Strengthening of corroded RC slab–column joints using thin-ply hybrid FRP under punching shear

A comprehensive investigation into the punching shear (PS) strengthening behavior of corroded two-way reinforced concrete (RC) slabs using thin-ply glass/carbon hybrid FRP was conducted through an integrated experimental, numerical, and theoretical program. Eleven slab–column joints (SCJs) were cast and tested under monotonic loading to assess different external strengthening configurations. The experimental results indicated that hybrid thin-ply FRP improved PS capacity more effectively than single-material GFRP or CFRP systems within the tested conditions. Numerical analysis using ABAQUS was then carried out to evaluate the influence of strengthening parameters. The simulations showed that increasing the FRP layers beyond two, or thickness beyond approximately 0.6 mm, resulted in limited additional capacity due to premature debonding. The parametric study identified a skewed strip layout with a 50 mm offset from the column face and two hybrid layers (0.6 mm each) as the most effective configuration for the investigated geometry and corrosion level. Strengthening efficiency was observed to decrease as corrosion severity increased, reducing from 50.77% at 5% corrosion to 24.76% at 30%. Overall, the findings suggest that thin-ply hybrid FRP, when optimally arranged, can be a practical option for partial recovery of PS capacity in moderately corroded RC slab–column joints; however, its effectiveness is influenced by corrosion extent, bond performance, and configuration, and therefore should not be generalized beyond the tested parameters.