Embodied carbon (EC) emissions, measured through Global Warming Potential (GWP) benchmarks and thresholds, are increasingly prominent in owners’ project expectations and in construction policies, certifications, and specifications. Great strides are being made to reduce the industry’s net carbon output and to find new materials and methods other than the carbon-intensive portland cement for producing lower carbon concrete. Prioritizing, evaluating and validating lower EC concrete necessitates new roles, responsibilities and changes to our structural engineering practice.
For decades, byproducts of industrial processes including fly ash and ground granulated blast furnace slag (i.e., slag cement) have been utilized as sustainable and cost-effective supplements and replacements for portland cement. A submitted and accepted concrete mixture utilizing these materials, or other supplementary cementitious materials (SCMs) like ground glass or natural pozzolans, may have earned project LEED points in prior projects.
Increasingly prominent in owners’ project expectations and in construction policies, certifications, and specifications, though, is the requirement to quantify the EC of products used in constructing a building project. For concrete, it’s no longer enough just to specify the use of SCMs; reporting concrete mixtures’ carbon footprint through Environmental Product Declarations (EPDs) is becoming the norm. Programs asking for EPDs and the estimation of a project’s EC impacts within their standards include the United States Green Building Council (USGBC) LEED v4.1, the International Living Futures Institute (ILFI) Net Zero Carbon Certification, and the forthcoming American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE)/International Code Council (ICC) Proposed Standard 240P.
Similarly, federal and state governmental policies and actions are emerging, with states like California, Colorado, Oregon, Minnesota, Maryland, New York, and New Jersey leading the way. Requirements to quantify the EC of products may be reflected in material intensity policies, building intensity policies, Whole Building Life Cycle Assessment requirements, EPD requirements, tax incentives and/or cement reduction requirements. While harmonization in approaches and criteria is lacking, the underlying math is straight forward: A project's EC = estimated material quantities times the estimated material’s EC. This is the underlying basis to help guide next steps.
Who Should Do What?
Ideally, the owner, at times aided by a sustainability consultant, should establish EC goals in the programming phase of a building project as defined within the Owner Project Requirements (OPR) documents, as well as the design team’s Basis of Design (BOD) documents. These goals then flow down to the project sustainability specifications (typically section 01 81 13). An advance summary report should define the total project GWP threshold for each major building component. This summary report typically requires advanced estimates of material quantities for the different building applications.
While concrete mixture GWP benchmarks may be established, only the project level GWP threshold, which is the summation of each specific concrete mixture quantity estimate times its GWP benchmark, should typically become a contractual obligation. When establishing concrete GWP benchmarks and thresholds, soliciting early input from potential ready-mix suppliers on their capabilities is encouraged.
As the responsible party for the development of project-specific structural concrete requirements, the structural engineer is responsible for aggregating this information within the project structural contract documents including the structurally relevant specifications and general notes, with the concrete GWP benchmarks set at the mixture level, and overall concrete GWP thresholds set at the project level. The National Council of Structural Engineers Association (NCSEA) Sustainable Design Committee has developed and recently published consensus-based recommended structural engineering general notes for this purpose.
Further, the architect may add finish and tolerance requirements, which may or may not impact the concrete mixture design. The construction teams may also specify construction-related performance requirements including but not limited to set time, pumpability, and finishing. This contractor information often will be identified within the concrete bid documents, constituting means and methods of construction, and appropriately should not become part of the structural contract documents.
Once the full concrete mixture requirements are complete, the construction team can proceed with material procurement. The concrete producer submits the proposed concrete mixtures including test data, for review and approval, demonstrating that the collective of the proposed mixtures meet or exceed the performance criteria and the established GWP project threshold set for concrete. The owner’s inspection and testing agency conducts responsible acceptance testing of the concrete delivered to the jobsite, in keeping with the project specifications. The construction team should be responsible for tracking and reporting as-built material volumes, the concrete GWP, and all other required material and product GWP thresholds. This information can then be checked against earlier estimates. Collectively, this allows for a thoughtful tracking and management of both the concrete and the other GWP impacts accumulated through construction.
But What About Lower Carbon Concrete?
There are numerous levers to pull to achieve lower carbon concrete starting with reducing the amount of portland cement in the mix. This can be achieved through improved quality control, better graded aggregates, and the use of SCM and alternative cements, including ASTM C595 blended cements. In limited markets, lower carbon hydraulic cements that satisfy ASTM C1157 may be available. Changing from “business-as-usual” concrete is often not as simple as a drop-in replacement. Short-term and long-term performance characteristics will likely change and some of these materials will require pilot test trials and mock-ups prior to their use.
When using any new concrete materials in a building project, including lower carbon, it is incumbent upon all parties to use project-ready concrete for all specified components of the final, non-temporary construction. Project-ready lower carbon concrete:
- Uses standard-compliant materials and mixtures supported with appropriate testing data validating use for the intended application.
- Achieves approval for use through the rules established within a project’s contract documents.
- Has sufficient material availability to meet the project demand and schedule needs.
Advancing innovation via pilot test trials of lower carbon concrete is an important step in a material’s transition toward becoming project-ready. Piloting concrete should be restricted to controlled conditions where tear-out can be tolerated, such as non-structural applications like sidewalks, or temporary blinding slabs. Concrete placement in trials should be under the supervision of the responsible parties for ultimately signing off on that final use condition.
Additionally, novel materials are coming into the market that don’t yet meet criteria of Standards Development Organization (SDO) such as ASTM or others, and/or project contract document approvals. These materials are only appropriate for testing in research and development efforts. Given their lack of standards, they typically should not be used as part of the final project.
Individual and team experience and sophistication with establishing, measuring, and validating EC will vary greatly now and in the future. Successful outcomes are improved by timely and effective communications with all impacted parties including the owner, architect, construction team, and the ready-mixed concrete producer. Informing the entire team of not only the EC ambitions and new responsibilities that come with it, but also the augmentations to the contract drawings and project specifications facilitates success. In doing so, the use of lower carbon concrete will yield substantial opportunities to reduce the EC associated with building construction. ■
About the Author
Don Davies, PE, SE, is Principal at Davies-Crooks Associates in Seattle.
(don@davies-crooks.com)
Anne Ellis, PE, Hon. M. ACE, F. ASCE, NAC, is a Principal with Ellis Global. (anne@anneellis.com)
Thomas Van Dam, PhD, PE, is a Principal with Wiss, Janney, Elstner Associates. (TVanDam@wje.com)