Use of Permeable Formwork in Placing and Curing Concrete

Book excerpt: Permeable framework is a special class of lined formwork used to produce improvements in the strength and durability of concrete. The bracing and the liner in the formwork are engineered to resist the pressure of plastic (or fresh) concrete, but to allow trapped air and excess water to pass through and be removed during concrete placement and consolidation. The objective in using permeable formwork is to eliminate voids on the surface of the concrete (bug holes) and to increase the strength and durability of the concrete surface immediately behind the formwork. A review of permeable formwork and its use in placing concrete was conducted. Methods, techniques, and materials are discussed, and example applications are described. Benefits of using permeable formwork include a reduction in bug holes and surface defects, improved resistance to freezing and thawing, reduced rates of surface carbonation and chloride-ion infiltration, increased surface strength, reduced form coating requirements, reduced efforts in curing, and reduced surface preparation for coating. The cost of using permeable formwork varies greatly among job sites. However, the cost of using permeable formwork will generally be double that for conventional impermeable formwork. Cost savings can be realized in the extended life of any wooden formwork used behind the filter fabric, the ability to proceed without applying form-release compounds, the decreased cost of final surface preparation (if coatings are to be applied to the finished concrete), and the increased service life of the finished concrete.