MICROSTRUCTURE OF FIBER MORTAR COMPOSITES UNDER FIRE IMPACT - EFFECT OF POLYPROPYLENE AND POLYACRYLONITRILE FIBERS

Two polymer fibre types (PP and PAN) and different microstructural met hods were used to compare the porosity of fibre mortar composites and the effect of rapid thermal exposure on the fibre structure. Polished thin sections were prepared from fibreless reference mortar and fibre mortar samples before and after fire impact. Polarizing light microsco py indicates increased air void porosity in the fibre mortars. Polariz ing light microscopy and scanning electron microscopy reveal that PP a nd PAN fibres react entirely differently to heat PP fibres melt, where as PAN fibres become dark in appearance, presumably due to chemical tr ansformations, and exhibit a zoned structure at the outer part of the fibre. A transition zone program shows that porosity detected by backs cattered electron images is higher at the fibre/matrix contact zone th an in the bulk matrix. Fire impact broadens this porosity region at th e contact zone. On the whole, the porosity level at the contact zone s eems to be higher in the PAN than in the PP fibre mortar.