High performance terminology is a new coined term. A concrete mix which possesses the high workability, high strength, high modulus of elasticity, high density, low permeability and low resistance to chemical attack and high dimensional stability is called high performance concrete.
In normal concrete, relatively low strength and elastic modulus are due to the high heterogeneous nature of the structure of the material, particularly the porous and weak transition zone, which exists at the cement paste aggregate interface. By densifying and strengthening the transition zone, many desirable properties can be improved many fold.
A substantial reduction of the quantity of mixing water is the fundamental step for making high performance concrete (HPC). The reduction in water/cement ratio will result in high strength of concrete. But the reduction in water/cement ratio to less than 0.3, will greatly improve the qualities of transition zone to give inherent qualities expected of HPC.
To improve the qualities of transition zone, the use of silica fume has been found necessary. For the strength of concrete above 80 MPa (800 kg/cm2) the use of silica fume as one of the ingredients has become necessary. The best quality fly ash may be used for other nominal benefits. Though these pozzolanic materials need more water, but their benefits will be more than their disadvantages. Thus the crux of the whole problem lies in using very low water/cement ratio consistent with high workability at the time of placing and compacting. NEVILE has suggested that the lowest water/cement ratio that could be used is 0.22.
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Adoption of water/cement ratio in the range of 0.25 to 0.3 and getting high slump is possible only by the use of super-plasticizers. Thus in the production of high performance concrete (HPC) the use of appropriate super plasticizer is essential. The super plasticizer and cement should be compatible and should retain the slump and other properties for sufficiently long time till the placement and compaction of the concrete is over.
Aggregate for HPC:
In normal concrete, the strength of aggregate particles usually is more than the strength of cement paste and thus the strength of aggregate plays a minor role. Any aggregate available at site may be used with little modification in its grading. However in case of High performance concrete the situation is altogether different. The bond between the hydrated cement paste and aggregate is so strong that significant stress transfer takes place across the transition zone.
Due to the low water/cement ratio, the strength of cement paste is very high and sometimes even higher than the strength of the aggregate particles. The fractured surface of the HPC has shown that very often the fracture takes place through the aggregate pieces rather than cement paste, where as in normal concrete fracture takes place at the interface of cement paste and aggregate piece i.e. it fails in shear. Thus in many cases the compressive strength of the high performance concrete is limited on the basis of strength of aggregate particles.
Size of the Aggregate for HPC:
On the basis of practical experience it has been found that for a strength upto 100 MPa the maximum size of aggregate may be 20 mm. For concrete higher than 100 MPa the size of coarse aggregate should be limited to 10 to 12 mm only.
Shape of Aggregate:
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Though crushed aggregate may be used but the aggregate should be cubic in shape, the flaky or elongated particles should be minimum.
In India High performance concrete of 60 MPa strength was used for the 1st time for the construction of containment dome at Kaiga and Rajasthan Atomic power projects.