India's First Long-Span Unbonded Post-Tensioned RC-UHPC Composite I-Beams

With ultra-high mechanical and durability characteristics, Ultra High-Performance Concrete (UHPC) offers structural solutions that compares well with structural steel. This paper illustrates the application of UHPC in a school building project where the functionality of the floor was altered from a set of classrooms to an auditorium at the top floor during the construction stage. The change required intermediate columns to be eliminated at the top floor of the three-story structure, thus increasing the span length from 7 meters to 14 meters. A conventional long-span reinforced concrete (RC) or post-tensioned beam would have added significant dead weight to the peripheral columns making them unviable. Aesthetically they would have also appeared to be bulky. Unbonded post-tensioned UHPC I-beams designed as composite with the RC-slab provided the apt solution to the project. The UHPC was prepared on-site using a dry premix and the beams were precast in a steel formwork and post-tensioned at the ground level with three un-bonded 0.5-inch diameter strands. The UHPC beams were lifted up and erected in place using a crane after which the RC slab was cast-in-place. The design resulted in about 30 percent lighter floor system as compared to the original framing with the 7 meters span. This led to a reduction in forces at the peripheral columns of top floor level. This paper describes the design and construction process of the RC-UHPC composite floor system using India’s first long-span UHPC I-beams.