Dual Confinement of Circular Concrete Columns Consisting of CFRP Sheets and Steel Ties or Spirals

It is well known, that confinement introduced by CFRP (Carbon Fiber Reinforced Polymer) sheets increases the ultimate compressive strength and ductility of concrete. In past various experimental research programs had been carried out by other researchers to express the increase in strength and strain by the use of CFRP jackets. But in the majority of cases the additional effects of reinforcing elements like ties or spirals are not analyzed very well. In this paper the investigations on wrapped short concrete columns with and without transverse reinforcement will be shown. In an extensive research program the volumetric ratio of the CFRP jacket and the ratio of the transverse reinforcement were varied. Thereby, columns with different geometrical shape, different CFRP thickness and with different transverse reinforcement elements (steel ties and spirals) were produced and tested in compression tests. As a main result it can be shown clearly, that a dual confinement consisting of CFRP and transverse reinforcement increases strongly the ultimate strength and axial strain. The comparison with existing guidelines and models also points out the necessity of new structural models to describe the influence of dual confinement in an exact way.