Yield Strength of Rebar: What the “500” in B500B Actually Means

Yield Strength of Rebar: The Core Concept

The yield strength of reinforcing steel is the stress at which the bar begins to deform plastically — the point beyond which deformation is permanent and the steel no longer returns to its original shape on unloading. In structural engineering, yield strength is the primary design parameter: Eurocode 2 bases all reinforced concrete design on the characteristic yield strength f_yk. The “500” in B500B refers directly to this: a minimum characteristic yield strength of 500 MPa (N/mm²).

Understanding this value matters for procurement because yield strength determines the cross-sectional area of steel required to resist a given load. Higher yield strength means less steel by weight for the same structural capacity — which is why the shift from older 400 MPa grades to 500 MPa grades reduced rebar consumption across Europe significantly.

Re vs Rp0.2: Two Ways to Measure Yield

Steel with a well-defined yield plateau exhibits a clear upper and lower yield point — labelled R_eH (upper yield strength) and R_eL (lower yield strength) in EN ISO 6892-1 tensile testing. Hot-rolled rebar in grades B500B and B500C typically shows this behaviour, and the relevant value reported on a Mill Test Certificate is R_e (yield strength, MPa).

Cold-worked products (B500A coil, some mesh) may lack a distinct yield point. For these, the proof strength R_p0.2 is used — the stress at which 0.2% permanent strain occurs. Both R_e and R_p0.2 serve as the measured yield strength; the standard and MTC will indicate which applies.

DIN 488 / EN 10080 Requirements

Note that DIN 488 also sets an upper bound of 650 MPa on yield strength. This cap prevents bars from being excessively strong, which is important in capacity-designed seismic structures and ensures consistency with Eurocode 2 design tables.

Characteristic vs Actual Yield Strength

The characteristic yield strength f_yk = 500 MPa is a statistical lower bound — defined in EN 10080 as the value below which no more than 5% of test results fall (5th percentile). The actual yield strength of any individual bar will typically be higher, often in the range 530–600 MPa for quality-produced B500B. This “overstrength” is a safety benefit in normal design but must be accounted for in seismic capacity design — which is why B500C has the additional upper cap on k.

When reviewing a Mill Test Certificate, you will see the actual measured R_e for the specific heat or delivery batch — not the characteristic value. Both values are important: the characteristic underpins design; the actual confirms the specific product meets the standard.

How Eurocode 2 Uses Yield Strength

In Eurocode 2 (EN 1992-1-1), the design yield strength of reinforcement is:

f_yd = f_yk / γ_s

Where γ_s = 1.15 is the partial factor for reinforcing steel (persistent/transient design situations). For B500B: f_yd = 500 / 1.15 = 434.8 MPa. This is the value used in all section design calculations — cross-sectional areas, beam bending, column interaction diagrams, and shear links. A higher f_yk means a lower area of steel required, but European practice has standardised on 500 MPa as the optimum balance of strength, ductility, weldability, and cost.

What to Check on a Mill Test Certificate

• R_e or R_p0.2: Must be ≥ 500 MPa and ≤ 650 MPa
• R_m: Must satisfy the k ratio for the declared grade (k ≥ 1.08 for B500B)
• A_gt: Must be ≥ 5.0% for B500B, ≥ 2.5% for B500A
• Standard reference: DIN 488 and/or EN 10080 should be cited
• Certificate type: EN 10204 3.1 — inspection by an independent third-party or mill inspector; 2.2 is mill’s own declaration (less rigorous)

All B500B rebar and related products supplied by Steel Rebar Germany are accompanied by EN 10204 3.1 Mill Test Certificates with full mechanical property reporting.

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