IS 456:2000 has recommended that minimum grade of concrete shall not be less than M20 in reinforced concrete work. Design mix concrete is preferred to nominal mix. If design mix concrete cannot be used for any reason on the work for grades M20 or lower, nominal mixes may be used with permission of engineerin-charge, which, however is likely to involve higher cement content. Accordingly all concrete above M20 grade for RCC work must be of design mixes.

Data for Mix Proportioning:

The following basic data are required for mix proportioning of a particular grade of concrete:

• Exposure condition of the structure under consideration (for guidance see Table 3 of IS 456-2000)
• Grade Designation: Minimum grade of concrete to be designed for the type of exposure condition of the structure under consideration (for guidance see Tables 3 and 5 of IS 456-2000). This has been reproduced in Appendix
• Type of cement, viz., Ordinary Portland Cement (OPC), Portland Pozzolana Cement (PPC), Portland Slag Cement (PSC) etc.
• Maximum nominal size of aggregate to be used, viz. 40 mm, 20 mm and 12.5 mm.
• Minimum cement content (for guidance see Tables 3,4,5 and 6 of IS 456-2000)
• Maximum water-cement ratio (for guidance see Tables 3 and 5 of IS 456-2000)
• Degree of workability desired (for guidance see Clause 7 of IS 456-2000) (this has been reproduced in Appendix as Table 5)
• Maximum temperature of concrete at the time of placing
• Early age strength requirements, if required
• Type of aggregate viz. Granite, Basalt, Natural River sand, Crushed Stone sand etc.
• Maximum cement content.
• Use of admixture, its type and condition of its use.

The step-by-step procedure of mix proportioning is as follows:

1. Target Mean Compressive Strength for Mix Proportioning:

Standard deviation shall be calculated for each grade of concrete using at least 30 test strength of samples (taken from site) when a mix is used for the first time. In case sufficient test results are not available, the values of standard deviation given in Table 1 may be assumed for the proportioning of mixes in the first instance. As soon as the sufficient test results are available, actual standard deviation shall be calculated and used to proportion the mix properly.

TABLE 1 - Assumed Standard Deviation

Note: The above values correspond to site control having proper storage of cement; weigh batching of all materials; controlled addition of water; regular checking of all materials, aggregate grading and moisture content; and periodical checking of workability and strength. Where there is deviation from the above, values given in the above table shall be increased by 1 N/mm2

2 .Selection of Water–Cement Ratio:

Concrete made today contains more than four basic ingredients. Use of both chemical and mineral admixtures has changed properties of concrete both in fresh and hardened state for good. Even quality of both coarse and fine aggregates in terms of grading, shape, size and texture has improved with the improvement in crushing technologies. With all these variables playing its role, concretes produced with same water-cement ratio may have different compressive strength. For a given set of materials, it is preferable to establish relationship between compressive strength and free water-cement ratio. If such a relationship is not available, maximum water-cement ratio for various environmental exposure conditions given in Table 5 of IS 456-2000 may be taken as a starting point. Any water-cement ratio assumed based on the previous experience for a particular grade of concrete should be checked against the maximum values permitted from the point of view of durability and lower of the two shall be adopted.

3.Selection of Water Content:

The quantity water considered per cubic metre of concrete decides the workability of the mix.. Use of water reducing chemical admixtures in the mix helps to achieve increased workability at lower water contents. Water content given in Table 2 of the standard is the maximum value for a particular maximum nominal size of aggregate (angular) which will achieve a slump in the range of 25 mm to 50 mm. Depending on the performance of an admixture (conforming IS 9103-19993 ) which is proposed to be used in the mix, a minimum of 20% of water reduction shall be considered in case of superplasticisers. Use of Poly Carboxylic Ether (PCE) based superplasticisers results in water reduction up to 30%.

Water content per unit volume of concrete is required to be reduced when there is increase in aggregate size, use of rounded aggregates, reduction in water-cement ratio and slump. Water content per unit volume of concrete is required to be increased when there is increased temperature, cement content, fine aggregate content, water-cement ratio.

TABLE 2 - Maximum Water Content per Cubic Metre of Concrete for Nominal Maximum Size of Aggregate

Note: These quantities of mixing water are for use in computing cementitious material contents for trial batches. 1 Water Content corresponding to Saturated Surface Dry aggregate Reduction in water content can be made in following cases:

• a) For sub angular aggregates, the water estimate can be reduced by 10 kg.
• b) For gravel with crushed particles, the water content can be reduced 20 kg.
• c) For rounded gravel, the water content can be reduced by 25 kg.
For higher workability (other than 25 mm to 50 mm) the required water content may be established by trial or an increase by about 3% for every additional 25 mm slump or alternatively by use of chemical admixtures conforming to IS 9103-1999 Use of water reducing admixture: If use of chemical water reducing admixture is permitted for the job in hand in proportioning of the mix, reduction in assumed water content shall be made depending on the type of admixture. Water reducing admixtures will usually decrease water content by 5 to 10% and superplasticisers decrease water content by 20% and above at appropriate dosages.

4. Calculation of Cementitious content:

Water content calculated in step V is divided by the water-cement ratio selected in step II, to arrive at cement content or cementitious content (if mineral admixtures are used). The total cementitious content so calculated should be checked against the minimum content for the requirements of durability and the greater of the two values adopted. The maximum cement content alone (excluding mineral admixtures such as flyash and GGBS) shall not exceed 450 kg/cu.m as per clause no. 8.2.4.2 of IS 456-2000. The total cementitious content so calculated should be checked against the minimum cement content for the requirements of durability for various exposure conditions and the greater of the two values adopted.

5. Estimation of coarse aggregate proportion:

Table 3 of the standard gives volume of coarse aggregate for unit volume of total aggregate for different zones of fine aggregate (as per IS 383-19704 ) for a water-cement ratio of 0.5 which requires to be suitably adjusted for other water-cement ratios. This table is based on ACI 211.1-19915 “Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete”. Aggregates of essentially the same nominal maximum size, type and grading will produce concrete of satisfactory workability when a given volume of coarse aggregate per unit volume of total aggregate is used. It can be seen that for equal workability, the volume of coarse aggregate in a unit volume of concrete is dependent on nominal maximum size, water-cement ratio and grading zone of fine aggregate.

Table 3 - Volume of Coarse Aggregate per unit volume of Total Aggregate for different Zones of Fine Aggregate

1Volumes are based on Aggregates in Saturated Surface Dry condition Note: Volume coarse aggregate per unit volume of total aggregate needs to be changed at the rate of -/+ 0.01 for every ±0.05 change in water-cement ratio.

6. Combination of different sizes of coarse aggregate fractions:

7. Estimation of Fine Aggregate Proportion:

In the steps mentioned above, all the ingredients of concrete are estimated except the coarse and fine aggregate content. These quantities are determined by finding out the absolute volume of cementitious material, water and the chemical admixture; by dividing their mass by their respective specific gravity, multiplying by 1/1000 and subtracting the result of their summation by unit volume. The value so obtained is the volume of total aggregate. Volume of coarse aggregate for unit volume of total aggregate is already estimated in step V. The contents of coarse and fine aggregate per unit volume of concrete are determined by multiplying with their specific gravities and multiplying by 1000.

8. Trial Mixes:

The calculated mix proportions shall be checked by means of trial batches as follows:

The calculated mix proportions shall be checked by means of trial batches. The concrete for trial mixes shall be produced by methods of actual production. Ribbon type mixer and pan mixer are required to be used to simulate the site conditions where automatic batching and pan mixers are used for the production of concrete.

Trial Mix No.1: Workability in terms of slump of the trial mix shall be carefully observed for freedom from segregation, bleeding and for finishing properties. If the measured slump of Trial mix No.1 is different from targeted value, Trial Mix No.2 shall be carried out as explained below

Trial Mix No.2: The water and/or admixture content shall be adjusted suitably in the Trial Mix No.1. With this adjustment, the mix proportion shall be recalculated keeping the free water cement ratio at the preselected value. With this trial more or less the stipulated value of slump will be obtained. In addition two more Trial Mixes No.3 and 4 shall be made with water content same as Trial Mix No.2 and varying the free w/c ratio by ±10% of the preselected value.
After laboratory trials field trial shall be carried out.

Will be Continued in Issue 10..

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References:
REFERENCE MANUAL FOR FIELD ENGINEERS ON BUILDING CONSTRUCTION

Published by:
Task Force for Quality Assurance in Public Constructions
GOVERNMENT OF KARNATAKA ISBN

Important Note:
This reference manual is intended for the use of Government Engineers.

Disclaimer:
Every effort has been made to avoid errors or omissions in this publication. In spite of this, some errors might have crept in. Any mistake, error or discrepancy noted may be brought to our notice which shall be taken care of in the next edition. It is notified that neither the publisher nor the authors will be responsible for any damage or loss of action to any one, or any kind, in any manner, therefrom.