What is the fc-value?
The fc-value is the characteristic compressive strength of concrete, expressed in N/mm² or MPa. It is the principal way to describe the strength of a concrete mix and the basis for reinforcement design, sizing of concrete structures and verification under the Eurocode.
How is fc determined?
The fc-value is established by testing standardised specimens:
- Cube test — 150 × 150 × 150 mm, tested after 28 days
- Cylinder test — 150 × 300 mm, also after 28 days
- Results — a mix is given two figures, e.g. C30/37 = 30 MPa cylinder, 37 MPa cube
Concrete strength classes
Under Eurocode 2 (EN 1992) and EN 206, concrete is classified as:
| Class | fc cylinder (N/mm²) | fc cube (N/mm²) | Application |
|---|---|---|---|
| C12/15 | 12 | 15 | Very light applications |
| C20/25 | 20 | 25 | General floors, simple construction |
| C25/30 | 25 | 30 | Standard housing |
| C30/37 | 30 | 37 | Prestressed concrete, roads |
| C35/45 | 35 | 45 | Bridges, high-rise |
| C45/55 | 45 | 55 | Special structures |
| C55/67 | 55 | 67 | High-strength concrete |
| C80/95 | 80 | 95 | High-strength concrete |
Application
The fc-value is decisive for:
Structural design
- Reinforcement quantity — stronger concrete, less rebar
- Cross-sections — higher fc, slimmer members
- Fire and durability — higher fc gives better performance
- Prestressed concrete — at least C35/45 required
Eurocode calculation
In Eurocode 2 design uses the design value fcd:
fcd = αcc · fck / γc
where:
– fck = characteristic compressive strength (fc)
– αcc = factor (typically 0.85)
– γc = safety factor (1.5 for concrete)
For C25/30 with αcc = 0.85: fcd = 0.85 × 25 / 1.5 = 14.2 N/mm²
Practice
- Foundation concrete — usually C20/25
- Building concrete — C25/30 or C30/37
- Industrial concrete — C35/45 and above
- Screed — C16/20 often sufficient
Cube versus cylinder strength
The cube gives a higher value than the cylinder. Friction between the cube and the press platens reduces actual stress, giving an apparently higher reading. The cylinder is closer to the real strength and is the international standard.
Other strength properties
Concrete also has:
- Tensile strength (fct) — about 1/10 of compressive
- Flexural strength (fctk,fl) — under bending
- Modulus of elasticity (Ecm) — stiffness
- Splitting strength (fct,sp) — under splitting
Conditions for validity
For a reliable fc-value:
- Curing — keep moist during hardening
- Temperature — preferably 20 °C
- Number of specimens — at least 6
- Statistical treatment — characteristic value = 5% fractile
Related terms
- Concrete
- Compressive strength
- Eurocode
- Concrete grade
- Cube strength
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