AS 3600 is the Australian Standard for concrete structures, covering the design of reinforced and prestressed concrete for buildings and civil structures.
It sets out requirements for strength, serviceability, durability, fire resistance, and reinforcement detailing, and is the primary reference for structural engineers designing in concrete across Australia.
Design Philosophy
AS 3600 uses limit state design: structures must satisfy both the ultimate limit state (ULS) and serviceability limit state (SLS).
$$ \phi R_u \geq E^* $$Where:
- $\phi$ = capacity reduction factor
- $R_u$ = ultimate (nominal) strength
- $E^*$ = design action effect from factored load combinations
Capacity Reduction Factors ($\phi$)
| Action | $\phi$ |
|---|---|
| Bending, with or without axial tension | 0.85 (well confined) – 0.65 (brittle) |
| Axial compression, spiral/tied columns | 0.65 – 0.75 |
| Shear and torsion | 0.70 |
| Bearing | 0.65 |
Material Properties
Concrete
| Property | Typical Range |
|---|---|
| Characteristic compressive strength $f'_c$ | 20–100 MPa |
| Modulus of elasticity $E_c$ | $E_c = \rho^{1.5}(0.043\sqrt{f'_c})$ MPa |
| Tensile strength $f'_{ct.f}$ | $0.6\sqrt{f'_c}$ MPa |
Reinforcement
| Grade | Yield Strength $f_{sy}$ |
|---|---|
| N-class (ductility class N) | 500 MPa |
| L-class (ductility class L, mesh) | 500 MPa |
Durability and Exposure Classification
AS 3600 links concrete cover and quality to an exposure classification (A1, A2, B1, B2, C1, C2, U), based on the environment the member is exposed to (e.g. inland, near-coastal, in contact with seawater).
| Exposure Classification | Example Environment | Typical Min. Cover (N32) |
|---|---|---|
| A1 | Inland, non-aggressive | 20 mm |
| A2 | Near-coastal (1–50 km) | 30 mm |
| B1 | Near-coastal (up to 1 km) | 40 mm |
| B2 | Tidal/splash zone | 45 mm |
| C1, C2 | Permanently submerged / severe marine | 50–65 mm |
Flexural Design (Rectangular Beams)
For a singly reinforced rectangular section:
$$ M_u = A_{st} f_{sy} \left(d - \frac{\gamma k_u d}{2}\right) $$Where:
- $A_{st}$ = area of tension reinforcement
- $d$ = effective depth
- $k_u$ = neutral axis parameter, $k_u = \frac{A_{st} f_{sy}}{\gamma \alpha_2 f'_c b d}$
- $\gamma$, $\alpha_2$ = concrete stress block parameters (depend on $f'_c$)
Ductility requirement: $k_u \leq 0.36$ for ductile (Class N) design.
Shear Design
$$ V_u = V_{uc} + V_{us} $$Where $V_{uc}$ is the concrete contribution and $V_{us}$ is the stirrup (shear reinforcement) contribution:
$$ V_{us} = \frac{A_{sv} f_{sy.f} d_o}{s} \cot\theta_v $$Serviceability
Deflection Limits (Span/Depth or Direct Calculation)
| Member | Deflection Limit |
|---|---|
| Total deflection | Span / 250 |
| Incremental deflection (affecting brittle finishes) | Span / 500 |
| Cantilevers | Span / 125 (total) |
Crack Control
Minimum reinforcement and maximum bar spacing rules apply to control crack widths under service loads, particularly for exposed or liquid-retaining structures.
Fire Resistance
Fire resistance level (FRL) is achieved through minimum member dimensions and axis distance to reinforcement: AS 3600 Section 5 gives tabulated solutions for slabs, beams, columns, and walls as an alternative to fire engineering assessment.
Related Standards
| Standard | Relevance |
|---|---|
| AS 1170 | Loads and load combinations |
| AS 5100.5 | Bridge design in concrete |
| AS 3610 | Formwork |
| AS/NZS 4671 | Steel reinforcing materials |
Practical Notes
- Cover, not strength, usually governs durability outcomes in Australian coastal projects: get the exposure classification right early.
- $k_u \leq 0.36$ keeps sections in the ductile range; over-reinforced sections fail in a brittle manner without warning.
- Deflection is frequently the controlling limit state for long-span, lightly loaded slabs: check SLS even when ULS strength is comfortably satisfied.