Rippling is normal natural swelling of wood or film faced plywood in a short time.
Edge rippling occurs on plywood frequently especially when the formwork plywood is used in heavy rain and harsh sunshine after rain. Plywood swells around the drilled holes too. These should be considered natural and not problematic.
Edge rippling is quite obvious on the black phenolic overlay.
Cause of Edge Ripping: Uneven Moisture Content
The double-faced film faced plywood has a moisture content ratio of 25-30% in use. It can create a smooth concrete finish in the formwork.
But the film faced plywood at the factory has a very low moisture content. The moisture content can be as low as 8% when they are shipped by the manufacturer.
In the first few casting, the moisture of concrete are going into the plywood little by little.
In the drilled holes and cut edges, the water goes into plywood faster than the film faces. This causes uneven results in different parts of the plywood sheet.
After 3-5 concrete forming, the moisture content of film faced plywood rise from 8% to around 27%. The film wrinkles and swelling finnally disappear.
During the process, the rippling won’t affect the concrete shuttering very much.
Vessels are like thin and long pipes in the hardwood. In a tree, vessels transport water along the grain direction.
Face veneer absorbs water in the concrete and causes rippling of film face. It absorbs water in every direction but it absorbs the most water in the vessel’s direction.
What is Rippling
Rippling appears when dry plywood absorbs water through some small damage on the phenolic film or edges in a short time.
Water moves along a vessel. Wood around the vessel gets wet.
- Dry plywood absorb water through a small damage or edges in film face.
- Water is transported by a vessel along grain direction.
- Wood around vessel gets wet and swells. Rippling appears.
- Later moisture is balanced in the plywood and rippling goes down.
Edge Rippling Steps
Edge rippling appears when dry plywood absorbs water from edges. Areas around vessels swell first. Later moisture is balanced in plywood and edge rippling goes down.
1. Edge of plywood absorbs water
2. Wood around vessels swell first (edge rippling)
3. Moisture is balanced in the plywood and rippling goes down
How to minimize the “rippling” effect?
As we know the rippling is caused because of the low moisture content of plywood. So in order to reduce the rippling effect, the phenolic film should be treated prior to first use.
How to do it?
Formwork workers will need to apply a wet cement slurry or grout wash on the plywood surface. The plywood edge swelling on the film will become less noticeable.
If you see the plywood edges swell at a concrete forming site, don’t panick. The swelling will disappear after several uses.