Cold-Weather Protection Decision on a Mat Foundation
A 3 ft thick mat foundation is placed in late November. Ambient temperature is 28 °F and falling; the forecast low is 18 °F. The mix has 500 lb/yd³ cementitious with 30% slag replacement, 0.45 w/c, and no accelerator. The contractor plans to place, screed, and cover with a single layer of plastic sheeting and one layer of blankets. The concrete temperature at placement is 55 °F. The spec requires the concrete to be maintained above 50 °F for the first 48 hours and above 40 °F until it reaches 500 psi. The contractor wants to strip forms at 24 hours to reuse them on another pour.
1. Evaluate the protection plan against the specification and the cold-weather requirements.
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The plan is likely inadequate. Key issues:
- Placement temperature: 55 °F is acceptable as a starting point, but with 28 °F ambient and an 18 °F low, heat loss from a 3 ft mat is significant, especially at edges and the top surface. A single layer of plastic plus one blanket is minimal for these conditions.
- The spec requires >50 °F for 48 hours. With 30% slag, early heat generation is lower and strength gain is slower; the mat may not hold 50 °F for 48 hours under one blanket at 18 °F.
- The spec also requires >40 °F until 500 psi. At 55 °F placement and slow slag hydration, 500 psi may not be reached quickly; the contractor needs a maturity or cylinder-based estimate, not a guess.
- Plastic directly on fresh concrete can cause surface defects and condensation; a curing/protection sequence should be planned.
Recommendation: increase insulation (two blankets, insulated edges and corners), monitor concrete temperature at the surface and interior, and use a maturity method or field-cured cylinders to confirm 500 psi before reducing protection.
2. The contractor wants to strip forms at 24 hours. What must be verified, and what are the risks?
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Form stripping at 24 hours is a strength and thermal-shock decision, not a schedule decision. Before stripping:
- Verify the concrete has reached the strength required for the member and for form removal (often 500 psi or as specified), using field-cured cylinders or maturity.
- Check the temperature differential between the concrete surface and the ambient air. Stripping a warm mat into 18 °F air can cause thermal shock and cracking, especially at edges. The surface-to-ambient differential should be limited (commonly ~20–30 °F depending on the reference); if exceeded, delay stripping or provide temporary enclosure/warming.
- Ensure continued protection after stripping - the mat still needs to stay above 40 °F until 500 psi and above 50 °F for 48 hours.
Risks: thermal cracking from sudden cooling, reduced early strength, and a nonconforming mat. The engineer should require documented strength and temperature data before approving early stripping.
3. Describe the temperature monitoring and contingency plan you would require.
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Monitoring:
- Place thermocouples or temperature sensors at the surface, mid-depth, and near the bottom of the mat, plus at edges/corners (the most vulnerable).
- Record ambient temperature and concrete temperature at least every 4 hours for the first 48 hours, and daily thereafter until 500 psi is confirmed.
- Use the maturity method (ASTM C1074) or field-cured cylinders to estimate strength gain.
Contingency:
- Pre-stage additional blankets, insulated tarps, and heaters (indirect-fired, vented) in case temperatures drop below the spec minimums.
- If concrete temperature approaches 50 °F, add insulation or heat; if it approaches 40 °F before 500 psi, stop and re-protect.
- Have a plan for wind: windbreaks and secured covers.
- Document all readings and actions; notify the engineer if limits are approached.
The goal is to keep the mat above the spec minimums with margin, not to react after the fact.