Concrete applications with Class F fly ash content in excess of 25 percent have emerged to meet the demands of a rapidly growing design and construction community, especially in the Western United States. What began as an effort to create low permeability concrete with early strengths for specialty road work now offers such performance criteria as low shrinkage and architectural finish.

The term "high-volume fly ash" as used here indicates fly ash percentages greater than 25 percent, reflecting those applications where cement replacement is not the only goal and, in fact, plays little or no part in the combination of materials. The end result is a large absolute volume of fly ash per yard of concrete in addition to a dosage greater than 25 percent of total cementitious material. Put it another way, the philosophy "it's difficult to have too much fly ash in concrete, but you can have too little cement" has been applied to meet project needs.

The combination of both Class F fly ash and water/cementitious material ratio in the 30 percent to 35 percent range is providing concrete that meets the engineer's specified low coulomb and 28-day shrinkage values, plus the contractor's need for high early strengths. More importantly, workability - hence, reduced effort to place, consolidate and finish - and economy accompany the specified performance criteria. While not always the least costly option available per delivered yard, high volume fly ash is becoming the engineer and contractor's choice for many higher performance, high profile projects.

Washington State DOT Ash Modified Overlay The development and application in 1995 of an Ash Modified Overlay, with 31 percent Class F fly ash as an alternative to silica fume and latex overlays, opened the door to promoting in the commercial sector the performance of concrete with fly ash contents greater than 25 percent. To understand the commercial acceptance of high volume ash mixes, a review of the performance targets and overlay results is in order. The Ash Modified Overlay mix has four primary performance targets:

1. Field strength of 3,000 psi in four days.

2. Rapid chloride permeability value of 750 coulombs or less at 150 days, per AASHTO T 277-93.

3. Placeability in the field from 1.25 in. to 1.5 in. thick.

4. Utilization of the existing preparation, placing and curing practice for silica fume overlays to minimize re-education and equipping of the contractor's labor force.

Measuring performance The initial field installation using the Washington State Department of Transportation (WDOT) Ash Modified Overlay occurred on the State Highway 9 overpass of the NPRR Overcrossing. All sample preparation and testing was performed by WDOT, with specimens obtained at the job site placed in an insulated cure box for the first 24 hours. The samples were then taken to the regional office and moist cured. The results in Tables 1 and 2 (page 65) show that field specimens attained the early-specified strength and low coulomb results required for the application.

Since the initial 1995 project, at least six other bridge decks have been overlaid with this mix and performed as WDOT officials expected. Unlike previous alternatives, the Ash Modified Overlay to date has not exhibited surface cracking and delamination from the deck. The repeated choice of Ash Modified Overlay by the same contractor on a number of projects is the best indicator of the material's superior field performance compared to that of silica fume or latex-based alternatives.

Retarder and slump control Since field observations indicated that retarder may not be required, only the NPRR Overcrossing utilized a small amount of the agent in the overlay mix. Subsequent bridge deck mixes have been placed without retarders. While not actively tested, the retention of slump in the absence of chemical retarders has been observed and planned for in all forthcoming applications. Ash Modified Overlay mixes' slump has been maintained for over 60 minutes without the use of retarder or added water despite the high cement content, low w/c ratio, and use of a mid-range or high-range water reducer.