The CarbonStar Standard (CSA/ANSI R118:24 – CarbonStar: Concrete Carbon Intensity Quantification and Verification) and the associated CarbonStar Calculator were developed by a broad range of industry, government, academic and NGO experts to guide building professionals in the specification and use of low embodied carbon and sequestered carbon concrete products for the construction of new buildings, renovated buildings, landscape paving, and infrastructure. There are many participants in the design and construction process and each participant has a role to play in specifying, purchasing, manufacturing, and using decarbonized building products. The major participants regarding the use of concrete include building owners (both public and private), architects, engineers, contractors, sub-contractors, and concrete manufacturers. These professionals can use this Standard and Calculator to facilitate procurement of environmentally preferable concrete using a standardized approach to carbon accounting thereby creating efficiencies during the building process.

As noted, this Calculator is based on CSA/ANSI R118:24, and the Standard should be used in conjunction with the Calculator. When used together, these help remove a major obstacle to decarbonizing concrete in that these provide a quick, rigorous, and standardized way to quantify and specify decarbonized concrete. These also can be used in conjunction with and in support of existing marketplace products, including environmental product declarations (EPDs), to provide a consistent and rigorous quantification of the carbon impacts of various forms of concrete and concrete mixes.

The CarbonStar Rating, which can be calculated using the Calculator, represents the net embodied carbon in a unit of concrete, presented as either a negative (net sequestered), zero (net neutral), or positive (net emitted) value. This calculation accounts for both emitted and sequestered carbon per unit of concrete: CO2CarbonStar = CO2Emitted – CO2Sequestered.

The tables below illustrate the methodology for calculating the carbon intensity of concrete production across three distinct processes: conventional production, utilization of recycled materials, and carbon capture. The tables illustrate how each process impacts the overall carbon intensity.

Table 1: Example of conventional concrete carbon intensity

Mix Details Mass, kg/m3 (lb/yd3) Carbon impact, kg CO2/m3(lb CO2/yd3)
Cement Portland Cement 356 (600) 370.20 (624)
Coarse aggregate Conventional aggregate 771 (1300) 14.65 (24.7)
Fine aggregate Conventional sand 890 (1500) 11.57 (19.5)
Water 178 (300) 0.035 (0.06)
A3 phase (production of concrete at ready-mix plant) 1.06 (1.8)
CarbonStar rating in kg CO2/m3 (lb CO2/yd3) concrete 397.52 (670.06)

The mix in table 1 is representative of a concrete mix that can be typically found within a ready-mixed plant. Traditionally, there is no carbon sequestering material within this mix, so the final CarbonStar rating will only consist of the sum of the carbon emitted values totaling to 397.52 kg CO2/m3 (670.06 lb CO2/yd3).

Table 2: Example of carbon intensity when recycled concrete aggregate replaces conventional aggregate

Mix Details Mass, kg/m3 (lb/yd3) Carbon impact, kg CO2/m3(lb CO2/yd3)
Cement Portland cement 356 (600) 370.20 (624)
Coarse aggregate Recycled aggregate 771 (1300) 3.08 (5.2)
Fine aggregate Recycled sand 890 (1500) 3.56 (6.0)
Water 178 (300) 0.035 (0.06)
A3 phase (production of concrete at ready-mix plant) 1.06 (1.8)
CarbonStar rating in kg CO2/m3 (lb CO2/yd3) concrete 377.94 (637.06)

The mix in Table 2 differs from the mix in Table 1 in that the coarse and fine virgin aggregate will be replaced with recycled concrete aggregate (both coarse and fine). Depending on the manufacturing process, recycled concrete aggregates often have a lower carbon intensity than virgin aggregate, resulting in a lower CarbonStar rating. The final CarbonStar rating will consist of the sum of the carbon emitted values totaling to 377.94 kg CO2/m3 (637.06 lb CO2/yd3). In this example, there is an approximately 5% reduction in CarbonStar ratings when compared to the mix in Table 1.

Table 3: Example of carbon intensity with carbon capture and geological storage at a cement plant

Mix Details Mass, kg/m3 (lb/yd3) Carbon impact, kg CO2/m3(lb CO2/yd3)
Cement Portland cement 356 (600) 37 (62.4)
Coarse aggregate Recycled concrete 771 (1300) 3.08 (5.2)
Fine aggregate Recycled sand 890 (1500) 3.56 (6.0)
Water 178 (300) 0.036 (0.06)
A3 phase (production of concrete at ready-mix plant) 1.06 (1.8)
CarbonStar rating in kg CO2/m3 (lb CO2/yd3) concrete 44.70 (75.34)

The mix in Table 3 is representative of a concrete mix that utilizes cement from a facility that has installed a carbon capture system that captures a net 90% of the carbon emissions. The actual capture percentage is a function of the technology employed and should be inclusive of the parasitic load (emissions from the heat and power required to run the capture equipment). The capture percentage is also net of any transportation-related emissions associated with moving the captured CO2 to the storage site. Note that CCS projects are sometimes developed to obtain carbon credits. In this example, the CarbonStar rating calculated above assumes that all credits accrue to the cement producer.

Using the CarbonStar Calculator

The functional unit shall be 1 m3 of concrete. If this calculator is used in the United States, the functional unit may be 1 yd3 of concrete.

There are four basic inputs to the calculator in terms of carbon emitted with the same four for carbon sequestered:

  • Mix details (specific concrete mix constituents, inputted separately)
  • Data sources for both mass and emission factors
  • Mass of the concrete mix constituent
  • Emission factor for the mix constituent

Once this data has been inputted, the CarbonStar rating can then be calculated.

Important: Users of the calculator should always refer to CSA/ANSI R118:24 for guidance and can use the examples detailed in CSA/ANSI R118:24 to provide metrics for the various concrete mixes. It is also important to have emission estimates verified and validated. Refer to Clause 7 and Annex B in CSA/ANSI R118:24 for details on verification and validation.

Disclaimer

The CarbonStar Calculator is meant to serve as a supplementary tool only and should not be used as a substitution for adherence to applicable standards, codes, laws, and jurisdictional requirements. CSA Group always strives to provide up-to-date and accurate services, tools, and information. However, no representation or warranty, expressed or implied, is made that the CarbonStar Calculator (the “Tool”) meets your specific needs and any reliance on the information provided by and recommendations made by this Tool, is at your own risk. Please contact CSA Group for more information about this Tool and our services.

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Total Carbon Emitted: Net Carbon Sequestered:

CarbonStar Rating

CarbonStar Calculations Input

Carbon Emitted
Mix Details Data source(s) Mass kg/m3 (lb/yd3) Emissions Factor kg CO2/kg (lb CO2/lb) Carbon Emitted

Carbon Sequestered
Mix Details Data source(s) Mass kg/m3 (lb/yd3) Emissions Factor kg CO2/kg (lb CO2/lb) Carbon Sequestered

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