Specifying Sustainable Concrete
LEED Projects
LEED, along with other green building standards, offers guidance for reducing the environmental footprint of building materials. NRMCA publishes a Guide Specification for Concrete for LEED Projects. The documents focus on the Material and Resources (MR) credits of LEED and how to specify concrete to meet the intent of the credits.
LEED Materials & Resources credits attempt to take a holistic look at materials by adopting life-cycle assessment (LCA) and product disclosure and optimization. LCA is the investigation and evaluation of the environmental impacts of a product, process, or service. LCA evaluates all stages of a product’s life to determine its environmental life-cycle impacts. LCA is the most comprehensive approach to determining the environmental impacts of a building. There is a credit in LEED called Building Life: Cycle Impact Reduction that rewards points if the building has lower life-cycle impacts than a baseline building.
Whole-building LCA credit: Specifications can require that product suppliers submit life-cycle inventory (LCI) data for their products or environmental product declarations (EPDs) to help the design team conduct a whole-building LCA. The concrete industry leads the way in conducting LCAs and publishing EPDs for its products. Many companies have published EPDs for concrete, and most would be willing to publish EPDs specifically for a project. NRMCA has published an LCA report and industry-wide EPD for concrete. Product-specific EPDs, the industry-wide EPD, and regional benchmark impacts for concrete can be found at www.nrmca.org/sustainability/EPDProgram. The Example 2 sidebar helps specifiers understand how concrete can contribute to lowering the overall footprint of a building and help meet the intent of LEED’s Building Life-Cycle Impact Reduction credit.
EXAMPLE 2: USING LIFE-CYCLE ASSESSMENT TO DEMONSTRATE LOWER IMPACTS
In this example, a cradle-to-gate life-cycle assessment (LCA) was conducted to determine the embodied impacts of concrete on a building to compare the GWP or carbon footprint of a reference building using typical concrete mixes with moderate amounts of SCMs, both fly ash and slag, and a proposed building using concrete mixes with relatively high volumes of fly ash and slag. The building is an 18-story residential tower located in the northeastern United States. For illustration purposes, only six different concrete mixes are selected for the project. In reality, a project of this size might have more concrete mixes. Compressive strengths and concrete volumes for each structural element are identified in Figure 5.
Figure 5: Shown here is the specified compressive strength of concrete for an 18-story residential tower.
The first step in the analysis is to identify typical concrete mixtures for the reference building. NRMCA publishes benchmark mix designs and their environmental impacts for eight different regions in the United States. This example uses the benchmark mix designs for the Northeast region.
The next step is to estimate mix designs that have significantly lower GWPs than the benchmark mixes that still meet the performance criteria. Keep in mind that concrete requiring high early strength should be limited to around 30 percent replacement of fly ash or slag. Concrete that does not require early-age strength, such as footings, basement walls, and some vertical elements like columns and shear walls, could have as much as 70 percent fly ash and/or slag and be tested at 56 or 90 days instead of 28 days to account for slower strength gain. These applications can also incorporate a significant volume of recycled aggregate with less risk. This example uses high-volume SCM mixes from the NRMCA Industry-Wide Environmental Product Declaration (EPD). A summary of the concretes selected for the reference and proposed buildings are provided in Tables 4 and 5.
Using the Athena Impact Estimator for Buildings (Athena IE) software, the reference building and proposed building were defined using the proposed mixes in Tables 4 and 5 respectively. Athena IE has the NRMCA benchmark mixes, and the NRMCA Industry-Wide EPD mixes are pre-loaded into the software. The software also permits the user to define new mixes based on the existing mixes in the library or completely new mixes if this information is available from a concrete producer.
Once all of the concrete information is defined for each project, the user can then run a report that will provide the estimated GWP, along with other impacts, for each building. The reference building will represent the largest impacts, and the proposed designs will represent lower impacts. The results for this example showed that the reference building has a GWP for concrete of 6.14 million kg CO2, while the proposed building has a GWP for concrete of 3.92 million kg CO2, meaning that the high volumes of fly ash and slag mixes resulted in a 36 percent reduction in GWP.
Proposed Specification Language
There are several ways that one could write a project specification that would result in a 30 percent reduction in GWP for concrete on a project.
The following are two options:
Option 1
Supply concrete mixtures such that the total GWP of all concrete on the project is less than or equal to 4.298 million kg of CO2 equivalents as calculated using the Athena IE, which is available at www.athenasmi.org.
Option 2
Supply concrete mixtures such that the total GWP of all concrete on the project is 30 percent or more below the GWP of a reference building using benchmark mixes as established by NRMCA and available for download at www.nrmca.org. Submit a summary report of all the concrete mixtures, their quantities, and their GWP to demonstrate that the total GWP of the building is 30 percent or more below the GWP of the reference building. Contractor may use the Athena IE or other similar software with the capability of calculating GWP of different mix designs.
Keep in mind that this example was simplified for illustration purposes. It only considered the effects of concrete during the material extraction and manufacturing stage on the environmental impacts of the building. For LEED, one must consider the impacts of all the materials and products associated with the structure and enclosure, including structural elements such as concrete, reinforcing steel and structural steel (including fireproofing), and exterior cladding, such as glass, aluminum, and insulation. The Athena IE software does contain environmental impact information for most materials and products used in buildings and outputs the six environmental impacts required for the LEED v4 LCA credit. In addition, the Athena LCA software allows the user to input energy-consumption data obtained from an energy analysis, making it an ideal tool for conducting a whole-building LCA.
Product disclosure and optimization credits: Product disclosure means reporting environmental, social, and health impacts through third-party-verified reports, including EPDs, corporate sustainability reports (CSRs), and health product declarations (HPDs), among others.
There are three Building Product Disclosure and Optimization credits, each having various options to lower impacts. The first option, Disclosure, requires that the project use a certain number of permanently installed products that disclose impacts using EPDs, CSRs, and/or HPDs. In LEED, a “product” is defined by the distinct function it serves. This means that concrete has the advantage of contributing significantly because of its wide range of applications and functions. For example, footings, foundations walls, shear walls, bearing walls, columns, beams, slabs, sidewalks, and parking areas, each with a unique mix design, would all be considered different products in LEED and therefore contribute significantly to the number of products required to meet the intent of the credit.
The second option, Optimization, requires a certain minimum value of building products to demonstrate that they perform better than previously disclosed impacts.
Figure 6: The Edith O’Donnell Arts & Technology Building at the University of Texas, Dallas uses a partially exposed concrete structure to take advantage of concrete’s thermal mass properties, which trap heat during cold months and keep structures cooler during warmer weather. Concrete’s long life and durability are important for academic buildings that must address the needs of the university and its students for many decades. Some of the same properties that make concrete strong (its mass and rigidity) also make it virtually soundproof. The beauty of concrete is on full display in the partially exposed structure, including polished concrete floors and beams in the lobby. The project achieved LEED Silver certification.
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