In this study, hybrid nonlinear finite element models (FEM) were developed to examine the flexural performance and the ultimate load capacity of bonded and unbonded two-way reinforced concrete post-tensioned (PT) slabs that were pre-strengthened with external carbon-fiber reinforcement polymer (CFRP) laminates. Full 3D simulations, using ANSYS models, have been created for five different slab samples that were selected from a previously available experimental study. The model results were assessed to enable further numerical analysis. The result calibration included measurements of first crack loads, ultimate loads, deflections, strains in the extreme fiber of concrete, strains in CFRP laminates, and failure modes. The results proved a good correlation between FEM output and experimental ones. Based on this, the influencing parameters that affect plate stiffness, as well as the bending capacity of PT slabs, were examined by performing a detailed parametric study. The parameters included real-life load simulation, cable-to-CFRP strength contribution, and CFRP laminate location selection. The results demonstrated that strengthening using CFRP laminates have significantly increased the ductility index of both bonded and unbonded PT concrete slabs by 62.18% and 59.87%, respectively. In addition, strip strengthening locations near supports are much more effective than in the middle of slabs. Additionally, the CFRP strengthening contribution is very considerable in slabs with low PT ratios.