Mix for High Strength Concrete
In order to obtain high strength of concrete, three methods are available in general. The first is to increase the strength of the binder, the second is to select aggregate with high strength, and the third is to improve the bond at the interface of aggregate and binder.
Among them ,the most popularly adopted is the first method. This is because of the fact that the binder strength of concrete in the ordinary strength range is smaller than the strength of aggregate, hence the strength of concrete is dictated by that of binder. The increase of binder strength requires the cement and mineral admixtures suitable for high strength, and reduction of water-cement ratio as the most effective means in terms of mix design. This is a well-known fact by the classical name of water-cement ratio theory. In addition, to maintain workability of concrete within the practical limit without increasing the unit water content while keeping the low water-binder ratio,that is, without increasing the unit binder content, it is necessary to develop chemical admixtures with high capability of dispersing cement and mineral admixtures.
The increase of binder strength naturally results in producing concrete whose strength is strongly affected by the aggregate strength. Hence the selection of aggregate suitable for high strength concrete becomes an important issue.
Finally, it is an established fact that the concrete strength depends microscopically on the structure of the transition zone between aggregate and binder. For the strength improvement of the transition zone,
not only the reduction of water-binder ratio but also the use of mineral admixtures with ultra-fine particle such as silica fume was found to be effective.
Based on these general considerations for high-strength of concrete, the selection of cement, chemical and mineral admixtures, and the selection of aggregate should be such that it is both suitable for high-strength, and achievement on the mix proportioning method of high-strength concrete.