Hot-Weather Slab: Cylinder Results and the Sign-Off Request
At 14:20 on an August afternoon a ready-mix truck arrives at a suspended slab pour. The batch ticket shows a 90-minute haul and a discharge temperature of 31.8 °C; the specification limits concrete temperature at placement to 32 °C and requires a slump of 100 ± 25 mm. The slump test reads 165 mm. The foreman tells the crew to 'tighten it up' and roughly 25 L of water is added to the 6 m³ load before a second test, which reads 120 mm. Placement finishes at 16:40. The 28-day cylinder results for the 25 MPa specified mix are 21.5, 22.0, 22.8, 26.1 and 27.0 MPa. The owner's inspector has already accepted the pour visually and asks the engineer to sign off because the mean of all five cylinders is 23.9 MPa, which he calls 'close enough'. The next pour of the same mix is scheduled in three days.
1. Evaluate the cylinder results against acceptance criteria. Is the concrete acceptable, and what follow-up is required?
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Test the governing criteria, not the overall mean. The usual acceptance rules for strength tests are: (1) the average of any three consecutive tests must equal or exceed f'c, and (2) no single test may fall below f'c − 3.5 MPa.
- f'c = 25.0 MPa, so the single-test floor is 21.5 MPa. The lowest result, 21.5 MPa, sits exactly on the floor and does not breach it.
- Consecutive three-test averages: (21.5 + 22.0 + 22.8)/3 = 22.1 MPa; (22.0 + 22.8 + 26.1)/3 = 23.6 MPa; (22.8 + 26.1 + 27.0)/3 = 25.3 MPa.
- The first average, 22.1 MPa, is below 25.0 MPa, so the strength requirement is not met. The mean of all five tests (23.9 MPa) is irrelevant to acceptance.
Follow-up. The failing test triggers core testing of the represented area: normally three cores, with acceptance requiring the core average to be at least 0.85 f'c (21.25 MPa) and no single core below 0.75 f'c (18.75 MPa). Cores must avoid reinforcement and be tested after curing. If cores pass, the concrete can be accepted; if not, the engineer needs a structural evaluation of load capacity and may require remediation or rejection.
Do not sign off on the inspector's logic. Document the nonconformance now, while the slab is accessible and before the next pour repeats the problem.
2. Assess the water addition and the temperature/slump conditions. What was improper, and what should happen before the next pour?
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Water addition. Adding 25 L to a 6 m³ load is about 4.2 L/m³. With roughly 350 kg/m³ of cementitious material, that raises the water/cementitious ratio by about 0.012. The direct strength penalty may be only a few percent, but the real issues are control and precedent: water added on site after initial mixing is normally prohibited unless the mix design and specification expressly allow it, it is recorded, and the required revolutions are applied. Here the slump moved from 165 mm to 120 mm, still outside the 100 ± 25 mm band, so the addition did not even bring the load into specification.
Temperature. At 31.8 °C the concrete was at the edge of the 32 °C limit with a 90-minute haul. High placement temperature accelerates hydration, increases water demand and early cracking risk, and makes slump loss harder to manage. The crew compensated for slump loss with water rather than controlling delivery.
Before the next pour: pre-cool or ice the mix, shorten the haul or adjust dispatch, use a approved retarder/plasticiser, reject loads that arrive out of slump or temperature, and record every site addition with quantity and mixer revolutions. Brief the batch plant and the crew, and require the inspector to document slump and temperature at discharge, not at the point of placement only.
3. The inspector wants the results accepted and the next pour to proceed on schedule. How should the engineer handle the schedule pressure?
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The engineer's obligation is to the specified performance and to the record, not to the pour schedule. A practical sequence:
- Issue a written nonconformance identifying the failing test, the affected placement, and the required core testing. This protects all parties and prevents later disputes about what was known.
- Hold the next pour of the same mix until the cause is understood. The five results are low as a group and the site water addition is a plausible contributing cause; repeating the mix without correction risks a second nonconforming element.
- Separate the schedule question from the acceptance question. Cores take time, but a slab that later requires demolition is far more expensive than a short delay. Ask the contractor for a revised sequence that pours unaffected areas first.
- Communicate in writing to the contractor and owner, stating the basis, the required tests, and the decision timeline. Offer to review core results promptly.
If the contractor proposes to proceed on a 'verbal OK', decline and state that only written acceptance after satisfactory results constitutes approval.