The Need For CPF
All concretes cast against wood or steel in the concrete cover zone, have a reduced cement content and increased water/cement ratio (i.e. less dense and more porous) compared to concrete located beyond the reinforcement. Within the core of any structural element the matrix is generally a denser and of better quality compared to the surface as a direct result of the concrete compaction. This compaction process drives excess mix air and water within the coverzone towards the formed surfaces.
As conventional wood or steel formwork is impermeable (IMF), the migration within the mix ceases as the concrete/formwork interface is reached. Visually, this may be observed on all concrete surfaces through the presence of blowholes following formwork removal.
This is a problem because the first line of defence of all structural elements against carbonatation, chlorides, frost and abrasion, is the cover zone. It is therefore imperative this region is durable.
Read more about this topic: Controlled Permeability Formwork