The easy window
At the Boise airport, the median last spring freeze is April 24 and the median first fall freeze is October 21, a frost-free season of about 179 days. Mid-April through October is the easiest time to pour in Meridian. Nights are above freezing, the ground is workable and concrete gains strength steadily. May, June, September and early October are the most comfortable months of all.
Cold weather
The American Concrete Institute defines cold weather for concrete as air temperatures at or expected to fall below 40°F during the protection period. New concrete has to be protected from freezing until it reaches about 500 psi. At 50°F most well-proportioned mixes get there within about 48 hours; colder, it takes longer. In Meridian, freezing nights are normal from November through March: about 16 in November, 25 in December, 25 in January, 20 in February and 11 in March.
We still pour in those months when the job calls for it. That means ordering a heated mix with an accelerator, never pouring on frozen ground or into a trench with frost in it, covering the slab with insulated blankets right after finishing, and moving the pour when a hard freeze is forecast. Footings for additions and garages are often poured in winter this way. Large decorative slabs are better left for spring.
Hot weather
The Treasure Valley summer is hot and dry. Boise averages about 55 days a year at 90°F or above and about 9 at 100°F or above, and July normally gets only about a fifth of an inch of rain. The airport has recorded 27 days of 100°F or more in a single year (2022) and an all-time high of 111°F. ACI's hot-weather guidance lists high air and concrete temperature, low humidity and high wind together as the conditions that cause trouble, and notes that inadequate curing at high temperatures reduces strength.
So summer pours start at first light. We wet the base so it doesn't pull water out of the concrete, keep the mix temperature down, finish promptly without adding water, and apply curing compound as soon as the surface allows. On very hot, windy afternoons, we don't start a big slab at all.
Wind
The National Weather Service notes that most of Boise's wind comes from the northwest or southeast, that March to mid-May is the windiest stretch, and that strong gusts can reach 40 to 50 mph. Wind dries the surface of fresh concrete fast and can cause small shrinkage cracks before it's even finished. We watch the forecast in spring and set up windbreaks or reschedule when gusts are expected.
How long before you can use it
| Use | Typical wait | Notes |
|---|---|---|
| Walking | Next day | Longer in cold weather |
| Cars | About 7 days | Keep to the center at first |
| Trucks, loaded trailers, RVs | 2 – 4 weeks | Pads for RVs: 3 – 4 weeks |
| Sealer | After cure | Before the first winter |
| Acrylic sport-court surface | About 4 weeks | Coating green concrete blisters |
| Deicers | Not the first winter | Sand only |
Common questions
Can you pour concrete in winter in Meridian?
Yes, with a heated mix, an accelerator, insulated blankets and no frozen ground. The concrete has to be protected until it reaches about 500 psi, roughly two days at 50°F.
Is summer a bad time to pour?
Not if it's done right. We start at first light, keep the mix cool, and cure it immediately. Hot, dry and windy afternoons are what to avoid.
What if it rains after the pour?
Light rain on a slab that's already been finished does little harm. Heavy rain on fresh concrete can mar the surface, so we cover it or reschedule. Summer rain is rare here.
Why did you move my pour date?
Usually a hard freeze or high wind in the forecast. A day's delay costs nothing; a damaged surface lasts for the life of the slab.
Related: Freeze-thaw and deicers · Concrete driveways · Foundations and footings · Sport courts