Eurocode Design Guide · Part 2 of 5

Material Selection per Eurocode

Concrete grades from EN 206, reinforcing steel from EN 10080, structural steel from EN 10025 — with design values, partial factors γ_M, elastic moduli, and exposure class guidance for durability.

Concrete — EN 206 & EN 1992
Reinforcing Steel — EN 10080
Structural Steel — EN 10025
Partial Safety Factors γ_M
Exposure Classes & Durability

1. Concrete — EN 206 & EN 1992

Concrete is specified by a strength class in the format C f_ck/f_ck,cube, where f_ck is the 28-day characteristic cylinder compressive strength and f_ck,cube is the equivalent cube strength. Cylinder strength governs EC2 calculations.

Class	f_ck (MPa)	f_ck,cube (MPa)	f_cm (MPa)	f_ctm (MPa)	E_cm (GPa)	ε_cu3
C20/25	20	25	28	2.2	30	3.5‰
C25/30	25	30	33	2.6	31	3.5‰
C30/37	30	37	38	2.9	33	3.5‰
C35/45	35	45	43	3.2	34	3.5‰
C40/50	40	50	48	3.5	35	3.5‰
C45/55	45	55	53	3.8	36	3.5‰
C50/60	50	60	58	4.1	37	3.5‰
C60/75	60	75	68	4.4	39	2.9‰
C90/105	90	105	98	5.0	44	2.6‰

Key Concrete Design Values (EC2 §3.1)

EC2 — Design compressive strength

SI f_cd = α_cc · f_ck / γ_C MPa

Where:

α_cc = 1.0 (EN recommended; many NAs use 0.85 or 0.8 for bending)
γ_C = 1.5 (persistent/transient); 1.2 (accidental)
Example C30/37: f_cd = 1.0 × 30 / 1.5 = 20.0 MPa

EC2 — Secant elastic modulus

SI E_cm = 22 × (f_cm/10)^0.3 GPa

f_cm = f_ck + 8 MPa (mean cylinder strength)

Concrete vs US Notation

EC2 f_ck and ACI f'_c are both 28-day cylinder strengths — numerically equal
C30/37 ≈ ACI 4,350 psi (30 MPa): a direct one-to-one comparison applies
ACI cube tests are not standard; UK-specified cube strengths (in the class suffix) exceed cylinder strength by ~25%

2. Reinforcing Steel — EN 10080

Reinforcement is classified by its characteristic yield strength f_yk, ductility class (A, B, or C), and weldability. B500B (500 MPa, class B) is the most common grade in Europe for building structures.

Designation	f_yk (MPa)	Ductility Class	ε_uk	k = (f_t/f_y)_k
B400A	400	A (low)	≥ 2.5%	≥ 1.05
B400B	400	B (normal)	≥ 5.0%	≥ 1.08
B500A	500	A (low)	≥ 2.5%	≥ 1.05
B500B	500	B (normal)	≥ 5.0%	≥ 1.08
B500C	500	C (high)	≥ 7.5%	≥ 1.15 and ≤ 1.35
B600B	600	B (normal)	≥ 5.0%	≥ 1.08

EC2 — Design yield strength of rebar

SI f_yd = f_yk / γ_S MPa

γ_S = 1.15 (persistent/transient); 1.0 (accidental)
B500B: f_yd = 500 / 1.15 = 434.8 MPa ≈ 435 MPa
E_s = 200,000 MPa (fixed by EC2)

EC8 ductility requirement: For seismic design (DCM / DCH), minimum ductility class B (B500B or higher) is required for critical regions of ductile walls and frames. Class A is only permitted for DCL or non-critical members.

Rebar: Eurocode vs US

EU B500B f_yk = 500 MPa vs US Grade 60 f_y = 420 MPa — European bars are 19% stronger
Design value B500B: f_yd = 435 MPa vs Grade 60 (ACI, no reduction): 420 MPa — approximately equal in practice
US ASTM A615/A706 bars; EU bars to EN 10080. Bar sizes differ: EU uses mm diameter (Ø8, Ø10, Ø12 … Ø32); US uses # designations (#3 = 9.5mm through #18 = 57mm)

3. Structural Steel — EN 10025

Structural steel grades are specified under EN 10025-2 (hot-rolled non-alloy) through 10025-6 (quenched and tempered). Grade designation: S (structural) + nominal yield strength + subgrade (A, B, C, D, E for impact test temperature).

Grade	f_y (t ≤ 40mm) MPa	f_y (t ≤ 80mm) MPa	f_u (MPa)	Charpy temp (D)
S235	235	215	360–510	–20°C
S275	275	255	430–580	–20°C
S355	355	325	490–630	–20°C
S420	420	390	520–680	–20°C
S460	460	440	550–720	–20°C

Thickness reduction: Yield strength decreases with plate thickness. EC3 Table 3.1 gives two tiers: t ≤ 40 mm and 40 mm < t ≤ 80 mm. Always check the actual thickness of flanges and webs against this table — hot-rolled wide-flange sections can have flanges > 40 mm in heavy sections.

Design Values (EC3 §6.1)

EC3 — Design strength of steel

SI f_yd = f_y / γ_M0 = f_y / 1.00 MPa

SI f_u,d = f_u / γ_M2 = f_u / 1.25 MPa (net tension)

E = 210,000 MPa · G = 81,000 MPa · ν = 0.3 · α = 12×10⁻⁶/°C

Steel Grade Comparison: EC3 vs AISC

EC3 Grade	f_y (MPa)	Closest ASTM	F_y (MPa)	Note
S235	235	A36	250	ACI A36 slightly stronger
S275	275	A572 Gr.42	290	US Gr.42 is closest
S355	355	A572 Gr.50 / A992	345	A992 W-shapes common; ≈ S355
S460	460	A913 Gr.65	448	High-strength, less common

Steel Flexural Design → Composite Beam →

4. Partial Safety Factors γ_M

Material partial factors reduce characteristic strengths to design values. EN 1990 Table A1.2(B) and the material Eurocodes specify recommended values; National Annexes may modify them.

Factor	Application	Persistent / Transient	Accidental
γ_C	Concrete (compression)	1.50	1.20
γ_S	Reinforcing steel	1.15	1.00
γ_M0	Steel — cross-section resistance	1.00	1.00
γ_M1	Steel — member instability	1.00	—
γ_M2	Steel — net section fracture, bolts	1.25	—
γ_M3	Slip-resistant connections	1.25 (cat C: 1.30)	—

UK National Annex note: UK NA reduces γ_M0 and γ_M1 to 1.00 (same as EN recommended) but requires α_cc = 0.85 for concrete in bending, giving f_cd = 0.85 × f_ck / 1.5 = 0.567 f_ck. Always confirm the applicable NA.

5. Exposure Classes & Durability

EN 206 defines exposure classes (XC, XD, XS, XF, XA) based on the environmental conditions causing concrete degradation. Each class requires a minimum concrete grade, maximum w/c ratio, and minimum cement content.

Class	Description	Typical Situation	Min. Grade	Max. w/c
X0	No corrosion risk	Dry concrete with no rebar; indoor	C12/15	—
XC1	Carbonation — dry or permanently wet	Concrete indoors, low humidity	C20/25	0.65
XC2	Carbonation — wet, rarely dry	Foundations, below-ground slabs	C25/30	0.60
XC3	Carbonation — moderate humidity	External concrete, sheltered	C30/37	0.55
XC4	Carbonation — cyclic wet/dry	External exposed surfaces	C30/37	0.50
XD1	Chloride — moderate humidity	Exposed to chlorides (not seawater)	C30/37	0.55
XD3	Chloride — cyclic wet/dry	Bridge decks, car parks	C35/45	0.45
XS1	Seawater — airborne salt	Coastal structures, not submerged	C30/37	0.50
XS2	Seawater — permanently submerged	Offshore, marine foundations	C35/45	0.45
XF2	Freeze-thaw moderate	Vertical surfaces in cold climate	C25/30	0.55
XF4	Freeze-thaw severe	Horizontal exposed (roads, parking)	C30/37	0.45

Cover Requirements

Minimum cover c_min,dur for corrosion protection (EC2 Table 4.4N, structural class S4, design life 50 years):

X0 / XC1: 10 mm (rebar) / 10 mm (prestress)
XC2 / XC3: 25 mm / 35 mm
XC4: 30 mm / 40 mm
XD1 / XS1: 35 mm / 45 mm
XD2 / XS2: 40 mm / 50 mm
XD3 / XS3: 45 mm / 55 mm

Nominal cover: c_nom = c_min + Δc_dev, where Δc_dev = 10 mm (recommended).

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Educational use only. Design values in this article use EN recommended partial factors. Always verify against the National Annex applicable to your project's jurisdiction. Material properties and limits given here are simplified for guidance; consult the full text of EN 206, EN 10080, EN 10025, EN 1992, and EN 1993 for complete requirements.

Material Selection per Eurocode

Contents

1. Concrete — EN 206 & EN 1992

Key Concrete Design Values (EC2 §3.1)

Concrete vs US Notation

2. Reinforcing Steel — EN 10080

Rebar: Eurocode vs US

3. Structural Steel — EN 10025

Design Values (EC3 §6.1)

Steel Grade Comparison: EC3 vs AISC

4. Partial Safety Factors γM

5. Exposure Classes & Durability

Cover Requirements

Related Tools & Guides

4. Partial Safety Factors γ_M