December 29, 2021
By Chris Bennett and Chris Flint Chatto
Concrete is a tremendously useful and flexible material: building foundations, roads, walkways, bridges, and other infrastructure utilize concrete for its strength, durability, and plasticity in formation. Concrete also has relatively low-embodied carbon per unit volume when compared to other building materials, but, because of its utility, modern construction uses a lot of it. When it comes to carbon impact, concrete is responsible for an estimated eight percent of global carbon dioxide (CO2) emissions. In the world of commercial architecture, concrete can represent 50 percent or more of overall global warming potential (GWP) of a building’s structure and envelope.
While some concrete applications have real alternatives with reduced impacts (e.g. sustainably harvested mass timber as a primary above-ground structural system), many other applications (e.g. foundational footings and slabs) do not. As well, even if the operational emissions associated with typical buildings constructed today will predominate over the decades, embodied emissions associated with materials and construction happen immediately and climate science shows the “time value” of early reductions is far more impactful. For these reasons, strategies to greatly reduce the GWP of concrete are a critical element to decarbonize individual building and infrastructure projects as well as global industry.
Concrete is a composite material that consists of cement, aggregates (rocks and sand), and water. The main culprit for CO2 emission during manufacturing is cement. While cement is typically a small portion of the overall mix, ranging from 20 to 35 percent of the material, it is responsible for around 75 percent of concrete’s GWP. When mixed with water, cement undergoes an incredibly complex reaction which produces a variety of compounds depending on the original composition of the cement, the most important being calcium silicate hydrate (C-S-H, the “glue” that holds the other components of concrete together and make it useful).
Concrete is also a local material. The reality of producing and shipping wet mixes to project sites typically limits potential suppliers to within 84.5 km (50 miles), forcing projects and specifiers to accept the range of materials and practices available and understood by local suppliers. The suitability of any concrete carbon reduction strategy is impacted not only by project benchmarks but by weather and raw material availability in a given region.
While there is no one-size-fits-all concrete carbon reduction strategy, there are numerous strategies that can be successful across a wide variety of scenarios.
An overview of strategies
Utilizing supplementary cementitious materials
With the demand from public and private sectors to reduce carbon emissions, the construction industry responded in a number of fruitful ways to make a positive impact. One of the most familiar methods for construction professionals to reduce cement content and GWP is by utilizing supplementary cementitious materials (SCMs).
Ground-granulated blast-furnace slag (GGBS) comes from the production of iron. Slag is a liquid containing impurities from iron and coke from the blast furnace process capable of forming into a material with latent hydraulicity. This material can be used effectively in concrete. However, slag can bring negative side effects in certain scenarios.
Fly ash has become a common SCM thanks to its ability to resist sulphate attacks, as well as chloride ion penetration, but recently fly ash has become less available in the market as coal power plants, a primary source for fly ash, are phasing out.
A newer SCM in the market is ground-glass pozzolans. Recovered consumer glass bottles are ground to a powder, providing performance similar to slag, with high level strength at mixes up to 50 percent replacement. ASTM standard 1866 addresses its use, but with limited suppliers now, its availability is currently limited regionally and will take time to scale.
It is also important to point out that SCMs can improve the sustainability of a concrete mix by replacing cement, not merely by adding it. One should never compare mixes by just looking at the SCM percentage; the content of cement is a far better benchmark.
Carbon sequestering technologies
Newer to the market are carbon-sequestering mixes. These products actively incorporate CO2 in their formulation though the specific techniques and carbon reduction impacts vary. New products on the market can directly inject CO2 into wet mix. The CO2, sourced and purified from the same captured emissions from power plants used in the beverage industry, chemically bonds the calcium oxide in the mix’s cement, creating additional strength and allowing for reductions in cement content. While the cement and GWP reductions are small (typically three to five percent), they are still significant, and one of the advantages of this process is it can accommodate most other additional strategies outlined in this article.
Another related strategy substitutes typical portland cement for portland cement products with higher limestone content. The two most common products are Portland-limestone cement (PLC) and limestone calcined clay cement (LC3), with the former much more common in North America (Europe has more experience with the latter). ASTM C595 guides PLC usage and increases the allowed limestone quantity in a cement mix from five percent to 15 percent, but the impacts to concrete formulation and properties are minimal. PLC replaces typical cement in the same quantity with an end product that measures and performs the same. The increase in ground limestone has a comparable 10 percent reduction in GWP reduction compared to ordinary Type I portland cement.
In Europe, some cement mixes may contain even higher PLC content (up to 35 percent of the overall cement), and LC3 combines both finely ground limestone and calcined clay to replace as much as 50 percent of the Portland cement. While calcined clay does require energy to heat and activate the material, the temperatures required are around 600 to 800 C (1112 to 1472 F) as compared to ordinary Portland cement where temperatures need to reach around 1450 C (2642 F).
LC3 also greatly reduces carbon emissions from the concrete’s chemical reactions as the clay contains very little carbon to begin with. The net effect is that LC3, as currently formulated, can impact concrete GWP reduction, approximately 30 percent.
In the future, high-reactivity metakaolin, derived from purified kaolin clay, will further push the boundaries of curing possibilities to produce high performance, lower carbon concrete in geopolymer concretes. However, it is not yet commercially available to all markets.
Nano infused cements (NICs) are from the family of nano silica admixtures technologies. Like silica fume, NICs are highly reactive when used in concrete construction and produce very durable, strong concrete with increased hydration cycles. However, while silica fume engages with cement on the micro scale, NIC—like all nano silica admixtures—operate in the nano scale, closing capillaries and reducing porosity to the point of slabs acquiring natural moisture mitigation attributes and thus shorter mobilizations to construct concrete on the job site. NIC is an SCM that can be used as a cement replacement, but also allows for increased use of other SCMs making it easy to reduce global warming potential (GWP) on a project. Labor and material cost impacts of NIC are par for the course with traditional methods, but with reduced schedules provide the opportunity to lower overall project costs. Combining internal curing with NIC, PLC, and increased SCMs would allow for GWP reductions well beyond 30 percent in most parts of North America.
As a locally produced and sourced product, strategies for reducing concrete’s footprint vary by region. In some of the largest and most progressively sustainable concrete markets (e.g. San Francisco and Seattle), many suppliers have commissioned Environmental Product Declarations (EPD) to measure the GWP of their products. The most typical and useful EPDs are third-party certified and utilize product category rules (PCRs) specific to concrete, to provide a reasonably accurate and comparable life cycle assessment (LCA) of their product. While it can be somewhat costly for a supplier to generate an EPD (some programs start at $3000), as it involves an assessment of each component of a concrete mix, once they do, the study can typically apply to all their products, as mixes typically just vary the quantity of the components.
For markets with sophisticated producers with available EPDs, best practice is to directly specify a maximum GWP, as evidenced by an EPD, like any other desired performance requirement. The Embodied Carbon in Construction Calculator (EC3), a free database of construction EPDs, is quickly becoming leveraged by the industry, and concrete is among the first of the material sections to contain a large amount of product EPDs. It is quite simple to register for the free web-based database tool, search for suppliers within a geographic limit, and identify not only the compliant producers, but to report median and achievable GWP values for particular concrete strengths and other properties.
In areas with less sophisticated producers, project teams will need to be more active. It is possible to specify maximum cement content limits as a proxy for GWP. Marin County, California’s concrete regulations do this, for example. Another approach is to work directly with suppliers to measure and optimize GWP of proposed mixes. The Concrete LCA Tool, a simple concrete GWP calculator does just this using general industry LCA data from Tally (the North American GaBI LCA database) for common concrete ingredients, which allows direct comparison of proposed mixes against published NRMCA regional averages for various strength classes.
While the results are not specific to particular material or supplier, they are good enough for comparing relative impacts of mixes from a single producer.
In addition to making sure what building are made with leads to a lower carbon footprint, how they are built must also guide design and construction decisions. Transporting a component product great distances may reduce or even eliminate its original low carbon benefit. Simple changes to execution of the exposed concrete finishes on millions of acres of concrete slabs worldwide can potentially improve carbon expenditure at installation, as well as improved life-cycles. Industry experience understands how micro fracturing of concrete can reduce matrix life-cycles and increase the need for repairs. Employing a laser screed to initially create flatter floors in these scenarios can reduce or eliminate the need to mechanically flatten a floor once it has hardened and plasticity is lost. Once mechanical grinding or scarifying has begun, the top layer of concrete loses an amount of integrity, often leading to additional mobilizations for patching, repairing, or petroleum formulated grout coats during initial construction and certainly throughout the slab’s lifetime.
Slag and recycled glass, while excellent choices in most cases for reducing carbon in concrete construction, may produce a similar effect in exposed concrete finishes. Because both materials are brittle, they will commonly fracture along with the portions of the concrete around them as grinders and trowel machines move over the surface of the concrete, creating voids. These, too, will not only require attention during initial construction, but may create additional material and energy demands throughout the concrete system’s life-cycle.
Curing is one of the simplest ways to reduce carbon expenditures, as cured concrete is good concrete. With cured concrete curling, shrinkage, cracking, and all manner of negative effects can be reduced. Wet curing is commonly recognized as one of the better ways of curing, but often not approached due to schedule and cost increases. Instead, common approaches will include cure and seal style applications, which are generally not as effective and in many cases contractors will not apply them when cutting corners. With this comes many physical defects and increased probability of short- and long-term concrete failure and increased energy requirements to keep the concrete matrix functional. Internal curing with nano silica admixture technologies like NIC maintains additional internal water to reduce early shrinkage and do so without need of cure and seals, membranes, and curing compounds. Increased hydration also means that a slab can be ready for framing or polishing a few days after placement, making it more cost and time effective approach than standard concrete construction.
Rebar is a necessary tension tool to reinforce concrete structures and aid the concrete system when it is under tension. However, when chloride ions migrate to steel inside of concrete, corrosion occurs and affects the surrounding concrete with detrimental results, lowering life-span and increasing the need for spending time, materials, and energy on maintaining the concrete structure.
Newer fiberglass and basalt rebar products provide an alternative. Both types of rebar offer the benefits of reduction in rust and corrosion. They can even be demolished along with the original concrete system and turned into aggregate for future projects. Fiberglass is more readily available in North America with basalt supply occupying a lower share of the market. However, the transportation weight of basalt can be lower and could alter availability in the future as the demand for lower carbon rebar solutions increase.
Lastly, specifiers must rethink of when and where to use rebar at all. While some structural reinforcement demands will never change there are areas of concrete construction where reinforcement is unnecessary. Many topping slab specifications will include rebar when perhaps the most sustainable approach would be to let the topping slab float unbounded to the structural slab. Removing rebar from certain topping slab situations allows concrete to move freely, unrestrained, and less likely to crack. Engineering and design teams will need to work together with builders to determine the need to include or exclude rebar with topping slabs, but in many cases project teams will find a simple way to prolong the life of the slab while eliminating the carbon cost of rebar.
Water is one of the four ingredients of concrete and its importance cannot be overlooked when making resilient, long-life cycle concrete. The quality of mixing water in fabricating concrete not only affects immediate behavioral differences such as workability, but also impacts the health of the concrete itself.7 As a general rule one can take the approach that if the water is fit for human consumption then it is also fit for concrete construction. Therefore, construction specifications will usually require potable water. Non-potable water can be used, but often carry impurities like dissolved salts, oil, and grease that can harm the concrete. Having discussions on water sourcing and water quality early on with constructor and supply teams is a simple way to increase the life-span of concrete. If non-potable water will be used, ensure appropriate testing is done to verify that harmful agents will not create legacy issues for concrete systems and shorten their life-spans.
Material science has neither found an alternative construction material to replace concrete nor is it likely in the near future. If the world is going to transition to a zero-carbon economy and mitigate the most severe impacts of climate change, concrete must be addressed by investigating greener methods of producing and building with concrete. This year the International Society for Construction Sciences (iSCS) will be hosting NCCS concrete symposium to explore new possibilities and frontiers of concrete research to make a direct impact on designing, specifying, and building more sustainable concrete.
Reducing the GWP of concrete is arguably the easiest, lowest cost, and most impactful way for a project to reduce its overall carbon impact. While most concrete today is not specified for lower GWP, this is rapidly changing as new technologies emerge in the market. Yet the strongest influence on the market will likely come down to economics. Older concrete technologies (steel rebar, silicates, cure and seals, etc.) are being replaced with new products that not only have a smaller carbon footprint, but are in many cases less expensive than traditional approaches. This has been especially true in recent years with traditional supply chains struggling to get product to market. At the same time, new supply chains are making these products available on job sites a la Amazon style delivery bypassing traditional distribution centers for admixture or simply because newer products, like reinforcement (being lighter weight) might cost less to transport. The immediate environmental benefits, coupled with time and economic factors are signaling to the market that lower GWP concrete systems are in demand.
Chris Bennett, CSI, iSCS, CDT, is president of a North American concrete consultancy that provides owner and designer representation in the development of sustainable concrete solutions and risk reduction. Bennett is the current president of iSCS and president-elect of CSI’s NEXT chapter. He can be reached at www.BennettBuild.us.
Chris Chatto is a sustainability savant who found his way to architecture after a decade of work in environmental activism. As a principal and one of ZGF’s resident sustainable design experts, Chris favors simple, yet powerful tools to analyze data and understand how a project will impact and respond to the natural environment. He also knows that data alone doesn’t create change. Chris appreciates that to be successful in sustainable design and ensure a meaningful impact, it’s important to meet clients where they are and find strategies that align with their values and missions.
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