What Is f_ck (Concrete Compressive Strength)?

Definition: f_ck Is the Characteristic Cylinder Strength

f_ck is the characteristic compressive strength of concrete, defined in EN 1992-1-1 (Eurocode 2) as the value below which no more than 5% of test results are expected to fall — i.e. the 5th percentile of the strength distribution. It is measured on standard 150 mm diameter × 300 mm height cylinders tested at 28 days, following EN 12390-3.

This definition matters enormously for procurement teams ordering rebar: the concrete grade (which expresses f_ck) determines the required reinforcement ratio, bar diameter, and cover — all of which feed directly into the bill of materials you issue to your steel supplier.

Cylinder Strength vs Cube Strength: f_ck vs f_cu

A frequent source of confusion in international procurement: two different test geometries produce two different numbers for the “same” concrete.

• f_ck (cylinder) — EN standard, used in Eurocode 2. Lower number.
• f_cu (cube) — British (BS 8110) and many international standards. Higher number because the platens restrain the cube, giving a higher measured failure load.

The empirical relationship is approximately: f_ck ≈ 0.80 × f_cu. This approximation is encoded in EN 206 and EN 1992-1-1, though precise ratios vary by mix and strength level.

EN 206 Concrete Grade Notation

Under EN 206, concrete is designated using both strength values: C{f_ck}/{f_cu}. The table below shows common grades used across reinforced concrete projects and their design compressive strength f_cd (= f_ck/γ_c, with γ_c = 1.50):

How f_ck Connects to Rebar Sizing

The concrete compressive strength and the rebar yield strength (f_yk) work together in the flexural design of reinforced concrete. A simplified illustration: for a singly reinforced rectangular beam, the required steel area A_s depends on the moment capacity equation that includes both f_cd (derived from f_ck) and f_yd (derived from f_yk). Increasing f_ck allows a smaller section or less rebar for the same load. This is why project specifications always pair a concrete grade with a rebar grade.

For B500B rebar (f_yk = 500 MPa, f_yd = 500/1.15 ≈ 435 MPa) paired with C30/37 concrete (f_cd = 20 MPa), the reinforcement ratio for a typical slab midspan moment will typically fall in the range of 0.3–0.8% — indicative values only; project-specific design is required.

See our B500B rebar product page for grade data, or our Standards & Certification guide for how these values appear in Mill Test Certificates.

f_ck in Mill Test Certificates and Supply Documentation

f_ck is a concrete parameter, not a steel parameter — it will not appear on your rebar Mill Test Certificate (MTC). However, understanding the concrete grade your engineer specified helps you confirm you are ordering the correct rebar grade and ductility class for the design. Key values that do appear on a DIN 488 / EN 10080 MTC include:

• f_yk (characteristic yield strength) — minimum 500 MPa for B500B
• f_t/f_y (tensile-to-yield ratio) — minimum 1.08 for B500B
• A_gt (total elongation at maximum force) — minimum 5.0% for B500B
• Chemical composition (C, Mn, S, P, Si, Ceq)

Steel Rebar Germany supplies B500B bars and reinforcing mesh with EN 10204 3.1 certificates covering all these parameters. Contact us via our quote form for project-specific documentation review.