Transfer Strength and Steam-Curing Temperature Nonconformance
A precast plant produces 400 mm deep prestressed hollow-core slabs with a mix containing 420 kg/m³ Type III cement, 7% silica fume, w/cm 0.32, and a polycarboxylate retarder. The specified transfer strength is 32 MPa. To meet a shortened schedule, the steam-curing chamber is raised to 85°C for 18 hours; the plant had been operating at 65°C. At transfer, three 100 mm × 200 mm cylinders break at 28.5, 29.8, and 30.5 MPa (average 29.6 MPa). The plant manager argues that the cylinders were probably damaged by stripping and wants to transfer tension based on the 85°C cure and a maturity meter reading. A walk-through shows fine map cracking on the top surface of several slabs and no recorded concrete temperature during the first 12 hours.
1. Can the plant transfer prestress based on the 29.6 MPa average and the maturity reading? Explain.
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No. The specified transfer strength is 32 MPa, and the average of the three cylinders is 29.6 MPa; the lowest is 28.5 MPa. Transferring at 29.6 MPa would violate the specified minimum unless the engineer approves a lower value after evaluation, which is not the plant manager's call. A maturity meter can be used only if it has been correlated with strength for the same mix, curing history, and materials, and the correlation is documented. A maturity index is not a substitute for the specified acceptance cylinders when those cylinders are below the required strength. The claim that cylinders were damaged must be investigated, but it does not convert failing results into passing results. Hold transfer, retest representative cylinders, and verify curing and test procedures.
2. What is the significance of the 85°C steam cure and the map cracking?
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The 85°C chamber temperature is above the roughly 70°C surface-temperature limit commonly imposed to reduce delayed ettringite formation (DEF) risk in steam-cured precast concrete. High early temperature can also cause thermal gradients, surface cracking, and later strength loss. The fine map cracking is consistent with DEF or thermal/plastic shrinkage cracking, but a visual pattern is not a diagnosis. Because no concrete temperature was recorded for the first 12 hours, the actual maturity and peak temperature are unknown. The combination of low transfer strength, high steam temperature, and surface cracking is a serious nonconformance. Suspend the affected production, review the curing records, and require petrographic examination and strength testing before accepting the slabs.
3. List the immediate actions and the investigation needed before production restarts.
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Stop transfer and steam curing on the affected units. Record chamber and concrete temperatures going forward. Reduce the steam temperature to the project limit and verify the curing cycle. Retest cylinders from the same batches, check cylinder fabrication and handling, and run a maturity correlation if the plant wants to use maturity. Quarantine the cracked slabs and those cured at 85°C. Notify the engineer and owner; issue an NCR. Do not ship or transfer until the engineer accepts the strength and durability evaluation. If the mix or curing cycle is changed, require trial batches and transfer-strength verification before restarting production.