High strength concrete offers many advantages in the construction of high-rise buildings and bridges.
In many countries, concrete strengths higher than 40 MPa are widely produced on commercial basis using normal aggregates and cement.
Use of chemical and mineral admixtures has become an integral part of producing high strength concretes.
Mix design of high strength concrete is influenced by properties of cement, sand aggregates & water-cement ratio have compressive strength above 40 MPa.
To achieve high strength, it is necessary to use lowest possible water-cement ratio, which invariably affects the workability of the mix and necessitates the use of special vibration techniques for proper compaction.
In the present state of art, a concrete with a desired 28 day compressive strength of up to 70 MPa can be made with suitably proportioning the ingredients using normal vibration techniques for compacting the concrete mix.
The trial mixture approach is best for selecting proportions for high-strength concrete. To obtain high strength, it is necessary to use a low water to cementing materials ratio and a high Portland cement content.
The unit strength obtained for each unit of cement used in a cubic meter of concrete can be plotted as strength efficiency to assist with mix designs.
The water requirement of concrete increases as the fine aggregate content is increased for any given size of coarse aggregate. Because of the high cementing materials content of these concretes, the fine aggregate content can be kept low.
However, even with well-graded aggregates, a low water-cementing materials ratio may result in concrete that is not sufficiently workable for the job.
If a superplasticizer is not already being used, this may be the time to consider one. A slump of around 200 mm (8 in.) will provide adequate workability for most applications.
ACI Committee 211 (1998) provides additional guidance on proportioning.
At present, normal strength concrete is used in reinforced concrete construction.
As the current trend for high rise buildings and the use of precast concrete is expected to increase, there will be a greater demand for high strength concrete.
Research has been done on various approaches for developing high strength concrete and to achieve concrete strengths higher than 40 MPa using only locally available materials.
High range water reducing admixture is needed to reduce water/cement ratio of the concrete mixes while achieving sufficient workability.
Compressive strengths up to 65 MPa were achieved using locally available cement, sand and coarse aggregates with moderate dosages of water reducing admixture.
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