Mechanical behavior of RC columns strengthened with thin UHPC jacket under cyclic loads

Ultra high performance concrete (UHPC) exhibits ultra high compressive strength and toughness due to its dense micro-structure and fiber bridging. This paper investigated the effectiveness of thin UHPC jacket for seismic enhancing of reinforced concrete (RC) columns with high/excessive axial load ratios (0.37 and 0.46). Seven columns, including two RC control columns and five UHPC strengthened columns were tested under combined lateral cyclic loads and constant axial loads, considering the effect of axial load level and jacketing scheme (fiber volume ratios and presence of mesh bars in UHPC). The failure modes, hysteresis response, skeleton curves, strains of steel bars, stiffness degradation and energy dissipation of the specimens were comparatively analyzed. Test results indicated that the proposed UHPC strengthening methods improved the load-carrying capacity and the energy dissipation at least 41.1% and 117%, respectively. With the increase of axial loads, the lateral load-bearing capacity of the UHPC strengthened specimens increased, while the effect of the UHPC jacket scheme on the load-bearing capacity of the strengthened specimens was small. The increase of axial load and decrease of reinforcement (steel fiber or mesh bar) in UHPC jacket reduced the post-peak ductility and energy dissipation. Besides, a design method was presented to predict the flexural capacity of the UHPC jacketed columns, and the average ratio of the predicted values to the tested values was 0.94.