Sustainably built environments: specifying low-carbon concrete

by arslan_ahmed | October 17, 2022 10:00 am

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By David Diedrick and Cecile Roman

According to climate experts, rising levels of carbon in the atmosphere will raise worldwide average temperatures by 1.5 degrees between 2030 and 2052. To the average person, a mere 1.5 degrees may seem inconsequential, but even a tenth of a degree can make a significant difference. For example, more heat trapped inside the atmosphere causes the air and water to hold more energy which, in turn, causes severe weather conditions and disasters.

This is far from being a possible concern as its impact is already happening. In 2021, the U.S. experienced an average temperature which was 2.5 degrees above the 20^th century average, with $145 billion in property damages due to hurricanes, tornados, wildfires, droughts, and flooding events–the third most costly year on record.¹ Over the last 10 years (2012-2021), there has been at least $1 trillion in disaster damage costs resulting from 142 weather and climate disaster events.

The global climate crisis has reached a critical point. As atmospheric carbon levels continue to increase, so will extreme weather conditions, which means a slew of threats to the health and wellness of people, flora, and fauna worldwide. With buildings accounting for nearly 40 percent of carbon dioxide (CO2) emissions, according to the Global Alliance for Buildings and Construction (GlobalABC), it is understandable why such an immense focus is being placed on them in the quest to achieve net-zero emission targets by 2050. The way the world constructs and manages its buildings is essential for mitigating global warming and its impact on the economy, communities, and ecosystems.

Sustainability attributes of concrete

Sustainable construction considers the entire life cycle of a structure and aims to reduce the environmental damage at every stage, from the initial choice of products to demolition. As the most widely used building material, concrete offers many advantages for meeting the challenges of sustainable development.

Not only is concrete entirely recyclable, but it also has a long lifespan. When produced and sourced locally, it has favorable CO2 and energy footprints in comparison to other building materials. Moreover, well-constructed concrete walls, floors, and roofing elements can be left in place when an existing structure is retrofitted; extending the life of existing buildings and eliminating the carbon footprint associated with new construction. Concrete’s strength, durability, and fire-resistance properties enable the design of high-performance structures by providing effective protection against natural hazards. These attributes add to the exceptional thermal inertia properties of concrete, as it absorbs heat during the day and gives it back at night—significantly lowering energy use by reducing heating and air conditioning requirements.

While concrete is considered a favorable choice for sustainable construction, the material has a high carbon footprint due to the energy intensiveness and generation of CO2 from Portland cement manufacturers. To pursue a net-zero future, concrete needs to have the lowest embodied carbon possible without affecting its performance. For specifiers, the most effective means of achieving this is by reducing the amount of clinker in a mix design by relying on blended cements. This can reduce the carbon emissions of structural systems by up to 40 percent.²

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Reducing embodied carbon

The use of supplementary cementitious materials (SCMs) as a partial replacement for Portland cement results in more durable and high-performance concrete and lowers energy consumption and greenhouse gas emissions. SCMs in concrete are used as a separate component or as a constituent of blended cement. The most common SCMs are slag (a byproduct of iron manufacturing), fly ash (a coal combustion byproduct from power plants), and silica fume (a byproduct of silicon and ferrosilicon alloy manufacturing).

Cementitious blends have many properties contributing to sustainable construction and stronger, longer-lasting concrete. They also reuse byproducts from other industries that might otherwise be disposed in landfills. Binary blends are a mixture of Portland cement and one SCM, and ternary blends are a mixture of Portland cement and two SCMs.

Considering how concrete is the world’s most widely used construction material, specifying cementitious blends can have a major impact on the environment and make a significant contribution to achieving sustainable building goals, such as those prescribed by the U.S. Green Building Council’s (USGBC) building rating system, Leadership in Energy and Environmental Design (LEED). SCMs contribute to LEED credits in the following categories: Sustainable Sites, Energy and Atmosphere, Materials and Resources, Indoor Environmental Quality, and Innovation.

EPDs and LEED v4

Environmental Product Declarations (EPDs) are registered documents that communicate the life-cycle assessment (LCA) of products based on product category rules (PCR).

EPDs must be consistent with the International Organization for Standardization (ISO) standards, such as ISO 14025, 14040, 14044, EN 15804, or ISO 21930. These standards address how to perform an LCA and what data to include in an EPD.

EPDs are important for providing transparency on products and to support design processes with innovative solutions that deliver structural integrity and eco-efficiency.

The trend of greater disclosure has been integrated into various green building standards, but particularly within the LEED v4 rating system. Within the materials and resources category, project teams can earn points for products that have verified EPDs.

SCM performance benefits

SCMs impart a wide range of exceptional performance properties. While there are many advantages of concrete in its plastic state, the greatest benefits and performance can be seen in the hardened properties.

Enhanced strength

With the addition of SCMs, flexural and compressive strengths can increase markedly at 28 days and beyond. Typically, slag cement and fly ash will lower early strengths (one to 14 days) but significantly improve long-term strengths (28 days and beyond), depending on the proportions used. Silica fume contributes primarily to strengths at 3 to 28 days.

Reduced permeability

Permeability is generally the critical factor affecting durability. SCMs significantly extend the life of concrete by reducing permeability to chlorides and other aggressive agents. Silica fume can provide as much as a five-fold reduction in permeability.

Resistance to alkali-silica reaction (ASR)

Some SCMs can prevent excessive expansion and cracking of concrete caused by ASR. In most cases, 50 percent of slag cement is sufficient with highly reactive aggregates. The amount of fly ash required typically ranges from 15 to 55 percent, depending on the calcium oxide (CaO) content of the fly ash. The lower CaO fly ashes are more efficient at mitigating ASR. Silica fume can control ASR, but the amount required generally results in poor constructability. Blends of slag cement and silica fume, as well as blends of fly ash and silica fume have a synergistic effect in mitigating expansion due to ASR.

Relevant ASTM Specifications

• ASTM C989, Standard Specification for Slag Cement for Use in Concrete
  and Mortars
• ASTM C618, Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete
• ASTM C1240, Standard Specification for Silica Fume Used in Cementitious Mixtures
• ASTM C150, Standard Specification for Portland Cement
• ASTM C595, Standard Specification for Blended Hydraulic Cements
• ASTM C1157, Standard Performance Specification for Hydraulic Cement

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Resistance to sulfate attacks

Sulfates, which are present in seawater, wastewater, and some soils, can react with the alumina in Portland cement and cause expansion. Some SCMs offer powerful resistance to these attacks because they contain fewer of the compounds which react with sulfates, and their low permeability keeps sulfate-bearing waters out. Typically, slag cement, silica fume, and Class F fly ashes are very effective in improving sulfate resistance.

Portland-limestone cement

One proven solution for reducing greenhouse gases per ton of cement—without sacrificing its performance—is Portland-limestone cement (PLC). Proven to serve as an effective alternative to ordinary Portland cement (OPC), PLC is a high-performance blended cement manufactured with up to 15 percent of finely ground high-quality limestone.

In the U.S., the American Society for Testing and Materials (ASTM) permitted up to five percent limestone in Portland cement in 2004 (ASTM C595). At the time, it was estimated that these changes would reduce energy consumption by 11.8 trillion Btu and CO2 emissions by more than 2.5 million tons (2.3 million tonnes) per year. In response to calls to further reduce the carbon footprint of the building sector, ASTM C595 was revised in 2012 to allow up to 15 percent limestone in Type IL blended cement. Type IL cement provides strength development, durability, workability, and other properties like Type I and II cements.

With Type IL cement, similar percentages of SCMs can be used in concrete mixes while also replacing up to 15 percent of the Portland cement with limestone. This results in an additional 10 percent reduction in greenhouse gas emissions associated with the production of Portland cement clinker. When combined with SCMs, the effective reduction in the CO2 footprint of concrete is highly significant.

Slag and PLC: Lowering carbon footprints of buildings in Seattle

Seattle has quickly become one of the fastest-growing technology hubs. In pursuit of its goal to become carbon neutral by 2050, the birthplace of the nation’s green building movement has adopted an ambitious climate action plan and was the first U.S. city to approve the use of PLC for structural concrete. With an ever-expanding presence in their hometown, tech leaders responded to the call to action on global warming with affirmative commitments to lower their carbon footprint.

When a prominent local technology and logistics company added 39,019 m² (420,000 sf) of new headquarters space and broke ground on a major building expansion, the construction used high-performance mixes to achieve a clinker factor of 16 percent, the lowest in the Seattle area.

The concrete used in the signature “spheres” of the headquarters was designed with 80 percent slag cement to achieve its sustainability goals, 55,158 kPa (8000 psi) strength requirements, and aesthetic needs.

Another iconic resident in Seattle’s business community has committed to removing more carbon than it emits by the end of the decade.

The addition of more than 1.86 million m² (2 million sf) of new office space at this tech giant’s 202-ha (500-acre) headquarters campus is relying on concrete mixes, designed with PLC and slag cement to reduce embodied carbon by at least 15 percent.

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Fly ash and PLC: The right prescription
for Batson Children’s Hospital

Finding themselves in dire need of expansion, Batson Children’s Hospital in Jackson, Mississippi, initiated the construction of a new seven-story medical tower and an adjacent five-level parking garage. With concrete representing a significant portion of the structures, the use of sustainable materials in the high-performance mixes was paramount to the project team.

The concrete used in the project incorporated PLC and 20 to 30 percent fly ash to meet durability requirements for moderate sulfate resistance and chloride exposure. It also included strengths of 31,026 kPa (4500 psi) for the foundations, 34,473 kPa (5000 psi) for the elevated decks, and 41,368 kPa (6000 psi) for the structural columns. The mix was also designed to achieve a high early strength in the elevated decks to keep the project on schedule.

The hospital expansion project was completed on schedule in October 2020. All application-specific performance targets aimed at durability, permeability, and strength were consistently met with the PLC/fly ash mix designs, and the embodied carbon of the concrete was reduced by as much as 35 percent.

New low-carbon concrete developments

To meet increasing demands to reduce embedded carbon in building materials, some cement producers have developed new low-carbon concrete products that offer 30 to 100 percent less carbon emissions than ordinary concrete. Up to 80 percent less carbon is achieved primarily with lower CO2 intensive materials. For a fully carbon-neutral solution, the last 20 percent is reached through offsets, with certified carbon projects. Where conditions allow, the low-carbon concrete can integrate construction and demolition waste, which closes the material cycle completely.

Washington, D.C., and Boston, Massachusetts, have pledged to be carbon neutral by 2050. Every large building under construction in these cities is a flagship project which requires the structural performance of high-quality concrete, aligned with the most advanced sustainability certifications from LEED. With concrete representing up to 80 percent of the mass of these buildings, the shift to low-carbon mixes played a key role in their sustainability profile by saving more than 700 tons (635 tonnes) of CO2. This is equivalent to taking 140 cars off the roadways for an entire year.

Exceeding LEED Platinum status in Washington, D.C.

Georgetown University is deeply committed to reducing the carbon footprint of its built environment. With the school pursuing LEED v4 Platinum status for its new student residence hall near the U.S. Capitol, the project team worked to achieve high levels of sustainability in construction.

The fast-track construction of the 12-story building by John Moriarty & Associates required the completion of one post-tensioned concrete floor deck each week. The challenge was to come up with a sustainable high-performance concrete solution that would consistently hit a high early strength of 20,684 kPa (3000 psi) in two to three days and a specified strength of 34,473 kPa (5000 psi) at 28 days.

To meet the specified strengths in the time needed while reducing the embodied carbon, the project team worked with the university to develop a low-carbon, high early strength mix containing high levels of SCMs. The high-performance concrete performed up to speed by hitting the specified high early strength, reduced the embodied carbon of concrete by 40 percent, and made a strong contribution to achieving the goal of LEED v4 Platinum certification.

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Building a net-zero future in Boston

Opening in 2023, Boston University’s new Computing & Data Sciences Center is essential to meeting the university’s climate action plan of net-zero emissions by 2040. Designed by KPMB Architects to attain LEED Platinum status, the 32,516-m² (350,000-sf) facility will be the largest carbon-neutral building in Boston.

With sustainability being a high priority, structural engineering firms LeMessurier of Boston, Massachusetts, and Entuitive of Toronto, Ontario, Canada, selected an eco-friendly concrete option. The concrete will reduce the building’s carbon footprint while attaining the best possible balance in meeting constructability and structural performance goals.

In the placement of the massive foundation alone, the eco-friendly concrete using SCMs provided emission savings, totaling 350,000 kg (771,617 lb) of CO2 reductions. Further, the innovative mix design offered the university a 30 percent reduction in CO2 emissions in comparison to traditional concrete.

Conclusion

The way buildings are constructed and managed will have a significant impact on mitigating current and subsequent climate change for many decades to come. It is important to preserve today’s infrastructure with more resilient and disaster-resistant structures, while building a better future by decarbonizing the built environment to curb global warming.

Cement is a crucial component in concrete, but it has a big footprint when it comes to emissions. Reducing the amount of Portland cement in concrete with alternative materials and other mix design strategies will lower the carbon intensity of concrete without hindering its structural performance. Often, the best approach is to move from a prescriptive-based specification to a performance-based specification. This allows concrete suppliers to optimize their designs to meet the specified performance requirements, avoiding unnecessary criteria, and optimizing the cement content of the concrete mix. Performance-based specifications generally result in more sustainable products being used, providing a greater value to all industry stakeholders.

Notes

¹ Consult the National Oceanic and Atmospheric Administration.

² Refer to “Achieving Net Zero Embodied Carbon in Structural Materials by 2050,” Structural Engineering Institute’s Sustainability Committee Carbon Working Group, 2020.

Authors

David Diedrick is the quality general manager of sales, North Central Region, for U.S. Cement at Holcim. He can be reached by email at dave.diedrick@holcim.com.

 

Cecile Roman is the innovation and sustainability manager for U.S. ready-mix concrete at Holcim. She can be reached by email at cecile.roman@holcim.com.

Source URL: https://www.constructionspecifier.com/sustainably-built-environments-specifying-low-carbon-concrete/

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