High-volume fly ash (HVFA) Concrete

Typically, concrete made with fly ash will be slightly lower in strength than straight cement concrete up to 28 days, equal strength at 28 days, and substantially higher strength within a year’s time. Thus, fly ash concrete achieves significantly higher ultimate strength than can be achieved with conventional concrete.

Fly Ash concrete is more durable than normal concrete made with ordinary Portland Cement.However, present specifications of 28 day strength do not allow taking full advantage of benefits that Fly Ash concrete offers. Developing sustainable concrete structures to last 100 years or more can require extending the common 28-day strength specifications. With extended age strength parameters, better, more durable concrete can be achieved.

It is important to note that extended strength parameters are not suitable for every application. However, if extended age strengths are acceptable, higher percentages of fly ash can be used. With the right expertise, mix designs can be amended to reflect a percentage of strength at designated time-frames with an ultimate strength overall. Utilizing the experience of the local ready mix producer, proper mix designs can be developed to optimize the projects timeline in order to achieve the highest quality concrete for the project.

In normal Fly Ash Concrete, the ash replacement (for Cement) is usually in range of 10-25%. IS 1489 ( Part 1 ) : 1991 (Indian Standard Portland-Pozzolana Cement – Specification : Fly Ash Based) specifies that the fly ash constituent in Portland-pozzolana cement shall not be less than 10 percent and not more than 25 percent by mass of Portland-pozzolana cement.

In High-volume fly ash (HVFA) Concrete, the ash replacement is more than usual, and in many cases, exceeds a 50% weight replacement for cement. So, technically, there is a higher than normal volume of ash within the mix design, and because of the specific gravity difference between cement  and a standard fly ash , the volume of total cementitious material can increase.

HVFA concrete (i.e. High Volume Fly Ash Concrete) can be used in a variety of structural applications. It has been proven as a technology that can accommodate sustainable development and improve the lifespan and performance of structures. With advances in admixture technology and proper proportioning, HVFA mix designs can have characteristics identical to typical concrete.

In 1981, long before using HVFA concrete in structural applications was an accepted practice, architects used concrete with 40% fly ash in the seawater holding tanks in the Monterrey Bay Aquarium (U.S.A) for durability considerations. Since then, concrete throughout U.S.A has regularly utilized 20% to 35% fly ash.

The desire from the environmentally conscious community to mitigate greenhouse gas emissions has resulted in a movement to allow higher fly ash contents than would have been thought possible a few short years ago. In fact, specifiers are now regularly asking for higher fly ash content for performance and environmental reasons. This movement is forcing the concrete industry to research the nature and performance of local materials that allow HVFA concrete to perform as needed.

Performance of HVFA concrete is directly related to performance of local materials. Because of this, proportions of HVFA concrete vary from location to location. As with all concrete utilized in construction, field verification testing and trial placements should be used to make sure HVFA concrete performance meets project needs.

Admixtures: Aggregate proportioning techniques that take advantage of the workability offered by HVFA generate a least voids condition in concrete, which complements the water reducing action of fly ash.

Concurrent with the demand for higher fly ash contents, mid-range and high-range chemical admixtures have been developed to further enhance water-reducing action without causing an unacceptable delay in setting time. Non-chloride set accelerating admixtures have proven useful for controlling set time within contractor requirements for finishing time.

Appropriate Applications: HVFA concrete can be used in a variety of structural applications. Fly ash has not only been used regularly in massive concrete applications, but has been used in columns, shear walls, and floor slabs as well. Even though technology has improved HVFA concrete performance, some mixtures may be inappropriate for certain applications requiring very early age performance. Mixtures with 25% to 55% fly ash may be acceptable, depending upon conditions, for mass concrete and foundation applications.

Where higher percentages of ash are used, the age of strength acceptance may need to be extended to 56 or 90 days. Column and shear wall work can generally accept 30% to 35% fly ash, especially when time of form-stripping is of concern. Higher cement content (20% to 30% depending on circumstances) may be required to provide appropriate finishing time for slab on metal deck exposed to cool temperatures and for achieving strength of 3000 psi (i.e. 21 N/mm2) at three days of age for post-tensioned concrete applications.