When concrete is fresh and in its liquid state it must be restrained within a mould in order for it to set in its required shape. Formwork is the term used to describe this mould.
For most in-situ pours, the formwork is made of wood. A smooth outer surface provides the main support for the concrete as it sets. This is normally made of plywood.
Concrete is approximately 2.4 times as dense as water, and in its liquid state, it imposes considerable forces on the formwork containing it. Consequently it is necessary to reinforce the plywood with horizontal wooden beams or wailings.
If constructed properly, this mechanism of a plywood skin supported by timber wailings provides ample support for the setting concrete. The mechanism of transferring these loads to a solid support can be performed by numerous methods which are collectively known as falsework.
There are two factors that decide how well the concrete will look in the final result.. one is how the concrete is placed and compacted, and the other is the quality of the formwork.
Formwork is made from expensive materials, and requires great skill and experience in its manufactures. Its importance is evident when you consider that the cost of fabrication, erecting and striking the formwork, often exceeds the cost of the concrete it is designed to shape and support.
Formwork usually needs to be used many times for it to be cost efficient. This can only be done if it is carefully handled, cleaned and stored, regardless of what material it is made from.
For anything other than most smallest of concreting jobs e.g. walls over 1m in height, a drawing should be provided, showing what formwork is required. This need not be an intricate design drawing, a simple sketch is usually sufficient.
The below picture shows part of a concrete structure soon after the formwork has been struck. Not the poor finish (circled). This is an imprint off the painted plywood formwork. Some of the paint has run, and this has been transposed to the concrete. It is important to remember that the face of the concrete is effectively a negative of the formwork face. Always inspect the formwork prior to pouring.
Good formwork should fulfill the following criteria:
It is strong enough to support the weight of fresh concrete during placing and compacting, and any other loads it may be required to take.
The face of the formwork is of sufficient quality for its use.
It can be easily erected and struck.
It is sufficiently stable in all weathers.
It can be handled safely and easily using the equipment available.
It provides suitable access for placing and compacting the concrete.
It conforms with appropriate safety regulations.
The joints between members are sound enough to prevent grout leakage.