Bond Performance of Interior Beam-Column Joints with High-Strength Reinforcement

Abstract

Reinforced concrete structures function by ensuring that the forces can be transferred between the concrete and reinforcement. One place this occurs is at the bond between the concrete and the reinforcement. This article considers the bond performance of interior beam-column joints, where reinforcement must transition from being at yield in tension on one side of the joint to being close to yield in compression on the other side. The authors report on a study of four beam-column joints, undertaken to assess the accuracy of New Zealand design rules relating to bond strength in beam-column joints when applied to large diameter, high-strength reinforcement. In their samples, the ratio of longitudinal beam reinforcement diameter to column depth intentionally did not meet the requirements of the New Zealand design standard for any of the units. The units were subjected to cyclic displacements up to interstory drift angles of 5%. Bond failure occurred in two of the four test units, at drift levels exceeding those allowed by international codes. The authors conclude that the unexpectedly good bond strength was caused by the large quantity of column longitudinal reinforcement included in these units. The reinforcement gave the columns a large excess of moment and vertical joint shear capacity over that required. This reduced damage (particularly the formation of splitting cracks) in the joint zone concrete as testing progressed, thus improving bond conditions.