Australian Rebar Sizes — AS/NZS 4671 Bar Dimensions
Complete reference for steel reinforcing bar sizes and properties per AS/NZS 4671:2001 (Steel reinforcing materials) and AS 3600:2018 (Concrete structures). Covers standard Australian bar diameters N10 to N40, Grade 300N and 500N mechanical properties, cross-sectional areas, mass per metre, development length calculations, weldability requirements, and practical detailing guidance for Australian concrete construction.
Quick access: Australian Steel Grades → | Australian Steel Properties → | AS 4100 Beam Design → | All Australian References →
Australian Rebar Size Classification — AS/NZS 4671
AS/NZS 4671:2001 classifies reinforcing bars by nominal diameter (N designation). The nominal diameter corresponds to the diameter of a round bar of equal cross-sectional area. Australian rebar designations are:
| Bar Designation | Nominal Diameter (mm) | Cross-Sectional Area (mm²) | Mass per Metre (kg/m) | Standard Length (m) |
|---|---|---|---|---|
| N10 | 9.5 | 71 | 0.56 | 6-12 |
| N12 | 12.0 | 113 | 0.89 | 6-12 |
| N16 | 16.0 | 201 | 1.58 | 6-12 |
| N20 | 20.0 | 314 | 2.47 | 6-12 |
| N24 | 24.0 | 452 | 3.55 | 6-12 |
| N28 | 28.0 | 616 | 4.83 | 6-12 |
| N32 | 32.0 | 804 | 6.31 | 6-12 |
| N36 | 36.0 | 1018 | 7.99 | 9-12 |
| N40 | 40.0 | 1257 | 9.87 | 9-12 |
The bar designation number is not exactly the nominal diameter in mm — it is a rounded identifier. For example, N10 has a nominal diameter of 9.5 mm, N28 has 28.0 mm. The tolerance on nominal diameter per AS/NZS 4671 is +/- 0.5 mm for bars up to N20 and +/- 1.0 mm for N24 and larger.
Rebar Grades and Mechanical Properties — AS/NZS 4671
AS/NZS 4671 specifies three standard reinforcing bar grades:
| Grade | Minimum Yield Strength fy (MPa) | Tensile Strength fu (MPa) | Elongation on 5d (%) | Bendability |
|---|---|---|---|---|
| 300N | 300 | 440 (min) | 12 | Standard |
| 500N | 500 | 540 (min) | 10 | Standard |
| 500E | 500 (earthquake grade) | 550 (min) | 14 (uniform elongation > 5%) | Enhanced |
Grade Selection Guide
| Grade | AS 3600 Design Strength fsy (MPa) | Typical Applications |
|---|---|---|
| 300N | 300 | Secondary reinforcement, stirrups, small elements, mesh replacement |
| 500N | 500 | Primary flexural and shear reinforcement in general construction |
| 500E | 500 (seismic) | Ductile frames, moment-resisting structures, plastic hinge zones |
Grade 500N is the current standard for most Australian reinforced concrete applications, providing a 67% higher yield strength than Grade 300N at approximately the same material cost. The transition from Grade 300 to Grade 500 in Australia occurred through the 1990s and was effectively complete by the early 2000s. Grade 500N is now the default grade specified on Australian construction projects.
Grade 500E is required for seismic force-resisting members in structures designed per AS 1170.4 with ductility class DC2 or DC3. The enhanced elongation requirement (uniform elongation > 5%) ensures stable inelastic cyclic behaviour in plastic hinge regions.
Weldability of Australian Rebar
| Grade | Weldability | AS/NZS 4671 Carbon Equivalent Requirement | Precautions |
|---|---|---|---|
| 300N | Weldable | CE ≤ 0.50 | Standard welding procedures |
| 500N | Weldable | CE ≤ 0.50 | Preheat may be required for bars > N20 |
| 500E | Weldable (controlled) | CE ≤ 0.45 | Preheat required, low-hydrogen electrodes |
All standard Australian rebar grades are weldable per AS/NZS 4671 provided the carbon equivalent (CE) is within limits. Welding must be performed in accordance with AS 1554.3 (Welding of reinforcing steel). Preheating to 100-150 degrees C is recommended for bars larger than N20 in Grade 500N, and mandatory for Grade 500E in all sizes.
Detailing Requirements — AS 3600:2018
Minimum Reinforcement Areas
AS 3600:2018 Clause 8.1.4 specifies minimum reinforcement areas for crack control:
| Element Type | Minimum Reinforcement Ratio | AS 3600 Clause |
|---|---|---|
| Beams (tensile reinforcement) | 0.0035 x b x D | 8.1.4.1 |
| Beams (shear reinforcement) | Ast/s = 0.06 x sqrt(fc') x b / fsy | 8.2.8 |
| Slabs (structural) | 0.0019 x b x D (each direction) | 9.1.1 |
| Walls (vertical + horizontal) | 0.0015 x b x D (each direction) | 11.4 |
| Columns | 0.01 x Ag (minimum) / 0.04 x Ag (maximum) | 10.7.1 |
Where b = section width, D = total depth, fc' = concrete compressive strength, fsy = steel yield strength, Ag = gross concrete area.
Cover Requirements
AS 3600:2018 Clause 4.10 specifies minimum cover to reinforcement for corrosion protection and fire resistance:
| Exposure Classification | Minimum Cover (mm) | Typical Environment |
|---|---|---|
| A1 (dry) | 15 | Interior of buildings |
| A2 (humid interior) | 25 | Bathrooms, laundries |
| B1 (exterior moderate) | 35 | Exposed to rain, sheltered |
| B2 (exterior severe) | 45 | Marine, industrial |
| C (aggressive) | 55+ | Tidal, splash zone, chemical |
Development Length — AS 3600 Clause 13.1
The basic development length for a straight deformed bar in tension per AS 3600 Clause 13.1.2:
Lsy.t = 0.5 x k1 x k3 x fsy x db / (k2 x sqrt(fc'))
Where:
- db = bar diameter (mm)
- fsy = yield strength of reinforcement (MPa)
- fc' = concrete compressive strength (MPa)
- k1 = 1.3 for bars with more than 300 mm of concrete cast below (top bar effect)
- k1 = 1.0 for all other bars
- k2 = 1.0 + 0.5 x db / (c + ct) (accounts for confinement)
- k3 = 1.0 - 0.007 x (c + ct) / db
Typical Development Lengths (Grade 500N, fc' = 32 MPa)
| Bar Size | Development Length Lsy.t (mm) — Bottom Bar | Development Length Lsy.t (mm) — Top Bar |
|---|---|---|
| N12 | 290 | 380 |
| N16 | 390 | 510 |
| N20 | 490 | 640 |
| N24 | 590 | 770 |
| N28 | 690 | 900 |
| N32 | 790 | 1030 |
| N36 | 890 | 1160 |
| N40 | 990 | 1290 |
These values assume standard cover (c + ct = 35 mm for bottom bars, 45 mm for top bars), straight bars without hooks. Development lengths may be reduced by 50% for standard hooks (Clause 13.1.2.7) or by using larger cover or confinement.
Worked Example: Rebar Selection for a Simply Supported Beam
Problem: Design the flexural reinforcement for a simply supported beam in an office building. Span = 8.0 m, width b = 400 mm, total depth D = 600 mm. Design bending moment M* = 450 kNm. Concrete fc' = 32 MPa, reinforcement Grade 500N. Exposure classification A2 (cover = 25 mm). Assume N20 stirrups (ds = 20 mm) for shear.
Solution:
Effective depth d = D — cover — ds — db/2 = 600 — 25 — 20 — 20/2 = 545 mm (assumes N20 main bars initially)
Required steel area (from flexural analysis): Ast-req = M* / (phi x fsy x 0.9 x d) = 450 x 10^6 / (0.85 x 500 x 0.9 x 545) = 450,000,000 / 208,463 = 2,159 mm²
Bar selection:
- 5 N24 bars: Ast = 5 x 452 = 2,260 mm² (sufficient)
- 7 N20 bars: Ast = 7 x 314 = 2,198 mm² (sufficient)
- 4 N28 bars: Ast = 4 x 616 = 2,464 mm² (sufficient)
Check minimum reinforcement (AS 3600 Clause 8.1.4.1): Ast-min = 0.0035 x 400 x 600 = 840 mm² 2,260 ≥ 840 ✓
Check maximum reinforcement (AS 3600 Clause 8.1.5): ku ≤ 0.4 (for ductility) With 5 N24: ku = Ast x fsy / (0.85 x alpha2 x fc' x gamma x b x d) Assume alpha2 = 0.85, gamma = 0.82 for fc' = 32 MPa ku = 2,260 x 500 / (0.85 x 0.85 x 32 x 0.82 x 400 x 545) = 1,130,000 / 4,133,975 = 0.27 ≤ 0.4 ✓
Select 5 N24 bars in two layers (3 + 2) as main flexural reinforcement.
Design Resources
- Australian Steel Grades — Rebar and structural steel grades
- Australian Steel Properties — Section property tables
- Australian Bolt Capacity — Bolt shear and tension values
- AS 4100 Beam Design — Steel beam design example
- Australian Weld Electrodes — Welding consumables
- All Australian References
Frequently Asked Questions
What rebar sizes are standard in Australia per AS/NZS 4671? Standard Australian rebar sizes range from N10 (9.5 mm diameter, 71 mm² area, 0.56 kg/m) to N40 (40.0 mm diameter, 1,257 mm² area, 9.87 kg/m). The full range includes N10, N12, N16, N20, N24, N28, N32, N36, and N40. N12, N16, N20 and N28 are the most commonly specified bar sizes for primary reinforcement in Australian construction. The bar designation N is approximately related to the nominal diameter but not exactly — N10 = 9.5 mm, N12 = 12.0 mm.
What are the standard rebar grades used in Australia? AS/NZS 4671 specifies Grade 300N (fy = 300 MPa, fu = 440 MPa min), Grade 500N (fy = 500 MPa, fu = 540 MPa min), and Grade 500E (fy = 500 MPa, fu = 550 MPa min, enhanced elongation for seismic). Grade 500N is the current standard for general reinforced concrete construction in Australia, providing 67% higher yield strength than Grade 300N at similar material cost. Grade 500E is required for seismic-resisting members in ductility class DC2 or DC3 structures.
Can Australian rebar be welded? Yes, all standard Australian rebar grades are weldable per AS/NZS 4671 provided the carbon equivalent (CE) does not exceed 0.50 for Grade 300N and 500N, or 0.45 for Grade 500E. Welding must comply with AS 1554.3 using low-hydrogen welding processes. Preheating to 100-150 degrees C is recommended for Grade 500N bars larger than N20 and mandatory for Grade 500E bars. Welding of primary reinforcement in plastic hinge zones requires special consideration and is generally avoided where possible.
How are development lengths calculated for Australian rebar per AS 3600? The basic development length Lsy.t = 0.5 x k1 x k3 x fsy x db / (k2 x sqrt(fc')). For a Grade 500N N20 bottom bar in fc' = 32 MPa concrete: Lsy.t = 490 mm (approximately 25 x bar diameter). Top bars (k1 = 1.3) require approximately 30% longer development: 640 mm for N20. Standard hooks reduce development length by up to 50%. Development length is inversely proportional to sqrt(fc') — higher strength concrete reduces required embedment.
What are the minimum and maximum reinforcement limits per AS 3600? AS 3600:2018 requires minimum tensile reinforcement of 0.0035 x b x D for beams, minimum structural reinforcement of 0.0019 x b x D for slabs (each direction), and minimum vertical and horizontal wall reinforcement of 0.0015 x b x D. Columns require between 1% and 4% of gross cross-sectional area as longitudinal reinforcement. Maximum reinforcement in beams is limited by the neutral axis parameter ku ≤ 0.4 for ductility. The maximum spacing of primary reinforcement is limited to 300 mm for beams and 450 mm for slabs in the primary direction.
Educational reference only. All design values must be verified against the current edition of AS/NZS 4671:2001, AS 3600:2018, and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.