Cold Weather Concrete and Schedule Pressure
You are pouring a 24-in. thick mat foundation for a warehouse in Flagstaff, Arizona, in November. The forecast shows daytime highs of 45°F and nighttime lows of 20°F for the next week. The mix design is a 4000-psi mix with 20% fly ash and a water-cement ratio of 0.45. The structural engineer specified a maximum temperature differential of 35°F between the core and surface to prevent thermal cracking. The owner is pushing to pour because the steel erection crew is scheduled to arrive in 10 days. You have one week to cure before they start. The ready-mix supplier can provide heated concrete at 65°F. You have insulated blankets and heaters available. The project is in Coconino County, which follows ACI 306 for cold weather concreting.
1. What are the cold weather concreting requirements and how should you plan the pour?
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ACI 306 requirements: Cold weather concreting is defined as when the air temperature is below 40°F or when concrete temperature is below 50°F. You must maintain concrete temperature between 50°F and 70°F during placement and protect it from freezing. For a 24-in. thick mat, the heat of hydration will be significant, but the cold ambient will cause rapid surface cooling.
Plan:
- Use heated concrete at 65°F as supplied.
- Preheat forms and subgrade to above freezing.
- Pour during the warmest part of the day.
- Immediately after finishing, cover with insulated blankets and use heaters to maintain surface temperature above 50°F for at least 7 days.
- Monitor temperature at the core and surface with thermocouples to ensure differential does not exceed 35°F. The core may reach 140°F due to hydration; if so, you may need to cool the concrete or use extra insulation on the surface to reduce differential.
- Do not remove forms or blankets until the concrete has reached 500 psi or the specified curing time, whichever is greater.
- Keep records of temperatures and curing conditions.
2. What are the risks of proceeding and how should you communicate with the owner?
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Risks:
- Thermal cracking: If the differential exceeds 35°F, the concrete may crack, compromising the mat's integrity. Repair could be costly and delay the project.
- Freezing: If concrete freezes before reaching 500 psi, it can be permanently damaged, requiring removal and replacement.
- Schedule: If the concrete does not reach sufficient strength in 7 days, the steel erection may be delayed. The mix with 20% fly ash may gain strength slower in cold weather; consider using accelerators or increasing cement content, but that may affect the mix design approval.
Communication: Inform the owner of the risks and the need for additional measures (heaters, blankets, monitoring). Provide a cost estimate for the cold weather protection (e.g., $15,000). If the owner insists on proceeding without adequate protection, document the owner's direction and note that the contractor is not responsible for resulting defects. Alternatively, recommend postponing the pour until weather improves, but that may delay the project more. Present a risk-based recommendation: proceed with full protection, but monitor closely and be prepared to adjust.