Abstract
Corrosion of steel bars embedded in reinforced concrete (RC) structures reduces the service life and durability of structures causing early failure of structure, which costs significantly for inspection and maintenance of deteriorating structures. Hence, monitoring of reinforcement corrosion is of significant importance for preventing premature failure of structures. This paper attempts to present the importance of monitoring reinforcement corrosion and describes the different methods for evaluating the corrosion state of RC structures, especially hal-cell potential (HCP) method. This paper also presents few techniques to protect concrete from corrosion.
1. Introduction
Deterioration of concrete structures due to harsh environmental conditions leads to performance degradation of RC structures, and premature deterioration of structures before completing expected service life is major concern for engineers and researchers. Deterioration rate of structures depends on the exposure conditions and extent of maintenance. Corrosion, a result of chemical or electrochemical actions, is the most common mechanism responsible for deterioration of RC structures which is mainly governed by chloride ingress and carbonation depth of RC structures. Usually, there are two major factors which cause corrosion of rebars in concrete structures, carbonation and ingress of chloride ions. When chloride ions penetrate in concrete more than the threshold value or when carbonation depth exceeds concrete cover, then it initiates the corrosion of RC structures. If the corrosion is initiated in concrete structures, it progresses and reduces service life of the structures and rate of corrosion affects the remaining service life of RC structures. However, these severe environments can cause corrosion of reinforcement only if required amounts of oxygen and moisture are available at the rebar level in concrete structures [1].
Corrosion of steel bars is the major cause of failure of concrete structures and about two tons of concrete is used per capita of the world population every year. Therefore, it has been realized that durable structures will reduce the cement consumption. Corrosion can severely reduce the strength and life of structures and in humid conditions pollutants from atmosphere percolate through the concrete cover and cause corrosion of steel. After the initiation of corrosion in reinforcing steel, products of corrosion expand and occupy a volume of about 6–10 times greater than that of steel resulting in the formation of cracks and finally in the failure of structures as shown in Figures 1 and 2.
Penetration of corrosion inducing agents such as chloride ions and carbon dioxide increased at the places of cracks, which further increases the corrosion [2]. Corrosion in concrete structures can be prevented by using low permeable concrete which minimizes the penetration of corrosion inducing agent, and the high resistivity of concrete restricts the corrosion rate by reducing the flow of current from anode to cathode [3].
2. Half-Cell Potential Method
Detection and evaluation of probability of corrosion in RC structures are essential. Proper corrosion monitoring of the concrete structures has been required for planning maintenance and replacement of the concrete structures. The most appropriate repair strategy can be selected for a distressed concrete structure by determining the corrosion status of reinforcing bars [4]. Repair of concrete structures without understanding the root cause of failure may be unsuccessful. If a cracked concrete patched without any treatment to the corroded steel, corrosion will likely continue and result in failure of patch work. Several methods for detecting corrosion activity discussed by authors in their previous paper [5] have been presented in Table 1.
There are several methods available for detecting and evaluating the corrosion in reinforcement steel as presented in Table 1. However, half-cell potential has been recognized by many researchers as the main method to detect the corrosion activity in RC structures [6]. In this method potential difference is measured between steel reinforcement and an external electrode with a voltmeter. The half-cell consists of a metal rod immersed in a solution of its own (Cu/CuSO4 or Ag/AgCl). The metal rod is connected with reinforcement steel by a voltmeter as shown in Figure 3. Some surface preparations including wetting to ensure good electrical connection are necessary. The main application of this method is in situ. External electrode and steel reinforcement are connected through a wet concrete cover as shown in Figure 3.
Interpretation of results of half-cell potential measurement for reinforced concrete structures required high skills and experience, as this only provides information regarding the probability of corrosion instead of rate and nature of corrosion [7]. Availability of oxygen, cover thickness, and concrete resistivity are few factors influencing the results of half-cell potential test. This method evaluates the potential difference on the exposed surface of concrete structures. The potential can be measured at any point on the surface or average of several measurements taken from different points on the same surface may be considered for evaluating the probability of corrosion. More negative value of measured half-cell potential indicates more probability of corrosion, as indicated in Table 2 according to ASTM C876 for Cu/CuSO4 half-cell.
This half-cell potential is also known as open circuit potential and is measured at several distinct points over a given area to be surveyed. Measured half-cell potential values can be used to plot a potential contour for the surface of reinforced concrete structure and this potential contour map as shown in Figure 4 can be used to evaluate the probability of corrosion at different points on the surface of the concrete structures. Portions of the structures likelihood of high corrosion activity can be obtained and identified by their high negative potentials.
3. Few Recently Conducted Corrosion Monitoring Activities
Several techniques have been reported in previous literatures that can be used for monitoring and evaluating the corrosion of rebars in concrete structures for diagnosing the cause and effect of corrosion. Few such studies performed by different researchers have been presented in Table 3.
4. Methods to Protect Structures from Corrosion
To increase the service life of RC structures, it is required to protect reinforcing steel completely from being corroded. Several chemical and mechanical methods are developed to prevent concrete structures from corrosion by retarding the corrosion rate and by controlling corrosion through reducing permeability of concrete and reducing the ingress of harmful ions such as oxygen and moisture, and some protective systems have been used in the form of coating. Different corrosion inhibitors and protecting systems have been discussed in Table 4.
5. Relative Limitations of Half-Cell Potential Method
Manually measuring potential values at different points on a large structure is tedious work. Therefore, automatic evaluating method is required. Half-cell potential measurements are widely used in structural engineering to assess the likelihood of corrosion. HCP measurements are found to be associated with several practical limitations such as (1) establishing connection with reinforcement, especially in structures with large concrete cover, (2) properly wetting the concrete cover for establishing proper connection between reference electrode and reinforcement, and (3) availability of oxygen, cover thickness, and concrete resistivity which can influence the results of half-cell potential test.
HCP method only provides the evaluation of the point likely to be corroded and no assessment of the corrosion rate. Half-cell potential values are indicative of the probability of corrosion activity of reinforcement located beneath the reference electrode only if the steel rebars are electrically well connected to the voltmeter. Half-cell potential method cannot provide reliable results with epoxy coated reinforcement or with coated concrete surfaces. Moist or wetting condition of concrete can influence the results of half-cell potential method, or it is important to assure the sufficient wetting of concrete to complete the setup for valid half-cell potential measurement. If measured value of the HCP varies with time, prewetting of the concrete is required. It is essential to thoroughly wet the concrete surface and allow sufficient time for the moisture to penetrate the surface layer to stabilize the potential. ASTM C-876 emphasizes that if the measured value of half-cell potential changes with time surface of concrete should be wet for at least 5 min.
It has been observed from literature that results of HCP mapping required careful interpretation. To interpret HCP data, factors such as variation in moisture content, chloride content, and concrete electrical resistance are required to be considered as all these parameters have a significant influence on the readings.
The major drawback is that HCP requires a localized breakout of the concrete cover to provide an electrical connection to the steel reinforcement. HCP results are highly influenced by the composition of the deteriorated concrete. Therefore, interpretation criteria might be different for different deterioration types. Shortcomings of HCP measurements result from the fact that the potentials are measured not near rebars but on concrete surface. Compensation is required to get more reliable results.
6. Conclusion
Failure of concrete structures due to corrosion of embedded rebars is a major problem causing significant loss of money and time. Hence, there is a need to fully understand the root causes of failure before the repairing for effective remediation. An effective method to measure corrosion is a fundamental requirement for planning maintenance, repairing, and removal for reinforced concrete structures. Information regarding corrosion state required three parameters: half-cell potential, concrete resistivity, and corrosion current density. Corrosion rate in a concrete structure is governed by several parameters such as moisture content, availability of oxygen, and temperature. So, for better results it is necessary to repeat corrosion rate measurement in regular time interval.
Half-cell potential measurement is the most widely used technique for the evaluation of corrosion of steel in concrete. However, in interpreting the data environmental factors should be taken into account. For interpretation of half-cell potential readings, it requires precise understanding of corrosion protection mechanisms and good knowledge and experience in half-cell potential mapping. In present research it has been observed that half-cell potential measurements are useful in the following purposes:(1)to assess the corrosion condition of the reinforcement by locating corroded bars,(2)for the condition assessment of a concrete structure,(3)to locate and decide the position of further detailed destructive and nondestructive testing,(4)evaluate the efficiency of repair work through corrosion state monitoring of repaired concreter structures.
In concrete with low resistivity potential distribution on surface represents potential at steel concrete interface. For better results interpretation of potential readings can be done in accordance with resistivity. With increase in concrete cover difference between surface and interface potential increases.
Content of this paper can be utilized to understand the principal of half-cell potential method, to plan investigation of corroded structures, and to select suitable corrosion monitoring technique.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.