Behaviour of two-way RC slabs strengthened with ultra fibre-reinforced concrete layers

Ultra fibre-reinforced concrete (UFRC) layers were used to strengthen two-way concrete slabs and their stiffness under monotonic four-point bending was evaluated. Separated into three groups with low, medium and high reinforcement ratios (groups W, M and S, respectively), a total of 15 slabs were cast and tested. N each group, one slab was not strengthened, while the rest were reinforced using UFRC layers containing 1% or 2% polypropylene fibres. These UFRC layers were applied either on the tensile side of the slab or on both tensile and compressive sides. The flexural behaviour, crack development and failure modes were examined. The results showed that the cast-in-place UFRC layers significantly improved the yield load, ultimate load and stiffness, while also reducing the final deflection. They also enhanced the ductility, energy absorption capacity and cracking strength of the slabs. Notably, displacement ductility increased in all slabs strengthened with 2% fibres and UFRC sheets on two sides. Compared with the respective control specimens, the ductility increased by factors of 2.75, 1.50 and 1.71 in groups W, M and S, respectively. Overall, the findings demonstrate that UFRC layers provide an effective strengthening technique for deficient slabs, offering improvements in load capacity, deformation resistance and overall structural performance.