Experimental pull-out behavior of helicoidal steel bars in masonry

When dealing with the retrofitting of historical heritage buildings, the improvement of the structural behavior should be weighed against the compatibility of the designed interventions, in order to preserve the architectural value, avoiding irreversible damages. Helicoidal stainless steel bars are becoming widely used for the retrofitting of historical masonry buildings for crack stitching or to improve the transversal connection between different structural elements, even if very few studies on their mechanical behavior are presented in the scientific literature. In this paper, an experimental study on the pull-out behavior of helicoidal stainless steel bars installed into the masonry is presented. The investigated parameters were the masonry wall strength (by considering two different bricks and two different mortar strengths), the embedment depth (100, 200 and 400 mm), the position of the bar (brick, T-joint, face or side of the wall) and the orientation of the bar (0° or 45°). The bar performances were assessed under monotonic tension and cyclic axial loading. The bar diameter adopted for all the tests was 12 mm. Results showed very good performance with reliable results associated to low coefficient of variation of the loads and with a very limited damage of the base material. The load-displacement curves showed a good ductility, an excellent superposition between monotonic and cyclic tests, with an extended plateau. Among the different investigated parameters, the position within the wall was the most influential one, with higher loads associated to the location of the bar in middle of the brick.