Cyclic behavior of RC beam-column joints with stone aggregate concrete in columns and brick aggregate concrete in beams

The Reinforced Concrete (RC) moment frame system is the most common building type in developing countries. The practice of using different types of coarse aggregates for beams and columns of the same frame is not uncommon in countries where stone aggregates are relatively expensive. This practice impacts the beam-column joint (BCJ) region where the different concrete mixes come together. These BCJ regions are the most susceptible zone under earthquakes in an RC frame structure, which warrants careful attention considering the increasing number of recent earthquake occurrences. However, there is very limited study in the literature that focuses on the seismic performance of these structures where dissimilar coarse aggregate concrete mixes are used in the same frame. The present study focuses on the structural behavior of RC joints cast with dissimilar coarse aggregate concrete mixes under the action of cyclic loading which are designed and detailed following the Codes ACI 318 and BNBC 2020. Four one-third scale exterior BCJ specimens were prepared to determine their behavior with respect to cracking and failure characteristics, hysteresis response, ductility, stiffness degradation, and energy dissipation capacity. The ductility of specimens cast with dissimilar coarse aggregate concrete mixes was 9.8-23.3 % lower compared to the specimens cast with uniform coarse aggregate concrete mixes, while the energy dissipation capacity was also 14.8-22.1 % lower. The cracks appeared earlier in lower displacements for specimens with dissimilar coarse aggregates, although the strength and elasticity of the concrete mixes were similar. Thus, the present study suggests that BCJs with dissimilar coarse aggregate concrete must be carefully analyzed and designed in terms of the seismic requirements of the structure before their use.