Mind the gap: Minimizing embodied carbon overages

by tanya_martins | December 18, 2024 1:53 pm

By Shane Mulligan, P.Eng.

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In the built environment, concrete is a critical construction material for futureproofing against the effects of climate change. Its strength, durability, and resiliency are integral to sustainable building practices and support a growing emphasis on climate resilience. Historically, a key drawback to the use of concrete has been the embodied carbon intensity of the product, which is attributed mainly to the cement content of a given mixture. Concrete is the most used substance in the world after water, and the industry is responsible for 5 to 8 percent of global greenhouse gas (GHG) emissions as a result of meeting this demand.

As green building practices evolve and a growing number of “buy clean” government policies are introduced to markets across North America, construction materials are under increasing scrutiny concerning their impact on embodied carbon. With concrete representing a significant portion of a typical building’s total embodied carbon footprint (often 50 percent or more), the cement and concrete industry is taking rapid and meaningful action toward net-zero concrete. Along the project value chain, however, other stakeholders are responsible for key decisions that impact concrete’s overall embodied carbon impact. Designers, engineers, architects, and general contractors play an integral, interconnected role in selecting concrete products to meet the performance, safety, and schedule demands of a given project.

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Concrete’s embodied carbon impact is typically estimated based on individual concrete elements specified by the design team, according to each component’s strength and exposure class. Industry best practices would see either a whole building lifecycle assessment performed or the preparation of a “carbon budget” for the concrete on the project. In either case, the calculations are informed by setting a global warming potential (GWP) target for each concrete element (kg CO2/m³), multiplied by the anticipated volume of each concrete mix. The sum of these products would then estimate concrete’s contribution to total carbon intensity for the building project.

However, depending on when these calculations are made in the design process—from early concepts to shovel-ready designs—there can be a wide variance in results. This is because the further along the design process, the more accurate the estimates are. Yet, the largest gap in total embodied carbon estimates is expected to be found between the final design and what actually gets built. The reason? Decisions that do not get made until construction—which are often predictable but likely not accounted for at the drawing table.

A prime example is suspended slabs, a common element in mid- to high-rise building types. Structurally, these slabs typically require 35 MPa (5 ksi) of strength and are not subject to freeze-thaw or chemical attack, meaning limited special considerations for the concrete mixtures required.

However, suspended slabs are often on the critical path of the general contractor’s construction schedule, meaning slabs must be poured and stripped before construction can progress to the next floor. The result is a demand for higher early strength concrete (i.e. 20 MPa [3 ksi] in 24, 36, or 48 hours), which requires a more cement-intensive, and therefore more carbon-intensive, concrete. This turns the required 35 MPa (5 ksi) concrete element into a 55 to 80 MPa (8 to 11 ksi) equivalent mixture, which may not have been captured when the carbon intensity target was set, despite how common this circumstance may be.

Concrete over-design has historically crept into a project in other ways. It is common for each stakeholder involved in a mix design to build in a factor of safety by increasing the required strength, leading to cumulative additions that can result in a concrete mix that is significantly over-designed. Concrete producers should be brought in as early as possible during design and construction decision-making processes to reduce the impact of deviations between design and as-built carbon estimates. Doing so allows the team to arrive at a better understanding of the project’s carbon budget, as well as incorporate plant- and mixture-specific environmental impact information.

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Exploring strategies for minimizing embodied carbon overages

One way to mitigate the impact of over-design is to budget appropriately. If construction schedules are likely to dictate the need for higher early strength concretes, this should be part of the embodied carbon target-setting process. In Canada, the “Guideline for Specifying Low Carbon Ready Mixed Concrete” by the Cement Association of Canada (CAC),  suggests 130 percent of the traditional concrete GWP values—as informed by the regional (provincial) industry average environmental product declaration values—should be used for all specialty concretes. Alternatively, working with local concrete producers to determine appropriate baselines may be possible. Lower carbon versions of high early-strength concrete are possible but should be measured against high early-strength baselines instead of traditional concrete.

Another way to reduce the occurrence of over-design is through maturity monitoring. Digital sensors are installed with the reinforcing steel of a given concrete element, and a pre-established temperature-strength relationship is calibrated to the concrete mix to determine the in-situ strength of the concrete as it cures in place. This live-look input provides a feedback loop for the general contractor and concrete supplier, enabling several possible benefits:

• Allows the general contractor to optimize their mixes. As general contractors work with repetitive construction cycles, they often have preferred concrete mixes they use as a force of habit; they know that a particular mix will deliver the strength in the required time and are generally not concerned with strength development beyond this timeframe. Maturity monitors can help demonstrate that a less carbon-intensive concrete mix can achieve the same high early strength results, resulting in economic and embodied carbon savings.
• Facilitates improved carbon reductions during construction. The contractor and concrete producer can monitor the live results of a given mix, specific to the curing conditions, live climatic conditions, etc., and make incremental improvements to the concrete mixes being delivered to the site, further reducing the carbon intensity of a given mix when measured against the original design target.
• Reduces emissions and other problems associated with traditional, destructive cylinder break tests. Traditional cylinder testing involves having onsite certified personnel cast cylinders from the pour, cure them onsite, transport them for testing, and crush them offsite to measure the pressure required to break the concrete in compression. Increasing the cement content in mixes has been used to avoid low cylinder breaks during testing. In addition to increasing carbon intensity, unnecessary cement can increase shrinkage, shrinkage cracks, creep potential, thermal cracking, and brittleness in finished concrete. Conducting cylinder break tests offsite also results in transportation-related emissions and multiple rounds of wasted concrete, which can be reduced by relying more heavily on digital maturity test results. Reducing the number of cylinder tests can have a positive effect on construction schedules as well since the time and costs associated with traveling, casting, collecting, delivering, testing, and reporting add up, and in-situ digital maturity monitoring validates strength sooner than possible with cylinder tests.
• Reduces waste and rework arising from sampling errors. Cylinder samples are not always consistent with the strength of in-place concrete since incorrectly prepared, stored, or tested cylinders can affect test results. Mishandling during transportation can also affect results by causing micro-fractures in the concrete that lead to a misleadingly low break strength. Testing concrete in place with digital sensors improves accuracy compared to using site-cast samples. Testing in place can also show local variations for different structural locations.
• Provides peace of mind amidst changing requirements. Rapid changes in mix design attributable to low-carbon concrete strategies may make designers and general contractors reticent about their use compared to traditional practices. Since they offer real-time insights into concrete strength gain, maturity monitoring and other digital tools can ease the shift toward reduced-carbon concrete.

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Probes placed inside concrete truck drums also give real-time insights into concrete mix conditions. Probes can measure mix uniformity, volume, slump, temperature, and drum speed and revolutions. They also constrain situations where human error might otherwise compromise quality, such as measuring the water/cement (w/c) ratio and deterring water additions that would put the mix over a specified ratio. Digital probes verify a load is within spec at the time of discharge, as well as proof that the product ordered is what was delivered.

Predictive artificial intelligence (AI) is improving transparency and supporting lower-carbon concrete mixes, particularly when paired with digital probes and/or maturity monitoring technologies. By accessing large data sets, AI can quickly generate numerous scenarios, leading to better mix designs. This type of mix optimization can occur not only at project outset but during later construction stages by incorporating variables that allow on-the-fly adjustments. For example, AI tools may be able to recommend mix adjustments and cement content reduction based on weather conditions during concrete placement.

To facilitate these responsive practices, specification writers should focus on performance-based specs rather than prescriptive ones (e.g. eliminating minimum cement content requirements or removing limitations on supplementary cementing materials [SCMs]). In a performance-based scenario, the general contractor and concrete producer must monitor, assess, respond to, and document changes throughout project delivery, but AI tools could also facilitate these efforts.

Design teams can achieve carbon, cost, and schedule efficiencies by supporting an industry-wide shift in standards and testing to incorporate digital measures and improve job-site safety. For example, digital data reduces the need for on-the-ground quality testing, requiring fewer inspectors to be physically present in dangerous work zones. At the same time, overall product assurance is improved. The data provided by digital tools may even reduce litigation since site-cast concrete specimens compromised on the job site are responsible for a significant number of presumed problems that lead to litigation.

Some carbon accounting categories currently estimated by design teams could be quantified by digital design tools, leading to more accurate numbers and better management of real-world conditions and emissions. For example, Environmental Product Declarations (EPDs) often account for concrete waste volumes by averaging them over the course of a year. Individual construction projects are assigned a certain percentage of waste based on estimated numbers. However, when concrete mixes are controlled using digital tools, not only does the improved quality effort reduce waste, but the exact amount of waste that does occur can be measured. This makes it possible to apply an actual number, not an estimated one, to the project’s carbon accounting.

If the power of digital tools and their ability to produce real-time data were to be harnessed, project-specific (or as-constructed) EPDs would be possible. Rather than using estimates based on aggregated data, as is currently the case, project-specific EPDs would represent real-time snapshots of the actual environmental impact of concrete on a given project.

Project-specific EPDs from concrete manufacturers could also provide information related to transport emissions, commonly referred to as Module A4 in a Lifecycle Assessment (LCA), which measure emissions associated with the distance a product travels from the production site to the construction site. Module A4 data is voluntary information reported on an EPD and often represents estimated values for these impacts as assigned during the design process. For construction projects where transport significantly contributes to the environmental bottom line, life cycle consultants often recommend producing more detailed data, including the distance traveled, number of deliveries and load weight.

Collectively, digital tools available throughout a project’s design and construction phases provide unprecedented transparency. There is room for improvement as teams learn to leverage and share data, use it to inform EPDs with an eye toward greater accuracy and specificity, replace standards and testing methods that are high-risk or inefficient with digital options, and prioritize performance specs over prescriptive ones to allow new paths toward achieving the same ends. As design and construction professionals progress in these areas, they will close the gap between embodied carbon estimates and the amount of embodied carbon in the final, as-built structure—the only emissions the environment counts.

Author

Shane Mulligan, P.Eng., LEED Green Associate, is the sustainability and technical marketing manager for Heidelberg Materials, where he is responsible for all aspects of sustainable sales and marketing efforts for the company’s concrete business in the Northwest region. This includes working with architects, engineers, and designers towards lower carbon, sustainable solutions in the built environment. Mulligan has more than 19 years of experience in a variety of civil engineering sectors, including the technical promotion of cement and concrete products, structural design of underground infrastructure, waste diversion planning, and stormwater management.

Key Takeaways

Concrete is essential for sustainable construction but has high embodied carbon. Stakeholders can mitigate carbon impacts through better design, digital monitoring, and performance-based specifications, leading to more accurate assessments and reduced emissions in building projects.

Source URL: https://www.constructionspecifier.com/mind-the-gap-minimizing-embodied-carbon-overages/

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