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
The easiest way to explain the CarbonStar Rating is through a few example applications and using the calculator on this page. While these examples are in imperial units, CarbonStar calculations can also be performed in metric units.
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 639.56.
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 639.56 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 an industrial waste product and do not have a carbon emitted value associated with them.
The reduction of cement used in a mix can significantly reduce the CarbonStar rating of the mix.
In this example 327.56 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 322.36 emitted, this results in a CarbonStar of -278.63.
This calculator can be used to see how altering the components of concrete and their relative proportions impacts the CarbonStar rating.
Emissions factors can be found in product specific LCAs or LCA databases. In the absence of product specific data industry/regional average data may be used.
The number generated by this calculator is not a certified CarbonStar rating as some elements, such as transport and energy use at the concrete plant, are not included.
To complete a full CarbonStar rating click here to access the technical specification document.