Cold-Weather Placement and Protection Timing
Concrete is placed at 4:00 p.m. in ambient air at 38 F with a forecast low of 22 F overnight and 30 F the following day. The mix contains no accelerator and no air entrainment beyond the 5% specified for the exposure class. The inspector observes that insulating blankets are on site but stacked in a corner, not deployed. The finishing crew plans to apply a curing compound at 7:00 p.m. and leave. The structural element is a 10 in. thick wall. The specification requires that concrete be maintained above 50 F for the first 48 hours.
1. What is the immediate inspection concern, and what should the inspector do?
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At 38 F ambient with a 22 F low, the concrete will lose heat rapidly, especially a 10 in. wall with high surface-to-volume ratio. The 50 F / 48-hour protection requirement is not being met if blankets stay stacked. The inspector should issue a nonconformance immediately, before the pour is covered up: require blankets to be deployed as soon as finishing permits, verify the concrete temperature at the surface and at depth, and document the actual protection timeline. Judgment: the concern is not the mix design - it is the protection plan. The inspector's job is to intervene in real time, not to note it in a report the next morning.
2. The crew applies curing compound at 7:00 p.m. and blankets at 9:00 p.m. Is this acceptable?
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Applying curing compound is necessary but not sufficient; compound alone does not provide thermal protection. Blankets at 9:00 p.m. leave a two-hour gap during which surface temperature can drop. More importantly, the inspector must verify that the blankets actually maintain the concrete above 50 F - measure, don't assume. A 10 in. wall in 22 F air may need more than a single layer. Judgment: acceptable only if temperature monitoring confirms the 50 F threshold is held for 48 hours. If monitoring shows excursions below 50 F, the event must be documented and the engineer notified, because early freezing can permanently reduce strength.