Enhancements in surface quality of concrete through use of controlled permeability formwork liners

This report presents results of a study to compare the performance of concrete wall surfaces cast against controlled permeability formwork (CPF) liners with that of concrete surfaces cast against conventional impermeable plywood formwork. Two formwork liners were used, namely `Zemdrain', a proprietary spun-bonded formwork liner manufactured by Dupont, and `Lotrak FL', a woven geotextile manufactured by Don & Low. A variety of test methods were used to assess performance, including surface hardness, initial surface adsorption, sorptivity, depth of carbonation, chloride penetration and freeze-thaw resistance. In addition, the quality of the surface finish was assessed visually. Two concrete mixes and two different curing regimes were tested. The variation in surface quality of concrete with depth from the top of the pour was also examined. Results demonstrate that surface qualities of concrete can be markedly improved by use of CPF liners. The two liners produced identical enhancements in surface quality. They differed in one aspect of performance only, as cement particles were able to travel through the Lotrak fabric and to bond to the backing plywood, necessitating cleaning of the plywood after each cast. This problem did not arise with Zemdrain. Effectiveness of CPF liners was slightly greater for the lower-grade mix and slightly lower close to the top of a pour. It is also observed that measured enhancements in performance may be strongly influenced by test duration.