Cold-Weather Placement and Protection Verification
You are the special inspector on a slab-on-grade pour in November. Ambient temperature is 38°F and falling; the forecast low is 25°F overnight. The specified minimum concrete temperature at placement is 50°F, and the contractor must protect the concrete from freezing. The concrete arrives at 52°F. The contractor plans to place, finish, and cover with blankets, but the blankets are on site and the crew is behind schedule. The mix contains no accelerators. The slab is 6 in. thick and exposed on all edges.
1. What cold-weather requirements must be verified before and during placement, and is the current plan adequate?
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Cold-weather concreting requirements (per ACI 306 and project specs) to verify:
- Concrete temperature at placement: must be at least 50°F (specified). Arrival at 52°F is acceptable, but temperature must be maintained through placement; cold ambient and wind can drop it quickly.
- Protection from freezing: concrete must be protected from freezing until it reaches sufficient strength (typically until it attains at least 500 psi or as specified). Overnight low of 25°F is below freezing, so protection is mandatory.
- Curing: maintain adequate moisture and temperature; blankets alone may not suffice if the slab is thin and exposed.
- No accelerators in mix: setting will be slow at 38°F, increasing risk of plastic shrinkage and freeze damage.
Assessment: the plan is not adequate as described. Blankets on site but not deployed, crew behind schedule, and no temperature monitoring plan create risk that the concrete freezes before gaining strength. The inspector should require: temperature monitoring (concrete and ambient), blankets applied immediately after finishing, and a contingency if temperatures drop faster than expected. Do not allow placement to proceed without protection in place and a monitoring plan.
2. Overnight the temperature drops to 22°F. The next morning the slab surface shows scaling and some cracking. What do you do?
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The surface damage (scaling, cracking) is consistent with freeze damage before the concrete gained adequate strength. Actions:
- Document the damage: photographs, location, extent, ambient and concrete temperature records, and protection timeline.
- Notify the contractor and EOR in writing; issue a nonconformance report.
- Do not allow the contractor to simply patch or cover up; the EOR must evaluate structural and durability implications.
- Determine whether the damage is cosmetic or structural. Cores or other testing may be required.
- Review whether protection was applied in time and whether temperature monitoring was performed; the record will show whether the contractor met cold-weather requirements.
The inspector's role is to document facts and report, not to prescribe repair. The EOR decides disposition.
3. What records should you have kept to support the cold-weather evaluation?
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Records needed:
- Ambient temperature at placement and overnight (with source/time)
- Concrete temperature at discharge and after placement
- Time of placement, finishing, and blanket application
- Protection method and coverage verification
- Any temperature monitoring data (thermocouples or probes)
- Batch tickets, mix design (no accelerators), and slump/air results
- Photos of the slab before and after damage
- Nonconformance report and communications with contractor/EOR
These records allow the EOR to determine whether the concrete was properly protected and whether the damage is attributable to a construction deficiency. Without them, the evaluation is speculative.