Cement concrete mix design
Cement concrete mix design means, determination
of the proportion of the concrete ingredients
i.e. cement , water, fine aggregate, coarse
aggregate which would produce concrete
possessing specified properties such as
workability, strength and durability with
maximum overall economy.
Design involves determination of the proportions
of the given constituents namely cement, water,
coarse and fine aggregates and admixtures
This would produce concrete possessing specified
properties both in the fresh and hardened states
with the maximum overall economy.
Workability is specified as the important property
of concrete in the fresh state
hardened state compressive strength and
durability are important.
Proportioning of the ingredients of concrete is an
important phase as it ensures the quality.
Design of plastic concrete of medium strength can
be based on following assumptions:
1) Compressive strength is governed by W/C ratio.
2) Workability of concrete is governed by water
Factors influencing the choice
of mix design
Type and grade of cement
Maximum nominal size of coarse aggregates
Grading of combined aggregates
It gives characteristic compressive strength
requirements of the concrete.
It’s the major factor influencing mix design.
Type and grade of cement
Choice of type of cement depends upon the
requirements at hand.
For very high compressive strength portland
cement of grades 43 and 53 used.
Where early strength is required, rapid
hardening portland cement is used.
For mass construction low heat portland
cement is used.
Cement content in concrete varies inversely
with the strength of cement used in
preparation of concrete, i.e. higher the
strength lesser will be the cement content.
Fineness of cement increases workability by
reducing friction between aggregates.
Max nominal size of coarse aggregate
Its determined by sieve analysis.
Max nominal size of aggregate used is
governed by size of the section and the
spacing of the reinforcement.
For the concrete with higher W/C ratio, the
larger max size of aggregate is used
For high strength concrete, 10-20mm size.
Grading of combined aggregate
Grading is one of the important factor
affecting strength of concrete.
The aggregates can be combined by:
The ratio of water-to-cement, or w/c, is the single
most important parameter with regards to concrete
Theoretically, about 0.22 to 0.25 is required for
Practically, the useful limit is around 0.33
Reducing the water for a given amount of cement
will move the cement particles closer together,
which in turn densifies the hydrated cement paste.
This increases strength and reduces permeability
It also makes the concrete more difficult to work
In combination, the w/c and degree of hydration
control many of the properties of the hardened
Steps to be followed in selecting the water cement
The strength of cement to be used is determined.
When cement strength data are available,
corresponding curve is chosen for the
determination of W/C.
The workability of concrete for satisfactory
placing and compaction is controlled by:
Size and shape of section to be concreted
Quantity and spacing of reinforcement
Methods employed for transportation,
placing and compaction of concrete.
Durability of concrete is the resistance to
deteriorating influences which may reside inside
the concrete itself, or to the aggressive
Durability are achieved by restricting the minimum
water cement content and the maximum water
cement ratio and the type of cement.