Applicability of FRP confinement to strengthen concrete columns

Artikel i vetenskaplig tidskrift, 2004

ABSTRACTThe bearing capacity of concrete cylinders in compression can be increased manifold by confining them with FRP (Fiber Reinforced Polymer) sheets or tubes. The stress-strain relationship for an FRP-confined cylinder is approximately bi-linear. The first branch may be simplified as linear with a slope E1 equal to the secant modulus of the unconfined concrete in compression. At the level of the concrete unconfined strength, a transition zone follows after which the tangent stiffness stabilizes at a constant value, E2, until reaching the ultimate strength. When transferring these data to the design of concrete columns, the stability problem must be accounted for. The buckling load is determined by the moment of inertia and the elastic modulus of the column. The moment of inertia is not increased by unidirectional tangential wraps around the column and the elastic modulus of the column decreases above failure load of unwrapped column. Consequently, the increase in strength due to wrapping cannot be utilized unless the slenderness ratio is lower than a limiting value; function of the confinement parameters and of the unconfined strength. Furthermore, when the confined strength can be used in design, it is necessary to ensure that the wrapping material cannot be damaged during the lifetime of column. Such damage can result, for instance, from the hit of a vehicle or from fire. In such a case the column will fail because the confined concrete within the wrap/tube in the second branch of the stress-strain curve is degraded. Therefore the load on confined column should be limited by long term sustained load on unconfined column.