Formwork for visual concrete
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 may be 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
again the designer should make it clear where they should be placed
and, if the holes are to be filled, the preferred method of filling
the holes after casting. This will require coordination with the
formwork designer to establish the number required and available
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
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 discoloration. 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 after
24-36 hours or so 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.
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