An investigation into factors affecting the performance of composite controlled permeable formwork liners: Part II – Filter medium

The aim of the filter medium study was to determine the influence that various filter fabrics, fixing methods and reciprocal usage have on the efficiency of air and excess mix water removal from CPF cast concrete. The goal is to significantly reduce the water/cement (w/c) ratio at the near surface of the slab so that specific surface performance characteristics could be monitored. A general factorial was used for the design of experiments with the results being statistically analysed using a one and two-way ANOVA and interaction plots. A range of tests including permeability, surface quality, surface hardness, surface strength and ISAT are used to determine the effectiveness of each system. Results are compared to a control sample which was cast against impermeable plywood formwork. The textiles were developed around pore size guidelines which were determined through a particle size distribution study for EN 197-1 cement (CEM). The influence of the textile permeability with reciprocal usage was a dominant factor in affecting near surface performance properties of the cast slabs. Surface hardness, surface tensile strength and absorptivity were shown to increase by various amounts depending upon the factors involved. It was concluded that an effective CPF liner can be constructed from a glued or stapled PP, PE or N6 filter medium with no drainage medium present. However, longevity of the system and optimum surface characteristics were found using a PE textile specifically.