M20 slab cubes fail the acceptance criterion
An RCC roof slab for a school building is specified as M20 concrete to a design mix with a free water-cement ratio of 0.50, cement content 340 kg/m³ and a sand content of about 700 kg/m³. Site records show that the sand delivered over three days had 6% bulking and 4% surface moisture, but batching was done by volume with the same gauge boxes as before, with no adjustment. The 28-day cube results from one lot of four specimens were 24.5, 23.0, 19.5 and 17.5 N/mm². The slump on the day of placing was 25 mm against a specified 50–75 mm, and the supervisor added about 8 litres of water per cubic metre at the pump to improve workability. Two of the cubes were taken out of the curing tank on the 26th day because of space constraints. The engineer must decide whether to accept the slab, and the contractor argues that the concrete had already set and nothing can be done.
1. Apply the compressive-strength acceptance criteria for M20 concrete to these four results and state, with numbers, whether the lot is acceptable.
Read the worked answer
For M20, fck = 20 N/mm². The acceptance criteria are applied to a group of four consecutive non-overlapping sample results:
- The mean of any four consecutive results must be at least the greater of (fck + 0.825 × established standard deviation) or (fck + 3 N/mm²). Where no established standard deviation is available, the criterion reduces to fck + 3 = 23.0 N/mm².
- No individual result may fall below fck − 3 = 17.0 N/mm².
Checking the results
- Mean = (24.5 + 23.0 + 19.5 + 17.5) / 4 = 84.5 / 4 = 21.1 N/mm²
- Minimum = 17.5 N/mm²
Verdict
- Individual criterion: 17.5 ≥ 17.0, so it is satisfied, but only just.
- Mean criterion: 21.1 < 23.0, so it is not satisfied. The lot is therefore not acceptable as M20 on the strength evidence; the deficiency is about 1.9 N/mm² on the mean, roughly 8% short.
Before acting on this verdict, the test records must be checked: sampling, cube moulding, curing and the testing machine's calibration, and whether the two cubes removed on the 26th day were actually tested at 26 days - if so, they are not 28-day strengths and the lot's result is incomplete rather than failed.
2. Working from the batching data, estimate the water-cement ratio actually obtained and use it to explain the cube results. Then state how batching should have been controlled.
Read the worked answer
Water actually in the mix
- Design free water = 170 L/m³ (0.50 × 340 kg)
- Water in the sand = 4% of 700 kg = 28 L/m³
- Water added at the pump = 8 L/m³
- Total = 170 + 28 + 8 = 206 L/m³
Actual water-cement ratio = 206 / 340 = 0.61, against a design value of 0.50 - a rise of 0.11, about 21%.
Why this explains the cubes. Strength falls steeply as the water-cement ratio rises: in the 0.45–0.60 range, roughly 10–15% of strength is lost for every 0.05 increase. A rise of 0.11 can easily cost 25–30%, which takes a mix designed to give about 27 N/mm² down to about 21 N/mm² - precisely what the lot averaged. The low slump of 25 mm was the visible sign that the aggregates were not being batched as designed, and adding water at the pump "cured" the workability while destroying the strength.
The bulking effect must be stated separately: at 6% bulking the same gauge box holds less solid sand, so the mix is short of fine aggregate - harsher, lower in yield and more prone to honeycombing - and the error was never corrected.
Control measures
- Batch by weight (weigh batcher) as required for a design mix; if volume batching is unavoidable, gauge boxes must be struck level and corrected for bulking.
- Test the sand's surface moisture at least once per shift and deduct it from the batch water.
- No water to be added after mixing; workability to be corrected by adjusting the mix, not the tap.
- Record slump for every batch, but remember slump is a workability check, not a strength test.
3. The contractor says the slab has set and nothing can be done. What investigation would you order before accepting or rejecting the slab, and what acceptance limits would you apply?
Read the worked answer
Cube failure is evidence, not a verdict on the structure. The slab should be investigated before any decision to demolish:
- Audit the test trail. Check sampling frequency, cube identification, curing history, the 26-day removal, and the calibration certificate of the compression machine. Cubes that were not cured or tested correctly may be the problem, not the concrete.
- Non-destructive testing. Rebound hammer (IS 13311 Part 2) and ultrasonic pulse velocity (IS 13311 Part 1) across a grid, to map uniformity and to find weak or honeycombed zones. NDT gives an indication only; it must be correlated with cores.
- Core testing (IS 516 Part 4). Take cores from representative, non-critical locations away from reinforcement and supports - typically three cores per test area. The commonly applied criteria are that the average equivalent cube strength of the cores is not less than about 85% of the specified grade (0.85 × 20 = 17 N/mm²) and that no individual core falls below about 75% (15 N/mm²).
- Structural check. If strengths are marginal but the slab is lightly loaded or generously reinforced, a reanalysis using the measured strength may show the section is still adequate; that decision belongs to the designer, not the site engineer.
- Load test. Only if the above is inconclusive, a load test as set out in IS 456 may be carried out, with deflection limits - typically the deflection must recover substantially on removal of load and must not exceed span/250 or 40 mm (whichever is less) under load.
Until this is done, no further concrete should be placed above the slab, and the batching and water-addition practices must be corrected immediately, since the same gang will cast the next pour.