage is not recommended simply because there are more effective alternatives. However, it is a common sustainability strategy in the marketplace and many green building rating systems utilize this approach for material related credits. Similarly, this strategy may be pursued on projects as an alternative to specifying GWP limits. AISC recommends minimum recycled content values of 75%, 60%, and 50% for hot-rolled sections, steel plate, and hollow structural sections respectively when this specification strategy is implemented.

Lower Embodied Carbon Steel Products

Nearly all of the domestic structural steel producers and manufacturers have published facility-specific EPDs, so it is becoming increasingly viable to directly specify a maximum GWP for these products. (AISC summarizes, links, and accurately interprets all relevant EPDs together at aisc.org/epd and provides specification guidance for maximum GWP approaches in their designer toolkits). A primary decision in this approach is where to set the GWP threshold. A common approach is to set thresholds at industry-average levels, which are based on industry-average EPDs. These EPDs also form the basis of the Carbon Leadership Forum’s material baseline publications. Setting GWP thresholds higher than industry average assures broad compliance while excluding only the highest emitters. Setting GWP thresholds lower than industry average may achieve better embodied carbon performance, but that approach may encounter supply-chain challenges.

Note – Third-party EPD database tools such as EC3 and the Transparency Catalog are other sources of EPD aggregation, but they should not be relied upon for statistical output or for regional benchmarking. In particular, 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.  

Although the fabrication stage only accounts for approximately 8% of structural steel’s cradle-to-fabricator-gate GWP, fabricators themselves are a key partner on projects pursuing sustainability goals because they are the entity that buys the project’s steel. Fabricators can leverage their supply-chain knowledge to achieve sustainable purchasing for projects and help engineers coordinate practical specification requirements. At the end of 2023, AISC launched a Sustainability Partner Program for their fabricator membership that educates fabricators on their own footprint, commits them to reporting environmental data, and has them create internal sustainability goals. AISC has created the program with the intent to make it easy for fabricators to participate, so it is encouraged to include this requirement in your specification. At a minimum, the program can be a starting point for a sustainability conversation on your steel projects with the fabricator.

Steel Resources:

• Specification Strategies for Embodied Carbon Reduction (Source: American Institute of Steel Construction, 2025)
• Making your steel specification more sustainable (Source: The Institution of Structural Engineers, 2021)
• How Clean is the U.S. Steel Industry (Source: Global Efficiency Intelligence, 2019)
• More than Recycled Content: The Sustainable Characteristics of Structural Steel (Source: American Institute of Steel Construction, 2017)

Performance Specifications

Historically, the structural engineering profession has relied on prescriptive specifications to ensure concrete performance, dating back to practices such as specifying minimum cement content and incorporating durability requirements into ACI 318. Minimum cement content was originally prescribed to ensure adequate strength, while prescriptive durability requirements, including maximum water-cementitious materials ratio (w/cm) and air content, served as proxies for long-term durability. Although these requirements have produced durable concrete structures, they prescribe how concrete should be proportioned rather than what performance the concrete must achieve. When considering embodied carbon, exposure class can significantly influence cement content by prescribing by lower w/cm ratios, higher minimum compressive strengths and air content requirements.

Over time, industry research and advancements in concrete materials have improved the understanding of how concrete mixtures achieve strength and durability. As a result, ACI 318 has evolved by introducing durability exposure classes, relaxing w/cm ratios and minimum compressive strength requirements for moderate freeze-thaw exposure, and more recently simplifying the freeze-thaw exposure classifications. Similar refinements have occurred for sulfate, water, and corrosion exposure provisions as the industry has continued moving toward performance-based durability requirements, some of which are offered as alternative compliance paths in the code. Despite these advancements, specifications are often copied from office master specifications or previous projects without considering project-specific exposure conditions, climate, or locally available materials, leaving room for unnecessary conservatism and increased embodied carbon. Structural engineers should carefully evaluate exposure conditions for each project and are encouraged to use the NRMCA Durability Exposure Class resources when selecting durability requirements that appropriately protect the owner’s investment.

As new blended cements, supplementary cementitious materials (SCMs), and admixture technologies continue to enter the market, performance-based specifications are becoming increasingly important. Rather than prescribing mixture proportions, performance specifications establish the required project outcomes while allowing the concrete supplier flexibility to develop the most effective mixture using locally available materials. Developing a successful performance specification requires coordination between the design team, owner, contractor, and concrete supplier to clearly communicate the project’s basis of design, including strength, durability, aesthetics, constructability, schedule, and sustainability objectives. Structural engineers are encouraged to advocate for early coordination between the contractor and concrete supplier during design to identify locally available materials, optimize mixture designs, and fully realize the benefits of a performance-based specification. Refer to the NCSEA Performance-Based Concrete Specifications resource for additional guidance.

Concrete Strength

Over-specifying concrete strength results in increasing the embodied carbon of a given concrete mix. Specify only the strength that is actually required in the design of members and when it is required. If durability provisions in the code require a higher strength, consider taking advantage of that in design. Certain applications may be more challenging to achieve carbon reductions, regardless of specified design strength, for constructability reasons, such as schedule, finishability, and pumpability. 

Requiring concrete to achieve its design strength at an early age often increases the embodied carbon because this is typically achieved through higher cement contents. Coordinate strength requirements with the construction team to ensure schedule critical elements achieve strength requirements (e.g. post tensioned members) and explore opportunities to utilize extended long-term strength during design.

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 SCMs, when properly cured. Specifying longer moist curing times can allow less over-design of the concrete mix by giving it more time to come up to strength.  Although not a substitute for final acceptance of standard-cured specimens, specifying the use of concrete maturity sensors to monitor early in-place strength can be helpful for advancing construction work, especially when stressing of tendons, removal of formwork, or column installation are on the schedule’s critical path. ASTM C1074 is standard practice for using the maturity method to estimate in-place strength. ACI PRC-228.1-19 is a useful resource with additional background on methods for estimating in-place strength, but cannot be incorporated by reference into specifications and other construction documents. 

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, however there are practical limits on how much the paste content can be reduced and still meet constructability requirements. Performance specifications for strength and durability can eliminate the need for water/cementitious materials limits, and designers should avoid specifying slump in most instances and let target slump be established by the contractor and concrete producer as needed for constructability. When using performance specifications, allow sufficient time for pre-qualification of mixtures through trial batches, placements, and testing.

Air content also impacts the ability to achieve design strength – in general, higher cementitious materials content will be required for air-entrained concrete. Avoid specifying entrained air except when required for durability purposes to protect against freezing and thawing exposure.  

It is also critical that specifications require the testing agency to provide curing on-site and at their laboratory in accordance with initial and standard curing requirements of ASTM C31. Failing to follow standard curing requirements for specimens (normally cylinders) designated for acceptance testing can lead to low strengths, prompting additional overdesign of the mixture to compensate for improper handling of these specimens. Improper initial curing and other testing variability can therefore increase both cement use and embodied carbon. Recent industry guidance and research from NRMCA and the MIT Concrete Sustainability Hub highlight reliable acceptance testing as an important, often overlooked opportunity to reduce concrete embodied carbon.

Supplementary Cementitious Materials and Blended Cements 

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. To allow the most flexibility, permit all types of SCMs in concrete and do not place prescriptive limits on their use, unless required by the code applicable to the project (note that prescriptive SCM limits are only required in ACI 318 for certain circumstances, for example concrete subject to the application of deicing salts).

Some commonly used SCMs used in concrete are fly ash (ASTM 618 Class C and Class F), ground granulated blast furnace slag (slag cement, ASTM C989), or raw or calcined natural pozzolans (ASTM C618 Class N and ASTM C1945), blended SCMs (ASTM C1697), and other pozzolans including ground recycled glass (ASTM C1866). ASTM C1912 is a new performance specification to cover novel SCMs that do not meet other standard specifications and is employed in combination with ASTM C1709’s guide to evaluating alternative SCMs. Regional availability is a factor, checking with local material suppliers to determine what SCMs are available can help engineers plan more achievable low embodied carbon strategies.

Permit the use of blended hydraulic cements (ASTM C595) and hydraulic cement (ASTM C1157) in the specification. The different types of ASTM C595 cements include:

Notes:

1. No stated maximum, but must satisfy limit on total limestone, pozzolan & slag cement.
2. In practice, Type IS>70 is not permitted for structural applications and has limited availability (applies to both Type IS>70 & Type IT S>70)

ASTM C595 cements with higher SCM contents (30%, or 40% for slag) have lower early-age strength requirements, reflecting the delay in strength development that may be expected.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. Types IP, IS, IT, and IC cements are also growing in availability, although the SCMs in these products vary regionally.

ASTM C1157 (performance hydraulic cements) also include blended cements containing SCMs, but without prescriptive limits on their composition. Blended cements with more ingredients or higher SCM contents than permitted under ASTM C595 can be sold as ASTM C1157 cements. 

It may not be feasible to reduce the portland cement content in all concrete mixtures used on a project due to lower strengths, greater setting times, or other factors. Allow suppliers the flexibility to tailor the SCM quantities in concrete mixtures to address sustainability requirements while balancing availability, cost, and performance.

When permitting the use of newer cements and SCMs, communication with the entire project team is critical. A supplier cannot simply substitute one cement type for another in an otherwise unchanged mix without treating it as a new mix design for review and approval.  Different cements have different chemistries and physical characteristics, that in combination with other mixture constituents, can affect setting, workability, and finishing characteristics. Material suppliers are often best positioned to manage the introduction of new cements and SCMs, drawing on their historical data and experience with similar mixtures. Engage suppliers early to determine whether a proposed mixture has an established performance history or requires additional validation. For mixtures developed specifically for the project or otherwise lacking sufficient prior testing, consider specifying pre-qualification testing and mockups. Testing and mockups should be coordinated with the builder, supplier, and design team to confirm that key performance criteria are met for the specific project and application.

Recycled Content 

Recycled water can be permitted by including a reference to ASTM C1602 (instead of specifying potable water). Recycled concrete aggregate (RCA) may also be appropriate for some members, such as footings and foundations. Research has found that up to 25% of natural crushed stone aggregate may be replaced with RCA without significantly affecting concrete strength and recommends partial replacement in moderate exposure conditions (link). However, avoid specifying a minimum recycled aggregate content for concrete as it can have an adverse impact on performance and GWP. ASTM C33, Standard Specification for Concrete Aggregates, now covers recycled aggregates in addition to traditional natural and manufactured aggregates, but notes there are precautions needed with its use, as it may necessitate greater attention to durability and can increase the water demand of mixtures. Refer to Chapter 7 of ACI PRC-221-25 for more information on the use of recycled concrete aggregate. ACI PRC-555-01 also provides guidance on the removal and reuse of hardened concrete. 

Note – the embodied carbon benefits of these strategies are hard to quantify since most publicly available concrete carbon calculators do not offer recycled water and recycled aggregate as options.  Moreover, they are not expected to change the GWP of the concrete mix significantly since the GWP factors for water and normal-weight aggregate are small compared to cement but are beneficial when considering resource conservation.

Global Warming Potential Limits:

Global warming potential (GWP) is becoming an increasingly common performance criterion as designers establish embodied carbon limits to meet project sustainability goals and policy requirements. There are several benchmarks and baselines available for establishing GWP limits; however, these datasets should be used carefully, as regional differences in material availability, cement production, and concrete manufacturing practices can create procurement challenges if limits are not tailored to the project location.

Concrete GWP limits may be established on a mix-by-mix basis or using a project-wide carbon budget as demonstrated in ACI 323-24. A project-wide carbon budget is generally encouraged because it provides concrete suppliers flexibility to use higher-GWP mixtures where functionally necessary, such as high-early-strength concrete or post-tensioned slabs, while offsetting them with lower-GWP mixtures where feasible elsewhere on the project. This approach encourages mixture optimization while maintaining the project’s overall embodied carbon target.

Regardless of the approach used, structural engineers are encouraged to advocate for early coordination with the contractor and concrete supplier. Early collaboration allows the project team to understand locally available materials, identify embodied carbon reduction opportunities, optimize mixture designs, and resolve potential procurement challenges before bidding, helping to fully realize the benefits of both performance-based specifications and project-specific GWP limits.

Concrete Baselines:

• NRMCA Member Industry-Average EPD for Ready-Mixed Concrete – Version 3.2 (Source: National Ready Mixed Concrete Association, 2021)
• A Cradle-to-Gate Life Cycle Assessment of Ready-Mixed Concrete Manufactured by NRMCA Members – Version 3.2 (Source: National Ready Mixed Concrete Association, 2021)
• Statistical Addendum for A Cradle-to-Gate Life Cycle Assessment of Ready-Mixed Concrete Manufactured by NRMCA Members – Version 3.2 (Source: WAP Sustainability, 2024)

Lower Embodied Carbon Steel Reinforcement (Rebar)

The embodied carbon of metals depends upon the electricity, fuel sources, and amount of scrap used in production.  In North America, rebar is typically produced from recycled steel in electric arc furnaces (EAFs). EAF steel generally has a higher recycled content and lower carbon footprint than blast oxygen furnace (BOF) steel, although its GWP is highly correlated to the emissions factors associated with the local electrical grid region.

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 determine appropriate GWP limits depending on their region and project goals. Because electrical grids can be more or less carbon intensive by region, the industry-wide GWP limits may not be appropriate.

Concrete Resources:

• Lower Carbon Concrete Guide (Lower Carbon Concrete Task Force, 2025)
• A Framework for Specifying Novel Concrete Technologies (Arup, 2026)
• Performance-Based Concrete Specification Guidance: Concrete Class Table (Source: NCSEA Sustainable Design Committee, 2024)
• Concrete – A Pragmatic Approach to Lowering Embodied Carbon (Source: ZGF, Fast+Epp, EllisDon, LaFarge)
• Guide to Improving Specifications for Ready Mixed Concrete: (Source: National Ready Mixed Concrete Association, 2026
• Guide Specification – Concrete for LEED v4 Projects (Source: National Ready Mixed Concrete Association)
• P2P Initiative: (Source: National Ready Mixed Concrete Association)
• Low Carbon Concrete Implementation Strategy: (Source: Magnusson Klemencic Associates)
• Blended Cements Website (Source: American Cement Association)
• Specifying Lower Embodied Carbon Concrete: Making Decisions at Each Step of the Project (Source: Simpson Gumpertz & Heger, 2026)
• Concrete EPD Availability (Source: Forrell Elsesser)

Seek Opportunities for Reclaimed and Salvaged Wood

The reuse of any structural material is often the lowest carbon solution available for structural engineers. The use of reclaimed or salvaged wood has low-carbon benefits not only from lower manufacturing emissions compared to equivalent virgin wood products, but also from the prolonged carbon storage in the wood element itself.  

The logging of virgin timber releases carbon stored by the soil (below ground biomass), which can contain twice as much carbon as trees; Carbon is also released due to the decomposition of roots, branches, leaves, and needles. This can be mitigated to some degree with sustainable forest management practices, but it remains an emissions source that is often overlooked when thinking about wood products.*  Additionally, there are emissions from the logging and manufacturing equipment associated with virgin timber products. The sum of the upfront manufacturing (A1-A3) emissions is typically greater (can be up to 3 times greater as shown in a study for framing lumber) than the emissions associated with deconstruction, regrading, and reprocessing salvaged lumber. The reuse of existing wood will also extend the life of a material that would otherwise be burned or allowed to decompose, which would release its stored biogenic carbon back into the atmosphere. Dimension lumber also does not make full use of the tree, leading to excess wasted material.  

For the structural designer looking to specify salvaged wood for their project, the design properties (primarily strength and stiffness) of each piece of lumber must be known. This requires identification of the species, and structural grading. Species identification in some cases can be completed by a wood expert analyzing the specimen under a microscope; another method gaining traction is acoustic grading using wave propagation measurements. Neither of these methods are perfect, and may sometimes result in a range of identified potential species. Grading pieces of salvaged lumber can also be challenging. Regrading a salvaged piece of lumber is not currently allowed by grading agencies. In some lucky cases, a salvaged piece may have an existing grade stamp, which can remain valid unless any replaning occurs to the member or excessive defects (such as nails) exist.  In other instances, lumber can be visually graded per ASTM D 245, which requires an examination of all 4 sides of the lumber piece for defects such as knots or nails.  Ultimately, the engineer of record does bear the responsibility for the species and grading determinations used in structural design.  More information on the structural wood reuse process can be found here.

Quality control measures are critical for successful implementation of this strategy, and the material may require additional testing or treatment before structural reuse is possible.  There may also be jurisdictional requirements prohibiting the use of salvaged or reclaimed wood, and engineers should review the applicable building codes to determine what is allowed for their project.

Recommendations:

• Investigate opportunities for procuring salvaged wood or wood coming from targeted harvests needed for forest health management by reaching out to local sawmills, foresters, and discussing with suppliers and manufacturers.
• Search for products through online reuse marketplaces like Heritage Lumber and Cambium Carbon.
• Engage with reuse communities such as BuildReuse, Unbuilders, UrbanMachine, and the Valley of the Sun Reuse/Deconstruction Working Group (VOSWG).

* See ‘Additional Considerations – Wood in LCAs’ section

Specify Sustainable Forest Management Practices

The net carbon benefit of wood products varies greatly with different forest management practices. Poor forest management practices (e.g., logging on illegal or protected land and clearing of forest for non-forest use) can lead to significant carbon emissions in addition to other negative environmental impacts on ecosystem and forest health. Sustainable forest management practices can vary regionally as different jurisdictions attempt to balance ecosystem health, forest resilience, biodiversity, and community needs. A well managed forest  can lead to win-win scenarios of maximizing ecosystem health and minimizing emissions into the atmosphere. It is crucial that sustainable forest management is a core criteria for the procurement of sustainable wood products. 

According to Think Wood, assurance for timber harvested sustainably can be provided by forest certification, responsible fiber sourcing standards, and/or Best Management Practices (BMPs, developed by every U.S. state). Currently, it is common practice among green building designers to specify certified wood (common programs include FSC, SFI, ATFS, and PEFC) as a means of assuring a baseline of sustainable forest management practices. State-developed Best Management Practices may not be as stringent as the requirements of forest certification, but a few strong examples of state codified BMPs include 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.

While forest certification is often seen as having a high level of quality assurance for sustainable forest management, direct climate benefits are not necessarily assured by wood certification since these programs are only recently beginning to specifically address climate mitigation. Additionally, certified products may come with a cost premium. Smaller forest owners and public forests can also demonstrate exceptional, sustainable forest management practices, but may not have the financial means for acquiring certification. The Climate Smart Wood Group provides guidance on additional criteria for climate-smart forestry operations outside of certification which includes non-industrial timber producers such as “US federal, indigenous or tribal, non-profit organizations and land trusts, and family forest owners.” Other recommendations are sourcing from ecological forestry operations where “restoration is occurring to enhance ecological resilience and integrity (which) by definition offer many climate-smart benefits,” and from forests with above-business-as-usual regulations and practices. Examples of the latter include forests governed by Habitat Conservation Plans and those that are third-party-certified carbon projects.

Given the complexity of wood sourcing impacts and challenges with transparency and traceability, practitioners are encouraged to engage with local foresters who are trained, deeply knowledgeable about local forest needs, and can help identify wood sources from forest management operations. Along with source forest disclosure as described in the previous section, it is recommended to request the disclosure of forest management practices.

While there is an ISO Standard (ISO 38200:2018) setting requirements for the supply chain of wood products, there is currently no consensus or widely-accepted standard for sustainable forest management practices. However, there are some clear wins for sustainable wood procurement that practitioners can consider. This includes salvaged and reclaimed wood products (more detail in next section), and feedstocks that are byproducts of targeted harvest, such as small diameter trees not suitable for typical dimension lumber, as part of forestry operations to improve forest health (some examples include Vaagen Brothers Lumber in Washington and Timber Age Systems in Colorado). 

Recommendations:

• Request information about forest management practices from manufacturers for wood products. 
• Avoid illegally sourced wood and wood sourced from deforested lands, old growth forests, and protected lands.
• Proactively engage with forest managers and sawmills, with support from the project team (client, designers, and contractors), to uncover opportunities for available feedstock as byproducts of local forest and ecosystem needs.
• Evaluate sustainable forest management practices against criteria provided by forestry experts, like the Climate Smart Wood Group procurement guide.
• Seek indicators of sustainable forest management practices such as forest certification, responsible fiber sourcing standards, Best Management Practices, ecologically-motivated forestry operations, and climate-smart forestry operations from non-industrial timber producers such as the US Forest Service, indigenous or tribal communities, non-profit organizations, and family forest owners.

Request Forest Sourcing Disclosure

The supply chain for wood products tends to be more opaque and harder to trace than for other structural products. Compared to commodity mineral supply chains, the wood supply chain is often more dispersed and with a larger variation in the involvement, attitudes, and intentions of stakeholders (such as local communities, land owners, loggers, mills, manufacturers, developers, the commoditized lumber market, etc.) while also varying from region to region.  Engineers can leverage their position as material specifiers by requesting a high degree of traceability and transparency within the forest industry. The World Resources Institute defines these terms as follows: 

• Traceability refers to the ability to link a product with information about its history of locations, owners, and transformations between points in the supply chain.
• Transparency refers to the making available of information by any stakeholder. This can include broader information that is relevant in the context of halting and reversing forest loss such as sustainability policies and practices, commitments, land use information, monitoring, or outstanding grievances.

EPDs of structural wood products currently do not include upstream primary data such as source forest disclosure and additional emissions sources.  As summarized in the wood chapter of a recent report by RMI, “Greater life-cycle impacts data and transparency of wood products such as EPDs are needed to make better informed choices. Wood products must be sourced and manufactured in ways that go beyond regulatory minimums and foster regenerative and climate-resilient solutions.” As such, it is necessary to request for traceability and transparency within the wood supply chain until EPDs are updated to reflect this information.

Source: Driving Action on Embodied Carbon in Buildings (RMI)

Some structural engineers are including questionnaires in their bid documents to collect sourcing information. For example, Davies-Crooks Associates provides one example:  

Disclosure Request Template Reference: MASS TIMBER FOREST SOURCING DISCLOSURE QUESTIONNAIRE (8/1/2023)

Introducing a detailed questionnaire during procurement is a relatively new practice for the industry, and therefore the extent of success and difficulty of this approach are not yet known. It is anticipated that the process will be continually refined for streamlined and practical implementation by users over time.The questionnaire may be included in bid documents to give project teams added transparency about the sourcing of mass timber products. The use of this questionnaire can also provide more accurate transportation data to evaluate the climate impacts of shipping timber products over large distances. 

The Climate Smart Wood Procurement Guide, linked in the Wood Resources section, provides more detailed recommendations and guidance on criteria for wood sourcing from a climate-focused perspective.

Recommendations:

• Aim to use the forest sourcing disclosure questionnaire (partially or fully) in your projects.
• At minimum, request information about the source forest from manufacturers for all wood products above a certain threshold (e.g., > 1% of total construction volume or cost). 
• Examine wood product EPDs and acknowledge any missing life-cycle impacts data.

Transportation Emissions

For wood products, transportation impacts can be a notable portion of the total embodied carbon impacts compared to other structural materials, in part due to the inherently lower carbon emissions associated with wood material production. Wood product specifiers should therefore pay particular attention to transportation. 

Engineers should research the availability and sustainable attributes of wood products in their local region, and specify product requirements that are consistent with the project’s carbon reduction goals. Transportation emissions can vary greatly between different wood products and are a function of the total distance traveled and the mode of transportation utilized.  A transportation consideration specific to CLT and some other mass timber products is if it will be sourced from North America or European sources.

Certain studies using WBLCA and EPDs have also indicated that transportation-related emissions can be the majority of the embodied carbon for a given mass timber product. For example, a UK-based study performed by Arup for CLT and Glulam indicated that the transportation of the original raw materials to the manufacturing facility (life cycle stage A2) accounted for 8-10% of the A1-A5 emissions.  Stage A4, transport of manufactured materials to the construction site, accounted for 50-55% of A1-A5 emissions.  It should however be noted that the emissions percentages in this study are representative of the UK market, which typically sources from Europe, and the results should not be directly applied to a North American project (where transport distances and modes will vary widely).  The results of any WBLCA study are dependent upon multiple factors such as the project location, proximity of mass timber suppliers, and modes of transportation, and the calculation should be performed for each project.

Typical carbon contributions to the supply chain for CLT and Glulam products. 

(Note: this study was performed in the UK.)

Source: Embodied Carbon, Timber (Arup)

Recommendations: 

• Weigh the transport emissions (life cycle stage A4) against the manufacturing emissions (A1-A3) to understand the full life cycle emissions associated with use of the wood product. Forell Elsesser Structural Engineers has created this tool to calculate A4 emissions.
• Prioritize locally sourced products whenever possible.
• Investigate whether lower carbon transport modes are available. Consider prioritizing suppliers that use lower carbon transport modes like rail and cargo ship whenever possible.  As the availability of electric vehicles continues to increase, investigate opportunities for electric semi truck transport.

Acknowledge the Impact of Adhesives

It is important to note that specification of adhesive types is beyond the reasonable scope of the structural engineer, as these are either prescribed by standards, performance requirements, or chosen by the supplier.  This section is instead provided to educate and provide insight into this lesser-known topic.

Many types of wood and mass timber products require the use of adhesive technologies.  While explicit information on the GWP of wood adhesives is limited, some research has shown that adhesives contribute significantly to the GWP of wood-based products.  

Adhesives are typically made from oil, which is energy-intensive to extract and refine. CLT and Glulam products typically contain at least 1 to 2.5% by volume of adhesive, yet the specific percentage of a product’s GWP that is due to adhesives is not consistently reported in EPDs.  Researchers at Arup have put together the graphic below from a variety of resources to provide a comparison of GWP/kg between common adhesives and common wood products that require adhesives.

Source: Embodied Carbon, Timber (Arup)

MUF = Melamine-Urea-Formaldehyde*

MF = Melamine Formaldehyde 

PF = Phenol Formaldehyde 

EPI = Emulsion Polymer Isocyanate

PEP = Polyurethane Emulsion Polymer  

PUR = Polyurethane* 

PVAc = Polyvinyl Acetate

* CLT most often uses Polyurethane (PUR), and Glulam most often uses Melamine-Urea-Formaldehyde (MUF).

Acknowledge the Impact of Kiln Drying

It is important to note that specification of kiln drying methods is often beyond the reasonable scope of the structural engineer, as these are either prescribed by standards or chosen by the supplier.  This section is instead provided to educate and provide insight into this lesser-known topic.

Wood is a hygroscopic material that absorbs and releases moisture into the surrounding atmosphere. Wood has improved strength and stiffness when dry compared to wet, and wood shrinks as it undergoes the drying process – causing checks. To ensure dimensional stability, avoid visual cracks, and benefit from the improved mechanical properties, wood is typically dried to approximately 12% moisture content – the typical equilibrium moisture content of a building. Wood for laminations (lamstock) in CLT and glulam is typically dried to a moisture content of less than 16% to ensure proper adhesion of the laminations.

Wood can be dried by air flow (wood stickered and stacked outside), in a solar kiln (wood stickered and stacked within a greenhouse), or dried in a fueled convective kiln. (Stickers are wood spacers placed between the boards to allow the air to circulate.) Air drying uses no additional energy but may take months to reach the desired moisture content. Solar kilns use a small amount of electricity to power fans to circulate air, but may take 4-6 weeks to reach the desired moisture content. Convective kilns require energy input to heat the air, but can dry wood in days. Convective kiln drying is the typical method for commercial wood and laminated wood products.

The kiln drying process consumes around 90% of the manufacturing energy of sawn wood products. The fuel used for the kiln depends on the facility location and operator, but typically will be a mix of incinerated biomass from the facilities process and natural gas. Some studies have indicated that between 35% and 40% of the Global Warming Potential of glulam can be attributed to lamstock production, which is associated with this drying process.

While it is often impractical to specify the drying method, this should be visible in the product EPD. Facilities that use a larger proportion of non-fossil fuels for their kiln or have a local renewable energy grid will have a lower carbon footprint. For unlaminated wood products in some environmental conditions, the emissions associated with drying can also be avoided by specifying green (undried) lumber with a moisture content of 20% or above and allowing the material to dry naturally.

Additional Considerations for Structural Engineers

SHEATHING | The 2020 AWC/CWC industry-average Environmental Product Declarations (EPD) indicates that OSB has over 10% more embodied carbon than softwood plywood. The latest OSB and plywood EPDs can be found through a search of the American Wood Council resources.

OTHER WOOD USES | The construction industry often requires the use of wood-based products for ultimately non-structural applications, most notably including concrete formwork and temporary structures such as shoring.  Structural engineers should consider these other uses of wood when revising procurement documents for sustainability, and apply the strategies listed in this resource where applicable (formwork is often specified in the Division 03 concrete specifications, for example).

WOOD IN LCAs | Always report biogenic carbon separately when performing LCAs (e.g., provide a total emissions value that does not include biogenic carbon storage).  

This is recommended because there is currently no consensus on the valuation of carbon storage for wood products.  There are two approaches: backwards accounting (taking credit for carbon absorbed in the last 40 years, which doesn’t affect our atmosphere today), and forward accounting (considering the carbon storage of newly planted trees as a direct result from harvest).  

It should also be noted that ISO 21930:2017 uses the +1/-1 approach for biogenic carbon in LCA accounting. When a bio-based material leaves nature, its stored biogenic carbon is reported as a negative emission.  When the bio-based material is converted back to emissions, via combustion or biodegradation at end of life, it is then counted as a positive biogenic emission.

Best practice is to report biogenic carbon separately in LCAs to enable effective comparisons between design options, and discourage inefficient use of timber.

OTHER BIOGENIC MATERIALS | There are many complexities associated with wood products from both a climate, environmental, and market perspective. However, there are many biobased materials with a more defensible low-carbon and carbon storage story since they can have shorter growth cycles and less nuanced land use impacts. Consider incorporating other biobased materials into your practice, such as bamboo, grass panels, or load-bearing straw and hemp assemblies.

Additional Wood Resources:

• CLF Wood Carbon Seminar: Trees, Forestry, and Carbon 101: https://www.youtube.com/watch?v=ciY6G5IbAYE (Source: Carbon Leadership Forum)
• Carbon Impact of Wood Products https://materialspalette.org/wood/ (Source: Carbon Smart Materials Palette)
• Forest Certification Update 2021: The Pace of Change https://www.dovetailinc.org/portfoliodetail.php?id=60085a177dc07 (Source: Dovetail Partners)
• Climate Smart Wood Procurement Guide https://www.climatesmartwood.net/ (Source: Climate Smart Wood Group)
• LETI Low Embodied Carbon Specification and Procurement Guide https://www.leti.uk/specification (Source: Low Energy Transformation Initiative)
• Embodied Carbon, Timber https://www.istructe.org/IStructE/media/Public/Resources/ARUP-Embodied-carbon-timber_1.pdf (Source: Arup)
• 10 Questions with Sustainable Forestry Expert, Dr. Edie Sonne Hall
• Carbon Leadership Forum, “Top 10 Questions and Answers from the Wood Carbon Seminars,” Question 10 (https://carbonleadershipforum.org/download/11317/).
• Differences between the Forest Stewardship Council (FSC) and Sustainable Forestry Initiative (SFI) Certification Standards for Forest Management: https://dovetailinc.org/upload/tmp/1581654356.pdf (Source: Dovetail Partners Inc.)
• AF&PA White Paper: Sustainable Forestry and Certification Programs in the United States: https://twosidesna.org/wp-content/uploads/sites/16/2018/05/sustainable-forestry-and-certification-programs-in-the-united-states.pdf (Source: American Forest & Paper Association)
• Wood: Is It Still Good? Part One: Embodied Carbon. https://www.buildinggreen.com/feature/wood-it-still-good-part-one-embodied-carbon (Source: Building Green) Note – purchase is required for this resource
• 2024 Embodied Carbon Data for Timber Products. https://timberdevelopment.uk/resources/embodied_carbon_data_for_timber_products/ (Source: Timber Development UK) Note – UK data only
• When to Include Biogenic Carbon in an LCA. https://www.woodworks.org/resources/when-to-include-biogenic-carbon-in-an-lca/ (Source: WoodWorks)

Supplementary Cementitious Materials

Using supplementary cementitious materials (SCMs) to replace a portion of the portland cement in the concrete block and/or grout can reduce concrete masonry’s carbon footprint. Both ASTM C90, Standard Specification for Dry-Cast Loadbearing Concrete Masonry Units, and ASTM C476, Standard Specification for Grout for Masonry, permit several options for including SCMs.

Some commonly used SCMs in the manufacture of concrete masonry block and grout are fly ash, ground granulated blast furnace slag (slag cement), raw or calcined natural pozzolans, and other pozzolans including ground recycled glass. Ground glass pozzolans conforming to ASTM C1866 may be used in grout. Blended cements may also be utilized, including Type IP (with pozzolans), Type IS (with slag) and Type IL (portland-limestone cement). Type IL is becoming more and more prevalent for both blocks and grout. If architecturally finished CMU are used on a project, SCM use may affect color. Regional availability is also a factor, so checking with local producers to determine what SCMs are available can help engineers plan for low embodied carbon strategies.

Natural Versus Lightweight Aggregates

Lightweight aggregate used to produce lower density CMU can be manufactured (such as expanded shale), naturally occurring (such as pumice or scoria), or derived from by-products of other industrial processes (such as slag or bottom ash). Manufactured lightweight aggregate has higher embodied carbon compared to naturally occurring lightweight aggregate because of the energy required to produce it. It is important to note that the type of lightweight aggregate available to CMU manufacturers varies by geographical region. As such, they should be consulted to determine what type of lightweight aggregates are regionally available when weighing the option of specifying lightweight block. Specifiers should also review EPDs for aggregate type when available. The industry average EPD provides separate categories for not only strength ranges, but also lightweight aggregate type, reporting natural lightweight aggregate and manufactured lightweight aggregate as separate categories.

Performance characteristics dependent on density should be considered when specifying the density of CMU, such as fire ratings, R-values, and STC ratings. Mason contractors may prefer lightweight units to increase their productivity. Unit compressive strength is independent of unit density as all units must meet minimum specified strengths regardless.

Carbon Reduction Technologies

Because of the way CMU are manufactured, there are several manufacturing innovations that can reduce the embodied carbon associated with CMU construction. Strategies include technologies that encourage more CO2 sequestration via concrete carbonation, such as using aggregates that are pre-carbonated, injecting CO2 directly into the mixture or utilizing specialized curing systems. Alternative cements are also being developed that lower embodied carbon by changing the way they are made.

Mortar

Mortar used in masonry construction falls into one of three categories based on cement type: portland cement/lime mortar, mortar cement mortar, and masonry cement mortar. Mortar cement and masonry cement mortars have much lower global warming potential compared to conventional portland cement/lime mortar because these cements already typically contain a high proportion of filler material such as limestone powder. Masonry cement mortars can be used in most regions, except in areas where Portland-lime mortar is required for high seismic structural masonry. Availability of mortar EPDs in the United States to quantify differences in emissions is limited at this time. To help estimate the embodied carbon of mortar, ASTM C270 proportion specification requirements can be followed. Depending on the mortar type, cements generally make up about 15% of the volume in a mortar mix.

Grout

It is recommended to specify the compressive stress method instead of the volume method to proportion grout mix. ASTM C476, Standard Specification for Grout in Masonry, permits the grout to be proportioned by volume (prescriptive) or using specified compressive strength in accordance with ASTM C1019 (performance). Using the compressive strength method generally results in less cement, thereby reducing embodied carbon.

Grout can be specified as either fine grout or coarse grout. Fine grout, with aggregate sizes typically less than 1/8 in., are appropriate when grouting highly congested masonry assemblies. The trade-off when using fine grout is that there is typically an increased demand for more cement in the mix due to the larger surface area of smaller aggregates compared to larger aggregate gradations.

Self-consolidating grout is also an option with masonry construction, however, to maintain a stable, flowable mix using self-consolidating grout, a very high cement content is required, often resulting in grout compressive strength in excess of 7,000 psi and an increase in embodied carbon.

Lower Embodied Carbon Steel Reinforcement (Rebar)

The embodied carbon of metals depends upon the electricity source used in production. Rebar In North America 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, see the steel specification guidance section for additional information.

Many rebar producers have published EPDs, so it is becoming increasingly viable to directly specify a maximum GWP for rebar. See the whole building strategy – promote transparency and disclosure section for more information on EPDs.

Joint reinforcement is less likely to be produced in an EAF and specifiers are urged to investigate the availability of lower-carbon products.

Resources:

• Concrete Masonry Assembly Embodied Carbon Calculator (Source: CMHA Concrete Masonry and Hardscapes Association)
• Evaluating The Compressive Strength Of Concrete Masonry (Source: CMHA Concrete Masonry and Hardscapes Association)
• Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (Source: CMHA Concrete Masonry and Hardscapes Association)
• Section Properties And Wall Weights Calculator (Source: BDC Block Design Collective)
• Do Manufactured Concrete Products Sequester CO2? (Source: CMHA Concrete Masonry and Hardscapes Association)
• Mortars For Concrete Masonry (Source: CMHA Concrete Masonry and Hardscapes Association)
• Grout For Concrete Masonry Walls (Source: CMHA Concrete Masonry and Hardscapes Association)
• High-Volume SCM Grouts for Masonry: (Source: Jamie Farny, Structure Magazine May 2018)