Chloride corrosion of reinforced concrete

Author: Efremov D.N.

It has long been recognized that chloride ions play a major role in the corrosion of steel in concrete. The chloride ions may originate from the ingredients of the concrete mixture, or may diffuse through the hardened concrete.

Most building codes attempt to control corrosion-induced deterioration by placing limits on the chloride ion content of the mix ingredients and by specifying the quality of the concrete and thickness of concrete cover appropriate to the exposure conditions.

Corrosion in Reinforced Concrete

O2H2O

Fe2O3H2O (rust)

- -- -electron transfer

anodic reaction

2Fe++

2Fe(OH)2

secondary reaction

O2

H2O

4(OH-)cathodic reaction

O2

H2O

4e-

anodic dissolution of iron cathodic region

CHLORIDE LIMITS IN CONCRETE Establishing limits for the chloride ion content of mix

ingredients in concrete is very difficult for three reasons: Chloride ions are present naturally in many concrete

ingredients and specifying a zero chloride ion content is therefore unrealistic.

There is no agreement on a test procedure which can be used to determine the chloride ions available to depassivate the steel.

The service environment often cannot be described with sufficient precision to determine the risk of corrosion.

Corrosion Threshold Concept• Although a zero limit for chloride ions is impractical, neither is it necessary because

it has been known for a long time that small concentrations of chloride ions in the concrete do not result in an unacceptable level of corrosion of embedded steel. This observation gave rise to the concept of the chloride corrosion threshold. The concept of a threshold implies that there exists a chloride ion concentration below which steel is 'safe' from corrosion and above which, an unacceptable level of corrosion may occur if other necessary conditions, principally the availability of oxygen and moisture, exist to support the corrosion reactions. The concept leads to the initiation-propagation model for corrosion damage but it must be remembered that the corrosion threshold value is not a constant value for all concrete mixtures. The quantity of chloride ions required to depassivate the steel depends on several factors, including the chemical composition of the cement the ratio of the concentration of hydroxyl ions to chloride ions, and the type of cation. The corrosion threshold value is also dependent on the method of testing for chloride ion content.

Shape of the chloride contaminated concrete zone in front of crack

Test Methods• Test methods for measuring the chloride ion

concentration in concrete or concrete ingredients consist of two parts, extracting the chloride ion into solution and measuring the concentration of chloride ions in the solution. Once in solution, the chloride ion concentration is measured by a conventional titration using a standard silver nitrate reagent or by a suitable, calibrated ion-selective electrode.

• Test methods therefore differ principally in the method used to extract the chloride ions from the concrete and variations on two basic methods are in common use: digesting in nitric acid and dissolving in water. An alternative approach is to extract the pore water by applying high pressure to the concrete, though this method requires sophisticated equipment, and there is concern as to whether the fraction of pore water extracted is representative of the pore water as a whole.

Service Environment• In many situations, both indoors and outdoors, the service

environment cannot be described with sufficient precision to determine the risk of corrosion. If the concrete will be exposed to dry conditions, controlling the chloride ion content of the concrete may be unnecessary. At the other extreme, if the concrete will be exposed , to chlorides during service, it is logical to limit the chloride contents of the ingredients to the practical minimum. However, there is a wide range of service conditions between these extremes where the risk of corrosion is not only uncertain at the time of construction, but may change during the life of the structure.

Early Screening Tests

• Routine testing of concrete for chloride ion content was not undertaken until the early to mid 1970s when the problems of corrosion-induced deterioration of highway structures were first recognized. It was at this time that the high chloride content of the dolomitic limestone aggregates was first documented.

• A simple test was conducted in the ministry's laboratories in 1975 on a sample of aggregate from the Amabel formation using particles in the 19 to 26 mm range. The stone was immersed in distilled water for a period of 90 days and the chloride ion content of the water was measured periodically. The measured chloride content was found to be 0.003% after one day and this did not increase over the 90 day period, indicating that the likely source of the chloride ions was the exposed particle surfaces and that chloride contained within the aggregate did not enter into solution. The rock in question was porous (about 2% absorption) and the pores were of a very large size but poorly interconnected. These findings diminished concerns that the aggregate might be contributing chloride ions to the concrete pore water.

Comparison of Acid-Soluble and Water-Soluble Chloride Ion Contents of Commercial Aggregates.

• In a 1982 experiment, samples of different particle sizes of the same aggregate as in the 1975 test, ranging from retained on 19 mm to passing 53 iim, were prepared and immersed in water for one week. The samples were agitated periodically. After seven days, the chloride ion content of the water was determined and the results are shown in Figure 2. As the particle size decreased, the chloride ion content increased and the relationship was almost linear when plotted on a log/log graph. In other words, as the rock was crushed to expose a greater surface area, the measured chloride ion content increased. This means that the measured chloride ion content is a function of the test method, specifically the preparation of a pulverized sample, and not a measure of the quantity of chloride ions available to depassivate the steel.

Effect of Particle Size on Measured Water-Soluble Chloride Ion Content

This approach is not valid if chloride- based admixtures have been used in the concrete. However, the presence of such admixtures is usually evident from the abnormally high chloride values which do not diminish with distance from the exposed surface.

The subtraction of the background value is a pragmatic approach which has been found satisfactory over a number of years. However, a test method which measures only those chloride ions available to initiate corrosion would be preferable, and research to develop such a test method was initiated in the early 1980s.

• The Soxhiet Test Method The research which led to use of the Soxhiet method for extracting

chloride ions from concrete has been described in detail elsewhere.The initial work involved the investigation of several hot and cold water regimes for extracting chloride ions from concrete samples and constituent materials. Tests were conducted on concretes which contained chloride-bearing aggregates, admixed calcium chloride, and chlorides resulting from vacuum saturation of hardened concrete with a 3.5% solution of sodium chloride. The results showed that the Soxhiet apparatus was capable of discriminating between chloride ions available for depassivation of the steel, and those which were unavailable by virtue of being contained in the aggregates or chemically bound in the cement paste.

The Soxhiet extractor consists of a heater, a lower flask, the sample compartment, and a condenser. The extractor contains approximately 100 ml of water in the lower flask. Heat is applied to this flask and vapour from the boiling water passes to the condenser, and the condensate collects in the sample compartment. The sample, consisting of pieces of concrete not larger than 13 mm in diameter, is contained in a porous holder. When the condensate reaches a critical height, the liquid is syphoned into the lower flask and the process repeats. The non-volatile components extracted from the sample accumulate in the lower flask, while each extraction involves fresh, hot distillate. The heat input should be sufficient to give about three extraction cycles per hour. Twenty four hours is normally sufficient to remove all the extractable chloride ions.

As noted elsewhere, the Soxhiet method of extraction was not intended to replace the acid and water extraction methods because, where chloride ion contents are low, or background levels are small, the conventional methods are quicker, less expensive, and adequate. Even where high levels are recorded, engineering approximations, combined with local knowledge, often suffice. However, where uncertainty exists, or there is a need for a discriminating procedure, the Soxhiet method is very useful.

Tests conducted in connection with the development of the Soxhiet method, using water extraction on pulverized samples of concrete, confirmed the effect of particle size on the measured value of chloride ion concentration. Larger differences in measured values were recorded as the particles were ground finer and finer. This explains why large differences in results can be obtained from duplicate tests, where in one case the powder may be finer than the other.

Experimental Design A detailed investigation was performed on a small number of bridge

structures which had been in service for at least 25 years. For each structure, the corrosion performance of components which had been exposed to deicing salts during service was compared with that of components not exposed to deicing salts. In this way, the service environments of the components were as similar as possible, except for the effect of an external source of chlorides.

The condition surveys of the structures included a visual inspection of the concrete surface, a delamination survey by sounding, corrosion potential measurements (according to ASTM C-876-87), rate-of-corrosion measurements (using a commercial three-electrode, linear polarization instrument), and removal of cores for determination of chloride ion content and examination of the condition of the bars.

• Test Sites The field investigations were conducted on the

components of three bridges built between 1959 and 1968: Sixteen-Mile Creek Bridge. The concretes contained dolomitic limestone coarse aggregate with a high chloride content. The 'background' levels of chloride ions in the concretes varied from 0.046 to 0.074% by mass, as measured by the acid-soluble extraction procedure.

• Sixteen-Mile Creek Bridge

Sixteen-Mile Creek Bridge

• Cracking and rust staining were observed on the barrier walls and sidewalks, and also on those areas of the abutments exposed to surface run-off through leaking expansion joints. All the twelve spirally reinforced columns (each with a surface area of about 10 m2) were in good condition and there was no cracking, rust staining or delamination. The columns were not exposed to surface run-off but were exposed to light splashing from the traffic.

Corrosion potential measurements were taken on a 10 m2 section on the west side of the north abutment and on the six columns located at the east and west sides of the bridge. The percentages of readings falling within each of the three ranges normally associated with passive, uncertain, and active corrosion activity are given in Table 1. The potential measurements indicated that the reinforcement in the columns was passive, whereas the steel in the abutment was corroding actively.

Rate-of-corrosion measurements were performed at selected locations on two of the six columns. Table 2 shows the percentages of the measurements falling within the ranges associated with insignificant, little and moderate corrosion activity.16 The readings showed that there was insignificant or little corrosion activity in the columns.

Severe corrosion was observed on the bars in the two cores removed from the north abutment The average chloride content at the level of the steel (with concrete cover of 45 mm) was found to be 0.252% by mass of concrete.

Corrosion Potential Measurements

Rate of Corrosion Measurements

The anticipated corrosion threshold level for the concrete was approximately 0.030% plus the background contribution of 0.074% by mass of concrete.

There was no corrosion on the bars in the three cores removed from the columns. The chloride content of the concrete in these cores was similar to the background chloride level of the concrete, indicating that chloride ions from de-icing salts had not penetrated the concrete to any significant degree. It is recognized that the absence of corrosion on the reinforcing bars is not sufficient proof that there is no corrosion activity because the cores could have been taken from the cathodes on the bars. The lack of corrosion is presented as corroborating the data from the corrosion potential, rate-of-corrosion, and chloride measurements.

CONCLUSIONS

• The high concentration of water-soluble chloride ions measured in the dolomitic limestone concrete aggregates is a function of crushing of the aggregates as required by standard test procedures. A direct relationship exists between the measured chloride content and the fineness of the aggregate particles, because of the greater surface area exposed.

• The Soxhlet procedure was identified as an effective method of discriminating between those aggregates which will contribute chloride ions to the concrete and depassivate embedded steel, and those in which the chloride ions remain bound within the aggregate particles.

• A field investigation of bridge structures, which had been in service for over 25 years, revealed corrosion only in components where deicing salts had penetrated to the reinforcement. In no case had chloride ions contained in the aggregates initiated corrosion of the reinforcement.