Concrete mix design
Concrete mix design is the
process of finding the proportions of concrete mix in terms of ratios of
cement, sand and coarse aggregates. For e.g., a concrete mix of proportions
1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or
the mix contains one part of cement, two parts of fine aggregate and four parts
of coarse aggregate. The concrete mix design proportions are either by volume
or by mass. The water-cement ratio is usually expressed in mass
Requirements for concrete mix design:
· The grade designation giving the
characteristic strength requirement of concrete.
· The type of cement influences the rate of
development of compressive strength of concrete.
· Maximum nominal size of aggregates to be
used in concrete may be as large as possible within the limits prescribed by IS
456:2000.
· The cement content is to be limited from
shrinkage, cracking and creep.
· The workability of concrete for satisfactory
placing and compaction is related to the size and shape of section, quantity
and spacing of reinforcement and technique used for transportation, placing and
compaction.
Procedure for Concrete Mix Design as per IS
456 : 2000 :
1. Determine the mean target strength ft from
the specified characteristic compressive strength at 28-day fck and the level
of quality control.
ft = fck + 1.65 S
where S is the standard deviation obtained
from the Table of approximate contents given after the design mix.
2. Obtain the water cement ratio for the
desired mean target using the empirical relationship between compressive
strength and water cement ratio so chosen is checked against the limiting water
cement ratio. The water cement ratio so chosen is checked against the limiting
water cement ratio for the requirements of durability given in table and adopts
the lower of the two values.
3. Estimate the amount of entrapped air for
maximum nominal size of the aggregate from the table.
4. Select the water content, for the required
workability and maximum size of aggregates (for aggregates in saturated surface
dry condition) from table.
5. Determine the percentage of fine aggregate
in total aggregate by absolute volume from table for the concrete using crushed
coarse aggregate.
6. Adjust the values of water content and
percentage of sand as provided in the table for any difference in workability,
water cement ratio, grading of fine aggregate and for rounded aggregate the
values are given in table.
7. Calculate the cement content form the
water-cement ratio and the final water content as arrived after adjustment.
Check the cement against the minimum cement content from the requirements of
the durability, and greater of the two values is adopted.
8. From the quantities of water and cement per
unit volume of concrete and the percentage of sand already determined in steps
6 and 7 above, calculate the content of coarse and fine aggregates per unit
volume of concrete from the following relations:
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