Factors affecting the resistance of cementitious materials at high temperatures and medium[0] heating rates (vol 39, pg 455, 2006)

The resistance of mortars made of dolomite and quartz aggregate with and without polypropylene fibers has been studied at a nominal heating rate of 30 degrees C/min from room temperature up to 1000 degrees C. It is demonstrated that the key parameters that affect the performance of plain mortars are volume instability, phase transformation, aggregate dissociation, and permeability. Experimental results point at the major role of aggregate type on mass loss, porosity, volume instability, microstructure, cracking pattern, and mechanical properties. Three thermal zones are identified; low (up to about 300 degrees C), intermediate (about 300 to 600 degrees C), and high (> 600 degrees C). It is shown that in the low thermal zone, the mechanical properties are about the same or better than those at room temperature. The intermediate thermal zone is characterized by a moderate decline of mechanical properties, whereas a rapid decline is registered in the high thermal zone. Explosive spalling due to pressure built-up of volatiles took place at temperatures over 200 degrees C. Addition of polypropylene fibers prevents spalling due to the occurrence of porous and permeable interface between the fibers and the matrix rather than fiber shrinkage or melting.