Recently a new term has come in the field of concrete technology “High Performance Concrete” or HPC. The properties of HPC are
- High Workability
- High Strength
- High Modulus of Elasticity
- High Density
- High Dimensional Stability
- Low Permeability and
- Resistance to Chemical Attack
There is a little controversy between the terms high-strength and high performance concrete. High-performance concrete is also, a high-strength concrete but it has a few more attributes specifically designed as mentioned above. It is, therefore, logical to describe by the more widely embracing term “High Performance Concrete” (HPC).
In normal concrete, relatively low strength and elastic modulus are the result of high heterogeneous nature of structure of the material, particularly the porous and weak transition zone, which exists at the cement paste-aggregate interface. By densification and strengthening of the transition zone, many desirable properties can be improved many fold. A substantial reduction of quantity of mixing water is the fundamental step for making HPC. With reduction of w/c ratio strength concrete will increase. But reduction in w/c ratio to less than 0.3 will greatly improve the qualities of transition zone to give inherent qualities expected in HPC.
Use of silica fume is also found to be necessary to improve the qualities of transition zone.Silica fumes becomes a necessary ingredient for strength above to 80 MPa. The best quality fly ash and GGBS may be used for other nominal benefits. Inspite of the fact that these pozzolanic materials increase the water demand, their benefits will out weigh the disadvantages. The crux of whole problem lies in using very low w/c ratio, consistant with high workability at the time of placing and compacting. Neville opines that the lowest w/c ratio that could be used is 0.227.9
Only with the use of superplasticizer, w/c ratio in the range of 0.25 to 0.3 can be adopted and a high slump is possible to achieve. Therefore, use of appropriate superplasticizer is a key material in making HPC. The associated problem is the selection of superplasticizer and that of cement so that they are compatible and retain the slump and rheological properties for a sufficiently long time till concrete is placed and compacted.
