Formwork is the temporary mould that shapes concrete until it gains sufficient strength to support itself. In Australia, formwork design and construction is governed by AS 3610.
Formwork failures are almost always due to inadequate design for fresh concrete pressure, premature striking, or poor propping/bracing: not the finished concrete itself.
Formwork Classification
| Category | Definition |
|---|---|
| Category 1 | Minor structures, low consequence of failure |
| Category 2 | Most building structures |
| Category 3 | High consequence of failure (e.g. supports public areas, high formwork) |
Higher categories require more rigorous design documentation and inspection.
Design Loads on Formwork
Fresh Concrete Pressure
Formwork must resist the lateral pressure of fresh (fluid) concrete before it sets:
$$ p = \rho_c g h $$ *(hydrostatic, for slow pour rates)*For faster pour rates, AS 3610 provides empirical formulae accounting for pour rate, temperature, and concrete type:
$$ p_{max} = C_1 C_2 \left[ 7.2 + \frac{790 R}{T + 18} \right] $$Where $R$ = rate of rise (m/h), $T$ = concrete temperature (°C), $C_1$, $C_2$ = coefficients for size/admixture type.
Other Loads
| Load Type | Description |
|---|---|
| Dead load | Self-weight of formwork and wet concrete |
| Live load | Workers, equipment, placement plant (e.g. concrete pump boom) |
| Wind load | On exposed vertical formwork (e.g. tall wall forms) |
| Impact load | Concrete discharge (pumping, chuting, skip) |
Falsework
Falsework supports formwork for elevated slabs and beams until the concrete gains strength. Key design considerations:
- Prop spacing and load path to ground or supporting structure
- Bracing against lateral instability (props are inherently unstable alone)
- Foundation bearing capacity beneath props (especially on suspended slabs below)
- Re-propping strategy if the slab below is not yet at full strength
Strike Times
Formwork and props must remain in place until concrete reaches sufficient strength to support itself and any construction loads above.
| Element | Typical Minimum Strike Time* |
|---|---|
| Vertical formwork (walls, columns) | 12–24 hours |
| Soffit formwork, props left in place | 3–7 days |
| Soffit props removed (spans < 3 m) | 7 days |
| Soffit props removed (spans > 6 m) | 14–21 days |
*Actual strike times depend on concrete strength development, span, loading, and back-propping: always confirm against measured/predicted strength, not calendar days alone.
Back-propping
Where a floor is loaded (by construction above) before reaching design strength, back-props transfer load down through multiple levels:
- Distributes load across several partially-cured floors rather than one
- Requires checking capacity of all floors in the back-propped stack, not just the immediate one
- Must be planned into the construction sequence, not improvised on site
Formwork Materials
| System | Application |
|---|---|
| Plywood/timber formwork | General purpose, complex shapes |
| Steel/aluminium modular systems | Repetitive elements (walls, columns) |
| Permanent formwork (e.g. steel decking) | Composite floor slabs |
| Climbing/jump formwork | Tall core walls, bridge piers |
Practical Notes
- Fresh concrete pressure calculations are highly sensitive to pour rate: a faster-than-planned pour (common with pump trucks) can exceed design pressure quickly.
- Prop and falsework instability, not overload, is the most common cause of formwork collapse: bracing design deserves as much attention as strength design.
- Back-propping schemes must be checked as a system across multiple floors, not floor by floor in isolation.