Method of concrete mix design and illustration
Generally, the proportions of ingredients of a concrete mix are indicated with the parts, ratios of cements and coarse aggregates. As for instance, if a mix of concrete is formed with the proportions of 1:2:4, it shows that cement, fine and coarse aggregate are in the ratio 1:2:4. On the contrary it can also be said that the mix consists of one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either developed with volume or mass.
The following considerations should be given for mix design.
. The grade designation providing the characteristic intensity essential for concrete.
. The type of cement affecting the rate of development concerning categorical strength of concrete.
. Utmost nominal size of aggregates for concrete may be considerably large inside the limits adhered to IS 456:2000.
. The cement content should be confined to shrinkage, cracking and creep.
. The applicability of concrete for positioning in an effective manner and compaction is associated with the size and shape of section, quantity and spacing of reinforcement and method applied for transportation, placing and compaction.
Method for Concrete Mix Design - IS456:2000:
1. Ascertain the mean target strength ft out of the designated characteristic compressive strength at 28-day fck and the level of quality control.
ft = fck + 1.65 S
where S denotes the standard deviation captured out of the Table of exact contents provided following the design mix.
2. Get the water cement ratio for the needed mean target through the empirical relationship connecting compressive strength and water cement ratio so selected is verified against the limiting water cement ratio. The selected water cement ratio is verified against the limiting water cement ratio for the essential strength provided in table and approves the lower of the two values.
3. Work out the amount of entrapped air for utmost nominal size of the aggregate from the table.
4. Opt for the water content, for the viability needed and highest size of aggregates (for aggregates in saturated surface dry condition) from table.
5. Find out the percentage of fine aggregate altogether aggregate by absolute volume from table for the concrete with crushed coarse aggregate.
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