Do the words “winter concrete mix design” mean anything to you? To many, those words mean nothing. To others, those words infer concrete that is poured in the winter. But to a select few, those words mean a whole lot more.

To construction project managers whose project schedules require concrete operations to commence in winter, winter concrete means longer curing times resulting in modifications to concrete mix design components. The time of year when concrete is poured can directly impact the project’s LEED® MRc4 – Recycled Content contributions.

In most large-scale commercial and residential building developments, concrete is among the largest and typically most expensive building materials. Therefore, it is often actively pursued as a significant contributor to the LEED MRc4 – Recycled Content Credit.

The Composition of Concrete

Slag and fly ash (otherwise known as supplementary cementitious materials or “SCM”) are typically found in standard concrete mix designs and are considered by LEED to be the recycled portion of the cementitious (cement) materials. The addition of SCM to “pure” concrete results in longer curing times for the entire concrete mix design.

Concrete cures faster in the summer. Therefore, if your concrete operations are scheduled in the winter (and you want to finish on time) you may be required to reduce or even eliminate the amount of slag and fly ash used. However, the absence of SCM in the mix designs can potentially derail your project’s ability to earn the LEED Recycled Content credit.

This absence means more “pure” concrete in the overall mix, which directly affects the use of sustainable (recycled) materials on a project. Ideally, project teams should factor this in as LEED credits are selected, credit achievability analysis is underway, and the construction schedule is finalized.

Earning LEED Credit

The intent of the LEED MRc4 – Recycled Content credit is to increase demand for building products that incorporate recycled content materials, thereby reducing environmental impacts resulting from the extraction and processing of virgin materials.

If you use materials with recycled content that constitute 10% based on value of the total project cost then you will earn 1 LEED point; for 20% you will earn 2 LEED points.

The exclusion of SCM may not affect your project if you are not pursuing the LEED MRc4-Recycled Content credit. However, from my experience as a LEED Construction Project Manager supervising multiple projects, it is a credit that we always count on for certification.

If your project is pursuing the LEED Recycled Content credit, you’re on a tight construction schedule and your concrete operations are scheduled in the winter, you may want to think about another way to achieve your recycled content contributions, before it’s too late.

LEED certification is often just one item on a long list of owner’s decisions. With that in mind, no developer is going to extend a construction schedule to incorporate recycled contributions during concrete operations. Every day the project is pushed back is another day the owner loses money from potential rent.

Recycled Content Credit Specifics

Below is how the recycled content calculation breaks out for this particular LEED credit:

Recycled Content Calculation

As you can see from the equation above, the numerator contains the sum of all the recycled content contributions from the whole project. Therefore, if concrete is taken out of this quotient you must use other materials to make up for that loss.

Evaluating the project’s material budget and construction schedule early on will allow you to better predict and plan for this loss in recycled content contribution. Typical materials you can turn to include steel, drywall or non-structural metal framing.

Planning Ahead

The absence of an action plan combined with inadequate forecasting is often responsible for projects missing the significant concrete recycled content contribution credit. A lack of SCM in the concrete mix designs due to winter concrete operations may force the team to look to other materials for contribution, which takes additional time and resources to evaluate and may be impossible on a tight construction schedule.

That’s why it’s important to plan and begin opening up the lines of communication between the developer, the architect and the general contractor once the project commences.

For projects I’m working on, we establish a “LEED Recycled Content Action Plan.” During this process, project teams work together to estimate the total material cost for the job, as well as identify the materials that will be major contributing factors in achieving the LEED Recycled Content credit.

The plan helps you to estimate when concrete operations will commence and make adjustments if you see that the concrete will not likely be a contributor in earning the credit.

The goal is to identify this issue early on so that your team is not blind-sided and “instant credit achievability panic” kicks in. Save yourself the time, resources and headache by understanding your construction budget, schedule and recycled content contributors before entering the submittal process.

Hawkins M. Thomas, LEED AP BD+C, is a Consultant with Paladino DC, the East Coast office of Paladino and Company.

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2 Comments

  1. Hawkins – Great post. I think these little bits of information are crucial because often certain credits are considered a “given” and these little nuances can have a monumental impact on the project.

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  2. Erika – I’m glad you enjoyed the post and thanks for the comment. I find it difficult to get General Contractors actually engaged in the LEED Construction Process especially once submittals begins flooding their desks. They typically see LEED as a nuisance, which is why I always try to get them initiated as early as possible. Have you had similar experience with this?

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    1. Yes! I really stick to my guns about having a pre-construction meeting where we sit down (Owner/Archictect/GC) and go over the sustainable requirements spec. Just so they know and I know they have at least looked at the document and have an overall sense of what is required of them. Still doesn’t mean they don’t see it as a nuisance.

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