Q. What are the effects of using fly ash in concrete pavements? Does it have an impact on abrasion resistance of the mix? Are case studies or field data available, especially where more than 30% fly ash was used in the mix?

A. Jerzy Zemajtis, of Construction Technology Laboratories, says an excellent starting point on this topic is "Fly Ash in Concrete," by V.M. Malhotra and A.A. Ramezanianpour, published by the Canadian Center for Mineral and Energy Technology (CANMET) in 1994. For abrasion resistance, the authors refer to a study by R.L. Carasquillo (1987) where a comparison is made between 35% Class C and Class F fly ashes to the control. Performance of the Class F mix was close to the control, but the Class C performed worse. In another study by T.R. Naik et al (1992), researchers found that abrasion resistance of Class C fly ash (using 15-70% fly ash and a w/c of 0.31-0.37) depended on the percentage of fly ash used. For 30% or less, abrasion resistance was comparable to that of the control mix; for 40% or more fly ash, abrasion resistance was lower.

Jennifer Johnson, of ISG Resources, notes that some state governments have established guidelines for fly ash use in pavements to address alkali silica reaction (ASR) and sulfate problems. In Washington, 31% fly ash is used for bridge overlays. Oregon’s spec calls for 30% or more fly ash. The Federal Aviation Administration spec for its northwest region requires 30% or more fly ash.