Experimental investigation on performance of interior beam column joints retrofitted with ferrocement and polyester fiber reinforced concrete

Abstract

Beam-column joints are recognized as the critical and vulnerable zone of a Reinforced Concrete (RC) moment resisting structure subjected to seismic loads. During an earthquake, the global response of the structure is mainly governed by the behaviour of the joints. If the joints behave in a ductile manner, the global behaviour generally will be ductile, whereas if the joints behave in a brittle fashion then the structure will display a brittle behaviour. The joints of old and nonseismically detailed structures are more vulnerable and behave poorly under the earthquakes compared to the joints of new and seismically detailed structures. Therefore, more often than not, the joints of such old structures require retrofitting in order to deliver better performance during earthquakes. Various researchers have proposed different methods to retrofit the beam-column joints of existing RC structures. This paper reports a few experimental investigations carried out for seismic retrofitting of RC beam-column joints using ferrocement and polyester Fibre Reinforced Concrete (FRC). The concepts behind each of them and their practical usefulness have been brought out in light. To achieve the objectives of the study, seven half scale columns were constructed. Three of them were retrofitted by Ferrocement with varying volume fractions (0.99%, 1.62% and 2.43%) and another three of them were retrofitted by FRC with varying dosages of polyester fibre (0.1%, 0.2% and 0.3% of concrete volume). The rest one was kept as control specimen. Then the specimens were tested under deflection control cyclic loading with a constant axial load. Their load-deformation behavior has been measured by dial gauges and video extensometer. Behavior of the strengthened joints is compared with that of the control specimens considering appearance of first crack, maximum load, deflection and failure patterns, rotation with respect to applied moment, stiffness and rotational stiffness. Diagonal cracks occurred under cyclic loading in interior joints which lack in shear reinforcement. Joints strengthened by Ferrocement exhibit better ductility and stiffness than those of the joints retrofitted by FRC. Joints strengthened by FRC exhibit better Rotational capacity and Rotational stiffness than those of the joints retrofitted by Ferrocement. The failure of the control specimen occurred at the beam-column joint due to the lack of shear reinforcement at the joint. On the other hand, the failure of the all retrofitted specimens occurred away from the beam-column joint which conclude that both ferrocement and FRC can be used satisfactorily as retrofitting materials for joints in seismic design.