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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.

**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.

Tags : Civil - Construction Materials: Concrete

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