Concrete Cube Failures on a Residential Frame
On a six-storey residential frame, 28-day cube results for a C25/30 concrete pour in the third-floor columns came back with a mean of 24 MPa and a lowest individual result of 19 MPa. The ready-mix supplier's delivery tickets show a water addition of about 25 litres per cubic metre on site, recorded as 'for workability.' The contractor wants to proceed with the floors above and proposes to 'accept the concrete because the columns are only lightly loaded.' The supervision engineer has not yet issued a non-conformance report. Cube specimens were stored on site in the open during a heatwave before curing.
1. Evaluate the possible causes of the low results and what must be verified before drawing conclusions.
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Low cube results can originate in the concrete, in sampling and curing, or both:
- Site water addition raises the water/cement ratio, reducing strength; 25 L/m³ is substantial and should be treated as a serious deviation unless authorised by the mix design.
- Specimen handling: cubes stored in the open during a heatwave and cured improperly will underperform regardless of the concrete in the structure. Curing and storage records must be checked.
- Sampling and testing: confirm that samples were taken per the standard, that the laboratory is competent, and that the testing machine was calibrated.
Before condemning the columns, verify curing and storage, review the batching records and any approved water allowance, and consider testing the concrete in place by cores or non-destructive methods. Only then can the cause be attributed.
2. Describe the acceptance decision process and the actions you would require on site.
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Process:
- Issue a non-conformance report and hold further pours on the affected elements.
- Establish the cause through records and, if needed, in-place testing; cores give the most direct evidence but must be located away from critical sections and made good afterwards.
- Judge conformity against the specified criteria, not against 'lightly loaded' arguments; the structure was designed for C25/30 and the analysis assumed it.
- If in-place strength is adequate, document the basis for acceptance; if not, require remedial measures such as enlargement, jacketing, or replacement, designed and checked.
Site actions: stop unauthorised water addition, require written approval for any admixture or water, tighten curing and specimen storage, and review the supplier's mix and batching. Record everything, because the decision will be scrutinised if a defect appears later.