Abstract: The effect and mechanism of chloride ion concentration of portland cement concrete and the structural steel reinforcement was investigated. At low concentrations, chloride ion has little or no effect on the physical stability of concrete structure but it causes the corrosion of the re-enforcing steel in the concrete but at high concentration and especially at low temperature, it causes expansion and cracking in the concrete. The degradation is usually accompanied by formation of calcium oxychloride salts. The mechanism for this destruction was discovered in this work as a result of the thermal transformations of the calcium oxychloride salts: 3CaO.CaCl2.15H2O to CaO.CaCl2. 2H2O.