US Standards Series · Part 2 of 9

Material Selection — Concrete, Rebar & Structural Steel

Concrete compressive strengths (f'c), rebar grades and moduli, structural steel grades (A36/A572/A992): the key properties, ACI 318-25 and AISC 360-22 material formulas, and selection guidelines in both US Customary and SI units.

Concrete — f'c and Ec
Reinforcing Steel — Grades and Limits
Structural Steel — A36, A572, A992
Comparison Table
Selection Guidelines
Durability & Exposure Requirements

1. Concrete — f'c and Elastic Modulus

Concrete strength in US practice is specified as the 28-day cylinder compressive strength f'c, measured on a 6×12 in (150×300 mm) standard cylinder. All ACI 318 design formulas are based on f'c.

Normal Weight

f'c = 3,000–5,000 psi

17–34 MPa

wc ≈ 145 pcf (2,320 kg/m³)

Typical slabs, beams

High Strength

f'c = 6,000–10,000 psi

41–69 MPa

wc ≈ 145 pcf (2,320 kg/m³)

Columns, high-rise

Lightweight

f'c = 3,000–5,000 psi

17–34 MPa

wc = 90–115 pcf (1,440–1,840 kg/m³)

Reduce dead load

Elastic Modulus Ec — ACI 318-25 §19.2.2

ACI 318-25 Eq. 19.2.2.1a — Normal-weight concrete

USEc = 57,000 √f'cpsi → psi

SIEc = 4,700 √f'cMPa → MPa

ACI 318-25 Eq. 19.2.2.1b — General (any density)

USEc = 33 wc^1.5 √f'cwc in pcf, f'c in psi → psi

SIEc = 0.043 wc^1.5 √f'cwc in kg/m³, f'c in MPa → MPa

Modulus of Rupture fr (Cracking)

ACI 318-25 §19.2.3 — Used for deflection calculations

USfr = 7.5λ √f'cpsi → psi

SIfr = 0.62λ √f'cMPa → MPa

λ = 1.0 (normal weight), 0.85 (sand-lightweight), 0.75 (all-lightweight)

Lightweight Concrete — Modification Factor λ (ACI 318-25 §19.2.4)

λ reduces the tensile strength contribution of concrete in shear, development length, and torsion formulas when lightweight aggregate is used.

Concrete Type	Unit Weight w_c	λ
Normalweight	≥ 135 pcf (2,160 kg/m³)	1.00
Sand-lightweight (sanded)	~115 pcf (1,840 kg/m³)	0.85
All-lightweight (unsanded)	~90–105 pcf (1,440–1,680 kg/m³)	0.75

Linear interpolation is permitted based on the volumetric fraction of normalweight sand replaced (§19.2.4.2). Alternatively, λ may be determined from split-cylinder test results (§19.2.4.3):

USλ = f_ct / (6.7√f'_c) ≤ 1.0psi

SIλ = f_ct / (0.56√f'_c) ≤ 1.0MPa

f_ct = average splitting tensile strength from ASTM C496 tests.

f'c (psi)	f'c (MPa)	Ec (ksi)	Ec (GPa)	Common Use
3,000	20.7	3,122	21.5	Slabs, footings
4,000	27.6	3,605	24.9	Beams, columns
5,000	34.5	4,031	27.8	Moderate columns
6,000	41.4	4,415	30.4	High-rise columns
8,000	55.2	5,098	35.2	Prestressed, special
10,000	68.9	5,700	39.3	HSC columns

2. Reinforcing Steel — Grades and Material Limits

ASTM A615 (carbon steel) and ASTM A706 (low-alloy, weldable, seismic) are the two primary rebar specifications. The grade number equals the minimum yield strength in ksi (US) or MPa (SI).

Grade	fy (ksi)	fy (MPa)	fu (ksi)	fu (MPa)	Es (ksi / GPa)	Use
Grade 40 (A615)	40	276	60	414	29,000 / 200	Light slabs (rare)
Grade 60 (A615)	60	414	90	621	29,000 / 200	Most RC design
Grade 60 (A706)	60	414	80	552	29,000 / 200	Seismic zones (required)
Grade 80 (A615)	80	552	105	724	29,000 / 200	High-rise columns
Grade 100 (A615)	100	690	125	862	29,000 / 200	ACI 318-19+ large columns

ACI 318-25 Yield Strength Limits

Flexure (§9.6.1): fy ≤ 80,000 psi (550 MPa) for As design unless special provisions met
Shear stirrups (§9.7.6.2.2): fy ≤ 60,000 psi (420 MPa) for conventional stirrups; ≤ 80,000 psi (550 MPa) for welded wire
Torsion (§22.7.6.1): fy ≤ 60,000 psi (420 MPa)
High-strength rebar (§20.2.2.4): Grade 80–100 permitted per ACI 318-19+ with elongation checks

Rebar Modulus of Elasticity

ACI 318-25 §20.2.2.2 — Constant for all steel grades

USEs = 29,000,000 psi = 29,000 ksi

SIEs = 200,000 MPa = 200 GPa

Rebar Sizes

US Bar #	Diameter (in)	Diameter (mm)	Area (in²)	Area (mm²)
#3	0.375	9.5	0.11	71
#4	0.500	12.7	0.20	129
#5	0.625	15.9	0.31	200
#6	0.750	19.1	0.44	284
#7	0.875	22.2	0.60	387
#8	1.000	25.4	0.79	510
#10	1.270	32.3	1.27	819
#11	1.410	35.8	1.56	1006

Seismic note: ACI 318-25 Chapter 18 requires A706 rebar (or A615 with additional testing) for special moment frames, special shear walls, and other seismic force-resisting members. A706 has a controlled yield-to-tensile ratio (fu/fy ≥ 1.25) that ensures ductile behavior.

RC Beam Calculator (ACI 318-25) →

3. Structural Steel — A36, A572, A992

AISC 360-22 specifies material requirements by reference to ASTM standards. The three most common grades cover general shapes, plates, and wide-flange sections respectively.

ASTM A36

Fy = 36 ksi (250 MPa)

Fu = 58–80 ksi (400–550 MPa)

E = 29,000 ksi (200 GPa)

Plates, angles, channels

General purpose

ASTM A572 Gr.50

Fy = 50 ksi (345 MPa)

Fu = 65 ksi (450 MPa)

E = 29,000 ksi (200 GPa)

Wide flanges, plates

Most common

ASTM A992

Fy = 50 ksi (345 MPa)

Fu = 65 ksi (450 MPa)

E = 29,000 ksi (200 GPa)

W-shape wide flanges only

Seismic preferred

Material Constants — AISC 360-22 §A3

Elastic modulus — same for all structural steel grades

USE = 29,000 ksi

SIE = 200,000 MPa (200 GPa)

Shear modulus

USG = 11,200 ksi

SIG = 77,200 MPa

A992 vs A572 — Key Differences

A992 adds requirements that A572 does not:

Yield-to-tensile ratio: Fy/Fu ≤ 0.85 (limits strain hardening — important for plastic design and seismic)
Carbon equivalent limit: Restricts weldability requirements for field connections
Chemistry limits: C ≤ 0.23%, Mn ≤ 1.60% — ensures consistent behavior

AISC 341-22 §A3.1 requires A992 (or equivalent) for wide-flange shapes in special moment frames (SMF) and intermediate moment frames (IMF).

Other Common Grades

ASTM Grade	Fy (ksi / MPa)	Fu (ksi / MPa)	Typical Products
A500 Gr.B	46 / 317	58 / 400	HSS (round)
A500 Gr.C	50 / 345	62 / 427	HSS (rectangular/square)
A1085	50 / 345	65 / 450	HSS seismic (tight tolerances)
A529 Gr.50	50 / 345	70–100 / 483–690	Structural shapes, plates
A514 Gr.B	100 / 690	110–130 / 758–895	HPS plates

Expected (Probable) Material Properties — Seismic Design

AISC 341-22 §A3.2 — for capacity design in seismic systems, the expected (probable) strength of yielding elements uses amplified values R_y·F_y and R_t·F_u rather than nominal values, to ensure capacity-protected elements (columns, connections) remain elastic.

ASTM Grade	R_y (yield)	R_t (ultimate)	Application
A992 / A572 Gr.50	1.1	1.1	W-shapes (SMF beams, columns)
A36	1.5	1.2	Plates, angles (avoid in seismic — high Ry)
A500 Gr.B/C / A1085	1.3	1.3	HSS braces (SCBF, BRBF)
A572 Gr.42	1.3	1.0	Plates

Used for: M_pe = R_y·F_y·Z_x (expected plastic moment in SMF beams), R_y·F_y·A_g (expected yield force in SCBF/BRBF braces), and R_t·F_u·A_n (expected rupture force for connection design).

Steel Beam Calculator → Steel Column Calculator →

4. Side-by-Side Comparison

Property	Concrete (f'c=4 ksi)	Rebar (Gr.60)	Steel (A992)
Design strength (US)	f'c = 4,000 psi	fy = 60,000 psi	Fy = 50,000 psi
Design strength (SI)	f'c = 27.6 MPa	fy = 414 MPa	Fy = 345 MPa
Elastic modulus (US)	Ec = 3,605 ksi	Es = 29,000 ksi	E = 29,000 ksi
Elastic modulus (SI)	Ec = 24.9 GPa	Es = 200 GPa	E = 200 GPa
Modular ratio n = Es/Ec	≈ 8 (f'c=4 ksi) — used in transformed section analysis
Unit weight (US)	145 pcf	490 pcf	490 pcf
Unit weight (SI)	2,320 kg/m³	7,850 kg/m³	7,850 kg/m³
Thermal expansion	5.5 μ/°F (9.9 μ/°C)	6.5 μ/°F (11.7 μ/°C)	6.5 μ/°F (11.7 μ/°C)

5. Material Selection Guidelines

Concrete Grade

f'c = 3,000 psi (21 MPa): Footings, lightly loaded slabs-on-grade
f'c = 4,000 psi (28 MPa): Standard for beams, slabs, and columns in most buildings
f'c = 5,000–6,000 psi (35–41 MPa): High-rise columns (5+ stories), prestressed elements
f'c ≥ 8,000 psi (55 MPa): Special high-strength applications; requires special mix design and QC

Rebar Grade

Grade 60 A615: Default for non-seismic design; widely available and economical
Grade 60 A706: Required for seismic force-resisting elements (SMF, SSW) per ACI 318-25 Ch.18
Grade 80/100: Reduces reinforcement congestion in heavily loaded columns; verify code applicability

Structural Steel Grade

A992: Default for all W-shape wide-flanges; preferred even in non-seismic zones
A572 Gr.50: Angles, channels, plates; also W-shapes in non-seismic non-ductility applications
A36: Limited use today; primarily for small plates or where 50 ksi is unneeded
A500 Gr.C / A1085: HSS members; A1085 for seismic

Cost note: A992 and A572 Gr.50 are typically the same cost from steel service centers, making A992 the economical choice for wide flanges. Using higher-strength steel (Fy = 65+ ksi) requires careful connection and compact section checks — the higher yield strength does not automatically improve LTB or local buckling capacity.

6. Durability & Exposure Requirements — ACI 318-25 §19.3

ACI 318-25 §19.3.1 requires that concrete be designed for the most severe exposure class anticipated. Exposure classes are assigned separately for each of four exposure categories; the most demanding requirement for any assigned class governs the mix design.

The Four Exposure Categories

Category	Protects against…	Classes
F — Freezing & Thawing	Freeze-thaw cycling damage, deicing chemicals	F0, F1, F2, F3
S — Sulfate	Sulfate attack from soil or water	S0, S1, S2, S3
W — Water Contact	Permeability control for below-grade and water-retaining structures	W0, W1, W2
C — Corrosion	Chloride-induced reinforcement corrosion	C0, C1, C2

F — Freezing and Thawing (ACI 318-25 Table 19.3.2)

Class	Condition	Min f'c (psi)	Min f'c (MPa)	Max w/cm
F0	Not exposed to freezing/thawing or deicing chemicals	2,500	17	—
F1	Moderate exposure: concrete exposed to freeze-thaw in moist condition or occasional deicing chemicals	3,500	24	0.55
F2	Severe exposure: frequent freeze-thaw cycles while moist or in contact with deicing chemicals	4,500	31	0.45
F3	Very severe: continuously wet and exposed to deicing chemicals (parking decks, bridge decks)	4,500	31	0.40

Air entrainment required for F1–F3. Total air content 4.5–7.5% depending on maximum aggregate size (ACI 318-25 Table 19.3.3).

S — Sulfate Exposure

Class	Soil SO₄ (% by wt)	Water SO₄ (ppm)	Min f'c (psi)	Min f'c (MPa)	Max w/cm	Cement
S0	< 0.10	< 150	2,500	17	—	No restriction
S1	0.10–0.20	150–1,500	2,500	17	0.50	Type II / MS or pozzolan
S2	0.20–2.0	1,500–10,000	4,500	31	0.45	Type V / HS
S3	> 2.0	> 10,000	4,500	31	0.45	Type V + pozzolan or slag

W — Water Contact

Class	Condition	Min f'c (psi)	Min f'c (MPa)	Max w/cm
W0	Concrete dry in service; no contact with water	2,500	17	—
W1	Concrete in contact with water; low permeability not required	2,500	17	—
W2	Concrete in contact with water; low permeability required (below-grade walls, cisterns, tanks)	4,500	31	0.45

C — Corrosion Protection of Reinforcement

Class	Condition	Min f'c (psi)	Min f'c (MPa)	Max w/cm
C0	Concrete dry or protected from moisture	2,500	17	—
C1	Concrete exposed to moisture but no external chloride source	2,500	17	—
C2	Concrete exposed to moisture plus external chlorides — parking structures, bridge decks, marine elements	5,000	35	0.40

Practical Design Guide: Exposure → Requirements

Structure / Location	Likely Exposure Classes	Governing Min f'c	Max w/cm	Min Cover (in / mm)
Interior slab, dry office	F0, S0, W0, C0	2,500 psi / 17 MPa	—	¾ in / 20 mm
Exterior slab, northern climate (deicing)	F3, C1	4,500 psi / 31 MPa	0.40	1½ in / 38 mm
Parking garage deck	F2–F3, C2	5,000 psi / 35 MPa	0.40	1½–2 in / 38–50 mm
Below-grade foundation wall (moderate sulfate)	S1, W2	4,500 psi / 31 MPa	0.45	3 in / 75 mm (against earth)
Marine pile (tidal zone)	F1, S2, C2	5,000 psi / 35 MPa	0.40	2–3 in / 50–75 mm
Water tank / cistern	W2, C1	4,500 psi / 31 MPa	0.45	1½–2 in / 38–50 mm

Cover values from ACI 318-25 §20.6.1. "Against earth" = cast directly against soil. Always use the most demanding class when multiple exposures apply simultaneously.

Rule: multiple categories apply simultaneously. A parking structure in Minnesota would be assigned F3 (freeze-thaw + deicing), C2 (chlorides), and possibly S0 (no sulfate). The governing requirements are: min f'c = max(4,500 psi from F3, 5,000 psi from C2) = 5,000 psi; max w/cm = min(0.40, 0.40) = 0.40.

Preliminary design only. This article is for educational purposes. Always verify designs with a licensed structural engineer and confirm local code edition and amendments.

← Previous Introduction to US Standards Next → Load Determination — ASCE 7-22

Material Selection — Concrete, Rebar & Structural Steel

Contents

1. Concrete — f'c and Elastic Modulus

Elastic Modulus Ec — ACI 318-25 §19.2.2

Modulus of Rupture fr (Cracking)

Lightweight Concrete — Modification Factor λ (ACI 318-25 §19.2.4)

2. Reinforcing Steel — Grades and Material Limits

ACI 318-25 Yield Strength Limits

Rebar Modulus of Elasticity

Rebar Sizes

3. Structural Steel — A36, A572, A992

Material Constants — AISC 360-22 §A3

A992 vs A572 — Key Differences

Other Common Grades

Expected (Probable) Material Properties — Seismic Design

4. Side-by-Side Comparison

5. Material Selection Guidelines

Concrete Grade

Rebar Grade

Structural Steel Grade

6. Durability & Exposure Requirements — ACI 318-25 §19.3

The Four Exposure Categories

F — Freezing and Thawing (ACI 318-25 Table 19.3.2)

S — Sulfate Exposure

W — Water Contact

C — Corrosion Protection of Reinforcement

Practical Design Guide: Exposure → Requirements

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