By Nick Lewis, LEED AP
The USGBC LEED rating system not only recognizes the amount of recycled content in concrete, it also recognizes pioneering and exemplary construction techniques through its four Innovation in Design credits. Reducing the use of portland cement by employing off-white rice hull ash as a pozzolanic material is just the type of innovation the USGBC is looking to encourage. While off-white rice hull ash is still in the laboratory phase, regular rice hull ash is available. With this in mind, specifiers could consider a combination of rice hull ash and coal fly ash to obtain even greater cement reductions in the precast manufacturers concrete mixes. The USGBC has previously approved an Innovation in Design Credit for a 40 percent reduction of CO2 by weight for all concrete on a project against standard baseline mixes. This credit would be in addition to the Material and Resources credits for recycled content.
The addition of rice hull ash will also increase the workability and durability of the concrete while permitting a high percentage of recycled content. Rice hull ash is classified as a Class N Natural Pozzolan under ASTM C 618. Rice hull ash is still not widely available on the market and, like coal fly ash, depends largely on the proximity to the plants producing the byproduct. Check with local suppliers to verify availability.
Project Highlight: The Environmental Technology Center (ETC) of Sonoma State University successfully used a concrete mix that included 8 percent rice hull ash and 42 percent coal fly ash that obtained compressive strengths of 3,000 psi at 56 days and 8,000 psi at one year, according to the U.S. Department of Energy. These results were achieved through the combination of proper planning, coordination and collaboration. Consider proposing a combination of rice hull ash and coal fly ash for your project’s concrete at your next LEED design charrette.
Nick Lewis, LEED AP, was formerly the director of sustainability for the National Precast Concrete Association.