A series of experimental and probabilistic investigations were conducted to determine characteristic values of durability properties, including carbonation rate and chloride migration coefficient, for actual structures. Five concrete bridges, in service for 43–45 years, were dismantled, and over 200 concrete cores were extracted from the slabs and girders. The carbonation rate and chloride migration coefficient of the core samples were measured using accelerated testing methods, and equations for calculating characteristic values from the raw data are proposed. Statistical simulation was performed with these equations, and the resulting data set was analyzed for discussion. The raw data of carbonation rate and chloride migration coefficient from the concrete core samples collected from existing structures exhibited a significant deviation compared with those from young concrete. Through probabilistic analysis using the data set from the statistical simulation, a protocol for obtaining more reliable and robust characteristic values of carbonation rate and chloride migration coefficient is proposed.