ACI 306 is the American Concrete Institute's guide for cold weather concreting. It is the winter counterpart to ACI 305R: where 305R protects concrete from drying out too fast, 306 protects it from the two things cold does to a fresh pour — freezing before it gains strength, and gaining strength so slowly that the schedule falls apart.
The core risk is simple. Fresh concrete is mostly water. If that water freezes before the concrete has developed enough strength, the expanding ice destroys the internal structure and the slab never reaches its design strength. ACI 306 exists to make sure that does not happen.
Cold weather is not a single cold night. ACI 306 defines it as a period of more than three consecutive days where the average daily air temperature is below 40°F, and the temperature stays below 50°F for more than half of any 24-hour period.
That “running condition” definition matters on the job: the decision is not about the temperature at the moment you pour, it is about the temperature across the whole protection window that follows. A mild afternoon pour followed by three freezing nights is a cold-weather pour.
ACI 306 sets a minimum as-placed concrete temperature that depends on the section size, because thinner sections lose heat to the environment faster and need to start warmer:
Sections under 12 in: 55°F • 12 to 36 in: 50°F • 36 to 72 in: 45°F • Over 72 in: 40°F
The surface the concrete is placed on must also be above freezing. Pouring onto frozen ground pulls heat out of the bottom of the slab and can freeze it from below — and the frozen subgrade will heave and settle as it thaws, cracking the slab regardless of how well you protected the top.
The single most important number in cold-weather concreting is 500 psi. That is roughly the compressive strength at which enough of the mix water has been consumed by hydration that the remaining water will not do freeze damage. Below it, a hard freeze can ruin the pour. Above it, the concrete can take a freeze and keep gaining strength.
So the protection window is: keep the concrete above freezing (and ideally above 50°F to keep it curing) until it reaches 500 psi. For conventional concrete that is typically a minimum of 48 hours, and can run 1 to 4 days depending on temperature and whether the mix included an accelerator.
Protection methods: insulating blankets, heated enclosures, ground thawing before placement, and mix adjustments like accelerators or a higher cement content. The goal is to hold the concrete's own hydration heat in long enough to get to 500 psi.
Pulling blankets too early is one of the most common — and most expensive — cold-weather mistakes. If protection comes off before the concrete reaches strength, or comes off so fast that the warm surface thermally shocks against cold air, you lose strength you never get back: 20 to 50 percent in bad cases.
Two rules: do not remove protection until the concrete has reached the required strength (verified by maturity method or field-cured cylinders), and when you do, remove it gradually so the surface does not drop more than about 40°F in 24 hours. The concrete should come back to ambient temperature slowly.
Because ACI 306 is about the whole protection window, cold-weather decisions get made the day before: order blankets, schedule the accelerator, decide whether to pour at all. The go / no-go depends on the multi-day low-temperature forecast, not just tomorrow morning.
PourDay reads the forecast for your exact job site so you can see the cold window coming and plan protection before the truck is scheduled — the same way you would check hot-weather conditions in summer. Cold weather is the other half of knowing before you pour.
See the cold window before you commit the truck. PourDay reads live forecasts for your job site — free on iOS and Android.
The field playbook for cold pours — blankets, heaters, accelerators, and timing.
Read articleThe hot-weather counterpart to ACI 306 — the standard for the other half of the year.
Read articleThe temperature window that keeps concrete out of both the freezing and the flash-set danger zones.
Read articleReal-time evaporation rate. 16-day forecast. Pour logging. Free for contractors.