Making reinforced concrete immune from chloride corrosion

Abstract

Chloride-induced corrosion is a major factor affecting the durability of reinforced-concrete structures. However, a high tolerance to chloride contamination has often been achieved under laboratory conditions. A significant factor affecting the chloride threshold level for corrosion initiation in concrete is the presence of defects, such as air voids, in the cement paste at the steel–concrete interface. To explain this observation it is noted that corrosion initiation and propagation involve the local production of acid at sites where the steel passive film breaks down in the otherwise highly alkaline concrete environment. Thus, solids with pH-dependent dissolution behaviour affect the corrosion process. Solid phase corrosion inhibitors would release hydroxyl ions to prevent this local reduction in pH. By reducing the entrapped air-void content at the steel to values below 0·2% it was possible to increase the chloride threshold level from 0·2% to above 2% chloride by weight of cement as the voids no longer dilute the inhibitive solid phases. A high tolerance to chloride contamination may also be achieved by reducing the electrolyte content of voids and by using electrochemical treatments to generate a reservoir of hydroxide-containing salts at the steel–concrete interface. Chloride contamination at levels above 2% chloride by weight of cement at a reasonable steel cover depth may not be possible in concrete exposed to many environments when the concentrating effects of evaporation are minimised. Thus, a chloride threshold level of 2% may effectively render steel immune to chloride-induced corrosion.

A new arrangement of galvanic anodes for the repair of reinforced concrete structures

“…As a result galvanic protection is not generally achieved by sustaining an adequate level of steel polarisation, as is the case for other electrochemical treatments [9,11].…”

“…The new International standard for CP of steel in concrete [17] has taken this into account and performance assessment of galvanic CP is preferably focused on corrosion risk assessment. In practice this is based on monitoring of changes in the condition of the reinforcement that arise as the result of the protective effects afforded by galvanic CP [11]. Examples include corrosion potential as a function of time and/or distance from an anode or edge of the repaired area and/or corrosion rate [5,22].…”

Responsive behaviour of galvanic anodes in concrete and the basis for its utilisation

“…The effects of pit re-alkalisation, chloride extraction from pits and polarisation have been compared by others and on balance pit re-alkalisation and pH maintenance appear to be the dominant protective effects at typical cathodic protection current densities [17]. Indeed brief high current treatments have been shown to raise the chloride threshold level for corrosion initiation to values substantially greater than 2% chloride by weight of cement [5]. It is postulated that this is due to the generation of hydroxide at the steel.…”

“…This is supported by the Pitting Potential/Re-passivation Potential hypothesis [1]. By contrast, it has been suggested that modifying the surface of the steel by raising the pH is the dominant protective effect, with reports that typical Impressed Current Cathodic Protection current densities are enough to halt corrosion when applied for long periods [2,3], or delay its onset in aggressive environments once the protective current has been removed [4,5]. These observations support the basis of the Pit Re-alkalisation/pH Maintenance hypothesis [6][7][8].…”

Assessing the long term benefits of Impressed Current Cathodic Protection

“…Figure 1 shows the results of a study in which a brief period of electrochemical treatment that would have generated hydroxide ions at the steel caused a substantial increase in the tolerance to chloride contamination [8]. The observation that electrochemically treated structures are more tolerant to chloride contamination is countered by an observation that in very heavily chloride contaminated concrete; corrosion may start again.…”