What is the impact of using seawater as mixing water in reinforced concrete?

Using seawater as mixing water in reinforced concrete can significantly increase the risk of corrosion of the reinforcing steel due to the high chloride content. Seawater contains a high concentration of chloride ions, typically around 19,000 parts per million (ppm). As previously discussed, chloride ions can penetrate the concrete and disrupt the passive layer that protects the reinforcing steel from corrosion. This accelerates the corrosion process, leading to cracking, spalling, and reduced structural capacity. While the use of seawater may not significantly affect the initial strength of the concrete, it substantially reduces its long-term durability in reinforced structures. The increased chloride content also raises the risk of chloride-induced stress corrosion cracking in prestressed concrete. For these reasons, the use of seawater as mixing water is generally prohibited or severely restricted in reinforced concrete construction, particularly in structures exposed to harsh environments. If seawater must be used due to the lack of alternative water sources, measures must be taken to mitigate the increased corrosion risk, such as using corrosion inhibitors, employing stainless steel or epoxy-coated rebar, reducing the permeability of the concrete mix through the use of supplementary cementitious materials, and ensuring adequate concrete cover over the reinforcement. However, even with these precautions, the long-term performance of the concrete may still be compromised. Plain (unreinforced) concrete may tolerate the use of seawater in some instances, but careful consideration of the long-term effects is still necessary.