Pathology of Fibre Concretes after Fire
Résumé
There are a lot of studies about mechanical behaviour of fibre reinforced concrete, but few works focused on the effect of fire attack. Our study is mainly experimental: fibre mortars and standard mortars are submitted to elevated temperatures and pathology is examined in terms of mechanical behaviour and microstructural observations. In order to better simulate fire, a flame test using propane gas was developed. 3 different mortars were submitted to this flame test: standard mortar, steel fibre mortar and hybrid mortar (½steel fibres + ½ polypropylene fibres). During flame test, the temperature evolution was measured at different points so as to evaluate the thermal gradient. After flame test, a punching shear test was conducted. Another test done in a muffle furnace has shown that: Between 400°C and 500°C, a 30 percent loss of flexural strength was measured, but no major thermal damage of fibre mortar was observed. In excess of 500°C, the damage was very important. Flexural strength is less than 50% of initial strength. However, between 400°C and 700°C, the addition of fibre provided a better ductility and a gradual breakdown. The rupture energy was increased. Microstructural observations were done using X-ray diffraction and SEM. The cement matrix was dehydrated during flame test, a debonding between the cement matrix and fibre were observed and an oxidation of steel fibre was detected. These different observations could explain the loss of strength.