Alkaline concrete

Very finely divided wax emulsions are effective concrete dampproofing agents and are formulated so that the emulsion breaks down after contact with the alkaline concrete environment and forms a hydrophobic layer. 

Very finely divided wax emulsions are effective concrete dampproofing agents and are formulated so that the emulsion breaks down after contact with the alkaline concrete environment and forms a hydrophobic layer. Waxes of melting point 57-60°C are used with an emulsifying agent based on sorbitan monostearate or ethoxylated sorbitan monostearate [2], The properties of a commercial product are given in Table 4.4 [5],... 

During hydration of cement a highly alkaline pore solution (pH between 13 and 13.8), principally of sodium and potassium hydroxides, is obtained (Section 2.1.1). In this environment the thermodynamically stable compounds of iron are iron oxides and oxyhydroxides. Thus, on ordinary reinforcing steel embedded in alkaline concrete a thin protective oxide film (the passive film) is formed spontaneously [1-3]. This passive film is only a few nanometres thick and is composed of more or less hydrated iron oxides with varying degree of Fe and Fe [4j. The protective action of the passive film is immune to mechanical damage of the steel surface. It can, however, be destroyed by carbonation of concrete or by the presence of chloride ions, the reinforcing steel is then depassivated [5j. 

The cathodic polarization curve has a trend similar to that observed in alkaHne concrete, and shows the same dependence on the moisture content in concrete. The curve, however, is shifted towards more positive potentials because the equiH-brium potential of oxygen reduction is approximately 200 mV higher at the pH of carbonated concrete than at the pH of alkaline concrete. 

Repassivation with alkaline concrete or mortar. This method is based on the appH-cation of a sufficiently thick (> 20 mm) cement-based layer of concrete or mortar over the surface of carbonated concrete. Only cracked or delaminated concrete has to be removed, while mechanically sound concrete, even if it is carbonated up to the reinforcement and thus in contact with corroding steel, will not be removed. The method relies on the diffusion of hydroxyl ions (OH ) from the new external alkaline layer towards the carbonated concrete substrate. In wet environments or in the presence of wetting-drying cycles (i. e. the worst conditions for corrosion), this can lead to realkalization of the carbonated concrete in time and thus to repassivation of the reinforcement. This method is mainly apphed in Germany and has proved to be effective in repassivating the reinforcement, usually within a few months, if the carbonation depth is not high [5]. This method should not be used if carbonation has penetrated behind the reinforcement more than 20 mm. 

Figure 20.3c shows the effect of application of cathodic protection on carbonated concrete. The applied cathodic current density, even if it brings about only a small lowering of the steel potential, can produce enough alkalinity to restore the pH to values higher than 12 on the reinforcement surface and thus promote passivation. The effectiveness of cathodic protection in carbonated concrete was studied with specimens with alkaline concrete, carbonated concrete and carbonated concrete with 0.4% chloride by cement mass that were tested at current densities of 10, 5, and 2 mA/m (of steel surface) [45]. Carbonated concrete specimens polarised at 10 mA/m showed that, although initially protection was not achieved since the four-hour decay was slightly lower than 100 mV, after about four months of polarization, the protection criterion was fulfilled and higher values, in the range 200-300 mV of the four-hour potential decay were measured (Figure 20.6). The same results were obtained on carbonated and slightly chloride-contaminated concrete.

When the steel is covered in other coatings, such as the zinc of galvanizing, then the potentials are created by the zinc, not the steel. This leads to very negative potentials (about 500 mV with respect to a Ag/AgCl or calomel electrode) when the structure is new. This drifts down to less negative values either as the zinc passivates in the alkaline concrete or as it is consumed and the steel becomes active. It can be impossible to distinguish the two... 

The corrosion potential of a non corroding, passive steel in concrete is determined by the passive current density and the current density of the oxygen reduction (Fig. 8-8). The passive current density is very low, thus for aerated concrete structures corr = o2- The main influencing factor is the pH with decreasing pH the corrosion potential becomes more positive. This is important for concretes with additives where the pozzolanic reaction takes place. In practice, variations in the corrosion potential of passive steel of up to 200 mV are observed due to changes in concrete humidity and pH. In aerated alkaline concrete values of... 

Thus the chloride transference number depends on the chloride concentration and on the amount of other ions present in the pore solution, especially OH and alkali ions. The composition of pore water in alkaline concrete corresponds essentially to a potassium hydroxide solution and transport numbers for the chloride ion tci of between 0.4 (Tritthart, 1996) and ca. 0.12 for lower chloride concentrations are reported (Polder, 1992). 

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