Abstract:
At room temperature, the addition of steel fibres in mortar conducts to an increase of mechanical strength and ductility. Exposed to
high temperatures, a cement based material undergoes deterioration to a greater or lesser extent. Heat induces various changes of
material properties; in particular changes to the micro-structure which are accompanied by loss of strength and mass. The aim our
study is an experimental contribution about behaviour under extreme conditions in terms of temperature (e.g. fire), of mortar reinforced
by different fibres: steel fibres, polypropylene fibres and a hybrid fibre combination (½ steel fibres + ½ polypropylene fibres). Three
types of standard mortar were studied, formulated with three different binders: CEM I, CEM I+SF and CEM III. Two types of cure were
used: a dry cure and a wet cure. For the mortars with fibre addition, the fibre content was 0.58% by volume. The exposure temperatures
were set at 400°C, 500°C, 600°C, 700°C, 800°C and 1000°C. The rate of temperature increase was set at 2°C/min and 5°C/min. The
mechanical properties (flexure strength, Young modulus, cracking energy, stress intensity factor, toughness factor, compression
strength) were analysed before and after heating. A specific test using propane flame was defined. The rate of temperature increase is
very elevated; the exposed face rapidly reaches a temperature of 1000°C. Samples were equipped with thermocouples to measure the
temperature changes within the sample at different depths. Subsequently, to enable a comparison between the mechanical strength of
samples a punching shear strength test was performed. A non-destructive characterisation of all mortars before and after heating was
conducted: water capillarity absorption, interconnected porosity, helium permeability, ultrasonic pulse velocity, ultrasonic attenuation.
Finally, our experimental study proposes a mortar formulation giving a good behaviour under severe temperatures and giving better
residual mechanical properties.
