Experimental characterisation of mechanical behaviour of concrete-like materials under multiaxial confinement and high strain rate

Concrete structures and rock engineering projects are generally in uniaxial (principal stresses σ₁ > σ₂ = σ₃ = 0), biaxial (σ₁ ≥ σ₂ > σ₃ = 0) and triaxial (σ₁ ≥ σ₂ ≥ σ₃ ≠ 0) stress states, and also frequently subjected to dynamic loads such as earthquake, high-velocity impact, drilling & blasting and explosive attacks. A series of experiments is conducted on cement and concrete specimens using a triaxial Hopkinson bar (Tri-HB) system, investigating mechanical properties and fracturing behaviour of concrete-like materials under the coupled static-dynamic loading conditions. Experimental results show that the values of dynamic uniaxial compressive strength (σ_UC ^d) of cement and concrete are much higher than those of static uniaxial compressive strength (σ_UC ^s), but decrease with increasing axial pre-stress from 0 to 30 MPa. The dynamic compressive strength under biaxial (σ_BC ^d) and triaxial (σ_TC ^d) static pre-stresses are around 2 and 3 times higher than σ_UC ^s, respectively. The values of σ_BC ^d are substantially sensitive to the pre-stresses (σ₁ and σ₂) and increase with the increase of σ₂. Whereas the increment of σ_TC ^d is not obvious at triaxial compression conditions. Tested specimens are broken into small pieces under uniaxial pre-stress conditions, but exhibit slant-shear-failure mode under biaxial conditions. The peak strains under triaxial pre-stress states are much higher than those under biaxial and uniaxial conditions. The cement under dynamic triaxial compression shows significant plasticity deformation and microscopic damage due to the constraint of three-dimensional confining pre-stresses. The X-ray computed tomography (CT) images show internal microcracks under triaxial conditions.