Formwork for visual in-situ concrete
The texture of cast in situ concrete is typically dictated by the formwork facing material providing an ‘as struck’ finish. The selection and construction of appropriate, good quality formwork is fundamental to achieving a good finish. Impermeable linings such as steel, polypropylene, glass reinforced plastic (GRP) and high density overlay (HDO) board produce a smooth, shiny surface whereas timber, unsealed plywood, paper-faced or medium density overlay (MDO) boards give a matt finish. Pattern or texture can be created by lining the formwork with a material to create the desired finish. Proprietary form liners are available with either established or bespoke textures or patterns. Timber boards are commonly used as a form liner to create a board-marked finish
Generally, the concrete contractor will determine the formwork required to meet the required finish or performance specification. There may be occasions when the designer wishes to have a particular finish that a form finish provides. In this situation it is prudent to specify the formwork rather than try to describe it. This prescriptive specification puts the onus on the designer to ensure that this form finish will give them the appearance required.
If the concrete is left exposed the formwork panel layouts will be visible and therefore there should be a clear understanding of the layout requirements.
No matter how good the workmanship, the joints between individual panels will be visible in the finished concrete. Therefore, the designer should clearly convey to the contractor the preferred panel layout to improve the visual appearance of the wall or soffit. The formwork sheet sizes are usually 1220mm by 2440mm and should be detailed by the designer as whole units with the minimum of cutting.
Wall elements usually require tie-bolts and the designer should indicate the preferred location of the ties and, the preferred method of filling the holes after casting. This will require coordination with the formwork designer/supplier to establish the number required and available positions once the actual formwork design takes place.
One technique for ‘disguising’ joints between panels and tie-bolt locations is to make a feature of them, although it is recognised this may not be a remedy for every project.
The formwork should be grout tight and all joints between panels sealed with an appropriate clear non-staining sealant to eliminate any grout loss.
The exposed formwork face should not have any nail heads nor screws fixed through the contact face nor allow hammers or metal objects to come in direct contact with the form face. Site operatives should take care not to damage the surface by scuffing with sharp tools or implements.
Release agents help prevent the concrete binding to the formwork. The release agent must be non-staining and not wash off during wet weather, nor break down over the concrete curing period. It should give a uniform finish to the surface with no dusting or discolouration. The release agents must be compatible with the admixtures in the concrete and the formwork used.
It is important to strike the formwork after a consistent period of time. When vertical formwork is removed, usually after approximately 24-36 hours, from concrete cast in the summer it will result in a lighter initial surface colour than a concrete cast in winter when formwork is maintained in positions for significantly longer. This contrast in colour tone between summer and winter casting will slowly even out as the concrete surface dries out and carbonates but it may take many months for this condition to stabilise.