Short Notes on Concrete Mix Design

CONCRETE MIX DESIGN:- The concrete mix design is a process of selecting suitable ingredients for concrete and determining their proportions which would produce, as economically as possible, a concrete mix of necessary workability in plastic stage and provide required strength and durability in hardened stage.

Cement , Sand, Coarse aggregate are mix in a certain proportion which can be 1:4:8, 1:3:6, 1:2:4, 1:1.5:3 i.e. M-10, M-15, M-20 grade of concrete such mixes are known as nominal mix and prepare by volume or weight batching. However if grade of concrete exceed M-20 than proportion of ingredients design in lab than implicated in filled. Such higher grads of concrete are complemented by inclusion of admixture so as to achieve higher compressive strength. Proportion of such admixture are essential in mix design to get more workability with less water cement ratio. Design mix is adopted in important nature of works and a weigh batcher is also must at site to mix proper weight of ingredients. 

FACTORS INFLUENCING THE CHOICE OF MIX PROPORTIONS:-

i. Grade Designation

ii. Type of Cement

iii. Maximum nominal size of aggregate

iv. Grading of combined aggregate

v. Water cement ratio

vi. Workability required

vii. Durability

viii. Quality control at site

MIX DESIGN BY INDIAN STANDARD RECOMMENDED GUIDELINES:- The procedure adopted for the selection of mix proportions is as follows:-

i. The Target Mean Strength (ft) is determined by using the relation Where fck is the characteristic compressive strength of concrete at 28 days. ‘S’ is the standard deviation ( the value of ‘s’ varies with the degree of quality control and grade of concrete)

ii. The water cement ratio for the target mean strength is determined from the standard curves released by IRC (IRC:- 44 -1972). The water cement ratio so choosen is compared with the maximum water-cement ratio specified for durability and the lower of the two values is used. 

iii. The degree of workability required in terms of slump, compacting factor or Vee- Bee time is selected. 

iv. Approximate air content is estimated from the available table corresponding to the maximum nominal size of aggregate used.

v. The water content per cubic meter of concrete and percentage of sand in total aggregate ( by absolute volume) are next selected from the available table for the standard reference conditions.

vi. The water content and percentage of sand in total aggregate are adjusted for any deference in workability, water cement ratio, grading of fine aggregate for the particular case from the reference values.

vii. Cement content is calculated using the selected water cement ratio and the final water content of the mix obtained after adjustment. The cement content so calculated is compared with minimum cement content from the requirement of durability and the greater of the two values is used.

viii. The total aggregate content ( saturated surface dry condition) per unit volume of concrete is determined by subtracting the air, cement and water quantities per unit volume of concrete. With the percentage of sand in total aggregate as already determined, the coarse and fine aggregate contents per unit volume of concrete are calculated.

Vab = 1.0 – [ W+C/Sc]1/1000 – Ѵ Where

Vab = Absolute volume of total aggregate per unit volume of concrete

W = Mass of water (kg)

C = Mass of cement (kg)

Ѵ = Air content per cubic meter of concrete

Sc = Specific gravity of cement

Also absolute volume of fine aggregate Vf(ab) = ƥ. Vab

and absolute volume of coarse aggregate Vc(ab) = (I- ƥ)xVab

ix. Now the mix proportions by mass (kg) can be calculated as under:-

Water : Cement : fine aggregate : coarse aggregate

W : C :[Vf(ab)xSfax1000] : [Vc(ab)xScax1000]

Where Sfa and Sca are the specific gravities of saturated surface dry fine aggregate and coarse aggregate in kg/liter respectively.

x. The adjustments for free moisture contents in fine and coarse aggregates are to be done in the calculated quantities and the final mix proportions by mass are calculated.

xi. The actual mix proportions are arrived at by means of tests on number of trial mixes prepared by varying the water-cement ratio by + 10% of the pre selected values.