High-volume fly ash concrete

Concrete Producer, The, Feb, 2003

Q. We're teaming with an architect who is in the early design stage of a project that the owner is considering to submit for a high LEEDS rating. To earn the maximum LEEDS design credit points, it's important to selection construction materials rated as environmentally friendly. To accomplish this, the project designer wants to use concrete with high percentages of fly ash.

Does anyone know of any research information outlining the optimum amount of fly ash in high-volume fly ash concrete? At what quantity does the fly ash become more than just a filler?

A. The use of high-volume fly ash is becoming very common to earn LEEDS credit points. What was once a question of economics is now becoming a tool of an environmentally concerned design community. Sustainability is firmly establishing itself as a key aspect of the green building movement.

So when this question was posed on the Aggregate Research Industry forum, it drew some very interesting responses.

One participant offered insights on defining the term high-volume fly ash mixers. He reported that he discovered a newly published book titled "High-Performance, High-Volume Fly Ash Concrete" written by V.M. Malhotra and P.K. Mehta. In it the authors report the term high-volume fly ash concrete (HVFA) was coined by Malhotra at an international meeting of concrete researchers help in the late 1980s. "High volume fly ash has very low water content and at least 50% of the portland cement by mass is replaced with ASTM Class F fly ash ..."

In regard to the second question of effectiveness, the respondent couldn't find an exact answer in the book. But the authors did provide some advice. The respondent reported quoted from the book, "The pozzolanic properties of a fly ash are not governed so much by the chemistry but by the mineralogy and particle size of the ash."

The respondent reported that the book is available from Supplementary Cementing Materials for Sustainable Development, Ottawa, Canada, for about $40.

Another respondent suggested the producer contact their local fly ash representative about this of replacement. He said to check out the Web sites of the 2 largest sources of fly ash--ISG Resources or Boral--to locate local representatives familiar with characteristics of the locally processed material.

He went to state that his design team has been working with 30-50% replacement values for the past 2 years. "Once you get over the 20% mentality and start performing trial batches with your materials, you will find out the fly ash allows you to further optimize your aggregates as long as is not classified to 3-5 microns," he suggests. "The increased fly ash volumes allow you to drop additional sand out further reducing your water requirement."

Another participant agreed, and offered another insight on the sand volume comment. "Using 3/8-inch materials to benefit your 3/4- and 1-inch typical gap grades also allows the mix designer to further reduce the sand requirement."

When considering using high fly ash mixers, many warned that portland cements also influenced design results. One producer said that he tries to select portland cements that run close to type III in their strength gain properties. He said that he found these cements allow for the release of calcium hydroxide much faster in the hydration process. "You also want to use a low dose of a Type A water reducer to further the initial hydration process in foundations and a midrange at 20-40 ounces per yard at a Type A water reduction level for flatwork to helps increase set times."

Another producer agreed with the idea on the importance of proper admixture selection. "I've tested mix designs for slabs using up to a 65% replacement with fly ash; the chemistry of the admixtures used is most critical." He reports that he's been able to stay within the 75% of standard (100% portland) by incorporating mixing various admixtures. The biggest problem is that the hydration reactions are temperature-dependent, and you will need to adjust the admixtures to ambient and aggregate temperature.

One producer wasn't very interested in the idea of high fly ash replacement. "I have looked at this from an economic standpoint with our materials and have found that somewhere around 15% replacement is the point where it is not cost-effective for the benefit in strength gain. We are basically adding to the cost of the concrete at a given strength to use more the 15% replacement."

Other factors that can be involved can be clays in the fine materials that can react quickly with the calcium hydroxide, eliminating it from reacting with the ash. Trial batches are really the only way to formulate your best mix designs.

An example of "green" design can be found at the University of Texas' School of Nursing facility in Houston. They sought environmentally friendly construction practices for the new building, promoting a clean atmosphere and good health.

They created a special concrete mix that was more favorable to the environment. A project team consisted of the University of Texas Health Science Center, BNIM architects, Jaster/Quintanilla engineers, and Jacobs/Vaughn acting as the construction manager.