Reinforcing decay

Chloride ions from de-icing salt can attack and break down the protection layer of the reinforcement in concrete structures. This leads to various forms of corrosion-induced damage such as cracking and spalling. A significant percentage of highway bridges suffer from serious premature decay, and in extreme circumstances, bridges have collapsed catastrophically.

Difficulty assessing

Since the early 1980s a frequently used method for identifying reinforcing steel corrosion in concrete structures, or for assessing the condition of concrete structures generally, has been the half-cell potential measurement. This method indicates the relative probability of corrosion activity by measuring the potential difference between a standard portable half-cell — usually a saturated copper/copper sulphate (Cu/CuSO4) reference electrode — and the reinforcing steel. It is unable, however, to determine corrosion rates or the degree of corrosion that has occurred.

Many factors, such as chloride and oxygen concentrations, temperature and moisture content, can affect half-cell potentials over a certain range. The data analysis guidelines described in American Society for Testing Materials/ASTM C8761 provide general principles for evaluating the probability of corrosion of reinforcing steel in concrete: there is a 10% probability of corrosion if half-cell potentials are more positive than -200 mV (millivolts), and a 90% probability if half-cell potentials are more negative than -350 mV (versus Cu/CuSO4).

In many cases the corrosion states predicted when using the guidelines are quite different from actual corrosion conditions. For example, in many bridges major discrepancies between the assessment of the corrosion state using the ASTM guidelines and the actual deterioration have been observed at the time of repair.2 Studies of bridge decks in Europe, where waterproofing membranes are used or where de-icing salts are applied less frequently, have resulted in a different set of interpretive guidelines.3

The effects of water

Half-cell potentials are very sensitive to the ambient environment, especially the oxygen concentration at the interface between the reinforcing steel and the concrete. Usually, a decrease in oxygen (O2) can drive the half-cell potential significantly towards more negative values. Completely water-saturated concrete can lead to oxygen starvation, resulting in potential values more negative by up to 200 mV.4 & 5 A test carried out on carbon steel in an electrochemical cell at the National Research Council of Canada’s Institute for Research in Construction showed that a significant shift of potential towards more negative values by 350 mV could be observed when oxygen was purged from the electrolyte by bubbling nitrogen gas into the cell.

If the concrete cover is saturated with water, oxygen is not freely available at the metal surface because oxygen is not very soluble in aqueous solutions. Under this condition, the corrosion rate is significantly diminished because it is controlled by the rate oxygen arrives at the metal surface. Because of the diminution in the concentration of oxygen, the corrosion potential will shift to more negative values. These values can be much more negative than -350 mV vs. Cu/CuSO4 in many conditions and can lead to an erroneous conclusion that there is severe corrosion, when in fact the reinforcement may still be in good condition. The result may lead to an engineer undertaking a rehabilitation that is not necessary.

When a bridge deck has been repaired many times the concrete cover usually becomes thicker than it was. The humidity at the surface of the concrete can change with the ambient conditions easily, but the relative humidity (RH) at the deep level is very difficult to change and remains at relatively high values. Under these RH conditions most pores in the concrete are saturated with water and the transfer of oxygen through the pores to the reinforcement takes place almost entirely through liquid-phase diffusion. Therefore the concentration of oxygen is reduced and the corrosion potentials (half-cell potentials) are shifted to more negative values.

The shifting of the half-cell potential by a change in the concentration of oxygen was confirmed from measurements conducted on the barrier walls of a highway bridge in Laval, Quebec. The results showed that the potentials were shifted to more negative values when the concrete cover was saturated by rainfall.

The effect of concrete resistivity

The corrosion of steel reinforcement arises through local structural or compositional variations within the concrete. Consequently, some areas of the steel become positively charged and others negatively charged. Iron dissolves at the positively charged anode, with the result that rust is deposited nearby. The electrical circuit is established through the movement of electrons within the metal, and hydroxyl and metal ions through the pore solution in the concrete. When the concrete resistivity is high, the electrically charged ions have more difficulty moving through the pore liquid, thus reducing the corrosion rate. The more corroded rebars are therefore usually located in areas where the concrete resistivity is low, while the less corroded bars are found in areas having a high resistivity.

Hawkesbury study

The Institute for Research in Construction carried out a study of reinforcement corrosion in repaired concrete slabs taken from an old bridge being dismantled at Hawkesbury, Ontario. The corrosion measurements on these slabs used techniques that included half-cell potential, linear polarization, concrete resistivity, and chloride ion concentration profile. The relative humidity and the temperature of the slabs were monitored throughout the project. After the corrosion measurements were completed, the concrete cover on the top layer of the reinforcement was removed with a jackhammer to expose the reinforcing bars for visual inspection. These visual results then were compared with those predicted by the corrosion measurements.

It was found that the state of corrosion of the reinforcement can be predicted reliably by analyzing the data from both half-cell potential and corrosion rate measurements, and by considering the effects of environmental conditions.

The study showed that when the concrete cover is very thick or saturated by water due to rainfall, the half-cell potential readings more negative than –350 mV should not be simply interpreted as a high probability of corrosion. In that case, the readings should be compared with the results of corrosion rate measurements. If the corrosion rates are low and stable in the area, then there is a very small probability that the reinforcement is corroded. If the half-cell potential readings are generally quite negative (say -450 mV), and corrosion rates are high (>0.5 uA cm-2) and fluctuate from one location to another, then one can confidently conclude that corrosion has already taken place. In areas with active corrosion, the section with more negative potentials was found to be more corroded, as confirmed by visual inspection.

The researchers also measured the concrete resistivity at various locations on reinforcing bars having different corrosion states. The results were analyzed based on the cumulative frequency versus resistivity. Large differences in resistivity were clearly observed, with more corroded rebars located in the areas where the concrete resistivity is low, and less corroded bars found in areas having a high concrete resistivity.

We concluded that a statistical analysis of a large number of half-cell potential values, results of concrete resistivity measurements and results of corrosion rate measurements taken together can provide a more reliable evaluation of the state of reinforcement corrosion than half-cell measurements alone.CCE

Dr. Shiyuan Qian is a research associate in the Urban Infrastructure Program of the National Research Council’s Institute for Research in Construction in Ottawa.

1 ASTM C876-99 (1999), Standard Test Method for Half-cell Potentials of Reinforcement
in Concrete, ASTM, Philadephia, 1999.

2 Elsener B. and Bohni H., ASTM STP 1065, N.S. Berke et al., ed., ASTM, Philadelphia, 1990, p. 143.

3 Raherinaivo A., Contrle de la corrosion des armatures dans les structures en bton arm. Bulletin de liason des Laboratoires des Ponts et Chausss, 158, Nov./Dec. 1988, pp. 29-38.

4 Vassie P.R., TRRL Application Guide 9, 1991, pp 30.

5 Naish C.C., Harker A. and Carney R.F.A., Concrete inspection: Interpretation of potential and resistivity measurements. Proc. Corrosion of Reinforcement in Concrete, Elsevier Applied Science, 1990, pp. 314-332.