Controlled permeability formwork

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 cover zone towards the formed surfaces.

As conventional wood or steel formwork is impermeable, 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.

CPF liners are typically constructed of 100% polypropylene fibres, spun and thermally bonded, with a woven texture of 0.7 mm thickness. Some systems may be laminated to a plastic latticed net to ensure drainage whilst providing stiffness to the liner.

They are robust and chemically inert and may be supplied in rolls of varying length to construction sites.

The basic elements of CPF systems are:

• Filter - to allow the passage of water and air away from the fresh concrete but retain cement and other fine solids.
• Drain - which transfers air and water from the filter to outside the formwork.
• Structural support - this is the formwork which supports the filter and drainage elements and also maintains the required formwork profile and resists the concrete pressure, commonly plywood/structural timber.

CPF liners are secured in place on vertical or inclined surfaces with staples or other fixing devices, having first been tensioned onto the formwork shuttering. Once attached, concreting is performed in the normal way. Release agents are not required as CPF liners easily debond from the concrete during formwork striking.

Throughout the concreting process and as a result of concrete pressures, entrapped air and excess mix water that would otherwise become trapped at the surface causing blemishes, can instead pass through the liner. A proportion of this mix water is held within the liner and under capillary action, imbibes back into the concrete to assist curing. Liners generally have a pore structure that is designed to retain the majority of cement and other small fines.

This results (for vertical and inclined surfaces) in the creation of a uniform surface relatively free from blowholes and other surface blemishes when compared to impermeable concrete. But more importantly the achievement of a cover area with significantly enhanced durability.

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