M25 Cube Failure at an Overhead Tank Site
During a 28-day quality audit of an M25 reinforced-concrete overhead tank staged in Jammu, the site engineer records three cube results from the same batch: 26.4, 24.1 and 18.8 N/mm². Batch records show OPC 43 grade cement at 380 kg/m³, fly ash added at 25% of cementitious content, water/cementitious ratio entered as 0.50 in the register, and sand batched by volume from a stockpile whose moisture test on the same morning reads 6.2%. The mixer operator confirms that the water box was set from the register value and no moisture correction was made. Slump at placement was 165 mm against a specified 75±25 mm, and the pour was placed at 34°C ambient without ice. Cube specimens were stripped at 20 hours and cured in a site tank that was emptied for two days during a water shortage.
1. Rank the probable causes of the low cube strength and link each to a specific number or observation in the record. Show any calculation you use.
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Ranking and evidence:
- Uncorrected sand moisture - the dominant cause. With 380 kg/m³ OPC forming 75% of the binder, total binder = 380/0.75 = 506.7 kg/m³; fly ash = 126.7 kg/m³; design water = 0.50 × 506.7 = 253 kg/m³. A typical sand content of about 700 kg/m³ at 6.2% moisture carries 43 kg/m³ of free water. Effective water becomes about 296 kg/m³ and effective w/c about 0.59, not 0.50. The 165 mm slump against a 100 mm upper limit corroborates the extra water.
- Volume batching of sand adds batch-to-batch yield variation and makes the moisture error variable, which helps explain the 18.8 N/mm² outlier.
- Curing interruption for two days at early age slows strength gain, especially with 25% fly ash.
- 34°C placement raises water demand and hydration rate; without ice or a retarder, workers may have added water at the mixer.
- 25% fly ash lowers early strength. It is contributory, not the primary cause: the mix should still reach M25 at 28 days if the w/c is controlled.
2. Apply the IS 456 acceptance rules to the three results and state whether the batch can be accepted. Identify the additional record you would demand before a final decision.
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Treat the three cubes as one sample: mean = (26.4 + 24.1 + 18.8)/3 = 69.3/3 = 23.1 N/mm².
IS 456 requires the individual variation within a sample not to exceed ±15% of the sample mean. Limits: 23.1 × 0.85 = 19.64 N/mm² and 23.1 × 1.15 = 26.57 N/mm². The 18.8 N/mm² cube is below 19.64, so the sample is invalid and cannot be used as an acceptance test.
Even if it were valid, 23.1 N/mm² is below fck (25) but above fck − 3 = 22 N/mm². The individual-test limit would be satisfied, but the four-consecutive-sample mean criterion (at least fck + 3 or fck + 0.825σ, whichever is greater) cannot be checked from a single sample. The batch therefore cannot be signed off on this record.
Demand: four consecutive sample results, batch-weight and moisture records, cement and fly-ash test certificates, the curing register, and a rebound-hammer/UPV survey followed by a core test plan for the suspect pour.
3. Propose a corrective action plan covering batching, placement, curing and retesting.
Read the worked answer
Batching: Stop using the register water figure. Weigh sand and test its moisture at least twice per shift; subtract free water from the added water. For 6.2% moisture and about 700 kg/m³ sand, reduce the water box by roughly 43 kg/m³. Recalculate the mix for saturated-surface-dry conditions and switch sand and fly ash to weigh batching. Calibrate the batching plant and keep signed batch tickets.
Placement: Hold slump at 75±25 mm. At 34°C, use ice or a retarder and shade the aggregates; never allow water to be added at the mixer. Reduce the time between mixing and placing.
Curing: Restore seven-day wet curing using gunny bags or a curing compound; do not empty the curing tank during a pour. Record curing dates and durations.
Retesting: Because the sample is invalid, carry out rebound hammer and ultrasonic pulse velocity tests, then extract at least three cores of 75–100 mm diameter from the suspect pour. If cores fall below 85% of fck, take a formal non-compliance decision and redesign the affected element. Retest the next four batches and maintain a strength register.