Dynamic mechanical properties and fracturing behaviour of concrete under biaxial compression

Heterogeneity is an important factor affecting the dynamic mechanical properties and failure process of geomaterials especially when considering the coupled effect of strain rates and confinements. In this research, dynamic biaxial compression tests are conducted on concrete by using a triaxial Hopkinson bar system with different biaxial confinements (i.e., pre-stress σ₁ and σ₂: 5–30 MPa) and impact velocities (i.e., 14–18 m/s corresponding to strain rates of 80-140 s^-1). High-speed three-dimensional digital image correlation (3D-DIC), synchrotron-based micro-computed-tomography (micro-CT) and a machine learning-based crack classification technique are adopted to quantify the dynamic deformation and fracturing properties. Experimental results show that both dynamic strength and peak strain decrease with increasing axial pre-stress σ₁, but increase with higher lateral pre-stress σ₂ and impact velocity. Fractures generally propagate from the surfaces of the specimen towards the centre along the impact direction, and appear at interfaces and in the matrix first and aggregates afterwards. Real-time surface deformation and post-failure fractures are aggravated with pre-stress σ₁ and impact velocity, but restrained by pre-stress σ₂. Statistical crack analysis indicates that different crack types (e.g., matrix crack, interfacial crack and transgranular crack) own distinct geometrical characteristics (e.g., orientation distribution, fractal dimension and position distribution) and are affected by the orientation and aspect ratio of aggregate. Moreover, transgranular crack ratio decreases with higher pre-stress σ₁, but increases with larger pre-stress σ₂ and impact velocity, consistent with variation of total stress and fracture energy, implying the significance of transgranular crack on mechanical properties and fracture energy of heterogeneous geomaterials under dynamic loadings.