Controlled permeability formwork (CPF) can increase concrete durability in the critical cover zone, improve surface appearance, and reduce formwork pressures. The system is comprised of a filter, drain, and structural support. While the concrete is being compacted, air and some of the mix water escape through the form leaving the concrete in the cover zone with a reduced water-cement ratio and significantly increased durability. Site measurements carried out by the Cement and Concrete Association in the early 1980s revealed that pore water is responsible for about 90 percent of formwork pressures. This explains why CPF gives substantially lower formwork pressures.
Most major Japanese contractors have developed CPF systems. They are, however, concentrating on the improved durability aspects and have so far ignored the cost-savings benefit of reduced formwork pressures. After citing benefits and drawbacks they received from the use of CPF, Japanese contractors noted that the drawbacks are more than offset by the advantages of CPF in appropriate situations.
Three types of Japanese CPF systems were studied. The textile form uses a single textile that combines the filter and the drain. The silk form system is made up of a polyester filter and a polyethylene drainage cloth tensioned over plywood and stapled to the back of the panels. SAP formwork is make of conventional formwork lined with superabsorbent polymer (SAP) sheeting.