LEED and Concrete Mixes 1

[coatsandrew]

We're starting to design a new school that will obtain a LEED Silver (or higher) rating. My question is what proportions/requirements for slag, fly ash, etc. are typically specified for concrete mixes in the 3,000 to 5,000 psi range? I would like to contribute to the LEED credit as much as possible, but I also don't want to go beyond "normal" proportions. Any link to articles or other backup information would be greatly appreciated. Thank you.

 

[Gumpmaster]

25% is normal.

50% is OK too.

Probably 25% max for flatwork, but some contractors may be able to finish 50%.

I don't know that you'll be able to get a specific innovation point unless you have a really large volume of concrete. You can help out with the recycled content though.

Check to make sure that the slag/fly ash is locally available and doesn't need to be shipped across the country.

 

[concretemasonry]

Depending on where you are located, there are several cement manufacturers that use local recycled materials (tires, waste, etc.) for 40% of the production energy. This could be a reason for an additional LEED credit.

Dick

 

[InDepth]

Watch out with the delay time for gaining strength. Also, the finishers need to be specially trained and possibly even use a mockup to test the finishing procedures, since high volume fly ash has different characteristics. Also be careful with curing as well as the color finish of the concrete. High volume slag can get a green color to it. The best place for high volume flyash is foundations and piles, since they don't need substantial strength gain early on. You need to be careful with slabs, because if the forms are pulled to early and there isn't enough strength slab cracking may occur.

A good person to talk to is Steve Ratchye of Thornton Tomasetti San Francisco. He's considered an expert when it comes to green concrete.

Bruce Kings book Making Better Concrete - Guidelines to Using Fly Ash for Higher Quality, Eco-Friendly Structures by Bruce King, P.E. is a good reference.

PCA's book on concrete design is good as well.

Just remember fly ash is not really green anyway...even though LEED says so...after all, if we elminated the high energy, high pollution, steel mills and concrete planst, we wouldn't have fly ash.

 

[steellion]

It's green in the sense that you're taking what would otherwise be waste and turning it into something useful.

LEED and Structure pretty much overlap on really only two principles: Recycled content and Regional materials. Steel will be 80-100% recycled (rebar ~60%), so require the contractor to obtain recycled content reports from the mill. It's hard to use steel for regional materials because it requires a chain of finding where the original raw material was harvested that's nearly impossible to obtain. Concrete will get you a lower % of recycled material, but you should realistically be able to get a good % of concrete within the 500-mi region.

 

[JedClampett]

I don't have the number in front of me, but reinforcing probably has a large percentage of recycled scrap in it. For structural steel, the number is 92% scrap.

 

[dakota99]

I have just completed a job with a very similar situation. We specified 30% for footings and foundations and 15% for flat work. I truly believe you could almost double those numbers with proper use of an accelerator and a knowledgeable contractor. But these are the numbers that local contractors felt comfortable with. I still got allot of phone calls and nasty emails from the contractor because they needed to leave there forms on long to get the required strength. So make sure your documents are clear on where to use what concrete and how long it should cure.Most of the 7 day breaks came in lower than I was use to seeing but almost all of the 28 day breaks were well over the specified strength.

 

[concretemasonry]

Dakota -

You are smart to survey the contractors for opinions.

The other resource you did not mention was the suppliers. In many areas, the concrete and cement suppliers have far more background in the testing and research of mixes that contractors have never seen or been forced/exposed to use. This is particularly true in the case of the vertical integrated companies (cement/aggregate/concrete) that individually spend far more more money on the product you are specifying than the entire contracting organizations spend including the "tag-along" support of some association programs without any real participation.

Contractors are often more interested in flipping forms and doing volumes instead of new combinations of raw materials, the performance and the methods to achieve the results.

If you are designing in a smaller market without good supplier connections. A local contractor is a good choice for a "seat of the pants" opinion for that situation.

Dick