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Many questions can arise when considering how to reduce carbon emissions in concrete. These frequently asked questions are provided as a guide for the user’s convenience, drawing from existing ACI documents and materials.

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  • How does concrete manufacturing affect global CO₂ emissions?
    Cement is the key ingredient in concrete that helps to achieve desired properties in concrete including strength and durability. The cement used in majority of concrete applications is portland cement, and manufacturing portland cement results in the majority of the contribution of concrete to global CO₂ emissions. Portland cement is manufactured from raw materials, typically limestone and clay that combine at high temperatures, approximately 2640°F (1450°C) to form calcium silicates that provide the binding properties of cement. Approximately 40 % of the CO₂ emitted during pyroprocessing is due to fuel combustion, with the remaining 60% driven off of the limestone during calcination (conversion of limestone to lime and CO₂). It is generally estimated that producing 1 ton (900 kg) of portland cement produces 1 ton of CO₂. However, the cement manufacturing industry has been active and innovative over the last few decades to reduce carbon footprints, and therefore, these general estimates may be higher than actual values. References: ACI PRC-130-19; SCG1; ITG-10R; ITG-10.1R-18; E3-13
  • What are different approaches for reducing carbon emissions in concrete?
    There are many ways to reduce carbon emissions in concrete manufacturing and construction including, Efficient use of materials and energy in cement manufacturing Alternative fuels in cement manufacturing Supplementary cementitious materials (SCMs) to replace portland cement content Blended cements Performance-based cements Alternative cements or binders Water-reducing admixtures to reduce cement content Optimization of aggregate gradation to reduce cement content High-performance concretes Efficient design to reduce total volume of materials used Carbon capture and sequestration technologies Reduce concrete waste Recycle concrete and wash water More information can be found in the following resources. References: SCG1; ACI PRC-130-19; ACI PRC-232.2-18; ACI PRC-232.5-21; ACI PRC-233-17, ACI PRC-234-06; ACI PRC-225-19; ACI PRC-212.3-16; ITG-10R; ITG-10.1R-18; ACI PRC-325.14-17; ACI PRC-555-01; ACI CODE-318-19;
  • What are the barriers to low carbon concrete?
    The cement and concrete industry, and researchers have been working on finding ways to reduce the carbon footprint of concrete while maintaining the expected performance of concrete for the past several decades. They have come up practical solutions to produce low carbon concrete and continue to work toward producing carbon neutral concretes. Several initiatives have been taken to resolve energy usage in cement kilns and reduce carbon emissions. Many alternative technologies, alternative cements and supplementary cementitious materials to reduce carbon footprints have been investigated and tested in laboratories. However, one of the major barriers to implement the most innovative methods is a lack of standards or acceptance criteria. Innovative approaches are sometimes expensive and require new capital equipment or processes that make them difficult to commercialize. For instance, some new technologies may require upgraded cement plants to produce low carbon cements or alternative cements as well as upgraded ready-mix plants to use the new materials. Some technologies may also require training for effective placement and curing of new types of concrete. References: SCG1; ACI PRC-130-19; ITG-10R; Thoughts Regarding Cement, SCMs, Concrete, and CO₂
  • What is Life-Cycle Assessment (LCA)?
    Life-Cycle Assessment (LCA) is a method to evaluate the full environmental impact associated with a process or a product over its lifetime. A credible LCA of a product or process helps to make awareness and transparency of its environmental impacts and provides the opportunity for improvements. The requirements and guidelines for conducting an LCA are described in international standard ISO 14044. An LCA consists of four iterative steps: Goal and Scope, Life-Cycle Inventory (LCI), Life-Cycle Impact Assessment (LCIA), and Interpretation. Cradle-to-grave LCAs are the most representative accounting of environmental impact of a product or process because they include all environmental impacts over its full life cycle. For example, an LCA of a concrete product provides a broad range of environmental impacts such as greenhouse gas emissions, toxic releases, energy use, and other resource use, considering several phases of the product including raw material acquisition, cement manufacturing, concrete production and construction, transportation, utilization, and end use. References: ISO 14044:2006; ISO 14040:2006; SCG1
  • What is an Environmental Product Declaration (EPD)?
    An Environmental Product Declaration (EPD) is a comprehensive summary of multiple sustainability measurements associated with a process or a product based on its Life-Cycle Assessment (LCA). For example, an EPD may include (but are not limited to) global warming potential, acidification potential, eutrophication potential, ozone depletion potential, photochemical ozone creation potential, energy and other resource consumption and generation of wastes. In order to fulfill the requirements for compiling and presenting data in the EPD, a Product Category Rule (PCR) must be established prior to developing an EPD to provide instructions on how to conduct an LCA. The principles and procedures for developing Type III environmental declaration programs and declarations are described in ISO 14025. This standard establishes the use of LCAs as per ISO 14040. An EPD must be peer reviewed and third-party verified. An EPD provides transparent data that allows architects, engineers, building owners, and other specifiers to better understand the environmental impacts of a certain product or process, and also helps to compare different products or processes. For cement and concrete, industry wide EPDs are commonly available. These EPDs represent average environmental impacts associated with a certain type of concrete or a group of concrete products. To have a better understanding of environmental impacts associated with a concrete product, regional EPDs are essential. References: ISO 14025:2006; ISO 14044:2006; ISO 21930:2017; ACI Concrete Sustainability Forum VI; ACI PRC-232.2-18
  • What is a Product Category Rule (PCR)?
    A product category rule (PCR) provides instructions as to how to conduct a Life-Cycle Assessment (LCA) to develop an Environmental Product Declaration (EPD) for a specific product or product category. A product category is a defined family of products that has similar functionality. PCRs are created or administrated by program operators. Core set of PCRs for developing EPDs of construction products or services are defined in ISO 21930:2017. This standard refers to ISO 14025:2006 for conducting a PCR review. The two key components of a PCR are to set rules for LCA and to set rules for what is reported in an EPD. Setting up rules for LCA includes specifying life-cycle stages for inclusion (e.g., cradle-to-grave), parameters to be covered and cut-off rules for data, and life-cycle impact assessment (impact categories and assessment method). Setting up rules for what is reported in an EPD includes information about program operator and verifier, required format for EPD label, impact categories to list and inventory items to list. A better understanding of PCRs is needed in order to compare EPDs from one product to another. Practitioners can use a PCR that has been established for a specific product or process to generate consistent results within the same product category for similar applications. Conversely, a single PCR cannot be used for different product categories. For example, PCRs for ready mixed concretes differ from PCRs for precast concretes or concrete masonry units. References: ISO 21930:2017; ISO 14025:2006;ITG-10.1R-18
  • What is the NEU technology validation/verification program?
    With increasing focus on climate change, there is a corresponding increase in product manufacturers and technology development companies making claims in the marketplace to position themselves and their products/technology as possible solutions to reducing carbon emissions in the built environment. With the wealth of information out there, it is not easy for decision-makers to decipher the best products/technologies to use. Therefore, it is important that as emerging products/technologies come forward, they are vetted and assessed for their efficacy. To facilitate that, NEU is developing a technology validation/verification program to assess and validate the claims of innovative and new products/technologies associated with low carbon cement and concrete production. The terms validation and verification are defined as below. Validation: Process for evaluating reasonableness of the assumptions, limitations and methods that support an environmental statement about the outcome of future activities. Verification: Process for evaluating an environmental information statement based on historical data and information to determine whether the statement is materially correct and conforms to relevant criteria. The product/technology categories for NEU validation/verification program will include, but not be limited to the following, Alternative Cements/Binders Supplementary Cementitious Materials (SCMs) Admixture / Additives Carbon Capture, Utilization and Storage (CCUS) Ultra-High-Performance Concrete (UHPC) Manufactured Aggregates Nanoparticles Low-Carbon Reinforcing Steel Reduced-Emissions Cement Manufacturing / Alternative Fuels and/or Energy Sources The process is conducted impartially by subject matter experts (SME) based on internationally accepted standards and will consist of several phases including application and initial review, independent SME committee formation, SME review and report drafting and public release or appeal. Pilot projects for this process are being identified by NEU to help facilitate the development and protocols needed for this program. The Validation/Verification program is a key function of NEU and plans are underway to start accepting applications in coming months. References: ISO 14065:2020 General principles and requirements for bodies validating and verifying environmental information; ISO 14066:2011 Greenhouse gases — Competence requirements for greenhouse gas validation teams and verification teams; ISO 14067:2018 Greenhouse gases — Carbon footprint of products — Requirements and guidelines for quantification; ISO/IEC 17029:2019 Conformity assessment — General principles and requirements for validation and verification bodies
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