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This article presents the behaviour under fire of a specific ultra-high performance concrete (UHPC) called BCV. Several compositions with synthetic additions such, as polypropylene (PP) fibres and powder or acrylic (PAN) fibres, are mixed to produce these specimens. As a first step, a blowtorch test on prismatic specimens shows that small PP fibres are more efficient in resisting fire. Next, scanning electron microscopic observations after heating reveal significant behavioural differences between PP fibres and PAN fibres during their vaporisation process. Lastly, a mercury intrusion porosimetry investigation following a heating cycle at 350°C indicates that concrete porosity is not a sufficient parameter for determining whether or not a given material composition is resistant to fire spalling. A critical factor dependent on pore size distribution, is proposed herein; its threshold value, i.e. under which the material exhibits a very high probability of being prone to fire spalling, is also estimated.

This paper presents an experimental study of concrete at meso-scale (aggregates, macro-pores and mortar matrix) in order to get a better understanding of the local failure mechanisms known to drive the macroscopic mechanical behaviour of the material. The main originality comes from conducting in-situ X-ray mechanical tests on micro-concrete samples of realistic composition (including cement, sand, aggregates and water), under uniaxial compression and, for the first time, under triaxial compression at 5 MPa, 10 MPa and 15 MPa confining pressures. A timeseries analysis of the set of 3D images coming from each test allows for the measurement of the 3D kinematic fields (displacement and strain fields) throughout the experiments. The different failure patterns observed for each loading path are discussed, along with a quantification of the 3D fracturing processes at the scale of the largest heterogeneities (aggregates and macro-pores). With an increasing level of confinement, the transition from brittle to ductile response is observed, as well as an increase of the strength of the material. The pronounced impact of the meso-scale heterogeneities of concrete on their local failure mechanisms is highlighted. It is shown that strain localisation mainly originates between aggregates and mortar matrix, with the shape and location of the largest aggregates and macro-pores essentially driving the propagation of the cracking network.