Question: I have heard about the greater influence temperature has on strength development with high volume fly ash concrete, but I can find little in the way of quantifiable data. Is there a greater sensitivity to changes in ambient conditions with HVFA? And is the relationship between test specimens and in situ concrete changed when using HVFA?
Answer: Craig Wallace of Headwaters Resources responded: In concrete with or without fly ash, changes in ambient temperature affect concrete strength gain exponentially, especially as temperatures drop below 70º F. For quantifiable data, you'll probably have to run your own tests on the materials you plan to use. Not all ashes and cements are the same and even if their chemical analyses are similar, it does not mean they will react the same.
Usually the concern with HVFA mixes is initial set times. Generally, the colder the ambient temperature (not below freezing) the slower the strength gain. Accelerating admixtures can offset cold ambient temperatures and slow strength gain, but even chemical admixture reactions are affected by ambient temperature changes.
Regarding in situ versus lab results, yes, there is a difference. Lab results merely measure the potential of a mix design, not the actual strength of the material used in application. The difference between the two will depend on the application's mass, exposed surface area, placement and consolidation techniques, and curing. In-place concrete could have higher or lower compressive strength. Talk with your fly ash supplier or concrete producer about the specifics of your application. I'm willing to bet that people in your area have the ingenious quantifiable data you are looking for.
Kit Badger responded that Concrete International published an article in July 2000 titled “Monolith Foundation: Built to Last 1000 Years,” which describes a concrete mix with 60% fly ash replacement. The article is available to ACI members at www.concreteinterational.com.
Note that CC will have an article in its January 2006 issue on construction using HVFA concrete.