Concrete Cube Failure in a Structural Slab
A 250 mm thick suspended slab was cast from ready-mix specified as C35/45 (characteristic cube strength 45 MPa at 28 days). At 28 days, three 150 mm cubes from the same batch returned 38.5, 41.0 and 44.2 MPa. The contractor argues the average (41.2 MPa) is close to 45 MPa and that the slab 'looks sound'. The engineer notes that the cubes were cured in the site tank, not under standard conditions, and that one cube was tested at 26 days. The slab supports a storage area and is already propped.
1. Assess the cube results against typical acceptance criteria and decide whether the concrete can be accepted as C35/45.
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Using a standard conformity approach (e.g. EN 206 / BS 8500 style criteria for a small number of cubes):
- Mean of the group must be ≥ f_ck + a margin (commonly f_ck + 4 MPa for 3 cubes). Mean = 41.2 MPa, required ≈ 49 MPa → fails.
- No individual result should fall below f_ck − 4 MPa (i.e. 41 MPa). One cube at 38.5 MPa → fails.
So on the face of the results the concrete does not conform. Two further issues undermine the contractor's argument:
- Curing - site-tank curing is not standard; results may be unrepresentative in either direction and cannot be used as the sole basis for acceptance without justification.
- Test age - a cube tested at 26 days is not a valid 28-day result.
Conclusion: the concrete cannot be accepted as C35/45 on this evidence. The engineer should require proper investigation rather than visual judgement - 'looks sound' is not a conformity argument.
2. Recommend the investigation and disposition steps before the slab is accepted or rejected.
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A defensible sequence:
- Check the records - verify the batch plant, mix design, water addition, delivery times, and whether the cubes were sampled and cured correctly. Confirm the 26-day cube is excluded or retested.
- Non-destructive testing - rebound hammer and ultrasonic pulse velocity across the slab, calibrated against cores, to map strength variation.
- Cores - take cores from representative locations (avoiding reinforcement and high-stress zones) and test. Core results, adjusted for orientation and moisture, give the most reliable in-situ strength.
- Structural review - if in-situ strength is below the assumed design value, re-run the slab capacity check with the measured strength and confirm the storage loading is still safe, including the propping condition.
- Decision - accept with documented justification, require strengthening, or reject and replace. Do not remove props until the capacity is confirmed.
Each step must be recorded; the engineer's decision must rest on measured in-situ strength, not appearance.