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Basics | The High-Volume Fly Ash Concrete

The High-Volume Fly Ash Concrete

The high-volume fly ash concrete is one specific type of fly ash concrete with higher fly ash contents, lower water-to-cementitious materials ratio (W/CM), and lower cement contents. This is to take full advantages of the increased workability and durability provided by fly ash and the low W/CM, and to produce a more environmentally friendly concrete by reducing its cement content.

The main difference between the High-Volume Fly Ash Concrete (HVFAC) and the usual fly ash concrete is that in the former concrete, the amount of ordinary Portland cement is minimized through proper mixture proportioning using large amounts of fly ash and judicious selection of materials and chemical admixtures while maintaining, and often improving its performance as compared to conventional concrete. There is no fixed percentage of ordinary portland cement replacement by fly ash in this type of concrete, but in many cases, percentages of 50 to 55% were found to be achievable. To obtain the superior performance of this type of concrete, it is recommended that the W/CM of the HVFAC be kept well below 0.40 and, preferably of the order of 0.35 or less. To produce a workable concrete at such low W/CM, the use of superplasticizer is most of the time, essential.

The HVFAC, defined as a concrete with more than 50% fly ash and low W/CM, was developed at CANMET, Natural Resources Canada in the mid 80s under the guidance of Dr. V.M. Malhotra, and has been studied extensively by the same organization for about 15 years. The purpose of the early development of the HVFAC was to design concrete for a massive application that required very low heat of hydration in order to minimise the thermal stress, and relatively high compressive strength and modulus of elasticity. The concrete element was made of the newly developed HVFAC, and its performance exceeded expectations. It appeared then that the concept of HVFAC could be used for a large number of applications. Following this, extensive investigation programs were undertaken to determine the mechanical and durability properties of the HVFAC made with cement and fly ash from numerous sources and covering a wide range of physical properties and chemical compositions. In parallel to the laboratory investigations, the HVFAC has been used in a number of structures in Eastern Canada since the late 80's. It was found from those investigations and field applications that properly designed and cured, HVFAC can demonstrate excellent performance, both in mechanical as well as in durability aspects, and this when it is made with a wide range of materials.

More recently, this technology has been used in Western Canada (British Columbia and Alberta) and Toronto, where case studies were documented, and which again, demonstrated the potential for a greater use of this type of concrete. However, there are limitations to its use in Canada related to the climatic conditions. Cold weather conditions in late autumn, winter and early spring in Canada limit the percentage of fly ash that can be used in concrete due to potential retardation in setting and slow strength development. Also, the HVFAC is not suitable for some applications that are subjected to high levels of deicing salts such as sidewalks during winter in northern countries.

In principle, it should be easier to apply the HVFAC technology in India than it is in Canada due to the favourable climatic conditions. The hot, humid climatic condition in India should be ideal for HVFAC. Literature review and experimental studies performed in this project have also indicated that there are plenty of sources of good-quality fly ash in India that could be used for HVFAC. However, in any cases, the project includes activities for adapting the technology to the Indian context.

More information about the HVFAC (mixture design, properties, concrete practice) is given in the “Technical Document” and “Publications” section.

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