The Experiment Research Of The Ultrahigh Toughness Cementitious Composite Beam-Column Connections Under Reversed Cyclic Loading

The beam-column connection is a vulnerable element in the reinforced concrete frames. And the failure mode of the joints does have significant influences on the sidesway mechanism of the entire building under earthquake. However, if the beam-column connections were fully designed and detailed according to the "strong column-weak beam" principle to withstand seismic actions, the longitudinal and transverse reinforcements would cross with each other. This would make the construction work extremely hard. What’s more, due to the congestion of the reinforcement in the beam-column joint region, it is more difficult to place and consolidate the concrete, therefore the quality of the whole construction cannot be guaranteed.It is reported that steel fiber reinforced concrete was used in the joint region and certain results were achieved by some researchers. In recent years, a kind of innovative fiber reinforced cement-based composite draws intensive attention, which is called ultrahigh toughness cementitious composite (UHTCC). UHTCC is a type of composite containing about2%volume fraction of short fibers, with properties such as pseudo strain hardening, superior deformation capacity and multiple cracking behaviors under uniaxial tensile loading, which promises an alternative for seismic design of structures.In this paper,9specimens of full-scale beam-column joints with different designs were tested under the earthquake-type reversed cyclic loading, in order to investigate the feasibility of reducing the transverse reinforcement in the beam-column connections by using UHTCC. The parameters to be researched in the whole experiment program include axial load ratio, the area where UHTCC was used, the stirrup ratio in the joint core and the stirrup spacing in the stirrup densification area of the beams.Of the total9specimens,3were made of common concrete, which are strictly designed according to the current codes in China. In the other6beam-column connections, UHTCC was used in the joint core region, as well as parts of nearby columns and beams. And in order to examine whether the UHTCC material could bear the shearing force instead of partial transverse reinforcement, or even improve the seismic-resistance performance of the structure element, less or even no stirrups were used in the joint core and nearby region. After the experiment, the hysteresis property, spine curve, stiffness degradation and the energy dissipation property of the9specimens was presented, analyzed and compared. It is noted that, the specimens with UHTCC, even though with less stirrups, still could undertake the reversed cyclic loading steadily. Moreover the ultimate bearing capacity of the UHTCC specimens was apparently higher than the normal concrete ones. The stiffness degradation process of the UHTCC specimens is basically stable, and the deformation performance of the UHTCC joints is better than that of the normal concrete ones. Besides, higher energy dissipation is obtained in the UHTCC specimens, too.