Biaxial testing machine for mixed mode cracking of concrete

Cracks in concrete may open in two fundamentally different modes in plane problems; normal to the crack plane (Mode I) and sliding of the crack faces (Mode II). A general loading situation consisting of an arbitrary combination of normal and shear stresses will invoke a combination of the two fundamental opening modes, i.e. leading to mixed mode cracking. Detailed experimental knowledge of the initiation and propagation of cracks in concrete under mixed mode loading is an important prerequisite for the formulation of mixed mode fracture mechanical models. For this purpose, an extremely stiff, bi-axial testing machine with mutually independent servo-hydraulic pumps has been developed. The setup is designed with non-rotating boundary platens, and the rotational stiffness has been measured to approximately 8000 kNm/rad. The crack initiation and crack propagation is measured locally on the surface of the specimen by photogrammetric techniques. The sample experiments presented cover both notched and un-notched specimens. The un-notched specimens were designed to study crack initiation, and it appears that cracks initiate purely in Mode I, while Mode II sliding displacements only grow slowly in magnitude. The notched specimens revealed that it is difficult to conduct mixed mode experiments for small crack widths since new cracks tend to initiate and since the crack may not necessarily have propagated all the way through the specimen.