Pull-out behavior of twisted steel connectors employed in masonry: The influence of the substrate

It is well known that multi-leaf walls, cavity walls as well as rubble masonry are prone to brittle collapse mechanism due to weak transverse connections. Twisted steel bars are widely used to strengthen such kind of masonry buildings, in particular to improve the transverse connection between different structural elements. Due to the shape of the twisted bar, the connector works as a self-threading screw, anchoring to the substrate material without any binder. The effectiveness of such technique mainly relies on the bond between the bar and the substrate, which relies on mechanical interlocking and friction mechanisms. In this paper, an experimental study on the pull-out behavior of twisted steel connectors inserted in units of different materials (bricks and stones) that can be commonly found in existing masonry buildings is presented. Mechanical and microstructural characterization of the substrate materials was conducted to investigate the influence of some key parameters (compressive strength, tensile strength, elastic modulus as well as abrasion resistance) on the pull-off load of the connector. To get a preliminary insight on the structural behavior of such connectors in presence of different loading regimes, the bars were also subjected to two cycles of unloading/reloading. Additionally, a 3D scanner was employed to geometrically characterize the inner surface of the hole left by the bar after its extraction. Results showed that among the different mechanical and microstructural parameters investigated, the substrate compressive strength plays a key role in the pull-out behavior of the connectors.