Controlled permeability formwork
Concrete cast against wood or steel formwork has a reduced cement content and increased water/cement ratio (concrete is less dense and more porous) compared to concrete located beyond the cover zone. During compaction of the concrete, excess water and air are driven towards the formwork where they become trapped, resulting in blowholes and other surface blemishes. Controlled Permeability Formwork (CPF) consists of a filter and drainage layer attached to the formwork, which allows the water in contact with the formwork to drain away. The result is a blemish-free surface that has the additional benefit of having a higher resistance to aggressive substances, increasing the life of the structure.
For further information see:
Controlled permeability formwork, Current Practice Sheet 144, Concrete, October 2005, pp. 28 & 30–31. Copies are available as a free download from the Members Area of the Concrete Society web site and from the Concrete Bridge Development Group web site (www.cbdg.org.uk).