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Recycled concrete contributes to sustainable construction.

Q: We operate in an area that does not have many LEED projects. However, we are now seeing more such projects under development, and specifications requiring LEED submittals are becoming more frequent. How can we manufacture concrete to meet these requirements, and how can we make our concrete more sustainable?

A: Your ambition to increase your concrete's sustainable aspects should be applauded, but also understand that concrete is a sustainable product already. Cement-based structures have been around for thousands of years. The Pantheon and the water viaducts in Rome have lasted many centuries. These structures are true examples of sustainable construction. They are durable, have served many generations, and have limited the amount of resources used over their life cycle.

I would start your exploration of meeting LEED project requirements by learning how concrete can help contribute additional credits. The Portland Cement Association and NRMCA have great documents and information on their websites, www.concretethinker.org and www.nrmca.org, respectively.

There are many ways to increase the recycled content of your concrete, although we recommend stepping delicately down this road. Long life and durability are the properties which make concrete a sustainable building material. If we sacrifice concrete's durability, we remove the most important sustainable property.

We have been in meetings with county and city officials whose 8-year-old concrete pavement is in very rough shape. We hear comments such as, “If it's going to last less than 10 years, we might as well use asphalt.”

Interesting developments are occurring worldwide to decrease the amount of portland cement in a yard of concrete. Several programs have eliminated the portland cement from the concrete mixture entirely. We are still many years away from seeing widespread use of these mixtures, as general acceptance from the construction industry will be slow. There are many things you can do to your own mixes to increase the use of supplementary cementitious materials (SCM).

The new I-35W Bridge in Minneapolis is a great example. A large portion of the concrete for this bridge contains 18% portland cement, based on the total cementitious content. This should not be your target for concrete flatwork, as it would likely create finishing issues, although there are concrete mixtures utilizing high SCM for flatwork.

Typical replacements are in the range of 25%, although innovative producers have been able to increase this to 40% or more. Working with admixtures, accelerators, and water reducers, you can maintain concrete set times and produce concrete which can still be placed successfully.

These methods can work, although as indicated above, there may be some downfalls. We were involved with one of these early this year. The specifications called for 50% fly ash replacement for all of the concrete. The ready-mix producer convinced the specifier to reduce it to 40%, but he was still uncomfortable, as they did not have any history with this mix.

A few placements went well but there were compatibility issues. The large slab-on-grade placement illustrated this with extended set times and slow strength gains. At seven days, it only achieved 200 to 300 psi. At 10 days, it bounced up to 3500 psi. The slab met strength requirements, but the damage was done. Dusting and a soft surface presented problems for the floor finishes. About three-quarters of the slab required grinding and sealing prior to any finishes being installed.

The intent of the specifier was noble, but the end-result was not sustainable. Extra labor, energy, and time went into the project. This would not have been required if a less risky mix was used or proper time and effort into understanding the mix was preformed prior to its use.

Producers should understand the risk involved with these mixes. Although higher SCM content is in the future for the U.S. concrete industry, better understanding of the mixes is required before using them. We can engineer a mix or a family of mixes with the intent to perform in any situation.

Crushed and reused concrete

Recycled aggregate can play an important role in earning LEED credit. Recycled aggregate can be from returned concrete or demolished structures.

Either way, the concrete can be crushed and reused in new concrete production. Again, a producer should not just jump into this. It is important to understand the potential issues with recycled concrete aggregate. Considerable research is ongoing and plenty of data is available through NRMCA.

In developing mixes with recycled content, it is important to understand how the amounts are calculated. Determining which mix proportion has the best bang for the buck can be difficult. However, there is a resource that can help with this.

Holcim (US) Inc. has a program available at its website which will calculate the recycled and regional contents of your concrete mixes. For more information, visit www.holcim.us/calq. This program allows users to enter their costs and mix designs to see how they provide value to the project.

Good luck with new mixes, but keep in mind that we are fortunate concrete is a sustainable construction material already.

Contributed by Braun Intertec. Visitwww.braunintertec.com.