Concrete Pour Day and Curing: What Happens, What Goes Wrong, and What the Code Requires

The week before

• Footings inspected and approved before the foundation forms go up, foundation forms or ICF inspected before the pour where the municipality requires it
• Rebar placed and tied as the engineer drew it; in ICF, horizontal bars in every course and verticals at the spacing on the drawings
• Bracing up and plumb; on ICF, alignment bracing on every wall with scaffold planks
• Openings bucked and braced; beam pockets and anchor bolt locations marked
• Pump booked, trucks sequenced, a plan for where each truck parks and where the first yard goes
• Weather checked for pour day and the three days after

Pour day, in order

1. Trucks arrive at 20 to 30 minute intervals; the first one is usually the slowest to place because everyone is finding their rhythm.
2. Slump is checked on the first truck; the right slump for walls is wetter than for footings but no water gets added on site beyond what the ticket allows. Water added at the site is the commonest way to turn 25 MPa concrete into 18.
3. Walls are filled in lifts, around the whole perimeter, not one wall at a time. On ICF that means a lift of about four feet all the way around, then the next, so the forms never see the full hydrostatic pressure at once.
4. Each lift is consolidated: internal vibrator on poured walls, a small vibrator or rodding on ICF, especially under window bucks where voids hide.
5. Anchor bolts, hold-downs and any embedded hardware go in while the top lift is wet; the top is struck level.
6. The crew walks the whole wall checking plumb and alignment while it can still be adjusted, then leaves it alone.
7. Slabs are different: screed, bull float, wait for the bleed water to leave, then trowel; finishing too early traps water under the surface and the slab dusts and flakes later.

What the Code requires

Part 9 of the Ontario Building Code sets the minimum compressive strengths at 28 days: 15 MPa for unreinforced footings and foundation walls, 20 MPa for interior floors and slabs, and 32 MPa with 5 to 8 percent air entrainment for garage floors, carport floors, exterior steps and anything else exposed to freeze-thaw and de-icing salt. Engineered designs usually specify higher, 25 MPa or more for ICF walls is common. The ready-mix ticket shows the specified strength and the slump; keep every ticket in the project file. The 2024 Code carries the tables; the concrete calculator sizes the order.

Curing: the part nobody budgets for

Concrete does not dry; it cures, which is a chemical reaction that needs water and warmth. Strength at 28 days depends on keeping the concrete moist and above about 10 degrees C for at least the first three days and ideally seven. Formed walls cure inside the forms; strip them too early in cold weather and the surface freezes and spalls. ICF walls cure inside the foam, which is one of the quiet advantages of the system in an Ontario November. Slabs need a curing compound, wet burlap or plastic, because a slab left to dry in the sun cracks and dusts.

The stripping and loading schedule follows: forms off, backfill, framing. Backfilling a wall before it has strength and before the floor system braces the top is how walls crack and bow; the engineer’s drawings or the ICF manufacturer’s guide give the wait.

Cold and hot weather

Cold. Below 5 degrees C, you need heated concrete from the plant, accelerators where appropriate, insulated blankets or tarps on exposed surfaces, and sometimes a heated enclosure. Concrete that freezes in the first day loses strength it never gets back. We pour foundations all winter in Simcoe County; it costs more and it works when the plan is made before the truck is ordered.

Hot. Above about 27 degrees C, concrete sets fast and dries faster than it cures. Pour early, keep the surface wet, use retarders if the plant recommends them, and never finish a slab by adding water to the surface.

Mistakes that show up years later

• Water added on site: low strength, dusting slabs, cracks
• Voids under window bucks on ICF from skipped vibration: air pockets found when the window is installed
• Footings poured on frozen or disturbed soil: settlement cracks
• No air entrainment in exterior concrete: scaling after the first winters of salt
• Slabs finished too early or cured dry: surface flaking
• Backfill before cure and before the floor is on: bowed or cracked walls
• Anchor bolts set after the fact with epoxy because nobody put them in the wet concrete

Cosmetic vs serious defects explains which cracks matter afterwards, and ICF foundation cost shows what a pour like this costs.

Related guides on this site

Sources and further reading

• The 2024 Ontario Building Code
• National Building Code of Canada 2020
• NRCan: Keeping the Heat In

Questions people ask