Out-of-plane non-intrusive seismic retrofitting of in-plane damaged masonry infill through 3D printed recycled plastic devices

Abstract

The current techniques used for strengthening existing buildings made out of masonry infilled frames are characterized by high impact on the daily internal activities, as people living or working in the building need to move out while the retrofitting activities take place.The seismic vulnerability of infilled concrete frames is related to their high stiffness that, being due to the presence of masonry infills, induces a very brittle failure mechanism. The available retrofitting techniques do not allow to decrease the frames stiffness without making demolitions, especially for what concerns the double layer infills, a typology that characterizes a large part of the Italian buildings built from 1970s. For such infills only traditional techniques are available, mostly lime/cement-based mortars strengthened with various fibers that increase the frame stiffness. This study proposes an innovative technique for the out-of-plane strengthening of masonry double layer infills. The system is based on a 3D printed system made of recycled plastic, placed between the two masonry layers composing the infill and acts: i) triggering preferential in-plane sliding surfaces for reducing stiffness and therefore damaging, ii) warranting out-of-plane stability avoiding the snap-through mechanism, iii) ensuring low environmental impact by using recycled materials and, iv) no disrupting the daily internal activities. The full scale out of plane test is carried out on an infill specimen previously damaged in the in plane direction, by using a hydraulic jack placed at mid-height of the infill. The experimental results are compared with analytical models available in literature combined with a model specifically developed for the resistant mechanism provided by the plastic devices. The obtained results suggest that plastic joints are effective in preventing instability and can be used as effective strengthening system for concrete frames with masonry infills.