What is the highest percentage of fly ash that can be used in ready-mixed concrete? Is durability the most important issue in determining the limit?
The answers to this question recently posted in the Aggregate Research Industries (ARI) Industry Forum ( www.aggregateresearch.com/forum) show just how difficult arriving at an answer can be because it really depends, in part, on the application for which the concrete is designed.

One forum participant related that his company replaced 50% of the portland cement with fly ash in a mix that was being used in the substructure of a large bridge project. Given the fact that the concrete contractor planned to place 2000-4000 cubic yards in each pour, the DOT engineers were concerned about thermal cracking.

A precast producer located on the East Coast reported that he has used a Class N pozzolan called XPM, which is produced by Lehigh Cement. XPM is like fly ash but reduces the air-entrainment requirement and yields better finishability. It appears that this pozzolan is available only on the East Coast. He wrote: "I've used the product for up to a 40% replacement of cement when using a Type III portland cement." He reports that permeability test results using ASTM C 1202 were great, yielding readings of less than 1000. When the producer tested for specimen length change using ASTM C 157, the results were -0.0420.But the real answer to the question of maximum replacement must be predicated on many factors, including the intended application, constructability, service condition, and exposure condition.One contributor indicated that on the factors related to durability-or more specifically, deicer exposure both ACI 318-99 and the International Building Code limit the maximum fly ash content to 25% if the concrete will be exposed to deicing chemicals. This restriction applies only to deicer exposure.

The respondent wrote: "I do not necessarily agree with the limitations because they do not take into account the base cement content, total cementitious content, or water-cement ratio. Higher percentages, either as a cement replacement or as an addition, could have other positive benefits."

A pavement engineer at the Naval Facilities Engineering Service Center, which has offices in Port Hueneme, Calif., and Washington, D.C., wrote that the center just completed a report on state-of-the-art practices regarding alkali-silica reactions (ASR) and fly ash while it has a high-volume fly ash project underway. He indicated that as a result of the report, the design engineers changed the concrete pavement specification to require 25%-35% Type F or N fly ash (or 40%-50% ground granulated blast-furnace slag) in all concrete for pavements, and they anticipate requiring the same percentages for all other concrete. "We have added this special chemical requirement for the fly ash with the goal of using higher volume," he wrote.

He indicated that the benefits should be threefold: reduced damage potential from ASR, increased pavement durability, and the public-relations benefits of using a waste product, which helps the environment.