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.

本文介绍了中尺度混凝土（骨料、大孔和砂浆基质）的实验研究，以便更好地了解已知驱动材料宏观力学行为的局部破坏机制。主要原创来自于现场进行在单轴压缩和首次在 5 MPa、10 MPa 和 15 MPa 围压下三轴压缩下，对真实成分（包括水泥、沙子、骨料和水）的微型混凝土样品进行 X 射线力学测试。对来自每个测试的一组 3D 图像的时间序列分析允许在整个实验中测量 3D 运动场（位移和应变场）。讨论了针对每个加载路径观察到的不同故障模式，以及在最大非均质性（骨料和大孔隙）尺度上对 3D 压裂过程进行量化。随着约束水平的增加，观察到从脆性响应到延性响应的转变，以及材料强度的增加。强调了混凝土的中尺度异质性对其局部破坏机制的显着影响。结果表明，应变局部化主要起源于骨料和砂浆基质之间，最大骨料和大孔隙的形状和位置基本上驱动了裂缝网络的传播。