Chloride influences the concrete cover for RC decks in cold regions. This research aims to investigate the probability of chloride-induced corrosion (PCI) versus various chloride concentrations applied to the top part of the RC deck using two methods: the Monte Carlo simulation (MCS) method and the proposed approximate algorithm method (PAAM). The RC deck element used in this research is made of concrete consisting of 50% fly ash and 50% slag with a concrete cover of 60 mm. Different chloride concentration scenarios were applied to the upper part of the RC deck. Furthermore, the chloride diffusion coefficient in the probabilistic model considers the impact of maximum temperature and relative humidity. Moreover, PAAM was introduced to obtain the PCI due to chloride, compared with the MCS method. It was observed that the likelihood of corrosion values obtained using the PAAM at various chloride concentrations is significantly higher than those obtained using the MCS. Finally, the PAAM isn’t accurate in conducting the PCI versus various chloride concentrations compared to the MCS.