Carbonation, chloride penetration and corrosion of reinforcement

Corrosion of reinforcement is probably the most widespread cause of deterioration in concrete. The expansion produced by rust formation causes the surrounding concrete to crack and spall. In a sound concrete, rusting is prevented by the high pH of the pore solution, which stabilizes an oxide film on the steel that inhibits further attack. This film is unstable at lower pH values, which can result from carbonation or leaching, or in the presence of Cl . Sources of the latter include sea water or salt spray, de-icing salts used on roads, certain aggregates, especially those available in desert climates, and CaClj used as an accelerator. Though now widely prohibited, this can 

Still affect older structures. Tensile stress can damage the protective lilm. and particular care must be taken to minimi/c ingress of CO, and Cl in prestressed structures. 

Carbonation begins at exposed surfaces, and spreads inwards at a rate proportional to the square root of time (K66). The thickness of the affected layer can be approximately determined by testing a section with phcnol-phthalein. Chloride ion from an external source similarly penetrates inwards. The age at which corrosion is liable to begin thus depends on the minimum thickness of concrete covering the reinforcement, the resistance to penetration and the conditions to which the concrete is subjected. The depth of cover needed to provide protection over a given period can be calculated (SI 17.B154). 

Roy (R67) reviewed carbonation and transport and reactions of chlorides. Hansson (H63) and Turriziani (T59) reviewed corrosion. 

Carbon dioxide dissolves in the pore solution of cement paste, producing CO3 , which reacts with Ca to produce CaCOj. The OH and Ca ions required by these reactions are obtained by the dissolution of CH and decomposition of the hydrated silicate and aluminate phases. C4AH. is quickly converted into C ACH, and ultimately into CaCOj and hydrous alumina monosulphate and ettringite yield CaCOj, hydrous alumina and gypsum. C-S-H is decalcified, initially by lowering of its Ca/Si ratio, and ultimately by conversion into a highly porous, hydrous form of silica. The 

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