# Resistance of Concretes to External Chlorides in the Presence and Absence of Sulfphates: A Review
## 2 Resistance of Concrete to External Chlorides  
Chloride-induced corrosion of steel reinforcement has been identified as one of the primary causes of early deterioration and failure of marine concrete structures, bridges, viaducts, tunnels on roads and highways where de-icing salts are applied during the winter period. This corrosion is a serious problem, not only because of the reduction in the effective area of the steel reinforcement and the corresponding reduction in the load-bearing capacity of the structure, but also due to the expansive formation of the rust products, whose volume can be 3-6-times more than the volume of steel. These rust products cause internal stresses and consequently result in the cracking and spalling of the concrete surrounding the reinforcement [19,20].  
Chloride attack can be considered as either internal or external. Internal sources of attack are thankfully rare, but can arise due to inappropriate admixtures, chloride-contaminated aggregates or chloride-bearing mix water. External chloride attack is more prevalent and arises through the application of de-icing salts or through the ingress of brackish water or seawater. Studies have shown that exposure conditions have significant effects on the rate of chloride ingress. The requirement for water as a transport medium means that airborne chlorides pose little danger, while cyclic wetting and drying allows for the accumulation of salts through crystallization during drying periods and more severe attack [21, 22, 23, 24]. Chloride ingress into permanently wet concrete lies somewhere between these extremes. For this reason, most national and international standards incorporate exposure classes in their specifications for concretes. For example, BS EN 206 [25] sets out minimum requirements for concretes to resist chlorides, other than from seawater (XD classes) and chlorides from seawater (XS classes). Each of these are sub-divided into three, with XD/XS1 for exposure to airborne chlorides, XD/XS2 for chlorides ingressing into permanently submerged concrete and XD/XS3 for structures exposed to cyclic wetting and drying, such as tidal, splash and spray zones.  
The rate at which external chlorides permeate into concrete is often used as an indicator of the concrete's durability. Lower rates indicate more durable structures and vice versa. The chloride diffusion coefficient (\(D_{c}\)) is a term mostly used to describe the rate of chloride ingress. The value of \(D_{c}\) can be obtained by fitting laboratory or field chloride ingress data into chloride diffusion models. Table 1 shows examples of such models, including the theory upon which they are based and their areas of application. The choice of the most appropriate model will depend on knowledge of the exposure condition, i.e., whether saturated or unsaturated. For saturated concrete (i.e., concrete continuously submerged in seawater), chloride ingress is assumed to be governed by pure diffusion [26], while for unsaturated concrete (i.e., concrete partially submerged in seawater or exposed to alternate wetting and drying cycles of seawater), chloride ingress is governed by a combination of diffusion and convection [27, 28, 29].