Concrete: Mix Design
Ø Mix design is the process of selecting suitable ingredients of concrete & determining their relative quantities with the objective of producing as economically as possible concrete of certain minimum properties such as workability, strength & durability.
Ø So, basic considerations in a mix design is cost & min. properties.
The Principles of Proportioning: The fundamental object in proportioning concrete or mortar mixes is the production of a durable material of requisite strength, watertightness, and other essential properties at minimum cost. To achieve these objectives, careful attention must be given to the selection of cement, aggregate, and water to the following considerations:
1. The mix must be workable so that it can be placed and finished without undue labour.
2. Since cement is the most costly ingredient in the mix, the proportion used should be as small as is consistent with the attainment of desired properties. Within wide limits, experiments have shown:
(a) The strength and degree of watertightness of mixes, having like constituent materials, density, and workability, increase with the cement content.
(b) With the cement content, materials, and workability all constant, the strength and
degree of watertightness increase with the density of the mix.
(c) For usual methods of placement, the strength and degree of watertightness of well-cured concrete and mortar are greatest when the mix is plastic (has a slump of approximately 50 mm). Drier mixes, although frequently as strong, are likely to be porous unless compacted by pneumatic rammers or electrically driven vibrators. Increasing the water content beyond that required for plasticity causes the strength to decrease rapidly.
(d) Concrete with 4-7 per cent, by volume, entrained air made by using an air-entraining cement or by adding air-entraining admixtures is more resistant to freezing and thawing action and also to scaling due to the use of salt for ice removal than concrete made with regular cement and without air-entraining admixtures.
In addition to the above, the following statements appear to be justified by the results of experience and tests:
To proportion concrete for the maximum resistance to fire, a porous non-combustible aggregate of high specific heat together with cement sufficient to provide the requisite strength should be thoroughly mixed and placed with as little ramming as possible to produce a porous concrete.
(f) In proportioning concrete or mortar which is to be subjected to freezing temperatures shortly after placement, a minimum amount of water and a quick-setting cement should be used.
(g) Concrete for road construction should be made from a carefully graded, hard tough aggregate bound together with as small a proportion of rich mortar as is consistent
with the required workability, strength, and imperviousness. In locations where resistance to freezing and thawing is required, the concrete should have 3-6 per cent of entrained air. The principal methods used in scientific proportioning of mixes are based upon relationships between properties and ratio of cement to voids in the mix, or on the relationship between properties and the ratio of water to cement in the mix.
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