Concrete Carbon Footprint Mitigation By Implementing Performance-Based Specifications

Concrete industry is the largest consumer of the natural resources, such as crushed rock, gravel, sand, and water. In addition, concrete has another two drawbacks: (1) Portland cement is both energy and carbon footprint intensive material. Every tonne of cement requires about 1.5 tonnes of raw material, and about 4000 to 7500 MJ of energy for production. More importantly, production of every tonne of cement releases 1.0 to 1.2 tonnes of CO₂ into the environment;  (2) Concrete deteriorates due to the load and some environmental factors, such as freeze-thaw cycling, de-icing chemicals, sulfate attack etc.,  which significantly influences its service behavior, design life and safety.  

This study reviewed concrete specifications from fourteen State Departments of Transportation in the U.S.  Prescriptive specifications, which define a concrete mixture in terms of its constituents and their proportions, are being used in all 14 states, which are combined with some kinds of performance criteria, which define a concrete mixture in terms of measurable plastic and hardened properties.  In Colorado, many concrete mixture requirements specified by Colorado Department of Transportation (CDOT) are generally governed by minimum cement contents for a given class of concrete and a range of water-to-cementitious materials ratio. This is prescriptive-based because the concrete mixture designer cannot go beyond these limits. These minimum values generally yield concrete strengths in excess of design compressive strengths on the order of 500 to 1,000 pounds per square inch (psi). Concrete mixtures that exceed CDOT strength specifications could be developed with less cement and more fly ash, which contributes greenhouse gas (GHG) reduction and sustainability of concrete industry. If concrete mixtures are accepted based on results from standard test methods, this provides data applicable to in-service durability and performance.  Ultimately, performance specifications are expected to provide better assurance of durable in-place concrete.

This study investigated the potential of concrete carbon footprint mitigation by implementing performance-based concrete specifications. Fifteen CDOT pre-approved Class D and Class P concrete mixtures were batched and tested under laboratory conditions. Class D concrete is a medium dense structural concrete.  Typical uses include: bridge decks, median barriers, and box culverts.  Class P concrete is used in pavements.  Concrete within this class are typically designed at low slumps for use in slip-form paving machines or curb and gutter machines.  The plastic and hardened concrete properties were performed on each of these mixtures in an effort to identify controlling test methods and results.  Data gathered from the test results was analyzed and recommendations were given on the test methods and acceptance criteria. In addition, the structural and durability performances of another nine concrete mixtures were investigated, which were batched with ASTM C618 Class C fly ash contents ranging from 15%-60% (weight ) of the cementitious materials. The mixtures with up to 50% fly ash met the developed CDOT performance-based Class D structural concrete specifications. The environmental life cycle assessments were completed using a regional "Cradle-to-Grave" model developed from an Economic Input-output model. The functional units of the output used in this model were selected as kilogram carbon dioxide equivalent mass (kgCO2E) for GHG emission and megajoule (MJ) for embodied energy. The GHG emission and embodied energy of the mixture with the least environmental impacts, while meeting the structural and durability requirements, are 35.2% and 32.8% less than the control mixture with most GHG emission and embodied energy.