Concrete GWP Demystified: From Mix Design to Real Numbers

By Jon S. Belkowitz, PhD, PE | Intelligent Concrete, LLC

The concrete industry is undergoing a major transformation with the introduction of standards such as ACI 323, which prioritize sustainability alongside traditional strength and durability requirements. A key component of this transition is the accurate calculation of Global Warming Potential (GWP), measured in kilograms of CO₂ equivalent per cubic yard of concrete.

This article provides a methodical approach to calculating the GWP of a concrete mix design based on standardized material data and real-world mix designs, specifically emphasizing mix adjustments for low-carbon concrete production.

1. Understanding GWP in Concrete Materials

The GWP of a concrete mixture reflects the total greenhouse gas emissions, expressed as kilograms of CO₂ equivalent (kg CO₂e), associated with the production of the mix constituents. Only primary contributors — namely cementitious materials and aggregates — are typically included in concrete mix-specific GWP calculations.

Admixtures and additives are not currently considered as part of the GWP calculation.

Based on industry-accepted values , the GWP factors for common concrete materials are as follows:

2. Concrete Mix Designs for Comparison

Three mix designs are evaluated — a traditional 4,500 psi concrete for a residential driveway and two modified low-carbon 4,500 psi concrete mixes designed to comply with reduced GWP thresholds.

Note: "pcy" = pounds per cubic yard.

3. Methodology for GWP Calculation

The GWP contribution for each constituent is calculated by multiplying the material mass by its respective GWP factor. The total GWP is the sum of all constituent contributions.

Calculation Example — Low-Carbon Mix:

The computed total closely aligns with the ACI Code 323 documented value for a normal weight concrete mix design with a 28-day compressive strength of 4001 to 5000 psi - 258 to 296 kg CO₂e/cy for the low-carbon mixes .

4. Comparative Analysis

Key performance metrics for the mixtures are summarized below:

Key Observations:

• The low-carbon mix achieved a 23% reduction in GWP.
• Cementitious content was significantly reduced, with cement partially replaced by slag cement (SCM).
• Coarse aggregate content was increased to maintain the mix's volumetric requirements.
• The paste volume was reduced by approximately 2% and 11%, with a corresponding increase in granular skeleton density.

These adjustments optimize the mix for both strength development and reduced environmental impact.

5. Conclusions

Calculating the GWP of a concrete mixture is an essential step toward producing environmentally responsible concrete. The outlined method demonstrates that:

1. Accurate material-specific GWP factors are critical.
2. Reducing Portland cement content has the most significant effect on lowering GWP.
3. Incorporating supplementary cementitious materials (SCMs) and optimizing aggregate proportions are effective strategies for achieving compliance with low-carbon standards like ACI 323.

Civil engineers and concrete technologists must now incorporate GWP considerations into routine practice — not only to comply with emerging codes but to contribute to the broader goals of sustainability in infrastructure development.

References

• American Concrete Institute. ACI CODE-323-24: Low-Carbon Concrete. ACI, 2024. https://www.concrete.org/store/productdetail.aspx?ItemID=323U24&Language=English&Units=US_Units
• Portland Cement Association. Environmental Product Declaration (EPD) for Portland-Limestone Cement (Type IL). NRMCA, 2021. https://www.cement.org/docs/default-source/sustainability-pdfs/pca_portland-limestone-cement-epd.pdf
• National Ready Mixed Concrete Association. Industry-Wide Environmental Product Declaration for Ready Mixed Concrete. NRMCA, 2021. https://www.nrmca.org/wp-content/uploads/2021/09/NRMCA_EPD_Industry_Wide_2021.pdf
• U.S. Environmental Protection Agency. Understanding Global Warming Potentials. EPA, 2022. https://www.epa.gov/ghgemissions/understanding-global-warming-potentials
• Obla, Karthik H. Improving Concrete Sustainability through Performance-Based Specifications. NRMCA, 2020. https://www.nrmca.org/wp-content/uploads/2020/10/P2P_Sustainability_Obla_2020.pdf