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Acceptance criteria of concrete strength

Short answer

As per IS 456: for compressive strength, the mean of four consecutive non-overlapping results shall be ≥ fck+0.825s (min fck+4 MPa for M20+) and any individual result ≥ fck−3 MPa. For flexural strength, mean of four results ≥ fck+0.3 MPa and each ≥ fck−0.3 MPa.

What are the acceptance (compliance) criteria while testing concrete? Write down the acceptance criteria of compressive and flexural strength according to IS 456.

The acceptance (compliance) criteria decide whether the placed concrete meets the specified characteristic strength fckf_{ck}. IS 456 (Cl. 16) deals with them; the strength for each sample is the average of three specimens (Cl. 15.4), a specimen differing by more than 15% from that average being rejected.

Compressive strength (IS 456, Cl. 16.1 and Table 11)

Concrete is accepted if both conditions hold:

GradeMean of group of 4 non-overlapping consecutive resultsAny individual result
M15≥fck+0.825×s\geq f_{ck} + 0.825\times s (min. fck+3f_{ck}+3)≥fck−3\geq f_{ck}-3
M20 and above≥fck+0.825×s\geq f_{ck} + 0.825\times s (min. fck+4f_{ck}+4)≥fck−3\geq f_{ck}-3

Here ss is the established standard deviation (Table 8 values may be used) and the value is rounded off to the nearest 0.5 N/mm2^2. The larger of the two limits is taken.

Flexural strength (IS 456, Cl. 16.2)

Concrete is accepted if

  • the mean of any group of four consecutive test results exceeds the specified characteristic flexural strength by at least 0.3 N/mm2^2, and
  • every individual result is not less than the specified characteristic strength minus 0.3 N/mm2^2.

If the criteria fail (Cl. 16.3 and 17)

The concrete may be accepted at a lower rate by the engineer, or NDT, drilled cores (average of cores at least 85% of fckf_{ck} and no core below 75%), or load tests on the structure may be done.

✓ Answer checked by Claude Sonnet 5.5

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