Tensile behaviour of mineral-based composites

Sammanfattning: Externally bonded fibre-reinforced polymer (FRP) systems have shown to be a robust and durable way to repair, or strengthen concrete structures. Epoxy, as the most common bonding agent, provides excellent force transfer, and bonds well to the base substrate to the as well as to the strengthening material. However, the epoxy-bonded systems exhibit certain inherent weaknesses, such as low compatibility with the concrete substrate, degradation in strength and stiffness around 85 °C, and toxicity both during application and when subjected to fire. Epoxies also require a minimum application temperature often above 10 °C, and create sealed surfaces, potentially resulting in moisture and freeze/thaw problems.In recent years, alternative, inorganic bonding agents have been in the focus of research. Cementitious bonding agents, when combined with the FRP, have the potential to become a high-performance strengthening system, without the drawbacks of the epoxy-bonded systems. Inorganic binders provide excellent protection to the FRP against UV-degradation, fire, or vandalism. Contrary to epoxy, they can be applied in colder temperatures or climates. They show a better compatibility with the base concrete in terms of chemical or thermal compatibility, shrinkage properties, and they do not create diffusion-closed surfaces.In this thesis, the mineral-based composite (MBC) strengthening system has been investigated, with focus on the tensile behaviour of the material. The MBC comprises of a carbon fibre polymer (CFRP) grid and an inorganic, mineral-based binder. Additionally, MBC has been placed in a wider context within the field of externally bonded, mineral-based strengthening systems. On the material side, MBC has been modified and enhanced by involving strain-hardening mortars. The experimental work presented in the thesis consists of two test series aiming to investigate the tensile behaviour. First, uniaxial tensile tests were carried out on dogbone-shaped specimens, to characterize the tensile properties of the bare composite strengthening material. Then, wedge-splitting tests were conducted to investigate the post-cracking behaviour, toughness, and ductility of the MBC, and the interaction between the MBC and the base concrete.The chosen test methods have proven to be suitable to characterize the tensile behaviour of the MBC. In all cases, the specimens failed with CFRP rupture, indicating good bond, both on the base concrete-mortar, and the mortar-CFRP interface. The MBC strengthening system performed excellent in terms of load-carrying capacity. Furthermore, the strain-hardening mortar has been found to enhance both the load bearing and in particular, the deformation capacity. It has also been shown that the pseudo-ductile mortar is capable to shift the overall behaviour from brittle towards a more ductile failure.The potential in such improved mineral-based strengthening systems is enormous. The ductility provided by the strain-hardening mortars together with the stiffness and strength from the FRP component could result in a high-performance strengthening material applicable in a range of different situations, from shear-sensitive structures through mining applications, such as tunnel linings.