Green Concrete
A New Sustainable Concrete Mix

Green concrete mixes made from recycled materials are being used on "green" sustainable construction projects to help owners qualify for LEED credits. Here is one green "unique" concrete mix described in detail.

This new sustainable concrete mix was designed by American Engineering Testing Inc. (AET) and Cemstone Products. The new concrete mix made with mostly recycled materials won the 2010 American Society of Civil Engineers' (ASCE) Charles Pankow Award for Innovation.

green concrete

Designing the Green Concrete Mix

green concrete materials

The new concrete mix was made using crushed recycled concrete aggregate for both the coarse aggregate and sand, recycled concrete washout water, fly ash, slag, admixtures, and only 2 percent cement. That's equivalent to 82 pounds of cement in a 3875 pound cubic yard mix. This concrete mix containing air-entrainment had a water cement ratio of 0.45 and reached a strength of 4000 psi.

The developers, AET and Cemstone, wanted to design a strong concrete mix that was made from as much recycled material as possible. By analyzing the recycled concrete aggregate through a microscope, they learned there is always unhydrated cement available to form cement paste in the new "green" concrete mix.

Knowing this, they decided to include both coarse and fine recycled concrete aggregate to expose more new surfaces for more complete hydration. By using the fines of the crushed concrete aggregate and recycled water, a better microstructure of the cement paste was produced from additional crystallization.

A reduced total cement content was made possible because of the cementitious material already available in the recycled concrete aggregate, this also allowed for more supplementary cementitious material (SCM) like slag and fly ash to be used further reducing the cement content.

Additional lime was created to react with the slag by blending the SCM and cement, increasing strength gain and density of the cement paste.

What was learned from using this "Green Concrete Mix"

  1. Achieving 3000 psi strength at 3 days is possible.
  2. Heat from hydration is relatively low.
  3. Virtually self-consolidating with a w/c ratio of 0.34.
  4. Easier to place with minimal vibratory equipment and fewer workers needed.
  5. Performs better than traditional mixes in reduced shrinkage, density, salt resistance, and resistance to alkali aggregate reactivity.

The need for designing these new "greener" concrete mixes is necessary because finding new virgin aggregate sources, especially close to major cities, is getting more difficult. The cost to transport virgin aggregate is also increasing, doubling every 12 additional miles from the source.

By recycling concrete and producing crushed concrete aggregate closer to where it will be used, the cost of transportation is lower, helping keep the cost of concrete lower. Coming up with new, innovative ways to make a greener concrete will benefit all of us in one way or another.

What is FLY ASH CONCRETE?


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