RC Column Retrofit with Ultra High Performance Concrete

Abstract

Ultra high performance concrete is a material that can exert a compressive strength of 180 MPa or more and a tensile strength over 10 MPa. Recently, researches and applications of UHPC as a structural retrofit material for existing structural members are actively proceeding. This study deals with a column retrofit with UHPC jacket that can efficiently utilize strong compressive capacity of UHPC. The UHPC column retrofit method can allow jacket designed in less thickness than ordinary concrete jacketing. Also, it is expected to have superior strengthening performance compared to other column retrofitting methods. Furthermore excellent performances are expected in durability, fire resistance, and abrasion resistance. The purpose of this study is to evaluate the strengtheningt performance of UHPC jackets. Considering the constructability for jacketing, any inserting additional flexural reinforcement or shear connectors were not considered and only pure jacketing performances were estimated. The main focus of this research was on the evaluation of strengthening performance for lateral load resistance capacity among various performances required for seismic strengthening of the column. For this purpose, experimental research and a theoretical analysis on the failure mode were carried out. In the experiment, UHPC jacket was found out to have excellent the strengthening performance in lateral load resistance. In the case of specimens with no additional stirrup reinforcement, the columns were destroyed by the massive diagonal cracks on the jacket and as the cracks increased, the performance of jacket has decreased very abruptly. It was confirned that UHPC jacketing may cause additional unexpected failure due to additional shear force applied to adjacent beams or slabs. Therefore, proper measures should be taken for this kind of unexpected failure. In theoretical failure mode analysis, three failure modes were considered . For each failure mode, corresponding failure curve was plotted against the axial load and lateral load. When the lateral load acts on the flexural member, the interface shear force is larger than that of the general RC composite section due to the characteristic of the section in which the UHPC having strong rigidity surrounds the inner column. When the interface shear strength is not sufficiently large, the composite column is prone to fail by diagonal tension cracks in jacket. Therefore, it is recommended to control the occurrence and opening of cracks in the jacket by inserting additional reinforcing bars or meshes in the jacket for better performance and further researches on this method and performance will be required.