CONCRETE SPALLING DUE TO FIRE EXPOSURE AND THE INFLUENCE OF POLYPROPYLENE FIBRES ON MICROCRACKING

摘要

This paper presents the results of fire tests with high strength concrete (HSC) elementsrnwith and without adding polypropylene (PP) fibres. PP fibres were generally used in concreternto prevent explosive spalling because they increase the permeability of the concrete duringrnheating in the relevant temperature range. As a result the moisture can escape fast enough outrnof the specimen and pore pressures are reduced. Without PP fibres the dense microstructurernof the HSC retards the moisture transport and pore pressures increase until explosive spallingrnoccurs. The reason for PP fibre induced increase in permeability is up to now not sufficientlyrnunderstood. Based on their research the authors come to the conclusion that microcrackingrnstarting from the fibres is the key mechanism of this phenomenon. Therefore the researchrnstudy is focused on the investigation of the influence of PP fibres on microcracking. For thisrnpurpose an acoustic emission (AE) analysis was performed on 600 × 600 × 300 mm~3rnspecimens during a hydrocarbon (HC) fire exposure. Through this innovative technique thernmonitoring of the crack and spalling development can be realized indirectly with highrnresolution. This experimental method is a new approach in the context of fire spalling. ThernAE results were validated by microscopic investigations after the fire tests. Supplementary tornthe AE analysis pore pressure measurements were performed in different depths of thernspecimens. These results will be related to the AE results for a better understanding of therninteraction between microcracking and pore pressure.