AbstractConcrete structures are exposed to water throughout their service life. Nowadays, fiber-reinforced concrete is the most preferred concrete due to its superior mechanical properties. However, good mechanical properties alone cannot protect concrete against the environmental conditions it may encounter during its service life. In particular, water is an inevitable factor faced by concrete elements. In this study, the permeability properties of concrete produced with different combinations of four different fibers were investigated. Hybrid fiber concrete (HFC) was made using the central composite design method. Steel, glass fiber, synthetic, and polypropylene fibers were hybridized among themselves, and the aggregate and paste phases of concrete were hybridized with electric arc furnace slag aggregate (EAFS) and fly ash (FA), respectively. Ultrasonic pulse rate, rapid chloride permeability (RCP), capillary water absorption capacity, and carbonation depth (CD) of HFC were determined. Statistically significant and nonsignificant parameters for each response were determined. Simultaneous substitution of EAFS and FA reduced the capillarity coefficient and CD of HFC. The RCP of HFC depends mainly on two factors: binder dosage and steel fiber ratio. The most suitable fiber for reducing CD was glass fiber. The estimated RCP, ultrasonic pulse velocity (UPV), and capillary water absorption capacity (CWAC) results were compared with the control sample, and the results were 81%, 114%, and 86% of the control sample.