N.R. Buenfeld

The presentation of the chloride threshold level for corrosion of steel in concrete

It has been argued that presenting the chloride threshold level as a free chloride content or chloride to hydroxyl concentration ratio in the pore solution of concrete is an improvement over the more commonly used total chloride content. In this review the basis for this hypothesis is examined. Contrary to expectations, an analysis of the literature suggests that, on balance, bound chloride presents a corrosion risk, an effect which may be due to its contribution to the reservoir of available chloride at the steel concrete interface. Furthermore, the soluble hydroxyl concentration in the pore solution, which is largely determined by the presence of alkali metals, is not an adequate measure of the inhibitive properties of the cement. Its most important property appears to be its ability to resist a fall in pH to values below that required to sustain a passive film. Thus, in terms of currently used representations, chloride threshold levels are best presented as total chloride contents expressed relative to the weight of cement. This may be viewed as the total potential aggressive ion content expressed relative to the total potential inhibitor content. It may be possible to improve this by, for example, expressing the total chloride content relative to the alkaline reserves of the concrete, but further work is needed to confirm this hypothesis.

FACTORS INFLUENCING CHLORIDE-BINDING IN CONCRETE

Abstract Chloride binding in concrete has been investigated by analysing pore solution expressed from cement paste specimens using a pore press. For chlorides introduced at the time of mixing, cement type, the type and proportion of cement replacement material, the chloride salt and total chloride content were found to be the most important factors governing binding. Binding also increased with increases in water/cement ratio, curing temperature and age. Similar, though less pronounced, trends were observed when chlorides were introduced externally by immersing thin discs of hardened, but relatively immature, cement paste, although SRPC gave similar results to OPC. Water/cement ratio and curing time prior to immersion had little effect, but the level of binding increased with exposure time and chloride concentration of the external solution. Solutions of calcium chloride, and particularly magnesium chloride, produced far higher levels of binding than sodium chloride.

The influence of chloride binding on the chloride induced corrosion risk in reinforced concrete

Chloride binding by the cement in concrete may affect the rate of chloride ingress and chloride threshold level which in turn determine the time to chloride induced corrosion initiation. In this work, a theoretical assessment of the influence of binding when chloride ingress results from diffusion, is presented. While chloride binding reduces the free chloride content within the concrete, it may increase or decrease the total chloride content depending on the distance from the concrete surface. The total chloride content at the transition between these effects is independent of the period of diffusion and value of the diffusion coefficient. The time to corrosion initiation of embedded steel is dependent on the corrosion risk presented by bound chloride. Bound chloride may participate in corrosion initiation when the establishment of pH gradients are required to sustain passive film breakdown. This is the result of the effect on the pore solution chemistry of the pH dependent solubilities of solid phases containing bound chloride that are very similar to that of calcium hydroxide. In some circumstances, the time to corrosion initiation may be reduced by an increase in binding because of the possible corrosion risk presented by bound chloride.

The participation of bound chloride in passive film breakdown on steel in concrete

Abstract It is well known that solid calcium hydroxide may act to inhibit chloride induced corrosion of steel in concrete. This hypothesis has been recently extended to include the inhibitive and aggressive nature of other solids that exhibit pH dependent dissolution behaviour. Concrete constituents that resist a local fall in pH may inhibit corrosion, while bound chloride released by such a fall may participate in corrosion initiation. In this work the pH dependent solubility of chloride in concrete is demonstrated. It is shown that most of the bound chloride is released as the result of the rapid dissolution of at least two hydrated phases in chloride contaminated OPC concrete. This occurs at pH values that are high (above 11.5) compared to that considered necessary to sustain local passive film breakdown at the site of a nucleating pit. Thus, in theory, the corrosion risk presented by bound chloride at the steel–concrete interface may be very similar to that presented by free chloride.

Potential of superabsorbent polymer for self-sealing cracks in concrete

Abstract Abstract The potential of conventional superabsorbent polymers (SAPs) based on partially neutralised acrylate and acrylate/acrylamide copolymers as an admixture for self-sealing of cracks in concrete is investigated. SAPs are cross-linked polymers that can absorb a disproportionately large amount of liquid and swell substantially to form a soft and insoluble gel. However, their swelling capacity is highly dependent on the alkalinity and ionic content of the solution. These characteristics may be exploited for self-sealing cracks in concrete. In this preliminary study, the mechanism involved is described and tests performed to determine the swelling ratios of SAP in various solutions including synthetic pore solution, groundwater and seawater are reported. Transport testing found that the flowrate through a 340 μm wide model crack is reduced substantially by using less than 1 vol.-% SAP. The reswelling capacity of SAP in cement paste and the effect of SAP on cement paste microstructure were investigated by microscopy.

Corrosion inhibition in concrete arising from its acid neutralisation capacity

Abstract It has been postulated that the most important inhibitive property of concrete affecting the level of chloride required to initiate corrosion is its ability to resist a local fall in pH that might otherwise sustain passive film breakdown at a developing pit. In this work a novel technique termed differential acid neutralisation analysis was used to characterise this property. It was noted that many solid phases in hydrated cement paste have pH dependent dissolution characteristics that may strongly influence the pore solution chemistry during corrosion initiation. While the important contribution made by calcium hydroxide has been widely recognised, other reactive hydration products contribute more than 75% of the resistance to a pH reduction to a value of 10. The resistance to a pH reduction (acid neutralisation capacity) to a value between 10 and 11 correlates reasonably well with the available chloride threshold level data. The inhibitive nature of the concrete environment, characterised by its acid neutralisation capacity, depends on the cementitious binder, decreasing in the order OPC > SRPC > PFA ≈ GGBS. Care is needed to minimise adverse effects occurring at transient pH values on the steady state data obtained in an acid neutralisation test. However the indications are that differential acid neutralisation analysis may prove to be very effective as an analytical technique.

Assessment of simple methods of determining the free chloride ion content of cement paste

Abstract A range of possible techniques for assessing the “free” chloride ion content of hydrated cement pastes dosed with sodium chloride or calcium chloride have been investigated. The techniques are based on mixing powdered samples with a solvent and measuring the amount of chloride passing into solution. The values obtained have been compared with the chloride ion content of pore solution expressed from parallel specimens. The results indicate that in the range of chloride additions considered several extraction techniques may be used to estimate the free chloride ion content of cement paste; the total chloride content will dictate the most appropriate extraction technique(s) to adopt.

On the corrosion risk presented by chloride bound in concrete

In previous work the influence of the solid phases of cement hydration on the pore solution chemistry during corrosion initiation has been discussed. It was noted that, because a fall in local pH is necessary for stable pits to develop on the passive steel, much of the chloride bound in concrete may participate in the process of corrosion initiation. At least two phases in hydrated ordinary Portland cement (OPC) will release such bound chloride before the pH falls to 11. In this work, these studies have been extended to include OPC blended with 10% calcium aluminate cement (CAC) and sulphate resisting Portland cement (SRPC). Evidence of a third phase that releases bound chloride was uncovered. Once again the data confirms that most of the bound chloride will be released by a relatively small reduction in pH. The release of chloride at such a high pH value compared to that required to sustain passive film breakdown suggests that the corrosion risk presented by bound chloride may be very similar to that presented by free chloride in concrete.

The inhibitive effects of electrochemical treatment applied to steel in concrete

Abstract It has been postulated that the ability of hydrated cement paste to resist a fall in pH at high pH values is one of its most important inhibitive properties, while the most important factor affecting the initiation of corrosion in chloride contaminated concrete is entrapped air voids. Air voids prevent the local formation of a calcium hydroxide rich layer at the steel surface. Such an inhibitive layer minimises the risk of corrosion initiation at relatively high chloride contents (typically 1.5% by weight of cement). This work considers the effectiveness of electrochemical treatment to produce an inhibitive layer at the steel–concrete interface. This method may be applied as a preventative measure to decrease the risk of corrosion initiation resulting from subsequent chloride contamination.

The resistivity of mortars immersed in sea-water

Abstract The resistivities of specimens of five different mortars were monitored during 18 weeks of exposure to sea-water. All specimens exhibited an increase in resistivity, 25mm thick specimens of the two more permeable mixes showing an increase equivalent to over 20mm of additional thickness after only 10 weeks of sea-water exposure. Measurement of pore solution resistivity, mercury intrusion porosimetry and surface examination and analysis were used to determine the mechanisms producing an increase in resistivity. Two distinct mechanisms were isolated, viz the formation of a discrete aragonite/brucite layer on the surface of the specimens and a more widespread bulk effect associated with a modification of the cement paste pore structure.