Polarization reinforced concrete

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

S. Feliu, J. A. Gonzales, C. Andrade, V. Feliu, On site determination of the polarization resistance in a reinforced concrete beam , Corrosionf... 

L. M. Ortega, A. M. Garcia, Corrosion rate measurements in reinforced-concrete structures by a linear polarization device , Int. 

Defining the area of measurement is important for accurate corrosion rate measurement. In other (non-reinforced concrete) applications linear polarization is carried out on a sample of known size in a pipe or tank or... 

Cathodic protection was applied at a very low level to avoid any risk of hydrogen evolution. As chlorides had not entered the structure the aim was to apply a small level of current to polarize the steel and effectively repel the chlorides. These systems were installed in the late 1980s and early 1990s. If properly maintained they should give excellent life as the anodes will be used at a very low rating. Another case of installation from new was a salt silo or chamber with a reinforced concrete floor. This was known to be at high risk from chloride penetration. Further examples are a palace in the United Arab Emirates (Funahashi, 1995) and a petrochemical plant in Saudi Arabia (Chaudhary, 2004). 

The protection current required for polarization of the steel-reinforced concrete foundation can be determined by a drainage experiment. In new installations this is relatively simple with individual electrically separated foundations [11]. In the foregoing case the results showed various current densities between 1.5 and... 

In the 1990s the most frequently used reinforced concrete protection criterion has been the second criterion, i.e., a decrease of the potential by 100 mV, determined after disconnection of the polarizing current. Measurement of the potential was carried out after 4 h from CP disconnection (NACE Standard RP0290-90 (1990)). 

Figure 8-21. Electrode arrangement in measuring polarization resistance on reinforced concrete structures (schematic), /fg reference electrode, CE counter electrode, WE working electrode (rebars). The diameter of the counter electrode is L, the length of current spread-out...

Broomfield, J. R, Rodriguez, J., Ortega, L. M. and Garcia, A. M. (1993) Corrosion Rate Measurements in Reinforced Concrete Structures by a Linear Polarization Device, Int. Symp. on Condition Assessement, Protection Repair and Rehabilitation of Concrete Bridges Exposed to Aggressive Environments, Minneapolis ACI Fall Convention. 

Macdonald, D. D., Urquidi-Macdonald, M., Rocha-Pilho, R. C., et al., Detemimation of the Polarization Resistance of Rebar in Reinforced Concrete, Corrosion, 47 330-335 (1991). 

Cement coatings are usually applied as linings for water pipes and water tanks, but occasionally also for external protection of pipelines [7]. Cement is not impervious to water, so electrochemical reactions can take place on the surface of the object to be protected. Because of the similar processes occurring at the interface of cement and object and reinforcing steel and concrete, data on the system iron/ cement mortar are dealt with in this chapter taking into account the action of electrolytes with and without electrochemical polarization. To ensure corrosion protection, certain requirements must be met (see Section 5.3 and Chapter 19). 

Several other factors, such as the temperature, the composition of the cement, the composition or surface roughness of the steel reinforcement, or the polarization with anodic or cathodic current may affect the chloride threshold. Some of these factors will be discussed later. Here it will be shown how the chloride threshold depends on properties of concrete and on environmental exposure (which, as discussed in Chapter 7, determines the potential of the steel). 

Polarization curves. The rate at which the anodic or the cathodic process takes place depends on the potential ( ). The corrosion behaviour of the reinforcement can be described by means of polarization curves that relate the potential and the anodic or cathodic current density. Unfortunately, determination of polarization curves is much more complicated for metals (steel) in concrete than in aqueous solutions, and often curves can only be determined indirectly, using solutions that simulate the solution in the pores of cement paste. This is only partly due to the difficulty encountered in inserting reference electrodes into the concrete and positioning them in such a way as to minimize errors of measurement. The main problem is that diffusion phenomena in the cement paste are slow (Chapter 2). So when determining polarization curves, pH and ionic composition of the electrolyte near the surface of the reinforcement may actually be altered. 

A V is the driving voltage that allows the current I to flow through the reinforcement, following the path X A B Y. This path presents, along with ohmic drops in the concrete cover (v cona.i + Vcona.i) and in the reinforcement (v rebai). the polarization contributions on the anodic and cathodic surfaces (y> + y>. Therefore the current / is determined by the relation ... 

In general, steel in concrete operates in the interval of potential and pH outside the critical ranges for hydrogen evolution. Under particular conditions, however, the situation may be different. Situations that make it jxtssible for hydrogen to develop are localized corrosion on the reinforcement that lead to oxygen depletion (and thus depresses the potential), acidity production at the anodic zones, and external cathodic polarization applied to the steel (due to, for example, excessive cathodic protection or stray currents). 

Corrosion rate measurements on-site. The main difference in measuring the polarization resistance Rp on site and not in the laboratory is the geometrical arrangement of the electrodes. In the laboratory a uniform geometry and thus uniform current distribution can be achieved usually with small specimens, whereas on site a there is a non-uniform current distribution between the small counter-electrode (CE) on the concrete surface and the large rebar network (WE). The current fed by the external counter electrode (Figure 16.15) not only polarizes the surface of the reinforcement below the CE itself but also spreads laterally or may reach... 

C. Andrade, ). A. Gonzales, Quantitative measurements of corrosion rate of reinforcing steels embedded in concrete using polarization resistance measurement . Materials and Corrosion, 1978, 29, 515-519. 

Repassivation with alkaline mortar or concrete. Repassivation of steel can be obtained by replacing the chloride-contaminated concrete with chloride-free and alkaline mortar or concrete. Because of the mechanism of chloride-induced corrosion, it is not sufficient to repair the concrete in the area where the reinforcement is de-passivated. The concrete must be removed in all areas where the chloride threshold has reached the depth of the reinforcement or is expected to reach it during the design Hfe of the repair. In fact, the concrete that surrounds the zones of corrosion usually has a chloride content higher than the chloride threshold, even though the steel remains passive because it is protected by the corroding site. In fact, a macrocell forms (Figure 18.6a) that provides cathodic polarization to adjacent steel and... 

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