Response of concrete cast in permeable moulds to severe heating

This paper evaluates the effect that a permeable mould, such as would be used to create fabric-formed concrete, may have on the heat-induced explosive spalling performance of cast concrete, using a novel experimental fire testing method and supported by scanning electron microscopy. Recent research suggests that a concrete cast using fabric formwork will gain durability enhancements at the cast surface that may negatively affect pore-pressure expulsion during severe heating. Six concrete samples were cast using high strength concrete including silica fume and tested using the University of Edinburgh's Heat-Transfer Rate Inducing System (H-TRIS), receiving thermal loading on one surface. Three samples were cast in permeable moulds, formed using a Huesker HaTe PES 70/70 single layer woven geotextile with a characteristic opening size (O90) of 0.1 × 10−3 m. Three samples were cast in conventional impermeable timber moulds. The tests showed no conclusive evidence of differences in thermal profile or differential thermal deflections between the two casting methods; no occurrences of heat-induced explosive spalling were observed for either casting method. However, scanning electron microscopy undertaken on additional samples showed that the test face of samples cast in permeable moulds were over four times less porous compared to their impermeably cast equivalents. This could increase the risk of spalling of samples, particularly in cases where pore-pressure spalling dominates the material response. However, additional fire testing using H-TRIS is needed under a range of heating and loading conditions, before definitive conclusions on the spalling propensity of fabric-formed concrete can be made.