What MPa actually measures
MPa (megapascals) is a unit of pressure. On a concrete quote it means compressive strength: how much crushing force a standard test sample withstands. The number is always tied to an age. “32 MPa” means the concrete is specified to reach at least 32 MPa when tested at 28 days. That is a test age, not the day the concrete is “done”.
If you’re used to American figures, 1 MPa is about 145 psi:
| MPa | Approximate psi |
|---|---|
| 15 MPa | 2,175 psi |
| 20 MPa | 2,900 psi |
| 25 MPa | 3,625 psi |
| 30 MPa | 4,350 psi |
| 32 MPa | 4,640 psi |
| 35 MPa | 5,075 psi |
Strength is the number homeowners hear most. For outdoor concrete in Ottawa, it’s only one third of the spec.
The CSA exposure classes, in plain language
The Canadian concrete standard, CSA A23.1, doesn’t start with strength. It starts with exposure: what the concrete will face. Each class sets a maximum water-to-cementing-materials ratio (w/cm), a minimum strength and an air-content category together.
| Class | What it’s exposed to | Max w/cm | Min strength (28 d) | Air | CSA examples |
|---|---|---|---|---|---|
| C-1 | Chlorides, structurally reinforced | 0.40 | 35 MPa | Category 1 | Parking decks, bridge decks |
| C-2 | Chlorides and freeze-thaw, plain concrete | 0.45 | 32 MPa | Category 1 (5–8 %) | Garage floors, porches, steps, pavements, sidewalks, curbs |
| F-1 | Freeze-thaw while saturated, no chlorides | 0.50 | 30 MPa | Category 1 | Pool decks, patios, tennis courts |
| F-2 | Freeze-thaw, unsaturated, no chlorides | 0.55 | 25 MPa | Category 2 | Exterior walls and columns |
| N | No chlorides, no freeze-thaw | — | — | — | Footings, interior slabs, walls |
Air-content category 1 means 5–8 % air for the 14–20 mm stone typical in residential mixes (4–7 % for larger 28–40 mm stone).
The key line for homeowners is C-2. CSA names steps and porches in it directly, and Concrete Ontario’s residential guide applies C-2 to “garage floors and all concrete exposed to freeze-thaw and de-icing salts, such as walkways, driveways, patios, steps”.
What the Ontario Building Code requires
The Ontario Building Code turns the same thinking into minimums for houses:
- OBC 9.3.1.6(1): unreinforced concrete must reach at least 32 MPa at 28 days for garage floors, carport floors and all exterior flatwork; 20 MPa for interior floors; 15 MPa for other applications.
- OBC 9.3.1.6(2): site-batched concrete for garage floors, carport floors and exterior flatwork needs 5–8 % air entrainment.
- OBC 9.3.1.7(1): the maximum water-to-cementing-materials ratio is 0.45 for garage floors, carport floors and all exterior flatwork; 0.65 for interior floors; 0.70 for other applications.
Concrete Ontario also notes that OBC 9.16.4.5 requires 25 MPa for a floor slab where damp-proofing isn’t provided (15 MPa is allowed with damp-proofing).
Concrete Ontario’s residential classes
Concrete Ontario (the Ready Mixed Concrete Association of Ontario, RMCAO) groups residential concrete into a few classes that line up with the Code:
| RMCAO class | Use | Strength |
|---|---|---|
| R-1 | Footings | 15 MPa |
| R-2 | Foundation walls | 15 MPa, 4–7 % air |
| R-3 | Interior slabs not exposed to freeze-thaw | 20 MPa |
| C-2 | Garage floors, walkways, driveways, patios, steps | 32 MPa, 0.45, 5–8 % air |
What each project needs
Here is how that comes together for common jobs. The MPa tool on this page lets you click through each one.
| Project | Class | Strength | Air | Max w/cm |
|---|---|---|---|---|
| Front steps | C-2 | 32 MPa | 5–8 % | 0.45 |
| Porch or landing | C-2 | 32 MPa | 5–8 % | 0.45 |
| Driveway | C-2 | 32 MPa | 5–8 % | 0.45 |
| Walkway | C-2 | 32 MPa | 5–8 % | 0.45 |
| Garage floor | C-2 | 32 MPa | 5–8 % | 0.45 |
| Patio or pad, never salted | F-1 | 30 MPa | 5–8 % | 0.50 |
| Patio or pad near a door | C-2 (our advice) | 32 MPa | 5–8 % | 0.45 |
| Foundation wall | R-2 | 15 MPa | 4–7 % | — |
| Interior slab | R-3 | 20 MPa (25 MPa without damp-proofing) | — | 0.65 |
We recommend C-2 for pads near doors because they almost always end up salted, even if that’s not the plan. That’s advice, not a Code requirement.
Why air and water-cement ratio matter as much as MPa
Think of the three numbers as a team:
- Water-cement ratio controls how porous the hardened paste is. Extra water makes concrete easier to place but leaves more pores for salty water to fill. That is why exterior concrete is capped at 0.45.
- Entrained air is billions of microscopic bubbles mixed in on purpose. When water in the pores freezes and expands, those voids give it somewhere to go. Concrete Ontario explains that at an early age the voids can still be saturated, which is why new concrete is vulnerable to de-icers.
- Strength is the number that gets tested, but CSA sets it alongside the other two for a reason: in every exposure class, the strength minimum, the water-cement ratio cap and the air category come as a package.
The National Research Council’s guidance for severe frost and salt exposure (CBD-116) lists air-entrained concrete and a water-cement ratio of 0.45 or less before anything else. It then adds the things no mix ticket can guarantee: careful compaction, finishing that doesn’t bring water and fines to the surface, good curing, and air-drying before salt exposure.
A higher number is not a substitute for good work
A 35 MPa mix finished with bleed water worked into the surface can still scale. A perfect mix on a soft base can still settle and crack. When concrete fails in Ottawa, the cause is usually one of these:
- the wrong mix, or water added on site to make it easier to place
- finishing errors that weaken the surface
- short or no curing
- salt in the first winter
- a poor base or missing support under steps
Our article on why Ottawa concrete cracks and scales walks through each one, and how concrete is poured shows where each step happens.
Questions to ask a contractor
Before you sign, ask any contractor, including us:
- What exposure class and strength are you ordering? For steps, porches, driveways, walkways and garage floors, the answer should be C-2, 32 MPa.
- What air content? It should be 5–8 %.
- What water-cement ratio, and will water be added on site?
- How thick is the slab and how deep is the base? The Code doesn’t set these for exterior flatwork, so get them in writing.
- How will it be finished and cured, and for how long?
- What do I do in the first winter? The answer should include no de-icer of any type.
Our contractor hiring guide has the full checklist.
The short version
For almost every exterior job at an Ottawa house (steps, porches, driveways, walkways and garage floors), the spec is CSA C-2: 32 MPa at 28 days, 5–8 % air and a water-cement ratio of 0.45 or less. The Ontario Building Code sets the same minimums. MPa alone won’t protect concrete from freeze-thaw and salt; air, water content, finishing and curing do the rest. Ask for all three numbers in writing. To see what the spec costs, start with the concrete cost guide.
Sources
Every technical claim in this article traces to a published source. We review articles when codes or standards change.
- CSA A23.1 exposure classes (via Atlantic Concrete)
- CSA exposure classes (via Thousand Islands Concrete)
- Ontario Building Code 9.3.1.6 — compressive strength
- Ontario Building Code 9.3 — materials (BuildCodes.ca)
- Concrete Ontario — Residential Concrete
- NRC — Durability of Concrete Under Winter Conditions (CBD-116)
- Concrete Ontario — De-icing Salt Usage