Concrete Cube Failures on a Suspended Slab
A 200 mm thick suspended slab is cast from C30/37 ready-mixed concrete supplied to a 300 m2 bay. At 28 days, three of six 100 mm cube samples fail to reach the specified characteristic strength; results are 24, 26, 31, 33, 35 and 38 N/mm2. The slab has been struck and is carrying only self-weight. The contractor argues the slab is acceptable because the average (31.2 N/mm2) is close to the target mean and only 'a few' cubes failed. The specification requires compliance with the standard conformity criteria, and the client wants to know whether the slab must be demolished.
1. Assess the cube results against typical conformity criteria and state whether the concrete complies.
Read the worked answer
Standard conformity criteria for C30/37 typically require both:
- the mean of the samples ≥ fck + 4 N/mm2 (i.e. ≥ 34 N/mm2 for the first few results), and
- no individual result below fck − 4 N/mm2 (i.e. ≥ 26 N/mm2).
Mean here is 31.2 N/mm2, which is below 34 N/mm2, so the mean criterion fails. Two individual results (24 and 26) are at or below the 26 N/mm2 lower bound, so the individual criterion also fails.
The contractor's argument is wrong on two counts. First, conformity is a statistical test, not a vote - 'only a few failed' is not a defence. Second, the target mean (fck + margin) exists precisely to absorb scatter; a mean near the target with a wide spread and low outliers indicates a non-compliant batch. The concrete does not comply with the specification.
Note that cube results are indicators; the in-situ strength may differ, and this should be established before any demolition decision.
2. Recommend the investigation and decision process before demolition is considered.
Read the worked answer
Demolition is premature. The correct sequence is:
- Check the cubes themselves: were they cured, stored and tested correctly? A single bad batch of cubes can explain low results. Review the testing laboratory's records and certificates.
- Establish in-situ strength: use indirect methods (rebound hammer, ultrasonic pulse velocity) to map variability, then calibrate with cores. Cores should be taken from representative locations, avoiding reinforcement, and tested to give an in-situ strength estimate. Apply appropriate core-to-cube correlations.
- Structural reassessment: if in-situ strength is, say, 26–28 N/mm2 rather than 30, re-run the slab design at the measured strength. A lightly loaded suspended slab carrying only self-weight may still have adequate capacity and deflection performance at reduced strength. Check flexure, shear and deflection.
- Decide by consequence: if the reassessment shows adequate capacity with acceptable reliability, the slab can be accepted with a documented concession. If not, options include local strengthening (e.g. FRP or additional reinforcement), partial reconstruction, or demolition of the worst bay only.
The key judgement is that conformity failure of cubes triggers investigation, not automatic rejection. The decision must rest on measured in-situ strength and structural capacity.