<p>The durability assessment of existing concrete structures, particularly under exposure to harsh environmental conditions, is of paramount importance. Additionally, the development of in-situ testing methods, which enable rapid and cost-effective evaluation of structures without the need for destructive intervention, plays a crucial role. Exposure of concrete to repeated freeze-thaw cycles leads to strength reduction, as the formation and growth of ice crystals toward low-pressure zones generates crystallization pressure, damaging the lateral walls of the concrete. Additionally, residual water in the pores causes expansive cracking. Incorporating polypropylene fibers into concrete restricts crack formation and hinders their propagation. In this study, the non-destructive cylindrical chamber method was used to assess the amount of water penetration in concrete. After validating the cylindrical chamber test, in-situ permeability evaluation of existing structures was carried out. The results demonstrated that the use of PP fibers reduced the permeability of concrete by approximately 20%. Microstructural evaluations revealed that after 100 freeze-thaw cycles, the capillary pore volume in C45 grade concrete increased, whereas the gel pores showed no significant changes. Furthermore, phase analysis of plain and fiber-reinforced concretes indicated that the presence of fibers enhanced the formation of calcium silicate hydrate gel during hydration, reducing the void spaces between cement particles and consequently improving the durability of the concrete.</p>