Clause 4.1 establishes the target mean compressive strength (f'ck) that the mix must be designed to achieve. The target strength is higher than the characteristic strength (fck) by a margin that accounts for variability in production. The margin is 1.65 times the standard deviation (s), ensuring that not more than 5% of test results fall below fck. When sufficient test data (≥ 30 samples) is not available, assumed standard deviation values from Table 1 are used.
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Key Requirements
•Target mean compressive strength f'ck = fck + 1.65 × s
•Standard deviation (s) shall be based on at least 30 test results from the same mix, materials, and conditions
•When adequate data is not available, assumed standard deviation values from Table 1 shall be used for preliminary mix design
•Once sufficient data is available, the mix design must be reviewed with actual standard deviation
•For field control with fewer than 30 samples, Table 1 values apply as initial estimates
Reference Tables
Table 1 — Assumed Standard Deviation (Clause 4.1)
Grade of Concrete
Assumed Standard Deviation (MPa)
M10 to M15
3.5
M20 to M25
4
M30 to M50
5
M55 and above
5
These values are for preliminary mix design when data from 30+ samples is not available. The mix design must be reviewed once adequate field data is obtained.
Formulas
f'ck = fck + 1.65 × s
Target mean compressive strength for mix design
f'ck = Target mean compressive strength in MPafck = Characteristic compressive strength in MPa (grade designation)s = Standard deviation in MPa (from test data or Table 1)1.65 = Statistical constant for 5% fractile (95% confidence)
Practical Notes
✓For M25 concrete with assumed s = 4.0 MPa: f'ck = 25 + 1.65 × 4.0 = 31.6 MPa — the mix must be designed for 31.6 MPa, not 25 MPa.
✓Higher standard deviation means poorer quality control, requiring a stronger (more expensive) mix to meet the same grade. Good batching plants achieve s ≈ 2.5-3.0 MPa for M20-M25.
✓The 2019 revision simplified Table 1 compared to the 2009 edition; values are now grouped into broader grade ranges.
✓For RMC plants with computerized batching, actual s is often 3.0-3.5 MPa for standard grades, well below the assumed 4.0-5.0 MPa.
Common Mistakes
⚠Designing the mix directly for fck instead of f'ck — this guarantees more than 5% failures in cube tests.
⚠Using assumed standard deviation indefinitely without collecting site data to refine the mix design.
⚠Confusing target mean strength (f'ck) with average cube test result — the mix design targets f'ck, but field acceptance uses IS 456 Clause 16 criteria.
⚠Applying cylinder strength values (ACI approach) when IS 10262 works with 150 mm cube strength.