8.1 Mix Proportion
The mix proportions 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 by designing the concrete mix; such concrete shall be called ‘Design mix concrete’.
8.1.2 Information Required
In specifying a particular grade of concrete, the following information shall be included:
a) Grade designation,
b) Type of cement,
c) Maximum nominal size of aggregates,
d) Minimum cement content (for design mix concrete),
e) Maximum water-cement ratio,
f) Workability,
g) Exposure conditions as per Tables 3 and 4,
h) Maximum temperature of concrete at the time of placing,
j) Method of placing, and
k) 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 conditions 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 section.
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 the appropriate
values given in Table 1. 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 shall be assumed to be 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.