Structural behavior of reinforced concrete beam column joints under cyclic loading

Abstract

Reinforced concrete frame structure is the most common building form in Bangladesh at present. It is commonly found in the construction industry that the different components of the frame is cast with concrete mixes made with different coarse aggregates, mainly brickbats and crushed stone. This practice has an impact on the joint region where the different concrete mixes come together. These joint regions are the most susceptible zone under earthquakes in a reinforced concrete frame structure which requires immediate attention with the alarmingly increasing number of earthquake occurrences. However, there is no study found in literature which focuses on the seismic performance of these structures where dissimilar coarse aggregate concrete mixes are used in same frame. The present study focuses on the structural behavior of reinforced concrete joints designed and detailed according to BNBC 2020 cast with dissimilar coarse aggregate concrete under the action of cyclic loading. Eight exterior beam-column joint specimens were prepared in order to determine their behavior with respect to cracking and failure characteristics, hysteresis response, stiffness degradation, ductility, and energy dissipation capacity. Along with conventional aggregates, the performance of wastes such as steel slag and recycled concrete aggregate in the joints has also been studied in this research to suggest a sustainable alternative. Ductility of specimens cast with dissimilar coarse aggregate concrete mixes was 9.8% to 23.3% lower as compared to the specimens cast with uniform coarse aggregate concrete mixes; while the energy dissipation capacity was also 14.8% to 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 beam-column joints with dissimilar coarse aggregate concrete must be carefully analyzed and designed in terms of the seismic requirements of the structure before their use. The energy dissipation capacity was 11.4% to 25.7% higher for specimens in which crushed stone aggregates were replaced by steel slag aggregates. Ductility was 3.7% to 11.4% higher while the energy dissipation capacity was 4.3% to 32.5% higher for specimens cast with recycled aggregate concrete compared to those cast with brick aggregate. The results establish that, steel slag and recycled concrete aggregate can be excellent alternative coarse aggregate choices in reinforced concrete frame structures.