Does rain right after a pour hurt Ottawa concrete?

We found no Building Code clause or neutral source that says how rain affects a fresh pour, so the protection plan belongs in your contractor's written specification. What the 2024 Ontario Building Code does say is related: its appendix note to 9.3.1.7 says adding water to concrete on site may make it easier to place but can reduce strength and increase porosity and the propensity to shrinkage cracking, and the Code caps the water-to-cementing-materials ratio at 0.45 for exterior flatwork and garage floors.

The Ready Mixed Concrete Association of Ontario's driveway maintenance guidance (industry guidance, not law) describes curing as maintaining a satisfactory moisture content and temperature for a minimum of 7 days. When the air is below 5 °C, the Code requires the concrete to be kept at not less than 10 °C for 72 hours after placing, and the National Research Council's Canadian Building Digest 116 says freezing of concrete in the plastic state usually results in permanent damage, even with a single cycle.

A few things are worth confirming with the contractor before the pour:

• what protection the contractor will have on site if rain arrives during or soon after the pour
• what the plan is if the air temperature drops below 5 °C during or after the pour
• that no dry cement will be added to the surface to absorb surplus water, which the Code (section 9.16.4.1) prohibits for floors-on-ground
• that the concrete strength for exterior flatwork meets the Code's 32 MPa minimum at 28 days

The contractor doing the work is required to hold a City of Ottawa hardscaping contractor licence for concrete patios, walkways, steps and pads on private property. Ask to see a copy of the licence and confirm the contractor appears on the City's published list of licensed hardscaping contractors before work begins.