ATERIALS IN RC BEAM-COLUMN JOINTS SUBJECTED TO CYCLIC LOADS

Abstract

There is a continuous research on the behaviour of joints particularly to increase shear strength. The conventional way to enhance the joint’s shear strength is to provide increased dimension with closely spaced transverse/shear reinforcement in the hinge region. The recommendations on the basis of confinement of joints are incorporated in national and international codes. This critical confinement in the joint region leads to steel congestion and also poses difficulty in construction. The provision of confinement at the joint region helps only upto a certain extent to increase the shear capacity of the joint due to the poor tensile property of concrete. The confined joints still behave inelastically under severe seismic loading in spite of acting as rigid panel zone for the development of desirable failure mechanism. In the recent years, the use of different high performance materials is being explored to improve the in-elastic behavior of joint without using special confinement reinforcement. These high performance materials are available in different forms namely (1) Fibers such as in discrete forms and as fabric, fiber reinforced polymer, (2) mineral admixtures in micro and nano size particles, (3) solid form material like plates/sheets/rods, carbon grids, geo-grids, shape memory alloys. Different chemical admixtures and adhesives are also widely used in reinforced concrete structural members to enhance the required strength at the concentrated regions. The research programme is focussed on different types of high performance materials used in the joint region of the external beam- column joints tested under cyclic loading. The feasibility of geo-grid as a reinforcing and confining material in reinforced concrete structural components is examined under monotonic and cyclic loading. The pre and post yield behavior of specimens with the derived parameters shows that the geo-grid can be effectively used as a confinement material in structural engineering. The geo-grid may become more effective with steel fiber to improve the damage tolerance capacity. The influence of different fibers with concrete is also investigated under compression and flexure. The possible use of different fibers and hybrid fibers (to reduce the higher volume of fibers) to improve the shear resistance capacity of beam-column joint without critical confinement is examined under cyclic loading. The enlarged hysteretic loop of FRC composite specimens establishes enhancement of shear strength and post iv yield stiffness retention of the joint. The energy dissipation and damping characteristics of FRC composite show that the discrete fiber reinforced concrete can be used in critical joint region without confinement. Influence of High Performance Fiber Reinforced Cement Composite (HPFRCC) in improving the post yield performance of reinforced concrete structural components is investigated under static and cyclic loading. The moment carrying capacity, post yield stiffness and energy dissipation behavior of RC specimen show that the HPFRCC can be effectively used in the RC components at hinge region without critical reinforcement. The use of fiber hybridization may become more effective in improving the ductile performance and damage tolerance capacity without increasing the fiber volume. The influence of precast SIFCON core enabled beam-column joint region is examined under cyclic loading. The hysteretic behaviour, energy dissipation and damage index of the specimens show a significant improvement of the post-yield behaviour of the joint with SIFCON core as compared to other conventionally confined joint specimens. An analytical model for predicting the shear strength of the beam-column joint is proposed under the unconfined and confined conditions based on existing experimental data base. A comparison is also made to predict shear strength with the shear strength of the model proposed in American Concrete Institute (ACI), New Zealand (NZ) and Architecture Institute of Japan (AIJ) codes. The proposed equations are closer to experimental test results of the joint with reasonable co-variance. The proposed model is further upgraded for SIFCON Core and the SFRC. The modified equation is able to predict the shear strength of specimen with reasonable accuracy. The performance evaluation of epoxy filled mechanical couplers connecting the two cut ends of a reinforcing bar is carried out by conducting the monotonic and cyclic testing of beam and beam-column joint specimens. The moment-curvature, hysteretic behavior, stiffness degradation, energy dissipation and damage tolerance capacity of this coupler enabled specimens are quantified and compared with the conventional specimens. The pre and post yield behaviour of specimens with couplers at different locations authenticates its usage in retrofitting work where the buckling of longitudinal reinforcement is possible. An experimental case study is also carried out on the use of mechanical couplers in plastic hinge region of columns and the results are compared with conventional techniques of retrofitting.