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High strength concrete

High strength concrete (HSC) may be defined as concrete with a specified characteristic cube strength between 60 and 100 N/mm2, although higher strengths have been achieved and used. Strength levels of 80 to 100 N/mm2 and even higher are being used for both precast and in-situ work in the USA, France, Norway and some other countries. The main applications for HSC in-situ concrete construction are in offshore structures, columns for tall buildings, long-span bridges and other highway structures. The main advantage is the reduction in size of compression elements and/or the amount of longitudinal reinforcement required.

The methods and technology for producing high strength concrete are not substantially different from those required for normal strength concrete. The target water/cement ratio should be in the range 0.30–0.35 or even lower. HSC can be produced with all of the cements and cement replacements (additions) normally available in the UK. A wide range of aggregates can be used though crushed rock aggregates (of suitably high crushing value) are preferable.

Superplasticisers / high range water reducers should be used to achieve maximum water reduction, although plasticisers may be adequate for lower strength HSC (C60 to C70). Silica fume (microsilica) or metakaoline can be used to enhance the strength at high levels (C80 and above), but is not needed generally at the lower end (C60 to C80).

The terms High performance concrete and High strength concrete are often taken to mean the same thing. However, as indicated, High performance strictly relates to a concrete that has been designed to have good specific characteristics, such as high resistance to chloride ingress or high abrasion resistance. As a result it may also have a high strength, but this is not the main consideration.

Acknowledgement: The Concrete Society

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TR49 Design guidance for high strength concrete