American Coal Ash Association*
Fly ash is the fine material that results from the combustion of pulverized coal in a coal-fired power plant and is captured by electrostatic precipitators. Fly ash in concrete is frequently viewed as just a way to reduce the concrete material costs. However, the use of fly ash can improve concrete properties in many ways.
Improved Early Age Properties
The spherical particles of fly ash contribute to improved concrete workability and improved packing. Consequently, the use of fly ash as a cement replacement almost always reduces the water demand for a given concrete slump. The use of fly ash increases the amount of fines in the concrete. This helps to increase concrete’s cohesiveness, improve pumpability, reduce segregation, and improve finishability. Cement replacement with fly ash reduces the build-up of heat in larger sections, thus reducing peak temperatures and the potential for thermal cracking.
Improved Hardened Concrete Properties
Fly ash improves the properties of hardened concrete by virtue of its pozzolanic reaction. The use of fly ash reduces permeability, chloride diffusivity, and increases resistivity of concrete thus making fly ash an essential component of concrete to protect reinforcement from corrosion. Fly ash also binds the cement alkalis, which helps concrete achieve excellent resistance to alkali silica reaction (ASR). Class F fly ash also provides increased resistance to concrete sulfate attack and contributes to the long-term strength gain of concrete.
Mix Design Considerations
Fly ash concrete can be designed to achieve a wide range of strengths at various ages. Some fly ashes may delay the concrete setting time. This can be offset, if necessary, by the use of an accelerating chemical admixture. Certain fly ashes, due to high amounts of un-burnt carbon may require a higher dosage of the air-entraining admixture to achieve a particular air content.
Ultra Fine Fly Ash
Ultra fine fly ash is an extremely fine material that has high pozzolanic reactivity. Its use in concrete increases concrete compressive strengths, reduces the rate of chloride diffusion, and lowers rapid chloride permeability values.(1)
High Volume Fly Ash Concrete
Fly ash is typically used in the range of 15 to 25 percent of the total cementitious materials content. High volume fly ash concretes, where the fly ash content is over 50 percent of the total cementitious materials, have been studied over the last two decades.(2) Adequate early strengths and setting times can be obtained by targeting a low water-cementitious materials ratio through the use of a HRWR. A number of structures have been built using high volume fly ash concretes.
(Photograph Courtesy of TXDOT)
Applications
Fly ash is frequently used in concrete for bridge decks to reduce the permeability of the concrete to penetration by chlorides. An FHWA compilation of HPC bridges lists information about materials used in 13 cast-in-place bridge decks.(3) Eight of the decks had fly ash in the concrete. For bridge deck overlays, the Washington State Department of Transportation specifies a concrete mix containing 31 percent fly ash and a total cementitious materials content of 885 lb/cu yd (525 kg/cu m).(4) For precast, prestressed concrete girders, fly ash is used to enhance the later age strength gains. In the San Angelo, TX, HPC bridge, fly ash at 32 percent of the total cementitious materials content was used in the girders to achieve a concrete compressive strength of 14,500 psi (100 MPa) at 28 days. In mass concrete bridge foundations and piers, fly ash is frequently used to reduce the heat of hydration while ensuring a low permeability concrete.
Further Information
Further information on the use of fly ash in HPC can be obtained from the American Coal Ash Association at 315-428-2400 or [email protected]
References
- Obla, K. H., Hill, R., Thomas, M. D. A., and Hooton, R. D., “Durability of Concrete Containing Fine Pozzolan,”Symposium Proceedings, PCI/FHWA/FIB International Symposium on High Performance Concrete, Orlando, FL, Precast/Prestressed Concrete Institute, Chicago, IL, 2000, pp. 125-134.
- Sivasundaram, V., Carette, G. G., and Malhotra, V. M., “Properties of Concrete Incorporating Low Quantity of Cement and High Volumes of Low-Calcium Fly Ash,” Proceedings of Third International Conference on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Publication SP-114, America Concrete Institute, Farmington Hills, MI, 1989, pp. 45-71.
- High Performance Concrete, Compact Disc, Federal Highway Administration, August 2001. (Available for downloading at www.nationalconcretebridge.org/hpconcrete.html).
- Halverson, R. R., “High Volume Fly Ash Mixes for Bridge Overlays,” Concrete Producer, January 1998, Volume 16, Number 1, pp, 27-31.
Editor’s Note
This article is the fifth in a series hat addresses the benefits of specific materials used in HPC. The benefits of silica fume, lightweight aggregate, different cements, and slag cement were discussed in previous issues of HPC Bridge Views.
*Written for the American Coal Ash Association by Karthik H. Obla of Boral Materials Technologies and Richard R. Halverson of ISG Resources with contributions from the Canadian Industries Recycling Coal Ash Association.