Pre-Concrete Checks for Formwork: Overview
Before any concrete is poured into the formwork, it must be inspected by someone who is professionally trained for this task. Depending on the complexity of the pour, the inspection could take anywhere from a few minutes to several hours. Once the formwork has been deemed safe, the pour can commence.
Formwork Pressure Considerations
Formwork pressures are determined by the height at which concrete is placed, as well as other factors such as concrete workability, rate of stiffening and rate of placement. The temporary works co-ordinator is responsible for considering ambient temperatures, concrete composition and other elements when calculating the maximum rate of concrete placement. Going beyond this limit may result in formwork deflections, gaps between grout and concrete, or even formwork collapse. The cost of fixing issues caused by formwork deflection is usually much higher than the cost of doing it correctly in the first place.
Below are the checks that should be verified before pouring begins:
- 1. Ensuring Formwork is Erected According to Approved Drawings: The formwork should be erected in accordance with the approved drawings to ensure that it is properly aligned and leveled. All props should be plum and spaced correctly, and bolts and wedges should be secure against any possible loosening. The number of ties used should be correct and placed in the right places and properly tightened.
- 2. Securing Inserts and Fixings: Inserts and cast-in fixings should be in the right position and secure. Stop ends should be properly secured to ensure that the formwork is restrained against movement in all directions.
- 3. Sealing Joints to Prevent Grout Loss: Joints should be sealed to prevent grout loss, especially where the formwork is against the kicker.
- 4. Preparing the Formwork for Striking: The formwork should be prepared for striking without damaging the concrete. All forms should be clean and free from rubbish such as tie wire cuttings, and odd bits of timber or metal. A release agent should be applied and checked to ensure it is the correct one.
- 5. Ensuring Access and Safety: Projecting bars should be straight and correctly positioned. There should be proper access for placing the concrete and compacting. Toe-boards and guard rails should be provided to ensure safety.
Release Agents for Formwork:
Release Agents for Formwork: Formwork needs to be treated with a release agent to ensure it can be removed easily after the concrete has set. Without the use of a release agent, the formwork can stick to the concrete, leading to surface damage when it is taken off. To ensure successful removal, a single application of release agent should be applied to cover all surfaces that will come into contact with the concrete. However, an excess of release agent can cause staining or retardation of the concrete.
Types of Release Agents: There are various types of release agents depending on the material used for the formwork. The three most common release agents for formwork are:
• Neat Oils with Surfactants: These are primarily used on steel surfaces, but are also suitable for timber and plywood.
• Mould Cream Emulsions: These are good general purpose release agents for use on timber and plywood.
• Chemical Release Agents: These are recommended for high quality work and should be applied by spray to all types of form face.