Mechanical characterization of the shear resistance of mineral-bonded composites under impact loading

Novel mineral-bonded composites such as strain hardening cement-based composites (SHCC) and textile-reinforced concrete (TRC) are promising solutions for strengthening existing buildings and designing new structures to resist impact loading. Under uniaxial tensile loading, these composites possess high strain capacity and energy absorption capability. SHCC and TRC exhibit a pronounced strain hardening behavior characterized by multiple cracking owing to the bridging capacity of the short fibers or continuous textiles, making them suitable for application as protective layers for existing structure and greatly increase the resilience and the impact safety of the built environment. Furthermore, mineral-bonded composites offer attractive pathways for enhanced sustainability standards, using e.g. alternative viable binders such as limestone calcined clay LC3. As structural elements and strengthening layers are subjected to a manifold of concurrent local and global loading conditions during impact, the comprehensive characterization of SHCC and TRC under various impact loading conditions is compelling. Indeed, a thorough understanding of their behavior enables the proper and safe design and application of mineral-bonded composites in the construction industry. Specifically, the rate dependent behavior of the composites and their constituents, i.e. matrix, reinforcement, and their bond, needs to be described. In this context, appropriate and robust impact testing techniques at the material scale need to be developed encompassing a variety of loading conditions, i.e. compression, tension and shear. A gravitational Split-Hopkinson tension bar (SHTB) was developed within the framework of the Research Training Group (GRK2250) for material characterization of ductile mineral-bonded composites under impact tensile loading. Several aspects were considered for the design of the SHTB, such as the ductility of the composites of interest and the influence that mechanical adapters have on the measurements of the material properties. The work at hand specifically focus on the material characterization of ductile composites under impact shear loading. To achieve this aim, a mechanical shear device, consisting of two adapters, was conceptualized and developed to be integrated into the gravitational SHTB. Several key aspects were considered in the design process of the shear device, such as (i) its influence on the wave propagation, (ii) the optimal specimen geometry to achieve a dominant shear fracture mode, (iii) the implementation of high-speed image acquisition, monitoring the fracture process during the experiment for further digital image correlation (DIC) analysis, and (iv) the suitability of the shear device in customary hydraulic testing machines as benchmark in quasi-static regime. Parameters influencing the shear behavior of composites such as the shear span length, notches depth, and lateral confinement were investigated using experimental and/or numerical techniques. An extensive experimental program of quasi-static and impact shear testing of various mineral-bonded composites was carried out using the developed shear device. The tested composites varied with respect to their constituent cement matrix, i.e. fully cement-based or sustainable limestone calcined clay cement and type of reinforcement, i.e. short fibers, continuous 2D- textiles, or 3D-reinforcement.