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 STRUCTURESC. Andrade, I. Martínez, Institute of Construction Science "Eduardo Torroja", CSIC, Madrid, Spain
M. Ramírez, M. García, Geocisa, Madrid, Spain
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.
2 ON-SITE TECHNIQUES FOR CORROSION MEASUREMENT
2.1 Corrosion Potential and resistivity maps.
The same that said for the potential can be stated on Resistivity, r , measurements, 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.
2.2 Polarization Resistance
2.2.2 On site measurements
Modulated confinement of the current (guard ring) method.
Potential attenuation method
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
2.3 New advanced technique of corrosion measurements
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.
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. 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.
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.
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.
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). 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 has confirmed the ranges of values previously recorded in laboratory experiments. 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.
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:
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,
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.