Concrete Mix Design based on (IS method)

Concrete Mix Design

INTRODUCTION

The common method of expressing the proportions of ingredients of a concrete mix is in the terms of parts or ratios of cement, fine 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 proportions are either by volume or by mass. The water-cement ratio is usually expressed in mass. 

Factors to be considered for 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 – IS456:2000 

1. Determine the mean target strength ft from the specified characteristic compressive strength at 28-day fck and the level of quality control. 

Target Strength = 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: 

Where, 

V = absolute volume of concrete = gross volume (1m3 ) minus the volume of entrapped air Sc = specific gravity of cement 

W = Mass of water per cubic metre of concrete, kg 

C = mass of cement per cubic metre of concrete, kg 

p = ratio of fine aggregate to total aggregate by absolute volume fa, 

Ca = total masses of fine and coarse aggregates, per cubic metre of concrete, respectively, kg, and 

Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates, respectively 

9. Determine the concrete mix proportions for the first trial mix. 

10. Prepare the concrete using the calculated proportions and cast three cubes of 150 mm size and test them wet after 28-days moist curing and check for the strength. 

11. Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.