Large-scale combined torsional and axial loading system (TorAx) - A new perspective on multiaxial testing of fracture and stress transfer in plain and reinforced concrete

Civil engineering structures are exposed to complex loading scenarios leading to multiaxial fracture and activation of stress transfer mechanisms along cracks controlled by combined deformations in two independent spatial directions (crack opening and crack sliding). Such multi-degree of freedom conditions cannot be properly investigated by state-of-the-art testing procedures. That is why - based on a thorough review of existing experimental procedures - this paper presents a novel testing methodology that applies shear via torsional loading and can simultaneously load specimens in axial direction (TorAx) to explore the material behavior before, during, and after macro-cracking. Independent control of both degrees of freedom allows the generation of arbitrary biaxial load or deformation paths. To prove the technical feasibility of the proposed testing method, several tests on thin-walled concrete tubes have been conducted. The obtained results proved uniformity of strain distribution along the tested ligament which guarantees several advantages (e.g., elimination of size/boundary effects and reduced scatter) over existing testing methods. With low costs (material/time) TorAx will serve as a benchmark for future studies under monotonic, cyclic, and fatigue loading. By experimentally quantifying all stress transfer mechanisms in one unifying test setup, an unbiased comparison of the individual mechanisms is possible, which is the first necessary step towards accelerated yet economical and reliable mechanical and numerical models.