What MPa means
MPa (megapascals) is a measure of compressive strength: how much load the concrete can take before it crushes, tested on a sample cured for 28 days. A 32 MPa mix is stronger than a 20 MPa mix. Strength also tends to track with durability, so higher grades resist wear, weather and surface damage better, which is why exposed and trafficked work uses higher numbers.
Concrete is governed by AS 1379 for supply and testing. When you order, three numbers describe the mix: strength (MPa), slump (how wet and workable, in mm) and aggregate size (20mm standard). Strength is the one most people focus on.
The usual choices
| MPa | Typical use |
|---|---|
| 20 MPa | Paths, footpaths, footings, blinding, kerbs |
| 25 MPa | Domestic slabs, shed floors, general slabs |
| 32 MPa | Driveways, exposed aggregate,outdoor and wear surfaces |
| 40 MPa + | Structural columns, beams, heavy-duty and engineered work |
These are general guides for typical work. Always follow the engineer’s specification where one exists; never go below the specified strength.
Slump and aggregate, briefly
The thing nobody tells you: scheduling
Concrete starts setting the moment it is batched, so the spacing of trucks has to match how fast your crew can place it. Too fast and trucks queue while the concrete ages on the back; too slow and you get cold joints between pours. The same logic applies to cement treated rock and stabilised sand, which is exactly why live truck tracking earns its keep on pour days.