Effect of Confinement on the Interaction Diagrams for RC Sections with CFRP Grids and Wraps

Application of CFRP composite materials is a popular method of strengthening reinforced concrete members. Wrapping columns with these materials is used in seismic retrofits because of the increase in the strength and ductility of concrete, and therefore, of the column itself. This is particularly beneficial for compression-controlled columns that fail by concrete crushing due to the fact that ductility and strength of the member is significantly improved. For the design of columns, interaction diagrams are used which define the maximum capacity of compression members that are subjected to axial force and bending moments. This work includes the development of interaction diagrams for circular compression members confined with CFRP composites. The concrete confinement can be light (low confinement pressure) or heavy (high confinement pressure). In this paper, three levels of confinement are evaluated: (a) zero confinement, (b) light confinement with the use of a CFRP composite grid, and (c) heavy confinement with the use of CFRP composite wraps with two different thicknesses of the wrap. A comparison of the unconfined section to the light and heavily confined sections shows a considerable difference primarily in the compression-controlled region where the axial compression and bending moment are significantly enhanced. The balance point for both light and heavy confinement has moved higher up on the interaction diagram, which changes the range of the compression and tension zones. This is evident for both light and heavy confinement. Also, the failure mode of compression-controlled columns is more ductile because of the change in the behavior of concrete due to confinement.