Low Cube Strengths in a Canal-Approach Bridge Deck
An RCC deck slab for a minor bridge on a canal approach road was cast with a designed M25 mix using OPC 43 grade, a water–cement ratio of 0.50, and a plasticising admixture. The contractor's 28-day cube results for three consecutive samples, each made from three cubes, were 19.5, 21.2 and 22.8 N/mm². Curing by ponding was interrupted for four days when the water tanker broke down. The contractor argues that the average of 21.2 N/mm² is ‘close to M25’ and that the slab should be accepted because the structure has not shown cracking. The assistant engineer asks you to decide whether to accept the slab, order demolition, or investigate further. The cube testing register shows that one set of cubes was transported to the laboratory without a curing tank and may have dried out. The mix design was not rechecked after the aggregate source changed two weeks before casting.
1. Assess the cube results against IS 456 acceptance principles. What can and cannot be concluded from only three sample results?
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For M25 concrete, fck = 25 N/mm². Under the IS 456 acceptance approach, an individual sample result should not fall more than 4 N/mm² below fck, i.e. not below 21 N/mm², and the mean of consecutive results must satisfy the characteristic-strength-plus-margin criterion. Here the three sample averages are 19.5, 21.2 and 22.8 N/mm²; the overall mean is 21.2 N/mm², below 25 N/mm², and the lowest sample (19.5) is below 21 N/mm². So the results do not demonstrate compliance.
However, only three results are available. The formal mean criterion is applied to four consecutive results with a standard deviation, so acceptance cannot be rejected on the mean alone. One cube set may also be invalid because it dried during transport. Conclusion: do not accept the slab as compliant, but do not order demolition on cube data alone. Treat the results as a red flag requiring investigation, especially because curing was interrupted and the aggregate source changed.
2. List the investigations you would order before deciding on demolition, and state the decision rule you would apply.
Read the worked answer
Investigate both the test and the structure. Check cube identity, sampling date, curing history, transport condition, laboratory calibration and the testing machine. Recheck the mix design for the new aggregate. Inspect the slab for cracks, honeycombing, cold joints and cover. Carry out non-destructive testing (rebound hammer and ultrasonic pulse velocity) on grid points, then extract cores from non-critical locations, avoiding reinforcement and prestressing zones. Test cores for compressive strength and examine carbonation/durability if relevant.
Decision rule: if the cores satisfy the applicable IS 456 core acceptance criteria - commonly, an average of three cores of at least 0.85 fck with no individual core below 0.75 fck - and the structure shows no distress, the slab may be accepted with a durability assessment and monitoring. If cores fall below this, refer the slab to a structural engineer for capacity checking. Repair or strengthening may be possible; demolition is justified only if the calculated capacity is inadequate and repair is not feasible.