How CarbonStar is Calculated
The CarbonStar Rating represents the net embodied carbon in a unit of concrete. This calculation takes into account both the emitted and sequestered carbon per unit of concrete:
CO2CarbonStar = CO2Emitted - CO2Sequestered
Portland Cement Concrete Mix
Portland cement concrete mix is representative of what is typically found in a ready-mix plant. Traditionally, there are no carbon-sequestering materials within this mix, so the CarbonStar rating consists only of the sum of the carbon emitted, a value totaling 519.56
The cement within this mix accounts for approximately 98% of the CarbonStar rating, further demonstrating the impact cement has on the embodied carbon of a mix.
In this example 519.56 lbs of CO2 was emitted into the atmosphere, for each cubic yard of concrete.
SCM Concrete Mix
SCM concrete mix differs from Standard Concrete Mix in that half of the cement is replaced with supplementary cementitious material (SCM). Currently, SCMs, such as fly ash, are viewed as industrial waste products and have low emission factors.
The reduction of cement used in a mix can significantly reduce the CarbonStar rating of the mix.
In this example 276.65 lbs of CO2 was emitted into the atmosphere, for each cubic yard of concrete produced.
CO2-Sequestered Concrete Mix
CO2-Sequestered Concrete mix demonstrates the benefit of incorporating several carbon sequestering technologies and cement reduction methods within a single mix. This mix includes: SCMs, CO2 injection, and Synthetic CaCO3 fine aggregate.
Because CO2-Sequestered Concrete mix significantly reduces cement usage and uses sequestered carbon components, the CarbonStar rating significantly decreases (lower is better) when compared to the other two mixes.
In this example 600.99 was sequestered in the concrete. When subtracted from the 270.65 emitted, this results in a CarbonStar of -330.43.