Seismic behavior of masonry-infilled reinforced concrete frames strengthened using ultra-high performance concrete diagonal strips

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An extensive experimental program was undertaken which aimed at investigating the lateral cyclic behavior of non-ductile masonry infilled RC frames strengthened with Ultra-High Performance Concrete precast plates. In response, a series of four ½ scale single-storey, one-bay masonry infilled RC frames were meticulously designed and constructed. Among these, one frame was tested as-build with masonry infill, another was strengthened with 300 mm wide CFRP diagonal sheets, while the remaining two were strengthened with 355 mm wide UHPC diagonal plates. CFRP and UHPC strengthening was applied to one side of the masonry infill and the main test variable was the thickness of the UHPC plates. Based on experimental results, it was found that UHPC-strengthened specimens exhibited approximately twice the peak lateral strength that of the as-built specimen and 1.3 times that of the FRP strengthened specimen. Stiffness analysis indicated that UHPC-strengthened frames possessed an initial stiffness approximately 71% higher than the as-built frames, contributing significantly to their enhanced seismic performance. Nevertheless, it is important to note that UHPC-strengthened specimens also experienced stiffness degradation, although at a slower rate when compared to the as-build and CFRP-strengthened specimens. Moreover, UHPC-strengthened frames dissipated roughly three times more energy than the as-built frame and twice as much as the FRP-strengthened frame. While doubling the UHPC plate thickness from 10 mm to 20 mm led to a significant increase in masonry tensile strength, it also introduced structural asymmetry between the strengthened and unstrengthened sides of the wall. This asymmetry resulted in masonry panel buckling, ultimately leading to the failure of both UHPC-strengthened specimens at the same peak lateral load. In summary, this investigation underscores the promising role of UHPC retrofitting as a highly effective and occupant-friendly strategy for enhancing the seismic performance of masonry infilled RC frames. These findings carry significant implications for enhancing structural safety and resilience in earthquake-prone areas. Furthermore, this study provides a solid basis for further research and practical implementations in the realm of seismic retrofitting, contributing to the advancement of safer and more resilient infrastructure globally.