Experimental investigation on seismic performance of columns enhanced by high-strength steel bars and ultra-high-performance fiber reinforced concrete

An experimental investigation was performed to study the seismic performance of columns enhanced by the 630 MPa high-strength steel bars (HSSBs) and ultra-high-performance fiber reinforced concrete (UHPFRC). Six HSSBs-UHPFRC enhanced columns and one normal concrete (NC) column reinforced with the HSSBs were subjected to static cyclic lateral loading. The effects of steel fiber volume ratio, axial compression ratio, longitudinal reinforcement ratio, and stirrup spacing on the seismic performance of UHPFRC columns were studied, and the differences in seismic performance between UHPFRC columns and NC column were compared and analyzed. The test results indicated that the UHPFRC columns exhibited superior flexural ductile failure with excellent integrity compared to the NC column, and the height of the damaged area was much smaller than that of the NC column. The tensile and compressive properties of the 630 MPa high-strength steel bars could be fully utilized in the UHPFRC columns, which had a good match with UHPFRC. The HSSBs-UHPFRC columns exhibited satisfactory bearing capacity, adequate ductility, and large energy dissipation capacity. Furthermore, the bearing capacity degradation of all specimens was greater than 0.85, indicating that the HSSBs-UHPFRC columns possessed robust bearing capacity stability. A higher volume ratio of steel fiber or denser stirrup spacing could effectively restrain UHPFRC and longitudinal reinforcement bars to enhance the ductility of the specimens. With the longitudinal reinforcement ratio increasing, the superior mechanical properties of the HSSBs and UHPFRC could be utilized better in the UHPFRC column and the seismic performance and flexural capacity of the UHPFRC specimen could also be enhanced correspondingly. A formula to calculate the flexural capacity of the HSSBs-UHPFRC columns was established based on the analysis of the specimens' flexural capacity. The calculated values were found to agree with the experimental values well. The results can serve as a reference for designing HSSBs-UHPFRC enhanced structures．