8.1 Mix Proportion
The mix proportion shall be selected to ensure the workability of the fresh concrete and when concrete is hardened, it shall have the required strength, durability and surface finish.
8.1.1 The determination of the proportions of cement, aggregates and water to attain the required strengths shall be made as follows:
a) By designing the concrete mix; such concrete shall be called 'Design mix concrete', or
b) By adopting nominal concrete mix; such concrete shall be called ‘Nominal mix concrete’.
PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5A PLAIN AND REINFORCED CONCRETE
Design mix concrete is preferred to nominal mix. If design mix concrete cannot be used for any reason on the work for grades of M 20 or lower, nominal mixes may be used with the permission of Engineer-in-Charge, which, however, is likely to involve a higher cement content.
8.1.2 Information Required
In specifying a particular grade of concrete, the following information shall be included:
a) Type of mix, that is, design mix concrete or nominal mix concrete;
b) Grade designation;
c) Type of cement;
d) Maximum nominal size of aggregate;
e) Minimum cement content (for design mix concrete);
f) Maximum water-cement ratio;
g) Workability;
h) Mix proportion (for nominal mix concrete);
j) Exposure conditions as per Tables 4 and 5;
k) Maximum temperature of concrete at the time of placing;
m) Method of placing; and
n) Degree of supervision.
8.1.2.1 In appropriate circumstances, the following additional information may be specified:
a) Type of aggregate,
b) Maximum cement content, and
c) Whether an admixture shall or shall not be used and the type of admixture and the condition of use.
8.2 Design Mix Concrete
8.2.1 As the guarantor of quality of concrete used in the construction, the constructor shall carry out the mix design and the mix so designed (not the method of design) shall be approved by the employer within the limitations of parameters and other stipulations laid down by this standard. If so desired, the employer shall be provided with supporting data including graphs showing strength versus water-cement ratio for range of proportions, complete trial mix proportioning details to substantiate the choice of cement content, fine and coarse aggregate content, water, mineral admixtures, chemical admixtures, etc.
8.2.2 The mix shall be designed to produce the grade of concrete having the required workability and a characteristic strength not less than appropriate values given in Table 2. Proportion/grading of aggregates shall be made by trial in such a way as to make densest possible concrete. The target mean strength of concrete mix should be equal to the characteristic strength plus
1.65 times the standard deviation.
8.2.3 Mix design done earlier not prior to one year may be considered adequate for later work provided there is no change in source and the quality of the materials.
8.2.4 Standard Deviation
The standard deviation for each grade of concrete shall be calculated, separately.
8.2.4.1 Standard deviation based on test strength of sample
a) Number of test results of samples — The total number of test strength of samples required to constitute an acceptable record for calculation of standard deviation shall be not less than 30. Attempts should be made to obtain the 30 samples, as early as possible, when a mix is used for the first time.
b) In case of significant changes in concrete — When significant changes are made in the production of concrete batches (for example changes in the materials used, mix design, equipment or technical control), the standard deviation value shall be separately calculated for such batches of concrete.
c) Standard deviation to be brought up to date — The calculation of the standard deviation shall be brought up to date after every change of mix design.
8.2.4.2 Assumed standard deviation
Where sufficient test results for a particular grade of concrete are not available, the value of standard deviation given in Table 8 may be assumed for design of mix in the first instance. As soon as the results of samples are available, actual calculated standard deviation shall be used and the mix designed properly. However, when adequate past records for a similar grade exist and justify to the designer a value of standard deviation different from that shown in Table 8, it shall be permissible to use that value.
8.3 Nominal Mix Concrete
Nominal mix concrete may be used for concrete of M 20 or lower. The proportions of materials for nominal mix concrete shall be in accordance with Table 9.
8.3.1 The cement content of the mix specified in Table 9 for any nominal mix shall be proportionately increased if the quantity of water in a mix has to be increased to overcome the difficulties of placement and compaction, so that the water-cement ratio as specified is not exceeded.
| (SI No. | Grade of Concrete | Assumed Standard Deviation |
| (1) | (2) | (3) |
| i) | M 10 } | 3.5 |
| ii) | M 15 } | |
| iii) | M 20 } | 4.0 |
| M 25 } | ||
| iv) | M 30 } | |
| v) | M 35 | |
| vi) | M 40 | |
| vii) | M 45 } | 5.0 |
| viii) | M 50 | |
| ix) | M 55 | |
| x) | M 60 | |
| NOTES1 The above values correspond to the 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, the values given in the above table shall be increased by .2 For grades above M 60, the standard deviation shall be established by actual trials based on assumed proportions, before finalizing the mix. | ||