Durability of Steel in Concrete
Nearly all structural concrete contains steel, either in the form of reinforcement to compensate primarily for the low tensile and shear strength of the concrete, or as prestressing tendons that induce stresses in the concrete to oppose those due to the subsequent loading.
Sound concrete provides an excellent protective medium for the steel, but this protection can be broken down in some circumstances, leaving the steel vulnerable to corrosion. Crucially, the corrosion products – rust in its various forms – occupy a considerably greater volume than the original steel.
Rusting within concrete therefore causes internal expansive or bursting stresses, which eventually will result in cracking and spalling of the concrete covering the steel.
Although unsightly, this will not immediately result in structural failure, but the remaining steel is then fully exposed, and undetected or unchecked the more rapid corrosion that results can lead, and has led, to collapse.
Although the processes involved are less complex than those of the various degradation mechanisms of the concrete itself, described above, they are much more difficult to avoid and control. Indeed corrosion of steel in concrete is the greatest threat to the durability and integrity of concrete structures in many regions.
In the last few decades the concrete repair industry has benefited considerably and is thriving. In this section we shall first describe the general nature of the phenomenon, and then consider the factors that control its onset and subsequent rate.
Principles of Corrosion of Steel in Concrete
The electrochemical nature of the corrosion of iron and steel was described in an another article, and the processes involved in the corrosion of iron in an air/water environment were illustrated in Fig. 1 of that article. In the corrosion cell shown in that figure the anode and cathode are close together, e.g. across a single crystal or grain. The oxide is formed and deposited near but not directly on the metal surface, allowing the corrosion to be continuous.
In concrete different conditions prevail. The electrolyte is the pore water in contact with the steel and, as we have seen, this is normally highly alkaline (pH = 12.5 – 13) owing to the presence of Ca(OH)2 from the cement hydration and the small amounts of Na2O and K2O in the cement.
In such a solution the primary anodic product is not Fe2+ but is a mixed oxide (Fe3O4), which is deposited at the metal surface as a tightly adherent thin film only a few nanometres thick. This stifles any further corrosion, and the steel is said to be passive.
Thus sound concrete provides an excellent protective medium. However the passivity can be destroyed by either loss of alkalinity by carbonation of the concrete, in which the calcium and other hydroxides are neutralised by carbon dioxide from the air, producing calcium and other carbonates, or chloride ions, e.g. from road de-icing salts or seawater, which are able to breakdown or disrupt the passive film (a process known as pitting).
Either of these can therefore create conditions for the corrosion reactions. The corrosion can be localised, for example in load-induced cracks in the concrete, or the corrosion cells can be quite large (‘macrocells’), for example if anodic areas have been created by penetration of chloride ions into a locally poorly compacted area of concrete.
However, it is important to remember that oxygen and water must still be available at the cathode to ensure that the corrosion continues.
As mentioned above, the corrosion products (ferric and ferrous hydroxide) have a much larger volume than the original steel, by about 2–3 times, and can eventually lead to cracking, spalling or delamination of the concrete cover. This damage can take various forms, as illustrated in Fig. 1.
Since carbon dioxide or chloride ions will normally have to penetrate the concrete cover before corrosion can be initiated, the total time to concrete cracking (the service life of the structural element) will consist of two stages, illustrated in Fig. 2.
- The time (t0) for the depassivating agents (the carbon dioxide or chloride) to reach the steel in sufficient quantities to initiate corrosion; t0 can be considered a ‘safe life’.
- The time (t1) for the corrosion to then reach critical or limit-state levels, i.e. sufficient to crack the concrete; this is the ‘residual life’, and depends on the subsequent rate of corrosion.
As we shall see, in many situations estimation of t1 is difficult and so design guidance and rules are normally framed so that t0 is a large proportion or even all of the intended service life. We shall now discuss the processes of carbonation-induced corrosion and chloride-induced corrosion separately.
Neutralising the hydroxides in the HCP by atmospheric carbon dioxide in solution in the pore water reduces the pH from 12 or more to about 8. There are also some reactions between the carbon dioxide and the other hydrates, but these are not significant in this context.
The carbonation reaction occurs first at the surface of the concrete and then progresses inwards, further supplies of carbon dioxide diffusing through the carbonated layer.
Extensive analysis by Richardson (1988) showed that the carbonation depth (x) and time (t) are related by the simple expression:
x = k.t0.5
where k is a constant closely related to the diffusion characteristics of the concrete. The form of this equation indicates that carbonation may be considered as a sorption process. The value of k depends on several factors, chiefly:
1. The degree of saturation of the concrete. It is necessary for the carbon dioxide to be dissolved in the pore water, and so concrete that has been dried at low relative humidities will not carbonate. At the other extreme, diffusion will be slow in concrete completely saturated with water, and so the fastest advance of the carbonation front occurs in partially saturated concrete at relative humidities of between 50 and 70%.
Thus concrete surfaces that are sheltered will carbonate faster than those exposed to direct rainfall (Fig. 3).
2. The pore structure of the concrete. Parrott (1987) suggested that relating carbonation depth to concrete strength, as in Fig. 3, is a useful way of combining the effects of water:cement ratio, cement content and incorporation of additions. Adequate curing at early ages is also an important factor.
Although additions can result in lower overall porosity with full curing, the pozzolanic reaction can also reduce the calcium hydroxide content before carbonation, and so additions do not necessarily have the same benefits as they do with other degradation processes.
3.The carbon dioxide content of the environment. Observed rates of carbonation, such as those shown in Fig. 3, are such that with high-quality, well cured concrete the carbonated region, even after many years’ exposure to normal atmospheric conditions, is restricted to less than 20 – 30 mm of the surface of the concrete.
It is difficult to estimate or predict the rate of corrosion once the steel has been depassivated, and therefore design recommendations are aimed at ensuring that the depth and quality of concrete cover are sufficient to achieve a sufficiently long initiation period (t0). BS 8500 (2006) gives combinations of required concrete quality and cover to steel for various exposure classes or conditions; the minimum values, summarised in Table 1, clearly show how the factors discussed above have been take into account.
It should also be noted that carbonation is not entirely detrimental. The calcium carbonate formed occupies a greater volume that the calcium hydroxide, and so the porosity of the carbonated zone is reduced, increasing the surface hardness and strength, and reducing the surface permeability.
There are four common sources of chloride ions:
- calcium chloride, a cheap and effective accelerator
- contamination in aggregates
- seawater, for coastal or marine structures
- road de-icing salts, a particular problem on bridge decks.
Calcium chloride, or any chloride-containing admixture, is normally no longer permitted in concrete containing steel, and aggregates, particularly from marine sources, should be washed before use to remove chlorides and other contaminants.
There has been considerable interest in the amount of chloride required to initiate corrosion, i.e. a threshold level that is required to depassivate the steel. In practice, corrosion in structures has been found to occur at a very wide range of total chloride content, but with increasing frequency with increasing chloride content.
For example in a survey of UK concrete highway bridges, Vassie (1984) found that only 2% of the bridges showed corrosioninduced cracking when the chloride content level was less than 0.2% by weight of the cement, but the proportion rose progressively to 76% showing cracking at chloride levels greater than 1.5% by weight of the cement.
It may, therefore, be better to think of the chloride content as giving a risk of corrosion, rather than there being an absolute threshold level below which no corrosion can ever occur. The reasons for such variations in behaviour are not entirely clear, despite much research effort.
One significant factor is that the C3A component of cement binds some of the chloride ions as chloroaluminates, thus reducing the amount available to depassivate the steel.
However, in a recent review, Page and Page (2007) concluded that many other factors are also involved, and there is no straightforward answer as to the effect of, for instance, variations in cement composition or blends of cement and various additions in this respect.
Despite this, standards and design recommendations have, since the 1970s, included allowable chloride levels. These have varied, but have generally been reduced as new or revised standards are published.
For example the current European Standard specification (BS EN 206) has chloride content limits of 0.2% by weight of cement for concrete containing steel reinforcement and 0.1% for concrete containing pre-stressing steel. If the chloride is included in the concrete on mixing, then the steel may never be passivated, and the initiation period, t0, will be zero.
Chlorides from external sources (seawater or de-icing salts) have to penetrate the concrete cover in sufficient quantities, however defined, to depassivate the steel before the corrosion is initiated: t0 is therefore finite in these circumstances. The transport mechanisms may be governed by: permeability in the case of, say, concrete permanently submerged in seawater; diffusivity, where salts are deposited onto saturated concrete; or sorptivity, where salts are deposited on to partially saturated concrete.
The corrosion risk in situations in which the salts are water-borne and deposited onto the surface by evaporation, such as in the splash zone of marine structures or on run-off from bridge decks, is particularly high as the reservoir of salts is constantly replenished.
An absorption mechanism may dominate in the early stages of such contamination, with diffusion being more important at later stages (Bamforth and Pocock, 1990). These processes result in chloride profiles such as those shown in Fig. 4. A large number of such profiles showing the effect of a large number of variables have been generated both experimentally and analytically.
In general, concrete with lower permeability, diffusivity or sorptivity will have lower rates of chloride penetration, and we have seen that these are achieved by lower water:cement ratios, adequate cement content, the use of additions and attention to good practice during and after placing the concrete. The amount of cover will also clearly affect the time needed for the chloride to reach the steel.
Although many recommendations for concrete cover and quality are aimed at extending the period t0 as far as possible, there are circumstances in which it is impossible to prevent corrosion being initiated. Much research has therefore been carried out to determine the factors that control the rate of corrosion during the residual-life period. These have been found to include the following:
- The spacing and relative size of the anode and cathode in the corrosion cell. Relatively porous areas of a concrete member, such as a poorly compacted underside of a beam, will allow rapid penetration of chloride, depassivating a small area of steel to form the anode. The reinforcement throughout the structure is normally electrically continuous, and so the remainder forms a largearea cathode, resulting in a concentration of corrosion current, and hence a high corrosion rate, at the anode.
- The availability of oxygen and moisture, particularly to sustain the cathodic reaction. If the supply of either is reduced, then the corrosion rate is reduced. Hence little corrosion occurs in completely dry concrete, and only very low rates in completely and permanently saturated concrete through which diffusion of oxygen is difficult, although localised depletion of oxygen at the anode can increase corrosion rates.
- The electrical resistivity of the electrolyte of the corrosion cell, i.e. the concrete. High resistivities reduce the corrosion current and hence the rate of corrosion, but increasing moisture content, chloride content and porosity all reduce the resistivity.
Analysis of the extensive and increasing amount of data on this subject has been used to produce guidelines to ensure adequate durability in all countries or regions where reinforced concrete is used. BS 8500 (2006) gives numerous combinations of required concrete quality and cover to steel for various exposure classes or conditions, which gives design engineers some flexibility of choice for the combination for each exposure condition; some of the minimum values are summarised in Tables 2 and 24.6.
Different exposure classes apply when the corrosion is induced by chlorides from road de-icing salts (Table 2) to those that apply when the chlorides come from seawater (Table 3), but the requirements for the concrete quality and cover are largely similar for the equivalent exposure class.
There are, however, circumstances in which protection against corrosion cannot be guaranteed by selection of the materials and proportions of the concrete, depth of cover and attention to sound construction practice.
These include, for example, marine exposure in extreme climatic conditions, and regions in which aggregates containing excess chloride must be used. One or more of the following extra protective measures may then be taken:
- the addition of a corrosion-inhibiting admixture such as calcium nitrite to the fresh concrete
- the use of corrosion-resistant stainless steel reinforcement bars, or epoxy-coated conventional bars
- applying a protective coating to the concrete, to reduce chloride and/or oxygen ingress
- cathodic protection of the reinforcement, i.e. applying a voltage from an external source sufficient to ensure that all the steel remains permanently cathodic.
Thanks for reading about “durability of steel in concrete”.