Recommended criteria to be concluded before designing concrete mix as per indian guidelines

In this article, an overview of concrete mix designing is based on various criteria and  must compulsory concluded finally as per indian provision code IS 456-2000

Nowadays concrete is produced in batch mixing plants located either at site of construction or away from the site in a location from where concrete is carried in transit mixers to the site. The later one is commonly called Ready Mix Concrete (RMC).

The proportion of various ingredients of concrete made in batch mixing plants mentioned above is usually determined in laboratory. This process is called designing (proportioning) of concrete mix and such a concrete is called design mix concrete. The designing process is a trial and error method in which right proportion of ingredients is sought to be determined so as to achieve targeted mean strength

Factors influence mix design is,

• To achieve minimum characteristic of compressive strength of concrete
• To maintain workability required throughout final placing of concrete 
• Selection of water content is based on environment factors
• Zone/Grading of coarse aggregate (crushed stone) and fine aggregate (sand)

Water Cement (W/c) Ratio

• It's ratio of the volume of water to the volume of cement used in a concrete mix 
• It governs the strength and durability of concrete 
• From graph,Strength is inversely proportional to W/C ratio 
• A required W/C ratio of 0.38 to complete hydration of cement 
• Every extra liter of water will approx. reduce design compressive strength of concrete by 2 to 3 N/mm2 and increase the workability by 25 mm of slump test 
• if water is used more, it may develop larger voids and honey-combing in the set concrete, in this way decreasing its density, durability, and strength, more permeable 
• Durability increases the less permeable of concrete set 
• Water permeability increases exponentially when concrete has a W/C ratio greater than .50 
• IS 456-2000 has restricted the maximum w/cement ratios for durability considerations under given table below

Table: Minimum Cement Content, Maximum Water-Cement Ratio and Minimum Grade of Concrete for Different Exposures with Normal Weight Aggregates of 20 mm Nomination maximum Size

Exposure	Plain Concrete			Reinforced Concrete
	Minimum cement content Kg/cum	Maximum Free Water-Cement Ratio	Minimum Grade of Concrete	Minimum Cement content Kg/cum	Maximum Free Water -Cement Ratio	Minimum Grade of Concrete
Mild	220	0.60	-	300	0.55	M 20
Moderate	240	0.60	M 15	300	0.50	M 25
Severe	250	0.50	M 20	320	0.45	M 30
Very Severe	260	0.45	M 20	340	0.45	M 35
Extreme	280	0.40	M 25	360	0.40	M 40

Cement Content

• If cement content is required to increase the workability of concrete mix for a given W/C ratio then compressive strength may increase with richness of mix.
• The workability of concrete is commonly measured by slump Cone Test(on right image)
• For pumped concrete, it is essential to have high workability to transfer concrete to greater heights with ease. This case also should be considered in the mix design.
• However, for a particular W/C ratio there would always an optimum cement content resulting in 28-compressive strength being the highest
• Increasing the cement content above the optimum value may not increase the strength of concrete specially for mixes with low W/C ratio and large maximum size aggregate.
• Minimum cement content required in cement concrete to ensure durability under specified condition of exposure is considered from above table 
• The cement content has to be reduced or increased as the nominal maximum size of aggregate increases or decreases, respectively






• Example:The minimum cement content is based on 20 mm aggregate from above table but for 40 mm aggregate, it should be reduced by about 10 percent of cement content; for 12.5mm aggregate, it should be increased by about 10 percent

Grading of Aggregate

• Aggregate are the important constituent in concrete. Aggregate are granular material,derived from the most part from the natural rocks, crushed stones, or natural gravels and sands
• Aggregate generally occupy about 70% to 80% of the volume of concrete and can therefore be expected to have an important influence on its property

Classification based on size

• Coarse Aggregate: aggregate which retained on the 4.75 mm sieve. The function of the coarse aggregate is to act as the main load-bearing component of the concrete
• Fine aggregate: Aggregate which passing 4.75 mm sieve and predominately retained on 75μ sieve. The fine aggregate serve the purpose of filling all the open space in between the coarse aggregate

Fineness Modulus

• Empirical factor called the fineness modulus is often used as an index of the fineness of aggregate
• The fineness modulus is computed from screen analysis data by adding the cumulative percentages of aggregate retained on each of a specified series of sieves, and dividing the sum by 100. The sieve used for determining the fineness modulus are : No.100 (150 μm), No.50 (300 μm), No.30 (600 μm), No.16 (1.18 mm), No.8 (2.36 mm), No.4 (4.75 mm), 10 mm, 20 mm, 40 mm etc.

Limits of Fineness modulus

• For all in aggregates or combined aggregates fineness modulus varies from 3.5 to 6.5
• Fineness modulus of coarse aggregate varies from 5.5 to 8.0
• Fineness modulus of fine aggregate varies from 2.0 to 3.5 
• Fine aggregate having fineness modulus more than 3.2 should not considered as fine aggregate