Crack-Width Control with Reinforcement: Eurocode 2 Methods & Worked Example

Why Crack Width Matters for Reinforced Concrete

Cracks in reinforced concrete are normal and unavoidable under service loads. Eurocode 2 does not require crack-free design — instead it limits crack widths to values that prevent functional damage. The two primary concerns are:

• Corrosion protection — wider cracks allow moisture, oxygen, and chlorides to reach the rebar surface faster. The crack width limit tightens with exposure class severity.
• Serviceability appearance — crack widths above ~0.3–0.4 mm are visually noticeable and can concern building occupants or end users.

Crucially, crack width control interacts directly with the bar diameter and spacing you specify. Using fewer, larger-diameter B500B bars at wide spacing produces wider cracks than using more, smaller bars at closer spacing to achieve the same steel area. This means crack control is a procurement-level decision as much as a design one.

Maximum Crack Width Limits — EN 1992-1-1 Table 7.1N

EN 1992-1-1 assigns w_max limits based on exposure class and the type of reinforced concrete:

* For XD and XS classes (chloride environments), 0.3 mm applies in conjunction with a minimum concrete quality and cover. Some National Annexes tighten this to 0.2 mm for XD3/XS3. The German NA applies 0.3 mm generally but requires checking with the project’s environmental engineer for marine and de-icing salt exposure. See Concrete Cover Requirements for exposure class definitions.

Two Methods in EC2 for Crack Width Verification

EN 1992-1-1 §7.3 offers two approaches:

• Direct calculation (§7.3.4) — compute w_k = s_r,max · (ε_sm − ε_cm). Requires calculation of the maximum crack spacing s_r,max, mean strains in steel and concrete, and the steel stress under the quasi-permanent load combination. Accurate but requires full section analysis.
• Simplified tabular method (§7.3.3) — limit bar diameter φ_s* or bar spacing s to values from Tables 7.2N and 7.3N based on the steel stress σ_s under quasi-permanent loads. This is the method used in practice for most in-situ reinforced concrete members.

Both methods are based on the same underlying crack model. The tabular method is conservative for members near the minimum reinforcement, and less accurate for unusual sections.

Simplified Bar Diameter Limit — Table 7.2N

If you choose to control cracks by limiting bar diameter (rather than bar spacing), EC2 Table 7.2N gives the maximum bar diameter φ_s* as a function of steel stress:

At a typical service stress of 200–240 MPa (common for beams designed to approximately 60 % of yield), bar diameters of 16–25 mm satisfy w_max = 0.3 mm. This aligns with our standard supply range of B500B in 16, 20, and 25 mm — the most commonly ordered bar sizes for building frames. See also Rebar Spacing Rules for the complementary spacing-based approach.

Worked Example: Beam Crack-Width Check

A simply-supported beam, 400 × 700 mm, C30/37 concrete, XC3 exposure (w_max = 0.3 mm), reinforced with 4 × 20 mm B500B bars (A_s = 1,257 mm²). Effective depth d = 630 mm.

• Design moment at ULS: M_Ed = 280 kNm
• Quasi-permanent moment (SLS): M_qp ≈ 0.6 × M_Ed = 168 kNm (approximate)
• Steel stress under M_qp: σ_s = M_qp / (A_s · z) ≈ 168×10⁶ / (1257 × 0.9 × 630) ≈ 236 MPa
• From Table 7.2N at 240 MPa, w_max = 0.3 mm: maximum φ_s* = 16 mm
• Actual bar diameter = 20 mm — exceeds the limit at this stress level
• Action: either reduce σ_s (increase A_s or reduce M_qp), or switch to 5 × 16 mm bars (A_s = 1,005 mm² — check ULS capacity) or 4 × 16 mm + verification via direct calculation
• Alternative: use bar spacing check from Table 7.3N — at σ_s = 200 MPa (if A_s increased to 1,570 mm²), max spacing = 250 mm; actual spacing with 5 × 20 mm bars in 400 mm width ≈ 70 mm — satisfies the spacing limit

This example shows why bar diameter choice matters at procurement stage — not just the total steel area. See also: Min & Max Reinforcement and Rebar Anchorage Length.

Practical Crack Control Strategies

• Use smaller bars at closer spacing — distributes strain more uniformly; more effective than a few large bars for crack width control.
• Increase concrete cover — improves durability but reduces effective depth; must be balanced against section efficiency.
• Increase A_s beyond ULS requirement — lowers service stress σ_s, directly reducing w_k.
• Use higher concrete strength — higher f_ctm reduces crack spacing s_r,max modestly.
• Specify crack inducers for walls — in water-retaining structures and retaining walls, controlled crack inducers at regular intervals are preferred to uncontrolled random cracking.