Pouring concrete on frozen ground in Ottawa: allowed?

Two clauses of Ontario's 2024 Building Code bear on this. Section 9.3.1.9 says no frozen material or ice shall be used in concrete, and that when the air temperature is below 5 °C, concrete must be kept at not less than 10 °C and not more than 25 °C while being mixed and placed, then maintained at not less than 10 °C for 72 hours after placing.

The Code's concern is the concrete itself, not just the air. The National Research Council's Canadian Building Digest 116 notes that freezing concrete in the plastic state usually results in permanent damage, even with a single freeze-thaw cycle. Frozen ground beneath a fresh pour is a separate problem: the 2024 Code, at section 9.12.1.3, requires that the bottom of excavations be kept from freezing throughout construction. We found no Code clause written for a patio slab placed on frozen ground, so ask your contractor and, for anything structural, the City's Building Code Services.

For a cold-weather pour the contractor has to meet those temperatures and the 72-hour period; the Code does not say how. Ask the contractor to describe in writing exactly how they will meet the 9.3.1.9 requirements for your pour date, including how the subgrade will be prepared and how the concrete will be protected afterward.

A few other points worth confirming before work starts:

• The concrete strength for exterior flatwork and garage floors is not less than 32 MPa at 28 days under section 9.3.1.6(1). Ask for this in the written specification.
• The maximum water-to-cementing-materials ratio for exterior flatwork and garage floors is 0.45 under section 9.3.1.7(1). The Code's appendix notes that adding water on site can reduce strength and increase the risk of shrinkage cracking.
• Since March 1, 2026 the City of Ottawa has required a hardscaping contractor licence for businesses installing concrete pads, patios, walkways, steps or retaining walls on private property. The City publishes a list of licensed contractors; ask to see the licence before signing.
• Whoever digs for footings or a base is an excavator under the Ontario Underground Infrastructure Notification System Act, 2012, and must submit a locate request before breaking ground.

Before committing to a winter pour date, ask the contractor to confirm in writing what temperature the subgrade will be at when the concrete is placed, and how they will verify it.