Cube Results Below Characteristic Strength: Accept or Reject the Slab?
During the 28-day compressive-strength testing of an M25 design-mix concrete lot used for a 150 mm thick slab, twelve 150 mm cubes return the following results in N/mm²: 27.5, 26.8, 28.2, 29.0, 24.6, 26.0, 25.4, 27.2, 23.8, 26.6, 25.9 and 27.8. The contract's acceptance clause states that the mean of any four consecutive test results must be at least fck + 0.825σ, where fck = 25 N/mm² and the established site standard deviation σ is 4.0 N/mm², and that no individual result may fall below fck − 4 N/mm². The contractor has already marked the next-floor columns and wants the slab to be treated as accepted so that the upper floor can be cast. The site engineer asks you to check the statistics before signing the pour card.
1. Compute the overall mean and the mean of the first four consecutive results. Does the lot satisfy both acceptance criteria?
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Sum of the twelve results = 318.8 N/mm², so the overall mean = 318.8/12 = 26.57 N/mm². The first four consecutive results are 27.5, 26.8, 28.2 and 29.0; their mean = 111.5/4 = 27.88 N/mm². The required mean is 25 + 0.825 × 4.0 = 28.30 N/mm². The lowest individual result is 23.8 N/mm², which is above the individual lower limit of 25 − 4 = 21.0 N/mm². Therefore the individual-result criterion is satisfied, but the mean criterion is not: even the most favourable four consecutive results give 27.88 N/mm², below 28.30 N/mm². The lot fails the contract acceptance rule and cannot be signed off as compliant M25 concrete on these cubes alone.
2. The contractor argues that because every individual cube is above 21 N/mm², the lot must be accepted. Evaluate that argument and state your decision.
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The argument treats one acceptance criterion as if it were the whole rule. The individual lower limit of 21 N/mm² is a guard against an isolated very low result; it does not prove that the concrete as a whole has the required characteristic strength. The mean-of-four criterion is the statistical check against a systematically low mix. Here the overall mean is only 26.57 N/mm² and the best four consecutive mean is 27.88 N/mm², both below the required 28.30 N/mm². My decision is to withhold acceptance and not sign the pour card for the next floor. Before rejecting the concrete permanently, I would verify cube identification, curing and testing records and check batching records for cement content, water/cement ratio and aggregate moisture, because a testing or sampling error could explain the shortfall.
3. Recommend a verification and remediation plan, with at least two options and the condition for releasing the next floor.
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- Issue a non-conformance report and place the slab lot on hold. 2. Carry out core testing from representative, structurally critical locations, following the applicable Indian standard procedure; three cores are normally a minimum, and acceptance should be based on the specified core-strength criteria, not on a single core. 3. Use rebound-hammer or ultrasonic pulse velocity testing only as supplementary uniformity checks, not as the sole basis for acceptance. 4. If cores meet the acceptance criteria and a structural check confirms adequacy, accept the slab conditionally and document the decision. 5. If cores fail, either carry out a load test or design strengthening, and obtain structural approval before loading. The next floor should be released only after the slab has been accepted in writing by the competent authority; additional cube sampling should be specified for the next pour.