Cube results below the M25 acceptance band on a framed structure
A residential frame uses M25 concrete supplied by a ready-mix plant. The site engineer has 28-day cube results for the third lift: 21.8, 23.2 and 24.6 N/mm², each the average of three 150 mm cubes. No standard deviation has been established from prior production, and the fourth consecutive sample is not due for another two days. The contractor wants to strike the props and proceed with the fourth lift tomorrow because each individual result exceeds 21 N/mm². The owner's engineer notes that the mix had a water-cement ratio of 0.50 on the batch sheet, slump 140 mm, and that two cubes were cured in a site water tank rather than the laboratory tank. The structural drawings specify M25 with a characteristic strength of 25 N/mm² at 28 days.
1. Evaluate the three cube results against the IS 456 acceptance criteria for M25 concrete when no standard deviation is established. State whether the lift can be accepted on these results alone.
Read the worked answer
Under IS 456, two conditions apply for M25 when no standard deviation is established:
- Individual result >= fck - 4 = 21 N/mm². All three results (21.8, 23.2, 24.6) pass.
- Mean of any four consecutive results >= fck + 3 = 28 N/mm². Only three results exist; their mean is (21.8 + 23.2 + 24.6)/3 = 23.2 N/mm², below 28 N/mm², and the fourth result is not available.
So the lift cannot be accepted on these three cubes alone. The individual pass is necessary but not sufficient; the governing group-of-four criterion is not met. Site curing of two cubes also weakens the test's validity and should be recorded as a non-conformance. Striking props and proceeding with the next lift would be premature.
2. What should the engineer direct before props are struck? Give the tests and acceptance values.
Read the worked answer
Direct that props remain and the fourth lift not start. Order:
- Core testing per IS 516: at least three 75-100 mm cores from non-critical locations. Acceptance: average of three >= 0.85 fck = 21.25 N/mm² and no individual core < 0.75 fck = 18.75 N/mm².
- NDT (rebound hammer and ultrasonic pulse velocity) to map uniformity and compare with cores.
- Audit cube curing, identification and batch records; retest remaining cubes.
If cores pass, the concrete may be accepted with a non-conformance report and enhanced testing of subsequent lifts. If cores fail, carry out a structural assessment or load test, and redesign the mix or repair as directed by the designer. Striking props is a strength-based decision, not a calendar decision.
3. Suppose the cores return 20.5, 21.8 and 22.4 N/mm². What decision follows?
Read the worked answer
Average = (20.5 + 21.8 + 22.4)/3 = 21.57 N/mm² >= 21.25; minimum 20.5 >= 18.75. The core criterion is satisfied, so the concrete can be accepted for strength, subject to NDT showing no voids and the designer confirming member capacity. However, the core average is only 86% of M25 and the original cubes were low, so the mix is variable. Require a mix review (water/cement ratio, batching, curing), increase sampling frequency to every 25 m³ or each lift, and retain the non-conformance record. If any core had fallen below 18.75, a load test or strengthening would be required.