Calcestruzzi ad Alte Prestazioni Soggetti ad Elevate Temperature: Influenza della Tipologia degli Aggregati e delle Fibre

The increasing use of High-Performance Concrete (HPC) in many structures exposed to extreme environmental conditions, bring in new problems, beyond the excellent knowledge on ordinary concrete in the same extreme conditions. HPC is definitely promising, but denotes a greater heat-sensitivity (in particular to spalling, namely the explosive expulsion of pieces due to thermal stress and vapour pressure in the pores). Its denser matrix and the ensuing lower porosity, in fact, are pros in ordinary conditions (since durability is improved), but bring in higher pore pressures due to water vaporization, being vapour release impaired. This is the reason why to understand the influence of fibre and aggregate on the heat-induced damage is a critical issue. Fibre, in fact, can bring in benefits on cement paste behaviour, while aggregate strongly affects the response of concrete since it is the largest fraction by volume. To this end, an experimental campaign has been launched at the Politecnico di Milano in collaboration with CTG-Italcementi Group (Bergamo, Italy) on 3 concrete grades (fcm,cube = 45, 70, 95 MPa), with 3 aggregate types (silico-calcareous, calcareous and basaltic), with/without polypropylene (monofilament or fibrillated) or steel fibres. Displacement-controlled compression tests have been carried out to evaluate strength and elastic modulus after heating to 5 different temperatures (T = 20, 105, 250, 500 e 750°C). Ultrasonic and microstructural investigations have been also performed. The experimental results give fresh and new information, instrumental to cement producers, designers and contractors for improving the fire performance of concrete mixes.