The high temperatures of fires affect the physical and chemical properties of the concrete and thus influence its mechanical properties. This paper presents the results of an experimental investigation on the compressive strength at high temperatures of high-strength fibre concretes. The influence of the high temperatures on the physical and chemical changes of the concrete was also analysed by Thermo Gravimetric Analysis/Differential Thermal Analysis (TGA/DTA), X-Ray Diffraction (XRD) and Scanning Electron Microscopy with Energy Dispersive Spectrometry (SEM/EDS). Five concrete compositions with different steel fibre contents and types have been tested: one without steel fibres (reference composition), two with Dramix 3D steel fibres and two with Dramix 5D steel fibres (45 and 75 kg/m3 ). This new type of steel fibres, the Dramix 5D, presents a double curvature at its ends, allowing a more efficient anchorage in the cementitious matrix. The behaviour at high temperatures of concretes made with these 5D fibres has been compared with the one of concretes made with the Dramix 3D steel fibres. Therefore, the impact of the high temperatures on the compressive strength and morphology of the high-strength fibre concretes made with the Dramix 3D and 5D steel fibres has been evaluated. The paper proposes also models for the compressive strength at high temperatures of the studied high strength fibre concretes. | Effect of the high temperatures on the microstructure and compressive strength of high strength fibre concretes
