WHY ISN’T EVERYONE USING HIGH-VOLUME FLY ASH CONCRETE?
The short answer to this question is simply a lack of knowledge, and outdated codes and standards. Local area engineers and ready-mixed batch plants typically design mixes with a maximum of about 25 percent fly ash, and this is not a 1:1 cement replacement. Much more common is for fly ash to be added to the mix. This makes the mix more workable and reduces cracking but costs more. This has caused most people to equate fly ash in the mix with more expensive concrete. Engineers and architects specify what essentially has existed for many years—a default of no more than 15 to 25 percent fly ash as a total of cementitious content. ACI 318, until recently, allowed a maximum of 25 percent fly ash for concrete. However, the current version of ACI 318 qualifies that the limit of 25 percent fly ash applies only to concrete that will be subject to de-icing salts and sulfates. That leaves most concrete used in buildings open to a much higher percentage of fly ash use. Structural engineers have tr
