Fracture of concrete cylinders subjected to torsion

In order to validate the 3D version of the 2D ’Delft’ lattice model some torsion tests were conducted on concrete cylinders, both notched and un-notched. A biaxial loading machine with rotational and vertical loading axes was used. Tests were conducted under different axial restraints. It was observed that a perturbation of the stress state induced by the torque may affect significantly the crack pattern. Under axial compressive stress, after a spiralling crack, an axial crack was detected in un-notched specimens. In notched cylinders, the failure surface is conical if an axial load is present, while it appears flat and localized within the notch height in case no axial load is applied.