Concrete Cube Failures on a Slab Pour and the Strength-Acceptance Decision
A 250 mm two-way suspended slab was cast from ready-mix C30 concrete (fck = 30 MPa, target mean ~38 MPa). At 28 days, three cubes from the same batch gave 31, 26, and 24 MPa. The specification requires the standard acceptance criteria (mean of any group of three ≥ fck + 4 MPa, and no individual result below fck − 4 MPa). The contractor wants to proceed with striking formwork and loading the slab, arguing the average of the three cubes is 27 MPa, 'close enough', and that cube results are 'always a bit low anyway'. The slab is a transfer slab carrying columns from two storeys above.
1. Apply the acceptance criteria to the three results and state whether the concrete is acceptable.
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Apply the criteria explicitly:
- Results: 31, 26, 24 MPa. Mean = 27.0 MPa.
- Criterion 1 (mean of group of three ≥ fck + 4 = 34 MPa): 27.0 < 34 → FAIL.
- Criterion 2 (no individual below fck − 4 = 26 MPa): 24 < 26 → FAIL.
The concrete fails both limbs of the acceptance criterion. It is not 'close enough' - it is non-conforming on the standard test.
On the 'cubes are always low' argument: cube results scatter, and a single low group does not by itself prove the structure is weak, because in-situ strength differs from cube strength. But the acceptance criteria exist precisely to trigger investigation, not to be argued away. The correct sequence is:
- Investigate the cause - batching records, cement content, water/cement ratio, curing, cube storage, and whether the cubes were from the correct batch.
- Do not strike formwork or load the slab on the basis of these results. Striking is governed by the actual in-situ strength at the time of striking, not by 28-day cube results that have already failed.
- Assess in situ - the next step is nondestructive or core testing, not argument.
Judgement. The concrete is non-conforming. The contractor's proposal to proceed is refused.
2. The contractor proposes core testing. Describe how you would use the cores to reach a final decision on the slab, and what you would do if the cores also fall short.
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Using cores correctly:
- Sample properly. Cores must be taken from representative locations - not from the strongest-looking areas - and must avoid reinforcement. Typically a minimum number per pour, located by the engineer, not the contractor. The transfer slab's critical regions (under the columns from above, and mid-span) must be included.
- Test and correct. Core strength is corrected for length/diameter ratio, moisture condition, and drilling damage. The corrected in-situ strength is compared against the acceptance criteria for cores, which are generally more lenient than cube criteria because in-situ concrete differs from cubes. Apply the code's core acceptance rule, not the cube rule.
- Decide on the corrected results. If cores meet the core acceptance criteria, the concrete can be accepted in situ and the slab released - with the cube failure recorded as a batch control issue.
If cores also fall short:
- Structural reassessment. Re-run the slab design with the measured in-situ strength. A transfer slab carrying columns is a high-consequence element; the reassessment must check flexure, shear (especially punching shear at the columns from above), and deflection.
- Options depend on the shortfall:
- If the shortfall is small and the reassessment passes with adequate margin, accept with documentation.
- If punching shear or flexure is deficient, strengthening (e.g., external FRP, additional reinforcement, or thickening) or local demolition and recasting may be required.
- Do not load the slab until the reassessment is complete and signed off.
Judgement. The decision must be evidence-based and documented. A transfer slab is not the place to accept 'close enough' - the consequence of being wrong is disproportionate.