Response of infilled RC frames retrofitted with a cementitious fiber-mesh reinforced coating in moderate seismicity areas

A significant number of Infilled Reinforce Concrete (RC) frame structures constructed worldwide are not designed to withstand seismic actions. An important contribution to the lateral resistance of the bare frame is usually provided by weak masonry infills, whose interaction with the frame has been frequently neglected by designers. In case low-rise buildings placed in low seismicity areas are considered, the moderate inelastic demand resulting from the seismic excitation allows implementing retrofitting techniques aiming at improving the structure resistance rather than its ability to dissipate energy by inelastic mechanisms. This paper studies a retrofitting approach aiming at exploiting the frame-to-infill interaction by using a thin layer of mortar coating connected to the outer surfaces of the perimeter walls of the RC buildings. The coating is applied on the existent plaster and is reinforced with an Alkali-Resistant glass fiber mesh properly anchored to the infill. A series of reverse cyclic tests on three hollow clay brick masonry infills, including a not strengthened wall as well as a strengthened and a repaired specimen, was carried out. A special RC frame provided with steel hinges at the columns edges was built to simulate flexural mechanisms generally occurring in weakly reinforced frames. Results proved the ability of the adopted technique to significantly improve both lateral stiffness and resistance of infills. However, the observed behavior suggested future improvements that may lead to a further increment of the infill capacity.