Investigation of force transfer mechanisms in retrofitted RC columns with RC jackets containing welded bars subjected to axial compression

Abstract

In the current research program the force transfer mechanisms are investigated in retrofitted columns containing bend down welded steel bars. It includes 6 specimens of square section (150×150×500mm) of 24,37 MPa nominal concrete strength with 4 longitudinal steel bars of 8mm diameter (500 MPa nominal strength) and different transverse reinforcement ratios (ωc= 0,075, ωc=0,15). All columns were subjected to initial axial compression up to maximum load. After repair with thixotropic high strength concrete, all columns were retrofitted with RC jacket of 80mm width of 31,8 MPa concrete strength, including 4 longitudinal bars of 8mm diameter (500 MPa nominal strength) and different confinement ratios (ωcj= 0,035/0,071/0,142). The jacket's longitudinal bars are welded to the ones of the core using steel bars of either 8 or 10mm diameter. To test the strengthened columns a loading pattern is selected in order to simulate the real structures, even though the experimental set up includes only axial loads (moments and horizontal loads are not examined). The behavior of specimens is investigated through P-δ, Σ - ε, energy absorbed diagrams and the levels of ductility achieved. The results indicate that the initial damages affect the total behavior of the column and the capacity of the interface to shear mechanisms and to slip: a) welded bars of higher diameter present early phenomena of buckling and as a result the retrofitted columns can bear lower maximum load, b) the presence of dowel action increases the capacity of the jacketed column to transfer load through the interface.