CSA A23.1 Curing Requirements for Canadian Projects

On a Canadian job the drawings rarely say “cure for 7 days.” They say “Class of exposure C-1” or “Curing type: Type 2, additional,” and leave you to know what that means. CSA A23.1 ties the curing type to the exposure class, and each type has a minimum time, a temperature and, for one of them, a required method.

The standard itself is sold by CSA Group and is not free to read. The numbers on this page come from published summaries that quote it, mainly slides by R. Douglas Hooton, chair of the CSA A23.1/A23.2 committee (Hooton, ACI). Treat them as second-hand and check the edition your contract names.

What are the CSA A23.1 curing types?

There are three: basic, additional and extended. Basic is the default for most concrete, additional is for severe durability exposures, and extended is for long service life where chlorides are the threat.

Source: Hooton, ACI, slide 17.

Older project documents word basic curing a little differently. A City of Winnipeg drawing quoting the 2009 edition reads “3 days >10°C and for a time necessary to attain 40% of the specified strength,” with “and” where the newer summary has “or” (Winnipeg drawing S1.1). If your spec uses “and,” cure for both.

Hooton also notes that “These limits are not based on durability performance.” They are minimums. The same slides stress that curing directly affects how well the cover layer resists chloride penetration, so a longer cure still pays.

Which exposure classes need additional or extended curing?

The curing type is part of the requirements for each exposure class in Table 2 of the 2019 edition. The C classes cover chlorides, F freeze-thaw, N concrete not exposed, A agricultural and biogenic acids, and S sulfates. Here is the curing column as shown on the committee chair’s slide.

Source: Hooton, ACI, slide 8, “CSA A23.1-19 Table 2 Exposure Class Req’ts.”

The designer can ask for more than the table minimum. The same Winnipeg drawing lists exterior concrete as class F-2, which the table puts at basic, but specifies “Curing type: Type 2, additional.” When the drawing asks for more than the table, follow the drawing.

Can you use curing compound to meet CSA A23.1?

Yes, for basic and additional curing. Canadian specs commonly call for ASTM C309 compounds. A City of Winnipeg cast-in-place concrete section specifies “Cure and sealing compound: to ASTM C309, Type 1” and says to cure and protect concrete to CSA A23.1, with “the curing regime… consistent with the Class of Exposure” (Winnipeg Section 03 30 00).

Extended curing is different. It has to be a 7-day wet cure by ponding, sprinkling or wet mats, so a membrane compound alone does not meet it. On C-XL work, plan for water, wet burlap and the labor to keep it wet for the full week.

On provincial highway and bridge work, check for a provincial specification or product list as well, the way state DOT curing compound specifications do in the US.

What temperature does CSA curing require?

The basic and additional times both count days at 50°F (10°C) or more. A Concrete Floor Contractors Association summary reads this as “a minimum concrete materials temperature of 10°C for the duration of the curing period,” and warns that “cold temperature substrates can also reduce the temperature of the concrete materials below 10°C” (CFCA).

So a day where the slab sits at 45°F (7°C) does not count toward the 3 or 7 days, even if the compound is on. A thermometer in the concrete, or a logger under the blankets, is how you prove the days.

If the weather turns cold during the curing period

Protect the concrete first and keep counting only the days it stays at 50°F (10°C) or more. The building codes add their own rule.

BC Building Code Article 9.3.1.9 says that when the air is below 41°F (5°C), concrete must be kept at 50 to 77°F (10 to 25°C) while it is mixed and placed, and at 50°F (10°C) or more for 72 hours after placing (District of Mission).

That 72-hour code rule lines up with basic curing, but it does not replace a 7-day additional cure. Use blankets, enclosures and heat as needed, and see curing concrete in cold weather for the protection side. For how long strength gain takes at different temperatures, use the concrete curing time chart.

How does CSA A23.1 curing compare with ACI 308?

They are close. FHWA summarizes ACI 308.1, the curing specification in the ACI 308 family, as a default of 7 days when the temperature is above 50°F (10°C), with early termination allowed at 70% of specified strength (FHWA). That is the same shape as CSA additional curing.

The big difference is structure. CSA picks the curing type from the exposure class, so the same crew can owe 3 days on an interior class N slab and a 7-day wet cure on C-XL work in the same pour week.