Book Description

High performance concrete (HPC) can be defined as the concrete, which meets special performance and uniformity requirements that can't always be achieved by conventional materials, normal mixing, placing and curing practices. The requirements may involve enhancements of characteristics such as placement and compaction without segregation, long-term mechanical properties, early-age strength, volume stability or service live in severe environments. Swamy states that High performance concrete (HPC) is that which is designed to give optimised performance characteristics for the given set of materials, usage and exposure conditions, consistent with requirements of cost, service life and durability. Architects, engineers and constructors all over the world are finding that using HPC allows them to build more durable structures at comparable cost. HPC is being used for buildings in aggressive environments, marine structures, highway bridges and pavements, nuclear structures, tunnels, precast units, etc. The major difference between conventional concrete and HPC is essentially the use of chemical and mineral admixtures. Use of chemical admixtures reduces the water content, thereby reducing the porosity within the hydrated cement paste. Pozzolanic materials are crucial to HPC as far as flowability is concerned. Mineral admixtures, also called as cement replacement materials, act as pozzolanic materials as well as fine fillers, thereby the microstructure of hardened cement matrix becomes denser and stronger. Fly ash has been used in blended cements, and it has been used successfully to replace Portland cement up to 30% by mass as pozzolanic materials in HPC. Because fly ash causes environmental pollution and the cost of storage of fly ash is very high, the utilization of fly ash in concrete technology, both in regard to environmental pollution and the positive effect on a country's economy are beyond dispute. The fly ash HPC system offers a holistic approach that can help us to achieve the goals of meeting the rising demands for concrete, enhancement of concrete durability with little or no increase in cost (in some instances reduced cost), and ecological disposal of large quantities of the solid waste products from coal-fired power plants. Several laboratory and field investigations reported that HPC containing fly ash had exhibited excellent mechanical and durability properties. Despite the benefits of fly ash, practical problems remain in field application. At early stages of aging, the strength of HPC containing a high volume of fly ash as a partial cement replacement is much lower than that of control concrete, due to the slow process of the pozzolanic reaction of fly ash, and its contribution towards the strength development occurs only at later ages. Different approaches have been used to accelerate the pozzolanic reaction of fly ash and, thereby, to increase the early strength of HPC containing fly ash. The activators have been used to improve the unsatisfactory early properties of Portland cement paste with fly ash added as a cement replacement.