Concrete Recycled Content

Please see Supplementary Cementitious Materials section under MRc4 in the LEED Reference Guide. It outlines how to deal with recycled SCMs. See the sample form for MRc4 to determine what documentation is required - http://www.usgbc.org/sampleforms/v3.

“Concrete Mix Designs” are “recipes” that list the mass of each component used in each batch of concrete. Typically, a structural or civil engineers writes the concrete specification and requires the concrete supplier to submit test reports & other documentation to demonstrate that the concrete meets specified requirements. Though submitted to show structural compliance, these documents can be invaluable for calculating recycled content & regional materials for LEED. These submittals usually include “Concrete Mix Designs”. Mix designs & test reports usually also identify material sources. Your structural engineer or concrete supplier may be able help you read the mix design and show you which quantities you need for MRc4 SCM & MRc5 assembly calculations. Just be aware that any one project typically uses a number of different concrete mix designs for various applications (foundations, slabs on grade, structural beams, roadway paving, etc.). Special mixes may be necessary to produce the required strength or appearance, to accommodate pumping, or to allow pouring concrete during cold or hot weather. The composition & SCS content of the various mix designs can vary widely. Therefore, you may need to do a separate SCM calculation for each design mix. For that, your contractor needs to break down cementitious material $ amounts for each mix design used on your project. For MRc5, for each mix, you also want a total cost that includes non-cementitious materials (aggregates). The volume (m3) of concrete used is not usually relevant to these calculations.

Hello, I have an additional question regarding calculating the recycled content value in concrete. I have a project that is using slag and flyash as a portion of the cementitious materials. If you only use the cementitious value of the concrete mix, we are using between 20 % and 50% of the cementitious material being fly ash/slag. I understand that LEED wants to encourage the use of fly ash/slag and is allowing the project teams to use this higher value for recycled content. My confusion is whether I use this percentage against the total cost of the concrete mix or if I need to break out the cementitious material cost of the concrete. If I have to break out the cost of the cementitious materials, I don't see the advantage over the typical method of calculating the percentage of fly ash/slag over the total mix design. It appears that the total recycled content dollar amount would be very similar in either calculation method. The only way I see an advantage is if you use the recycled content of the cementitious materials only against the total dollar amount of the concrete materials cost. Any help would be greatly appreciated.

Pound for pound, concrete’s non-cementitious components (stone, sand, gravel, water) are cheap. Cement, fly ash, & slag tend to be more expensive. In a typical concrete mix design, cementitious material might be only about 14% of the mix’s weight, but cement & SCMs could comprise 42% of the cost for the same mix. By using the SCM method to calculate recycled content for this mix design, we effectively triple its MRc4 contribution. Pretty good trick! But it’s more than just a trick. Using slag & fly ash in concrete recycles these hazardous industrial waste products, rendering them inert. Substituting SCMs for cement reduces the carbon emissions that result from cement production. USGBC allows this “trick” to encourage the use of SCMs. Since we use so much concrete in construction, the overall advantage of using the SCM calculation method can be significant. Break out the costs of cementitious materials if you can get them from your concrete supplier. You will be glad you did.

Jonathan - Thanks for chiming in on this for Lisa. I appreciate your assistance. Lisa - I hope you got your question answered. If not, please post again!

Hi All,  Does Flyash used in a concrete mix or a CMU classify as preconsumer recycled content or postconsumer content?  Regards, Ruju