Cube compressive strength is the compressive strength of concrete as determined on a cube-shaped test specimen. The standard cube has 150 mm sides. The strength is expressed in N/mm² (the same as MPa).
How the test works
- During the pour, test cubes are filled with concrete from the same batch and compacted.
- The cubes cure under standardised conditions: 28 days at around 20 °C, in water or a saturated environment.
- On day 28 the cube is crushed in a testing machine. The maximum force divided by the loaded area is the cube compressive strength.
The 28-day age is the convention against which all strength classes are calibrated. Concrete does keep gaining strength after that, but far more slowly.
Cube or cylinder?
Internationally, cylinders are also tested: 150 mm diameter and 300 mm high. The same concrete gives a lower value on a cylinder than on a cube — in the order of 15 to 20 percent — purely because of the shape and the height-to-width ratio of the specimen.
That is why a strength class designation carries two numbers:
| Designation | Meaning |
|---|---|
| C20/25 | Characteristic cylinder strength 20 N/mm², characteristic cube strength 25 N/mm² |
| C30/37 | Cylinder 30 N/mm², cube 37 N/mm² |
The C stands for concrete; lightweight concrete uses LC. Both numbers refer to the same concrete — they are two ways of measuring, not two requirements.
Why “characteristic”?
The stated value is not the average of the tests but a lower bound: statistically only a small proportion of results may fall below it. The mean strength of a C20/25 batch is therefore noticeably higher than 25 N/mm² in practice.
In practice
- Ask for the test results. On structural work the supplier should be able to demonstrate strength development.
- Strike to strength, not to the calendar. What matters for removing formwork and props is the strength the concrete has actually reached, not the number of days. In cold weather strength development lags well behind.
- Curing is decisive. Concrete that dries out too fast will not reach the specified strength. Covering, keeping it damp or applying a curing compound is not a luxury.
- Assessment of existing work is done with drilled cores; these give different values again from test cubes and have to be converted.
Related terms
- Compressive strength
- Concrete strength class
- Concrete
- Ready-mixed concrete
- Formwork
- Flexural strength
- Initial strength
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