Stainless twisted bars for masonry structures retrofitting

The demand for reversibility, compatibility and limited damage in masonry retrofitting led to the use of twisted stainless-steel bars in many applications. However, limited studies are available in literature both on local and structural behavior. This paper presents the results of an experimental study conducted to investigate the local behavior under tension load and the structural behavior of stainless twisted bars embedded in masonry walls. The local behavior was assessed by performing pull-out tests considering different parameters (embedment depth, position within the wall, and monotonic or cyclic loading). Experimental results showed very good performance with reliable results associated to low coefficient of variation of the loads and very limited damage of the base material. The load-displacement curves showed a good ductility and an excellent superposition between monotonic and cyclic tests. Among the different investigated parameters, the position within the wall was the most influential one, with higher loads associated to mid-brick location of the bar. On the basis of the tension test results, the structural behavior was assessed considering T- and C-shaped masonry specimens realized with a “weak” connection between the front and the back walls. The specimens were tested in unreinforced and strengthened configurations, considering different retrofitting layouts. An out-of-plane load was applied on the front wall, together with a vertical prestress to simulate the gravity load coming from the floors and walls above. Each specimen was tested in two consecutive runs: a monotonic one on the unreinforced masonry, and a cyclic run with the wall intersection reinforced with twisted bars. The twisted bar retrofitting resulted in a significant increase of the dissipative capacity of the wall despite the number of adopted bars, and, when properly designed, a certain increase of strength with respect to the undamaged and unreinforced configuration was noticed.