According to Indian Standard: 456-2000 and Indian Standard 1343-1980, the most important Factors affecting Concrete Mix Design are:
- Grade of Concrete
- Type of Cement
- Maximum nominal size of aggregate
- Grading of combined aggregate
- Maximum water/cement ratio
- Quality Control
#1 Grade of concrete
- Concrete Mix is categorised by its grade which indicates it’s comprehensive strength at a 28 day
- It’s the most important factor which highly influencing the mix design of concrete
- The grade M25 denotes crushing strength (fck) of 25 N/mm^2
- Depending upon the degree of quality control, materials, the technology available at the site, the mix design of concrete has to be designed for a target mean comprehensive strength (fck) with applying suitable standard deviation
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#2 Type of cement
- Category of cement highly affected the rate of development of compressive/crushing strength of concrete.
- If you use high strength cement than low cement content needed.
- The use of 43 grade of cement and 53 grade of cement, provide savings in the use of cement-like as 15 per cent and 25 per cent respectively, as compared to 33 grade of cement.
- It is suitable to use 43 or 53 grades of cement of reputed brands because the good fineness of 43 and 53-grade cement help in increases the Workability of the concrete mix.
#3 Maximum nominal size of aggregate
- A sieve analysis test is carried out for determining the maximum size of C.A. because reinforcement spacing control by aggregate size which alternatively control the size of the structural member.
- IS 456:2000 and IS 1343:1980 provided detail guidelines about the nominal size of aggregate.
- The maximum nominal size of coarse aggregate should not be greater than one-fourth of the minimum thickness of the element.
- For heavy reinforced concrete element members in the case of the rib of the main element, the nominal maximum size of the coarse aggregate should generally be limited to less than the minimum clear distance between the main reinforcement bars or 5 mm less than the minimum cover. For steel reinforcement, whichever is smaller.
- The workability of mixing concrete increases with increasing the maximum size of aggregate but the smaller size of the aggregate offers a more surface area that is much more responsible for the bonding with the mortar matrix which gives higher strength.
#4 Grading of combined aggregate
- In a concrete mix design, the relative proportions of the fine aggregate and the coarse aggregate are among the most important factors that greatly influence the strength of concrete.
- It is necessary that the aggregates (fine aggregate and coarse aggregate) are well graded.
- In the case when aggregates taken from natural sources do not conform to the specified grading in such a situation, a ratio of more than two aggregates becomes necessary.
- Their ratio should be such that the combination provides specified grading.
#5 Maximum water/cement ratio
- Abram’s water/cement ratio law states that for any test condition, the strength of green concrete depends only on the w / c ratio. The lower the water/cement ratio, the higher the compressive strength and vice versa.
- The workability of green concrete determines the ease with which the concrete mixture can be mixed, transporting, moulding, condensing, and separating and finishing capacity without segregation and bleeding.
- A concrete mix that is hard to place and compaction will not only increase the cost of handling but will also have poor strength and durability.
- The cost of concrete not only increase if it is hard to place and compaction but will also result in poor strength and durability.
- On the other hand, a concrete mixture that has separation and bleeding are more expensive to repair and will produce less durable concrete
- Thus, workability can affect both the quality and the cost of concrete.
- The ability of concrete to withstand adverse environmental conditions is called concrete durability.
- The durability of concrete required a lower water/cement ratio.
- Generally, it is achieved not by increasing the cement quantity, but by lowering the water quantity at a given cement content.
- You can control water demand by taking care of aggregate grading and also by using water-reducing admixtures.
- This reduces the chances of cracking due to high heat of hydration and drying shrinkage.
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