Effects of Midlayer Used in FRP Confinement of RC Columns

Fiber-reinforced-polymer (FRP) composites are widely used currently as strengthening materials for the repair and strengthening of important elements of concrete structures, especially columns. This study investigated the effects of a novel technique that involves placing a thin galvanized midlayer between the substrate concrete and the overlying polymer composite sheet to improve the confinement quality of RC columns under axial loading. The technique is meant not only to prevent the transfer of concentrated stress from the concrete cracks to FRP sheets but also to decrease the transfer of biaxial stresses to FRP sheets. For this purpose, 24 RC columns, each 150 mm in diameter and 500 mm in height, were strengthened with one or two layers of FRP strips and loaded under axial compression. The effects of confinement with intermittent strips of carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) were investigated in columns with and without the thin galvanized midlayer. The results were compared with those obtained from loading control specimens lacking any external strengthening. The midlayer was found to increase maximum load capacity and energy absorption by 16% and 53%, respectively, in specimens strengthened with one layer of carbon fiber relative to those recorded for the control. These enhancements recorded for the specimens strengthened with one layer of glass fiber were 17% and 117%, respectively. (C) 2019 American Society of Civil Engineers.