Destructive and non-destructive experimental investigation of polypropylene fibre reinforced concrete subjected to high temperature

Concrete is an inherently fire-resistant material, which in most cases requires no additional fire protection. However, a long-term exposure to high temperature results in the change in both strength properties and mechanical behaviour of concrete. Most of the experimental studies conducted so far investigated the fire resistance of concrete composites after the fire exposure. Whereas the aim of this paper was to focus on the performance of selected concrete composites, particularly plain concrete and fibre reinforced concrete with polypropylene fibres, exactly at the elevated-temperature state. Namely it concerns reference plain concrete and polypropylene fibre reinforced concrete tested up to 800 °C. Standard destructive tests and non-destructive tests were conducted on cylindrical specimens at hot state immediately after heating-up by using a specific heat treatment. The system used for the heat treatment was composed of ceramic pads, thermocouples the Mannings HTC 70 kW heating and control machine, which provided and regulated the heat transferred to the pads. The main objective of the experimental study was to observe actual material strength during fire situation as well as the reliability of the Schmidt's rebound hammer test when used for testing concrete at elevated temperature. The obtained results are in good agreement with the existing result database of polypropylene fibre reinforced concrete and plain concrete subjected to elevated temperature. The results obtained by the two different testing methods also show good agreement in a certain range of temperatures. As a consequence, in scientific manner, it seems that conducting the Schmidt's rebound hammer test on hot specimen with subsequent standard destructive compression test on the identical specimens cooled down to ambient temperature enable the observation of the compressive strength at hot state and the residual compressive strength after fire exposure on the same specimen. The investigation also demonstrates the benefit of the polypropylene fibres when added to a concrete composite subjected to elevated temperatures. Their presence in a mixture improves concrete spalling resistance and ensures a stable environment with low pore pressure in the concrete microstructure when subjected to elevated temperature and as a consequence the deviation in concrete compressive strength is small. The upcoming experimental investigation will focus on the effect of fibre content on the strength properties of concrete under elevated temperature.