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International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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NON DESTRUCTIVE TECHNIQUES FOR ON SITE MEASUREMENTS OF REINFORCEMENT CORROSION IN CONCRETE STRUCTURES

C. Andrade, I. Martínez, Institute of Construction Science "Eduardo Torroja", CSIC, Madrid, Spain
M. Ramírez, M. García, Geocisa, Madrid, Spain

1 INTRODUCTION

It is recognised that reinforcement corrosion is the main distress behind the present concern regarding concrete durability. In spite of the very numerous papers published on the subject, relatively few are devoted to the development of measurement of on-site techniques in general, and even less to the measurement of corrosion. However, it is known the importance of an accurate (non destructive) on-site identification of the zones suffering corrosion, and in these zones to appraise the importance of this corrosion, that is, the achieved loss in cross section and the rate of its progress. Corrosion measurements techniques are needed or may be applied for identifying corroding zones, predicting the rate of corrosion of reinforcement, predicting the residual life of the structure and monitoring the efficiency of repair systems.

In the present paper, a review is made on existing on-site techniques, and on the recent development of new ones, including application examples in reinforcement structures. Embedded sensors, as well as the needed treatment of their expected evolution and integration on time in function of the changes in the climatic conditions in the environment surrounding the concrete structure, are commented too.

Key words:
electrochemical techniques, corrosion, polarization resistance, on-site values

2 ON-SITE TECHNIQUES FOR CORROSION MEASUREMENT

2.1 Corrosion Potential and resistivity maps.
Up to the present the main techniques used on-site for appraising corrosion of reinforcements are of electrochemical nature due to that is the basis of the corrosion process. Because of its simplicity, the measurement of Ecorr (rest or corrosion potential) is the method most frequently used in field determinations. From these measurements, potential maps are drawn which reveal those zones that are most likely to undergo corrosion in the active state[1]. However, such measurements have only a qualitative character which may make data difficult to be interpreted[2]. This is due the potential only informs on the risk of corrosion and not in its actual activity. In addition, the developing of macrocells may as well mislead the deductions because corroding zones polarize the surrounding areas which may seem corroding as well, when they are cathodic areas of the macrocell. In spite of which potential mapping still has a function to accomplish as a qualitative indication of the general performance and a complement of the other on-site techniques.

The same that said for the potential can be stated on Resistivity, r , measurements[3], which sometimes are used jointly with Ecorr mapping. The r values indicate the degree of moisture content of the concrete, which is related to the corrosion rate when the steel is actively corroding, but which may mislead the interpretation in passive conditions. In figure 1 is represented a corrosion risk map of a slab. The corrosion risk level has been calculated by a combination of these two parameters: Ecorr and r.

Fig 1: Corrosion risk map on a reinforcement slab calculated from the combination of Ecorr and r measurements.

2.2 Polarization Resistance
2.2.1 Laboratory measurements
The only electrochemical technique with quantitative ability regarding the corrosion rate is the so called Polarization Resistance, Rp[4]. This technique has been extensively used in the laboratory. It is based on the application of a small electrical perturbation to the metal by means of a counter and a reference electrode. Providing the electrical signal is uniformly distributed throughout the reinforcement, the D E/DI ratio defines Rp. The corrosion current, Icorr, is inversely proportional to Rp, Icorr= B/Rp where B is a constant. Rp can be measured by means of D.C. or A.C. techniques[5], both of which have specific features in order to obtain a reliable corrosion current value in agreement with gravimetric losses.

2.2.2 On site measurements
Direct estimation of True Rp values from D E/D I measurements is usually unfeasible in large real concrete structures. This is because the applied electric signal tends to vanish with distance from the counter electrode, CE rather than spread uniformly across the working electrode, WE. Therefore, the polarization by the electric signals is not uniform, and it reaches a certain distance that is named the critical length, Lcrit. Hence, DE/D I measurements on large structures using a small counter electrode provides an apparent polarization resistance (Rpapp) that differs from the true Rp value depending on the experimental conditions[6]. Thus, if the metal is actively corroding, the current applied from a small CE located on the concrete surface is 'drained' very efficiently by the metal and it tends to confine itself on a small surface area. Conversely, if the metal is passive and Rp is high, the current applied tends to spread far away (e.g., around 50 cm) from the application point. Therefore, the apparent Rp approaches the true Rp for actively corroding reinforcement, but when the steel is passive, the large distance reached by the current needs a quantitative treatment.

Modulated confinement of the current (guard ring) method.
There are several ways of accounting for a True Rp value, among which the most extended one is the use of a guard ring[6], in order to confine the current in a particular rebar area, as Figure 2 depicts. The measurement is made by applying a galvanostatic step, lasting 30-100 seconds, from the central counter. Then, another counter current is applied from the external ring, and this external current is modulated by means of the two reference electrodes called "ring controllers" in order to equilibrate internal and external currents, which enables a correct confinement, and therefore, calculation of Rp. By means of this electrical delimitation to a small zone of the polarized area, any localised spot or pit can be first, localised, and second its measurement can be made by minimising the inherent error of Rp. Not all guarded techniques are efficient. Only that using a "Modulated Confinement" controlled by two small sensors for the guard ring control placed between the central auxiliary electrode and the ring, shown in figure 2, is able to efficiently confine the current within a predetermined area. The use of guard rings without this control leads into too high values of the Icorr for moderate and low values, and the error introduced in the case of very localized pits, is very high.

Fig 2: Modulated confinement of the current (guard ring) method.

Potential attenuation method
When the concrete is very wet, its resistivity may be so low that the confinement by the guard ring of the current cannot be well achieved because the area polarized is very large. For these conditions, another measurement method has been developed, the so called measurement of the potential attenuation with the distance[7], which is based in the direct measurement of the critical length. The sensor is formed on this case, by a small disc acting as the only counter electrode which has in its center the reference electrode for the recording of Ecorr. Other three reference electrodes are placed aligned with the Ecorr one at fixed distances. For the measurement, a potentiostatic step, lasting between 30-100 seconds, is applied to the bar. This applied potential step attenuates with the distance as observed in figure 3. From the distance (Lcrit) reached by the signal and certain geometrical considerations of the bars diameter, it is possible to calculate the True Rp (referred to a particular steel area). This method is not applied for normal non-wet concretes due to it cannot, in these cases, localize well the isolated corroding areas. When the potential step is applied, the corroding spots "drain" the current and therefore, high values of Icorr are measured in spite the sensor is not placed just above the corroding zone.

Fig 3: Potential attenuation with the distance in active and passive concrete structures.

Being the concrete very wet seldom occurs that the corrosion is very localized, and therefore the method of potential attenuation can be applied without significant errors.

Galvanostatic pulse methods
These methods consist in measuring the current or potential just after application of the electrical signal[8] instead of waiting the usual 30-100 seconds needed for achieving a certain steady-state period[4,5]. Although they can be accurate when applied to small specimens, for on-site measurements they are not appropriate as the analogue model used for the Rp calculation is Randles one, which does not reproduce the potential attenuation with the distance, and therefore gross errors can be obtained. The attempt to improve the galvanostatic pulse method by means of using a guard ring (not modulated) has not introduced any improvement but sometimes even introduces larger error due to overconfinement.

2.3 New advanced technique of corrosion measurements
A new technique has been recently developed. It has been called "passivation verification technique", PVT. It can be applied when the reinforcement or the steel is cathodically polarized, or when there are doubts on whether the rebar is actively corroding.

The cathodic protection is the only method able to stop an on-going corrosion. By means of applying a cathodic polarization, the corrosion potential is moved to the region of "inmunity" of Pourbaix's diagram and the corrosion is stopped from a practical point of view.

Fig 4: Frequency scan on AC current.

Until now the only methods available for verifying the efficiency of the cathodic protection need to switch off the applied current for measuring the potential without the ohmic drop (instant off potential method) or to record the depolarizing curve during some hours (potential decay), in order to measure the difference in potential between this switch-off and that recorded after 4-24 hours or more (100 mV criteria, for instance). Any of these methods are fully reliable due to their empirism and a controversy on the optimum measurement parameters is been continuously in the literature[9]. The PVT has been developed precisely for being applied without switching off the current. It uses the confinement sensor for delimitating the area and is based in applying an A.C current (instead of a D.C step) with the central counter electrode. The response is analysed at a set of different frequencies, as is shown on Figure 4.

This new technique has been applied on different structures, one of them is placed in Algeciras, in the south of Spain. This concrete structure is a market built by Eduardo Torroja in 1933 (Figure 5). It has a 48 m. diameter space shell placed over eight pillars joined by a prestressed tie beam. The market had some damages caused by the sea environment and recently has been repaired and the tie beam has been cathodically protected. Measurements with PVT has been made.


Fig 5:
Algeciras Market.
Fig 6: Algeciras Market. Location of the points measured by means of PVT.

In Figure 6 is shown the measurement points inside the market. There were measurements in points close to the current entry and others far away, so it's possible to see if the current is coming properly into the tie beam. The expression of the results is given in Figure 7 as percentage of protection. Each point represents an area over the pillars and the tie beam, so, in point 0 (current entry) the level of the protection is good, but going far away the current applied is not enough, and there are non protected points. All these changes in the protection state on the different points are detected by the PVT.

Fig 7: PVT results obtained in Algeciras Market.

Figure 8 shows the representation of numerous measurements in other structures as "protection level" versus potential (including ohmic drop). The threshold level of protection has been fixed in around 90%, considering that lower protection levels represent a not full efficiency of the cathodic protection.

Fig 8: General results obtained in PVT.

The PVT can be also applied when no cathodic protection is operating for simply verifying if the reinforcement is actively corroding or not. Although still more results are needed, the PVT may be used in the future to complement Rp measurements in order to find out whether a particular result is reliably informing on the corrosion state.

3 EMBEDDED SENSORS

The introduction of small sensors in the interior of the concrete, usually when placing it on-site is being one of the most promising developments in order to monitor the long term behaviour of the structures. The most usual, as in the case of non-permanent on-site techniques, is to embed reference electrodes or resistivity electrodes. They can inform of the presence of moisture and on the evolution of corrosion potential. Others events that can be monitored are the advance of the carbonation or chloride fronts, the oxygen availability, temperature, concrete deformations and the corrosion rate.

A particular example of the use of embedded sensors is the case of storage facilities of low and medium radioactive wastes in El Cabril (Córdoba)[10]. There, a pilot container has been instrumented from 1995 by embedding 27 set of electrodes. The parameters controlled are: temperature, concrete deformation, corrosion potential, resistivity, oxygen availability and corrosion rate. The impact of temperature on several of the parameters is remarkable, and therefore, care has to be taken when interpreting on-site results.

4 RANGES OF CORROSION RATE VALUES MEASURED ON-SITE

The experience on real structures[11] has confirmed the ranges of values previously recorded in laboratory experiments[4]. In general, values of corrosion rates higher than 1mA/cm2 are seldom measured while values between 0.1-1mA/cm2 are the most frequent. When the steel is passive very low values (smaller than 0.05-0.1mA/cm2) are recorded.

A comparison of on-site Icorr values to electrical resistivity has allowed the authors to also rank the resistivity ones.

5 TRANSFROMATION OF ICORRVALUES INTO CALCULATIONS OF LOSS IN BAR CROSS SECTION

Corrosion leads into four main structural consequences: 1)reduction of bar cross section, 2) reduction of steel ductility, 3) cracking of concrete cover and, 4) reduction of steel/concrete bond (composite effect). All these effects occurring in isolation, or simultaneously, will result in a loss in the load bearing capacity of the structure[12].

The primary information obtained from corrosion measurements is that concerning the loss in cross section of the bar. This parameter informs about all the other effects of the corrosion process. The attack penetration Px is defined as the loss in diameter as shown in Figure 9. It is obtained through the expression:

(1)

being tp= the time in years after corrosion started and 0.0115 a conversion factor of mA/cm2 into mm/year (for the steel). This expression implies the need to know when the corrosion has started in order to account for tp.

When the corrosion is localised (right part of figure 9), the maximum pit depth is calculated by multiplying expression (1) by a factor named a which usually takes a value of 10. Hence expression (1) above becomes,

(2)

Fig 9: Residual steel section loss considered for the cases of uniform and localized corrosion.

6 FINAL COMMENTS

The corrosion of reinforcements is one of the justifications that most often is found in the numerous studies being at present developed related to durability, however, in very few occasions the corrosion is correctly measured and interpreted, because there are very scarce the specialists that know thoroughly how to measure Rp in the laboratory and, there are even more scarce, the researchers that have been studied on-site results. This relatively small number of studies where, corrosion measurement techniques have been applied, is one of the reasons why, in spite of the numerous papers, the general advance is small. It is then necessary that specialists in electrochemical corrosion techniques work together with the other specialists in the subject.

Concerning the state of the art on on-site corrosion techniques, themselves, it is necessary to remark that the advances achieved are much more important than in other metal-electrolyte systems. In spite of it, however, several aspects remain to be improved in order to achieve the goal of making measurements of reinforcement corrosion a necessary and routine technique for any structural assessment of corroding structures.

7 REFERENCES

  1. ASTM C876-91. "Standard Test Method for Half Cell Potentials of Uncoated Reinforcing Steel in Concrete".
  2. Elsener, B and Bóhni, H. Corrosion Rates of Steel in Concrete, N.S. Berke, V.Chaker and D. Whiting (Eds.), ASTM STP 1065, pp. 143-156. (1990)
  3. Millard, S.G. and Gowers, K.R., "Resistivity assessment of in-situ concrete: the influence of conductive and resistive surface layers", Proc. Inst. Civil Engrs. Struct. & Bldgs, 94, paper 9876, pp.389-396. (1992)
  4. Andrade, C. and Gónzalez, J.A., "Quantitative measurements of corrosion rate of reinforcing steels embedded in concrete using polarization resistance measurements", Werkst. Korros., 29, 515 (1978).
  5. Andrade, C., Castelo, V., Alonso, C. and González, J.A., "The determination of the corrosion rate of steel embedded in concrete by the Rp on A.C. Impedance methods," ASTM-STP 906, pp. 43-64. (1986)
  6. Feliú, S. , González, J.A., Feliú, S.Jr., and Andrade, C., "Confinement of the electrical signal or in-situ measurement of Polarization Resistance in Reinforced concrete," ACI Mater. J., 87, pp 457. (1990)
  7. Feliú S., Gonzalez J.A., Andrade C., "Multiple-electrode method for estimatinfg the polarization resistance in large structures". Journal of applied electrochemistry 26. Págs 305-309. (1996)
  8. Elsener, B., Klinhoffer, O., Frolund, T., Rislund, E., Schiegg, Y., Böhni, H., "Assessment of reinforcement corrosion by means of galvanostatic pulse technique" International Conference on Repair of Concrete Structures. From theory to practice in a Marine Envionment . Svolvaer. Norway 20-30 may 1997. A. Blankvoll, Norwegian Public Road Administration, pp 391-400.
  9. Andrade C., Martínez I., Ramirez M., Jiménez F. "Medida de la eficacia de la protección catódica en estructuras de hormigón". Congreso internacional Colloquia 2001.
  10. Andrade C; Sagrera J.L; Gonzalez J.A; Jiménez F; Bolaño J.A; Zuloaga P. "Corrosion monitoring of concrete structures by means of permanent embedded sensors". Niza. Eurocorr'96
  11. Rodriguez, J., Ortega, L.M. García, A.M., Johansson, L. Petterson, K. "On-site corrosion rate measurements in concrete structures using a device developed under the Eureka Project EU-401-Int. Conference on Concrete Across Borders, Odense, Denmark, vol.I, pp.215-226.
  12. Rodriguez, J., Ortega, L.M., Casal, J., Díez, J.M., "Assessing Structural conditions of concrete structures with corroded reinforcements", Conference on concrete Repair, Rehabilitation and protection, Dundee (U.K.), Edited by R.K. Dhir and M.R. Jones, Published by E&FN Spon in June 1996, pp.65-77.
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