3action: Advances in direct-tensile testing of concrete

Gaining a deeper insight into stress vs crack opening curve of concrete is essential, especially when crack stability governs the durability and the capacity of a structure. Despite its rationality, the direct-tension test faces the challenges entailed by strain-softening materials, namely axial and flexural instability. To address these issues, an innovative frameless testing apparatus-nicknamed 3action-was developed. The system employs three symmetrically arranged electromechanical actuators that directly apply tensile force to short, notched cylindrical specimens. The samples are first bonded to thick steel platens and then bolted to the machine's base and moving head. The actuators, optimised for stiffness and responsiveness, operate through three parallel control loops that ensure uniform crack opening, monitored by three LVDTs positioned astride the notch. Following each tension test, the two halves of the fractured specimen were used to perform the splitting (Brazilian) tests. This paper presents and analyses results obtained from a highly brittle high-performance concrete mix, investigating the influence of polypropylene fibres and thermal exposure up to 250 degrees C. Finite element modelling is also employed to validate the reliability of the direct tension test in capturing the constitutive response of the material without resorting to inverse analysis. The study demonstrates the potential of transforming advanced experimental setups into standardised testing methods, offering valuable insights into the fracture behaviour of concrete under diverse mechanical and thermal conditions.