Concrete chlorides attack

The most widely used anodic inhibitors are calcium and sodium nitrite, sodium benzoate and sodium chromate. With the exception of calcium nitrite, no other chemical is available in North America as a proprietary product. Nitrites have been used in the USA for more than 14 years and for nearly 40 years in Europe. Calcium nitrite is marketed as a non-chloride accelerator, as well as a corrosion inhibitor. For 25-30% solids in solution, dosage rates range from 2 to 4% by weight of cement depending on the application [50]. Calcium nitrite has been used in bridges, parking and roof decks, marine and other prestressed concrete structures that are exposed to chloride attack. 

By consuming the Ca(OH)2 the pozzolan makes it unavailable for sulphation and carbonation reactions and increases the density of the concrete, which lowers its permeability and reduces the risk of chloride attack. 

In many cases, any attack on reinforced concrete will be on the concrete itself. However, there are two species, namely, chloride and carbon dioxide that penetrate the concrete and attack the reinforcing steel without breaking the concrete. The chloride and carbon dioxide penetrate concrete without causing significant damage and then promote corrosion of the steel by attaeking and removing the protective passive oxide layer on the steel created and sustained by the alkalinity of the concrete pore water. 

Reactions in Eqs. (12.19) and (12.20) destroy both ferric oxide and magnetite (Fc304) protective layers on the rebar. Hence, research is directed to improve inherent steel corrosion resistance, corrosion control parameter evaluation, implementing corrosion inhibitors that influence physical and chemical concrete and steel properties, chloride attack evaluation under severe chloride and temperature and improving corrosion, and pore solution models. 

Figure 12.15 shows corrosion initiation time as a function of water/cement ratio in the presence of an inhibitor. The simulation was carried out for a 2.54 cm concrete cover and 7.6 kg/m surface chloride concentration. Comparison of Figs. 12.13 and 12.15 shows that rebars better resist chloride attack with an inhibitor for every water/cement ratio. 

Figure 12.18 shows the dimensionless threshold chloride concentration as a function of time for different inhibitor concentrations. The required time to exceed chloride threshold value increases by increasing inhibitor concentration. Corrosion initiation time increases from 20 years without inhibitor to 60 years for 9.9 L/m inhibitor. These results show inhibitors increase the threshold hmit to a much higher value, but under aggressive chloride attack this threshold limit will eventually be reached. This dependence is given in Fig. 12.19 which illustrates the corrosion initiation time for different slab thicknesses as a function of nitrate inhibitor concentration. The plot shows corrosion initiation time as a function of concrete cover thickness for diflerent inhibitor concentrations. 

However, the passivating environment is not always maintained. Two conditions can break down the passivating environment in concrete without attacking the concrete first. One is carbonation and the other is chloride attack. These will be discussed in Chapter 3. In the rest of this chapter we will discuss what happens when depassivation has occurred. 

Once the passive layer breaks down then areas of rust will start appearing on the steel snrface. The chemical reactions are the same whether corrosion occnrs by chloride attack or carbonation. When steel in concrete corrodes it dissolves in the pore water and gives up electrons ... 

Corrosion is often local, with a few centimetres of corrosion and then up to a metre of clean passive bar, particularly for chloride induced corrosion. This indicates the separation of the anodic reaction (2.1) and the cathodic reaction (2.2) to form a macrocell . Chloride induced corrosion gives rise particularly well defined macrocells. This is partly due to the mechanism of chloride attack, with pit formation and with small concentrated anodes being fed by large cathodes. It is also because chloride attack is usually associated with high levels of moisture giving low electrical resistance in the concrete and easy transport of ions so the anodes and cathodes can separate easily. 

There are two main causes of corrosion of steel in concrete. This chapter will discuss how chloride attack and carbonation lead to corrosion and how the corrosion then proceeds. There will also be discussion of the variations that can be found when carrying out investigations in the field. 

In Chapter 2, we discussed the corrosion of steel in concrete and the effectiveness of the alkalinity in producing a passive layer of protective oxide on the steel surface which stops corrosion. In the previous section we observed that alkalinity is neutralized by carbonation. The depassivation mechanism for chloride attack is somewhat different. The chloride ion attacks the passive layer although in this case (unlike carbonation) there is no overall drop in pH. Chlorides act as catalysts to corrosion. They are not consumed in the process but help to break down the passive layer of oxide on the steel and allow the corrosion process to proceed quickly. This is illustrated in... 

As stated in Chapter 2, corrosion proceeds by the formation of anodes and cathodes (Figures 2.1 and 2.2). In the case of chloride attack they are often well separated with areas of rusting separated by areas of clean steel. This is known as the macrocell phenomenon. Chloride induced corrosion is particularly prone to macrocell formation as a high level of water is usually present to carry the chloride into the concrete and because chlorides in concrete are hygroscopic (i.e. they absorb and retain moisture). The presence of water in the pores increases the electrical conductivity of the concrete. The higher conductivity allows the separation of anode and cathode as the ions can move through the water filled (or water lined) pores. 

We have considered the main mechanisms of corrosion in Chapter 2. We have seen that the chemical process is the same regardless of whether the cause is carbonation or chloride attack as described in Chapter 3. But if we are to perform an effective repair we must fully understand the cause and extent of damage or we risk wasting resources with an inadequate or unnecessarily expensive repair. This chapter explains how to evaluate the condition of corroding reinforced concrete structures. 

These are the logical extension of the waterproofing membrane. Often one of the simplest ways of reducing the deterioration rate due to chloride attack is simple deflection of chloride laden water away from the concrete surface. This can sometimes be done with the introduction of guttering and... 

A simplified method of calculating the initiation time for chloride attack is to look at the progress of the chloride threshold through the concrete. By taking samples with depth it is possible to fit a parabolic curve to the chloride concentration (or more simply to fit a straight line to a plot of depth vs. the square root of chloride concentration) and to find the depth... 

Most of this chapter has discussed bridges, with peripheral comments about other civil engineering structures. This is partly because most of the emphasis in the field of durability has been problems with bridges caused by chloride attack. Many of the discussions are relevant to all reinforced concrete structures. However, buildings are constructed to withstand less severe environments and conditions than bridges. There are also lower life expectations on buildings which may become obsolete before serious deterioration occurs. 

The chemical test for this type of corrosion is very straightforward. A Imnp of concrete is freshly broken out from the corroding area and sprayed with a 1% phenolphthalein solution in isopropyl alcohol. If the phenolphthalein remains colorless, then the alkalinity of the concrete has been neutralized. If the phenolphthalein goes purple (i.e., the concrete is sufficiently alkaline) but reinforcement corrosion is observed, then the far more serious problem of chloride attack is likely to be occurring. An explanation of the mechanism of chloride attack follows. 

The corro. iion rates associated with carbonation rarely exceed 0.5. A cm ivhile chloride attack can give corrosion rate. of more than 1. A cm equivalent to 12.5. m section loss per year metal (Broomfield el aL, 1993, 1994 and Figure 4.15). This may be due to the fact that carbonated concrete drie." out more rapidly than chloride contaminated concrete and surveys are normally done in the dry, when the rate would be lower in the carbonated condition. 

Rebars are polymer fibre reinforced-concrete composites, and they are used as primary structures. It is estimated that replacement of steel reinforcing bars by non-corrosive polymer fibres, i.e., by Kevlar or carbon fibres (which gives rise to Kevlar or C-composite bars) for concrete structures produces structures with one-quarter the weight and twice the tensile strength of the steel bar. It is known that, corrosion of steel reinforcement from carbonation or chloride attack can lead to loss of the structural integrity of concrete structures, and such a danger is non-existent for rebars. Thermal expansion coefficient (TEC) values of these fibres are closer to concrete than that of steel, which provides an another advantage and they have the same surface deformation patterns as the steel bars. In addition, they can provide more economy than epoxy-coated steel bars. 

The primary cause is the chloride ion that results from the use of deicing salts on roadways or the presence of seawater. The chloride ion penetrates the concrete and attacks the carbon steel rebar forming rust, which expands the volume approximately 3-7 times causing the concrete to spall. 

The main causes of corrosion of steel in concrete are chloride attack and carbonation. These two mechanisms are unusual in that they do not attack the integrity of the concrete. Instead, aggressive chemical species pass through the pores in the concrete and attack the steel. This is unlike normal deterioration processes due to chemical attack on concrete. 

---