Pullout behaviour of shape memory alloy fibres in self-compacting concrete and its relation to fibre surface microtopography in comparison to steel fibres

Superelastic nickel-titanium shape memory alloy (SMA) can recover large strains. Hence, SMA fibres (SMAFs) have been incorporated into concrete to provide re-centring and crack-closing ability to the concrete. Since the structural performance of fibre-reinforced cementitious composites is significantly influenced by the fibre pullout mechanism, this research was focused on the pullout behaviour of SMAFs and their associated parameters. Single fibre pullout tests were performed on straight and hooked-end SMA and steel fibres (SFs) embedded in self-compacting concrete. The experimental investigation considers three variables: the type of hooked-end, embedded length, and loading rate. The full range of pullout load-slip behaviour was obtained under two different loading rates. The results were analysed in the slip corresponding to maximum pullout load, maximum tensile stress induced in fibre, material use factor, pullout energy, average and equivalent bond strength. Straight SMA fibres showed a weaker interfacial bond with concrete than steel counterparts. SMA fibre with 45° hooked-end showed the highest bond strength among SMA fibres considered in this study. The increase in the embedded length of most of SMAFs and SFs caused a significant increase in the pullout load without changing the configuration of the pullout load-slip curve. The pullout behaviour of SMAFs was almost rate-insensitive, whereas SFs exhibited evident rate sensitivity in the pullout. The observed differences between SMA and steel fibres were clarified based on fibre surface micro-topography and roughness and matrix damage. This difference was explained based on the surface micro-topography of fibres and matrix damage.