Design Requirements For Concrete Formwork

When designing formwork, consider the following:

1. Strength

Forms and shutters have to be designed to support dead weight, live load and hydrostatic pressure. Sheathing must be rigid enough to resist bulging. Formwork for vertical concrete elements i.e. columns and walls are subject to pressures on the form face. This is caused by the fluid action of the fresh concrete. The pressure of the fluid concrete on the vertical faces increases proportionately with the depth of concrete. The maximum pressure being at the bottom of the form. This maximum pressure for the full depth fluid concrete is the hydrostatic pressure for concrete and usually occurs when the concrete is placed very quickly. It should not be possible for the bracing to be dislodged by impact, or wind, acting from any direction.

2. Speedy erection and dismantling

The formwork design and the methods of assembly must be as simple as possible to reduce time spent in erection and dismantling. The formwork should be simple to remove without causing damage to the concrete.

3. Tightness of joints

The liquid retaining properties of the formwork must be adequate to prevent leakage of cement and fine aggregate from the concrete.

4. Rigidity

Brace formwork and support to ensure no movement may take place under wind pressure, or when the concrete is being placed and vibrated. The shutters must be rigid enough to keep the concrete member within the allowable tolerances.

5. Reuse

Design for unit construction, if possible, so that you can strike and reuse as soon as possible. Use clamps, wedges, and similar devices to hold sections of formwork in place. Avoid nailing as much as possible, nail holes and bruising of the timber will spoil formwork for further use. The formwork material must be durable and capable of producing a good surface finish.

6. Ease of handling

Forms and shutters must be of a size and weight that can be handled by the labour and plant available on site.

Adjustment: Arrange all props, shores, and struts so that they can be properly adjusted. They must bear on sole plates, so that the load is safely distributed on to the structure below.

Removal of debris: Provide all formwork with special cleanout holes to allow for the removal of sawdust, shavings, and other debris from the bottom of the formwork before the pour begins.