Experimental study on the spalling behaviour of ultra-high strength concrete in fire

High strength construction materials are now attractive owing to their economic and architectural advantages. The higher the material strength, the smaller member size is required. Ultra-high strength concrete (UHSC) encased columns are being developed for the erection of high-rise buildings due to their higher load bearing capacity and smaller cross section size compared to normal strength concrete encased columns. When the UHSC is subject to elevated temperature, explosive fire-induced spalling is more often observed than in normal strength concrete. The consequence of spalling could cause serious life loss and damage to the close key infrastructure. Spalling is mostly due to the UHSC increased density, lower permeability and brittleness. Most of the previous studies show that polypropylene fibres have been found effective in preventing fire spalling. The aim of this experimental study is to discover the minimum polypropylene fibre dosage to control the fire spalling of steel fibre reinforced concrete of 115-135 MPa strength. The experimental study was carried out on 15 concrete specimens with different parameters and two fibre-reinforced concrete encased columns exposed to ISO 834 fire. The study indicates that a polypropylene fibre dosage of 1.365 kg/m(3) can prevent the 115-135 MPa ultra-high strength concrete from explosive fire spalling. This polypropylene fibre dosage is lower than that proposed in Eurocode 2, which is 2 kg/m(3). The proposed lower polypropylene fibre dosage can potentially bring sustainability (use less polypropylene fibres that are made of crude oil) and economy, as well as improve constructability by improving the workability of fresh concrete. It is also found steel fibres may relieve the fire spalling but not adequate to prevent spalling. Moreover, there is no significant effect of the size and inner temperature of the centre of the concrete specimen on spalling. (C) 2020 Elsevier Ltd. All rights reserved.