M25 slab cubes fail the IS 456 acceptance test after hot-weather concreting
A 24 m × 12 m two-way slab (span 3.6 m c/c, M25, Fe500) is being cast for a government school building in Tiruchirappalli in May. The approved mix has cement 400 kg/m³, w/c 0.48, and a retarder dosed at the supplier's maximum recommended rate. Batching slump is 90 mm; the truck reaches the pump after 50 minutes and the slump at placement is 40 mm, so the site engineer adds 25 litres of water per 6 m³ drum. After 28 days, six standard cubes from two pours are tested: 27.5, 26.0, 24.5, 23.4, 22.1 and 20.4 N/mm². The contractor struck the slab soffit formwork and removed all props on day 5 to reuse material on another block; a 6 mm sag and edge spalling are visible at midspan, and the block is due for handover in 10 days. Ambient shade temperature at placing was 38 °C.
1. Apply the IS 456 acceptance criteria to the six cube results and state whether the concrete can be accepted as M25. Show the arithmetic and say what the failing result obliges the engineer to do next.
Read the worked answer
Criterion 1 - individual result: every individual cube must be not less than fck − 4 N/mm² = 25 − 4 = 21.0 N/mm². The lowest cube, 20.4, fails by 0.6 N/mm².
Criterion 2 - group mean: the mean of any four consecutive results must be ≥ fck + 0.825σ. Only six results exist, so σ is not 'established' (IS 456 expects about 30 results); the assumed σ for M25 in mix design is 4.0 N/mm², giving a requirement of about 25 + 3.3 = 28.3 N/mm². The three possible groups give:
- (27.5+26.0+24.5+23.4)/4 = 25.35
- (26.0+24.5+23.4+22.1)/4 = 24.00
- (24.5+23.4+22.1+20.4)/4 = 22.60
Even the best four are 25.35, well below 28.3, and the original design target mean was 31.6 N/mm². Verdict: the concrete cannot be accepted as M25.
Next steps: withhold acceptance of the two pours; order three cores from each pour (mean ≥ 0.85 fck and no core below 0.75 fck is the usual working rule), plus UPV/rebound for uniformity mapping; verify batching records and cement content; then a structural review and, if capacity is doubtful, a load test before handover. The 10-day handover date is irrelevant to the technical decision.
2. Diagnose the slump loss and the site water addition, and set out corrective measures for the remaining pours - separating specification changes from site discipline.
Read the worked answer
Diagnosis. Slump fell 90 → 40 mm in 50 minutes at 38 °C. Contributors: high cement content (400 kg/m³) generating heat and rapid hydration; evaporation from the drum and pump line; absorption by dry aggregate; and retarder already at the maximum dose, so the only remedy left on site was water. The 25 L added per 6 m³ = 4.17 L/m³, raising w/c by 4.17/400 ≈ 0.01 - a small strength penalty (a few per cent) that cannot by itself explain a fall from 27.5 to 20.4. The declining trend across pours points to a batching or materials problem (cement content, aggregate moisture not corrected, or over-retardation), not merely site water.
Specification changes (engineer's approval needed): re-design with corrected aggregate moisture; replace part of the retarder with a mid-range water reducer; consider PPC/slag blend or chilled water; restrict pour timing to night; specify a slump band at batching and at placement.
Site discipline (immediate order): no water addition at site; reject any truck below the placement slump and re-dose only by the supplier's approved method; limit transit to about 30 minutes; cover and wet burlap the chute; maintain batch and cube registers; re-test every pour until three consecutive groups comply.
3. The contractor wants to keep the props off and patch the sag. Give your decision on the day-5 depropping and the structural check you would require before handover.
Read the worked answer
On the depropping: IS 456 clause 11.3 permits removal of soffit formwork to slabs after 3 days provided props are left under, but the props to a slab of span up to 4.5 m may be removed only after 7 days. This slab is 3.6 m span, so removing all props on day 5 is non-conforming by two days, and it was done to reuse material, which is not a permissible reason.
On the sag: 6 mm over a 3600 mm span is about L/600, within the L/250 total deflection limit, but a sag appearing immediately after early depropping signals either green concrete overloaded by construction loads or settlement of the props. Patching hides the symptom and leaves the cause in place.
Decision: reject the patching proposal. Order (i) immediate re-propping of the span, (ii) core tests and UPV on the slab to confirm in-situ strength, (iii) a level survey of soffit against design camber, (iv) a check of the slab for construction-stage loading, and (v) a load test if the core results or the survey are marginal. Only the engineer can accept the element, and all rectification and testing costs rest with the contractor.