How to minimise Distortion (Bow and Twist) in
Formply
- Do not use any new panels showing bow or twist out of the pack. Where
distortion exceeds 30mm, these panels fail the product standard. They should
not be used and returned to the manufacturer. - Prevent/Minimise Exposure to Moisture
Where formply panels that have been supplied flat on site distort, there is only
one cause, this is exposure to moisture, or a change in moisture content.
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No formply manufacturer provides any warranty where flat plywood sheets
distort on a jobsite due to exposure to moisture or weather.
Once installed, it is the responsibility of the formworker to ensure that the
formwork plywood is maintained flat to achieve the desired off-form finish.
The following is good practice to minimise distortion due to moisture or weather
exposure. - Always use form oil. Form oil provides supplementary protection to the
phenolic surface film and greatly reduces water permeability and
penetration. - Remove ponded water as soon as practically possible. Significant
distortion takes place where ponded water is allowed to stand for an
extended period. - Where an extended period of wet weather is expected, or the job site is
shutdown (i.e. Christmas shutdown), consideration should be given to
protecting the deck from weather by tarping. While this may not be
practical for large areas, smaller areas can be protected. - Consider the grade of formply. Formply can be manufactured with
thicker less permeable phenolic overlays. The standard weight of a
phenolic surface film overlay is 60gsm (60 grams grain per meter
squared).
Formply can be manufactured with heavier overlays (up to 400gsm) that
are less permeable to moisture.
The construction of the formply should also be considered.
Thicker formply with thin face veneers is less prone to distortion. Where
adverse weather conditions are expected, using a 19mm or 21mm formly
with thin face veneers i.e. 1.0mm or 1.5mm will give superior
performance to standard 17mm. - Always inspect the deck for distorted sheets immediately before pouring.
Any distorted sheets should be nailed or if necessary screwed flat. Do
not rely on the weight of the concrete to flatten the sheets. - Dealing with cupping
Cupping occurs where the surface veneer (upper most) has lower
moisture than the back veneer. The face veneer shrinks and “pulls up”
the ends of the sheet causing cupping. Cupping is most common after
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heavy rain followed by very hot days. The surface veneer dries and
shrinks, but the back veneer remains at higher moisture content.
Cupping can be remedied by hosing the upper surface of the formply
sheet to increase the moisture content. This should be done 2-3 hours
prior to pouring.
