Pre-Concrete Checks for Formwork
Before the concrete is poured into the formwork, it must be checked by someone who has been trained to inspect formwork. Depending on how big or complicated the pour is, the inspection may just take few minutes or it could take hours. Only when the formwork has been approved, may the pour take place.
Formwork pressures are function of height (including the height from which concrete is dropped into the forms) and are affected by concrete workability, rate of stiffening and rate of placing. One task of the temporary works co-ordinator is to consider such factors as ambient temperatures and concrete composition, when calculating maximum permissible rate of concrete placing.
Exceeding this limit may lead to unacceptable formwork deflections, loss of grout / concrete at joints, or even collapse. The cost of remedial work due to formwork deflection will usually exceed the original cost of doing the job properly.
Below are the checks that should be verified before pouring begins:
- Is the formwork erected in accordance with the approved drawings?
- Is the formwork restrained against movement in all directions?
- Is it correctly aligned and leveled?
- Are all the props plum, and at the right spacing?
- Are bolts and wedges secure against any possible looseing?
- Has the correct number of ties been used? Are they in the right places and properly tightened?
- Are all inserts and cast-in fixings in the right position and secure?
- Have all stop ends been properly secured?
- Have all the joints been sealed to stop grout loss (especially where the formwork is against the kicker)?
- Can the formwork be struck without damaging the concrete?
- Are the forms clean and free from rubbish such as tie wire cuttings, and odd bits of timber or metal?
- Has the release agents been applied, and is it the correct one?
- Are all projecting bars straight and correctly positioned?
- Is there proper access for placing the concrete and compacting?
- Have all the toe-boards and guard rails been provided?
Release Agents for Formwork:
Formwork needs to be treated with a release agent so that it can be removed easily after the concrete has set. Failure to use a release agent can result in the formwork sticking to the concrete, which may lead to damage of the concrete surface when it is prised off.
A single application of release agent is all that is required when forms are then used. Care must be taken to cover all the surface that will come in contact with the surface of concrete. However, if there is an excess of release agent, it may cause staining or retardation of the concrete.
There are different release agents depending on what material is used for the formwork. The three most common release agents for formwork are:
- Neat oils with surfactants: used mainly on steel surfaces, but also suitable for timber and plywood.
- Mould cream emulsions: good general purpose release agents for use on timber and plywood.
- Chemical release agents: recommended for high quality work, applied by spray to all types of form face.