This study aims to understand how moisture, CO₂, and chloride ions penetrate usable postfire concrete by using electrical resistivity data. Mortar and concrete specimens with water-to-cement ratios (W/Cs) of 0.35 and 0.45, respectively, were prepared for testing. Polypropylene (PP) fibers were added to the specimens with W/C of 0.35. One surface of each specimen was placed on a hot plate and exposed to 350°C for 15 min. The moisture transfer in the specimens stored at 20°C and 60% relative humidity was assessed by measuring their electrical resistance. Then, the penetration of CO₂ and Cl ions in the mortar specimens was examined. The electrical resistance of the specimen surfaces was found to be greater than that at greater depths, and the unheated specimens were found to have the lowest resistance. This implies that the regions of the specimens heated with and without PP fibers were drier than those of the unheated specimens. In addition, the heated mortar specimens exhibited greater penetration of CO₂ and chloride ions compared to the unheated specimens. Thus, PP fibers and high W/C increased the rate of carbonation and chloride penetration into the mortar specimens after heating.