WO2007096668A1

WO2007096668A1 - Monitoring method

Info

Publication number: WO2007096668A1
Application number: PCT/GB2007/050072
Authority: WO
Inventors: Gareth Glass; Nigel Davison; Adrian Roberts
Original assignee: Gareth Glass; Nigel Davison; Adrian Roberts
Priority date: 2006-02-24
Filing date: 2007-02-19
Publication date: 2007-08-30
Also published as: WO2007096668B1

Abstract

A method of monitoring the protection delivered to steel bars (1) in concrete construction protected by a discrete sacrificial anode cathodic protection system comprising measuring potentials at potential measurement points located at close intervals in a representative area while the installed sacrificial anodes (2) are operating wherein the potential measurement points include at least three points (3) all located between but away from the same nearby installed anodes (2) and are all preferably located between the same pair of adjacent parallel steel bars. The results are used to identify the position of local anodes. The identification of anodes at locations where none are installed indicates that the steel may not be protected and the discrete sacrificial anode cathodic protection system is not operating effectively.

Description

MONITORING METHOD Technical Field

[0001] This disclosure is concerned with the electrochemical protection of steel in reinforced concrete construction and in particular a method of monitoring the performance of discrete sacrificial anode cathodic protection systems in reinforced concrete structures.

Background Art

[0002] Cathodic protection is an established technique covered by the European Standard EN 12696:2000. Performance is assessed by determining steel potential changes arising from the delivery of a protection current on interrupting this current. This requires some control over the current output delivered to the steel. However, in a discrete sacrificial anode cathodic protection system the anodes are connected directly to the steel in the concrete and the potential difference between the sacrificial anodes and the steel drives a galvanic protection current. The ability to control the current output is only possible with the installation of a more complex system. As a result, equipment is provided by the proponents of sacrificial cathodic protection to assist with the interruption of the protection current delivered by discrete sacrificial anodes embedded in concrete (Data sheet for the Galvashield XP monitor unit, Fosroc International Ltd., December 2004).

Summary of the Invention

[0003] Existing methods of monitoring the performance of cathodic protection systems in concrete construction require a facility to measure and interrupt the protection current. However, in discrete sacrificial anode cathodic protection systems, it is preferable to connect the anodes directly to the steel. This simplifies the installation of a reliable system but disables the use of conventional monitoring methods. To maintain the simplicity of the system and avoid the complications associated with installing a facility providing at least some control of the current output from the sacrificial anodes, a working-copy method of monitoring performance is required.

[0004] In summary this new method comprises preferably selecting one or more areas of a protected concrete structure within the discrete sacrificial anode system and measuring potentials at close intervals in the selected area while the sacrificial anodes are operating and using the results to assess the electrochemical condition of the steel between the anodes, preferably at areas that receive the least protection. The potential data may be used to identify the location of anodic areas on the protected steel. To identify one anodic area on the steel between the installed sacrificial anodes, at least three potentials measured at closely spaced points between but away from the same nearby installed anodes are required. One of these three measurement points will preferably be closer to the other two measurement points than to any of the installed sacrificial anodes. The potential measurement points will preferably be surrounded by neighbouring anodes adapted to protect all the steel between them and the midpoint of the potential measurement points will preferably be above steel that receives the least protection current. This will generally be above steel that is as far as possible from the neighbouring anodes that are adapted to protect the steel at this location. The identification of anodes at these least protected areas indicates that the steel may be corroding and the system is not performing adequately, while the identification of cathodes at these least protected areas indicates that the system is performing adequately. In the more widespread collection of potential measurement data, the identification of anodes only at the positions of the installed anodes indicates that the sacrificial anodes are operating effectively. Failure to identify all installed anodes indicates the presence of inactive sacrificial anodes.

[0005] This method of assessing the performance of a discrete sacrificial anode cathodic protection system in a reinforced concrete structure may be undertaken without any disturbance to the function of the system. The method does not require any form of control of the current output of the sacrificial anodes nor does it require reliable embedded probes when the concrete surface is accessible. Indeed, connections to the protected steel or to the anode system are not essential when the concrete surface is accessible. This reduces the complexity of the monitoring system when compared with existing systems for monitoring the performance of cathodic protection of steel in concrete.

Brief Description of Figures in the Drawings

[0006] The invention is described below with reference by way of example to the figures in which: [0007] Fig.l shows an illustrative plan view of steel bars, embedded sacrificial anodes and selected potential measurement points in a concrete structure. [0008] Fig.2 shows a table of manganese dioxide reference electrode potentials on a 50mm grid on a concrete surface above a discrete sacrificial anode cathodic protection system expressed relative to the embedded steel. [0009] Fig.3 shows a potential map using the data in Fig.2 where the potential contours are at there default spacing of 10OmV. [0010] Fig.4 shows a potential map using the data in Fig.2 where the potential contours are at a spacing of 1OmV. [0011] Fig.5 shows an equivalent table of manganese dioxide potentials to that in Fig.2 that would be recorded on a 100mm grid. [0012] Fig.6 shows a graph of the potential as a function of distance of the data in row A of

Fig.2. [0013] Fig.7 shows a section through a reinforced concrete beam containing sacrificial anodes and a repair to the underside of the beam. [0014] Fig.8 shows the potentials of the steel in the reinforced concrete beam in Fig.7 expressed relative to a reference electrode as the reference electrode is moved from just above a sacrificial anode on one side of the beam, around the underside of the beam to just above a sacrificial anode on the other side of the beam. [0015] Fig.9 shows the potential of a reference electrode measured relative to a stationary reference electrode as the electrode is moved over a line of weak sacrificial anodes in the side of a reinforced concrete beam.

Detailed Description of the Preferred Embodiments

[0016] The conventional understanding of cathodic protection is that it operates by shifting the steel potential in the negative direction. This understanding is enshrined in some of the definitions of cathodic protection. Thus performance monitoring has preferably been undertaken using steel absolute potentials or steel potential shifts or steel potential decays. For reinforced concrete structures potential decays are generally used and these are measured after interrupting the protection current (EN 12696:2000).

[0017] However work has shown that a negative potential shift is unlikely to be the most important factor resulting in steel protection in reinforced concrete (Glass, Roberts and Davison, Corrosion 2004, NACE, Paper No. 04332, 2004). Protection is more likely to occur because of a change in the environment induced at a steel cathode in concrete. It is proposed that the principle protective effect is the re-alkalisation of corroding sites to arrest corrosion and that steel passivity is sustained in the longer term by sustaining the alkaline environment at the steel (Glass, Roberts and Davison, Recent Advances in Cathodic Protection, An International Symposium at the University of Manchester, Manchester, 6 - 7 February 2006). The origin of the conventional criterion for reinforced concrete, commonly referred to as the 10OmV potential decay criterion is pragmatic, but a theoretical basis for this has been found by viewing it as a form of open circuit corrosion rate measurement (Glass, Roberts and Davison, Proc. 7th Int. Conf. Concrete in Hot and Aggressive Environments, October 2003, Volume 2, p.477-492, 2003). The criterion is generally only achieved in atmospherically exposed concrete at practical applied current densities when the steel is in a near passive state. It has been noted that relatively low protection current densities are capable of achieving and sustaining long term protection.

[0018] As the result of the above observations a new method of monitoring the performance of a discrete sacrificial anode cathodic protection system is proposed. It is supported by the proposed hypothesis that, when the steel is a net cathode in a galvanic couple, it is likely that it will be protected. One aim of the method is to ascertain whether the steel is a net cathode at areas where it receives the least protection current from the discrete sacrificial anodes. These areas will generally be close to points midway between nearby discrete sacrificial anodes. A very close interval potential survey of the steel may be used to ascertain if any local steel anodes exist.

[0019] The principle of the method of assessment is based on identifying the characteristic electric field that surrounds a net anodic area. The electric field or potential (voltage) gradient gives an indication of the location of current sources (anodes) and current sinks (cathodes). Data collection and analysis may be simplified by using potential differences as opposed to potential gradients.

[0020] A spontaneous anode (not driven by being connected to a power supply) such as that which occurs during the corrosion of steel or an anode in a sacrificial cathodic protection system, typically dissolves to form positive ions that enter the concrete leaving behind negative electrons in the anode electrode. A spontaneous anode therefore has a relatively negative potential while the potential of the adjacent concrete is relatively positive. Current flowing through the concrete from anodes to cathodes results in a potential difference in the concrete and large currents are associated with large potential differences. The concrete in the vicinity of the anodes is more positive than concrete in the vicinity of the cathodes and an anode will be associated with a local positive peak in concrete potential (a local negative peak in steel or anode potential).

[0021] The method of assessment preferably includes comparing the location of identified anodes with the location of the installed discrete sacrificial anodes. Active installed anodes will be represented by strong anodes in the measured potential data. Disconnected or inactive installed anodes will be more difficult to detect in the measured potential data. The identification of an anode where no sacrificial anode is installed gives an indication of a location where the steel is corroding. A sacrificial anode system that is performing well with active anodes and protected steel is expected to produce potential measurement data that only shows anodes at locations associated with the presence of the installed anodes.

[0022] The method of assessment is a refinement of a method previously used to identify an installed sacrificial anode in a concrete patch repair area in laboratory tests and to demonstrate the function of the anode in this area (Sergi and Page, Eurocorr '99 Proceedings, European Federation of Corrosion, Section 10, Paper 12, Aachen, Germany, September 1999). In this work it is argued that a sacrificial anode installed in the area of a concrete patch repair provides a benefit that comprises preventing the steel in the concrete adjacent to the repair area from rising to a value where it would be at risk from pitting corrosion. The benefit is derived from preventing a positive shift in steel potential. Most of the current flows to the steel near the anode inside the patch. However to maximise the benefit derived from a current induced change in the environment at the steel in a more powerful cathodic protection system, it would be preferable to locate the anodes at areas of high corrosion risk. In the patch repair example this would be just outside the area of the patch repair. The disclosed method of demonstrating the function of an anode installed within a patch repair is modified in this work to identify whether or not the steel is a cathode at areas of high corrosion risk between neighbouring discrete sacrificial anodes in a discrete sacrificial anode cathodic protection system.

[0023] The method of assessment is also a refinement of the use of potential measurements to determine the risk of steel corrosion in concrete. These measurements would typically be made at points spaced 500mm apart (DMRB Vol. 3 Sec. 3 Part 2 - BA 35/90 - Inspection and Repair of Concrete Highway Structures, UK Highways Agency Advice Note, http://www.standardsforhighways.co.uk/dmrb/vol3/sect3/ba3590.pdf) and would be used to identify the strongest or most significant anodic areas in the concrete representing areas associated with a high risk of corrosion damage. A local steel anode will conventionally be represented by a trough (local negative area) in steel potentials. In a discrete sacrificial anode cathodic protection system, the installed sacrificial anodes should be the strongest anodes and should have a dominant influence on the potential data obtained. These are intended to corrode and therefore do not represent any risk. Indeed strong active installed anodes could indicate that the steel is being protected. Furthermore, at a spacing of 500mm, it is very unlikely that the standard potential survey will produce sufficient data to identify all the installed sacrificial anodes in the survey area and it would be extremely difficult to identify anodic areas on the steel between the installed anodes.

[0024] The standard potential survey technique needs to be refined by using more closely spaced potential measurement points and the sensitivity of the data analysis needs to be improved to identify weaker steel anodes between the installed anodes. However, reducing the spacing between measured data rapidly increases the quantity of data per unit area. Thus by reducing the spacing from 500mm to 50mm the number of measurements per square meter increases from 4 to 400. This makes it practically difficult to survey large areas and small areas at high corrosion risk need to be selected for monitoring purposes. This work discloses a method of using just three potential measurements at points between but away from neighbouring installed anodes at an area of high corrosion risk in the protected structure to identify the presence or absence of one anode on the steel. [0025] Accordingly the present invention provides in one aspect a use of measured potentials to identify the presence or absence of a risk of steel corrosion as a performance assessment criterion for a discrete sacrificial anode cathodic protection system comprising a plurality of individually distinct sacrificial anodes adapted to protect steel in reinforced concrete construction which use comprises measuring potentials while protection is delivered at a minimum of three potential measurement points located within an electric field surrounding steel between neighbouring sacrificial anodes designed to protect all the steel between the neighbouring sacrificial anodes wherein the three potential measurement points are located between but away from the neighbouring sacrificial anodes and at least one of the three measurement points is located closer to the other two measurement points than to any of the neighbouring sacrificial anodes.

[0026] The three potential measurement points are located within an area of the sacrificial cathodic protection system containing steel that is intended to be protected by the system. The measured potential data is used to assess whether the sacrificial cathodic protection system is functioning adequately by determining whether the steel is or is not protected. The measured potentials are preferably interpreted using at least one criterion from the list comprising; the absence of an anodic area between the discrete sacrificial anodes indicates that the steel is protected, the presence of an anodic area between the discrete sacrificial anodes indicates that the steel is not protected.

A discrete sacrificial anode system that is functioning adequately will result in protected steel.

[0027] The three potential measurement points will preferably span a distance of no more than 400mm and more preferably span a distance of no more than 200mm and more preferably span a distance of no more than 100mm. It is preferable that there are at least 5 closely spaced measurement points between but away from the same neighbouring sacrificial anodes.

[0028] It is preferable that the steel in the concrete is located using a non-destructive method and the potential measurement points are located close to the steel. Knowledge of the location of the steel and of the installed anodes allows areas where the steel is likely to receive the least protection to be identified and targeted for monitoring and allows the number of potential measurement points to be reduced. An area of the structure surrounded by neighbouring sacrificial anodes may be selected for performance assessment of the discrete sacrificial anode cathodic protection system and it is preferable that no more than 8 and preferably no more than 5 potential measurement points are located within the selected area.

[0029] When the location of the steel is not known, an area of the structure containing neighbouring sacrificial anodes may be selected for performance assessment of the discrete sacrificial anode cathodic protection system and potentials may be measured on a closely spaced grid of potential measurement points within this selected area. It is preferable that the average distance between adjacent potential measurement points between neighbouring sacrificial anodes is no more than 100mm and more preferably no more than 50mm.

[0030] The measured potentials are potential differences between two electrodes where one electrode may be a removable electrode located on the concrete surface. The other electrode may, for example, be the protected steel. Alternatively the potentials may be measured between two removable electrodes on the concrete surface that are removed from the concrete surface after taking the measurements. When at least one electrode is located on the concrete surface, it is preferable that the concrete surface is wetted prior to measuring the potentials.

[0031] The potentials may also be measured using at least three electrodes embedded within the concrete at the potential measurement points.

[0032] In another aspect this invention provides a combination of a discrete sacrificial anode cathodic protection system applied to protect steel in reinforced concrete construction and a method of assessing the protection delivered for use in any of the above uses wherein the discrete sacrificial anode system comprises a plurality of discrete sacrificial anodes embedded within a concrete structure and the method of assessing the protection delivered comprises measuring potentials at potential measurement points to identify the presence or absence of a risk of steel corrosion within a section of a discrete sacrificial anode cathodic protection system wherein potentials are measured at a minimum of three potential measurement points that are located between but away from neighbouring sacrificial anodes designed to protect all the steel between the neighbouring sacrificial anodes and one of the three measurement points is located closer to the other two measurement points than to any of the neighbouring sacrificial anodes.

[0033] It is preferable that the discrete sacrificial anodes in the combination are located in a spaced relationship in the concrete outside concrete patch repairs.

[0034] In another aspect this invention provides a method of assessing the protection delivered to steel in reinforced concrete construction by a discrete sacrificial anode cathodic protection system that comprises applying any of the above uses.

[0035] In a discrete sacrificial anode cathodic protection system applied to reinforced concrete, the sacrificial anodes are normally installed less than 600mm apart. A more typical spacing would be 400mm. To obtain sufficient information to identify anodic areas between the installed anodes, at least three potential measurements would be required between the installed anodes. It is therefore preferable that the potential measurement points should on average be spaced no more than 200mm apart. A smaller spacing of no more than 100mm or even no more than 50mm would be more preferable.

[0036] Fig. 1 shows an example of a schematic plan view of steel bars [1] in a concrete structure containing sacrificial anodes [2] located in a regular pattern between the steel. In this example the steel furthest from the nearby sacrificial anodes would be expected to receive the least protection and is therefore likely to be at a high risk relative to other protected steel in the system of supporting net anodic activity (significant corrosion). It is preferable to target such high risk steel areas when selecting the potential measurement points. This will be aided by identifying the location of the steel bars between the installed sacrificial anodes using a non destructive technique such as that used to measure concrete cover to the steel.

[0037] In the example in Fig.l at least three potential measurement points have been located at two areas [3] and [4] of relatively high corrosion risk. If the potential of the central measurement point is positive relative to the potential of the points on either side, it indicates a high risk of anodic activity below the central measurement point. It is preferable to locate the central measurement point above the selected steel bar. The three potential measurement points may be located between neighbouring steel bars that cross the selected steel bar either side of the area of high risk.

[0038] It is also preferable to confirm the presence of the local anode by taking at least two more measurements either side of the anodic area away from the steel bar making a total of five measurements to identify one anodic area as is illustrated at one location [4] in Fig.l. If the measurement points either side of the steel remain more negative than the central measurement point above the steel, a local steel anode is probable. However such confirmation is not necessarily essential. If three measurement points follow a line that lies close to the line connecting the nearest neighbouring anodes and the potential of the central measurement point indicates the presence of an anode on the steel below it, it is likely that this represents an anode away from all the installed sacrificial anodes as it indicates the presence of an anodic area away from the nearest installed sacrificial anodes.

[0039] In cases where the concrete surface is accessible, it may be preferable to obtain the potential data by measuring the potential of a movable reference electrode located on the concrete surface relative to another stationary electrode with a stable potential that may be mounted on the concrete surface or embedded within the concrete. One such electrode is the steel in the concrete. However a convenient stationary electrode may be a partially embedded metal fixing that protrudes from the concrete surface. The use of a movable and stationary electrode on the concrete surface allows the data to be obtained in the absence of any connection to the embedded steel reinforcement as well as in the absence of any other electrode embedded in the hardened concrete for the purposes of potential measurement. Thus no special electrodes or connections need to be installed within the concrete to obtain the potential data. If no convenient metal fixing protrudes from a nearby concrete surface to form a stationary electrode, a second electrode may be placed in a fixed position on the concrete surface and used as a stationary electrode. Such an electrode may be removed at the end of the data collection process. The use of movable, portable electrodes for potential measurement allows the calibration of the electrodes to be checked in the laboratory shortly before use.

[0040] In cases where it is necessary to monitor the performance of the sacrificial anode system remotely or to monitor the performance of a system that is installed under a coating on the concrete surface or to monitor an inaccessible system, it may be preferable to embed at least three electrodes in the concrete at a targeted area of relatively high corrosion risk. The targeted area preferably contains a section of steel receiving a minimum level of protection by the sacrificial anodes. The position of the targeted area may firstly be estimated from an analysis of the position of the anodes and the position of the steel. The position of the targeted area is then preferably confirmed by undertaking a local potential survey on the concrete surface. The potentials determined on the embedded electrodes may be obtained by measuring the potential differences between embedded electrodes to reduce the number of measurements taken. It is preferable that one of the electrodes is used as a reference against which the potentials of all the other electrodes can be measured.

[0041] In the above illustrations, potential measurements are taken at a relatively small number of carefully selected measurement locations. However useful data may also be obtained by recording potential measurements on a grid of closely spaced measurement points. Because weaker anodes on the steel induce small potential changes on top of large potential changes induced by strong installed anodes, potentials need to be recorded at closely spaced intervals and the analysis of the potential data needs to be sensitive to small differences between closely spaced adjacent recorded potentials.

[0042] The small spacing of the measurement points could result in a very large number of measurements that would be practically onerous to obtain. It is therefore preferable to select one or more small areas of concrete surface above the installed discrete sacrificial anode system on which to obtain closely spaced potential data. The size of the selected area(s) will be determined by the number of installed anodes per unit area of concrete surface and it is preferable for the selected area to include at least three and more preferably four installed sacrificial anodes located on its perimeter. If such potential data is plotted as a potential contour map, it is preferable to force the contours to be closely spaced, particularly if the potential data includes a strong influence from the installed anodes.

[0043] Errors in the interpretation of the potential data may arise from measurement errors in the data and other perturbations of the measured potential arising from membrane and junction potentials. Membrane potentials in particular may vary with moisture gradients and could give rise to weak artificial anodes appearing in the potential map. To minimise this effect it is preferable to measure the potentials at the different measurement points with similar moisture conditions at all points. This may be achieved by wetting the concrete surface and leaving it to allow the moisture content to approach a stationary state. To check that the anode is not an artefact of membrane potentials, it is preferable to check for the presence of steel within the anodic area. This may be achieved non-destructively using a steel - concrete cover meter.

Example 1

[0044] Fig.2 shows a table of manganese dioxide reference electrode potentials obtained on a 50mm grid of points on a concrete surface expressed relative to the embedded steel electrode. The data are presented in rows A to E and columns 1 to 17. The position of the data points in the table corresponds to the position of the measurement points on the concrete surface with rows and columns representing horizontal and vertical lines on the concrete surface. The data were produced in part by measuring the potential difference between a portable manganese dioxide reference electrode and the embedded reinforcing steel using a portable voltmeter over a selected area of concrete surface above an embedded discrete sacrificial anode system in a concrete structure. The sacrificial anodes were nominally placed in rows 200mm apart and in columns 400mm apart, although this was varied to accommodate a variable steel spacing. The relative anode locations [20] were in row A column 3, row A column 11, row A column 17, row E column 7 and row E column 14. The measured data has been modifi ed to illustrate the possible effect of a theoretical local steel anode location [21] in row C column 9.

[0045] Fig.3 shows a potential map generated using the data in Fig.2. The potential contours are 10OmV apart. This is the default contour spacing for a contour map of this kind in a common spreadsheet software package like Microsoft Excel when the potential readings range between -200 and -800 mV. A key showing various potential ranges corresponding to various shading patterns is included. The map discloses the location of the installed sacrificial anodes as peaks in a contour plot. However the map is not sufficiently refined to disclose the location of the relatively weak steel anode.

[0046] Fig.4 shows another potential map produced using the data in Fig.2. In this case the potential contours are only 1OmV apart and a key showing a subset of the potential ranges present corresponding to various shading patterns is included. The location of the weaker steel anode [21] is now evident and is surrounded by one contour showing the potential rising to a local small peak. The presence of such and anodic area may easily be lost in a less refined potential map.

[0047] Fig.5 shows an equivalent table of manganese dioxide potentials to that in Fig.2 that would be recorded on a 100mm grid. In effect every second row or column of data has been deleted. The presence of the weak local steel anodic area [21] is no longer distinguishable in the data. However the strong installed anodes are still evident in the courser potential data on the larger grid spacing, although they are less well defined.

[0048] Fig.6 shows a graph of the potential as a function of distance for the data in row A of Fig.2. Strong anodes are represented by strong peaks with very positive potentials in the graph and the installed anodes are clearly evident at 100, 500 and 800mm in Fig.6. A weaker anode would be represented by a small peak in the potential data and a weak anode on the steel would be most likely to occur at sufficient distance from the installed sacrificial anodes to be outside the influence of the installed sacrificial anodes. The absence of any such peak in Fig.6 suggests there are no steel anodic areas along the line corresponding to the data in row A in Fig.2.

[0049] It should also be noted that, when analysing a single line of data like that in Fig.6, a small peak in the graph does not necessarily indicate an anodic area. A small peak will also occur when a line of data crosses a saddle in the contour map which may be present between two nearby adjacent strong installed anodes. It is therefore preferable to align three potential measurement points between neighbouring anodes with a line that connects the nearest and most influential installed sacrificial anodes to avoid the false identification of an anode in a line of potential measurement data.

[0050] The above example shows that the potential measurement points need to be closely spaced relative to the anode spacing to be able to resolve anodic areas on the steel between the installed sacrificial anodes. Furthermore sensitive inspection of the data is required to locate any possible relatively weak anodes on the steel between the much stronger installed sacrificial anodes.

Example 2

[0051] In another example, sacrificial anodes were installed on either side of a patch repair to the underside of a reinforced concrete beam. The anodes were installed on either side of the beam. Steel potentials were recorded relative to a portable copper/copper sulphate reference electrode from just above an anode on one side of the beam, down over one anode to the underside of the beam, across the underside of the beam and up the other side of the beam passing over another installed anode. Potential measurements were made at 100mm intervals. The layout is illustrated in Fig.7. This shows a section through the reinforced concrete beam with anodes [31] installed on either side of the beam, the repair area [32] on the underside of the beam and the line followed by the potential measurement points [33].

[0052] The potential measurements are plotted in Fig.8 as the potential of the steel relative to a reference electrode on the y-axis against the distance of the reference electrode from the starting point on the line on the x-axis. The location of the two installed anodes is indicated by two negative peaks [34] in the steel potentials measured. The whole of the patch repaired underside of the beam appears to be cathodic relative to these installed anodes and there is no indication of the presence of any areas at risk of corrosion. By locating the anodes where they are most needed in the parent concrete either side of the patch repair material, the average anode spacing in a discrete sacrificial anode cathodic protection system may be increased while still maintaining protection.

Example 3

[0053] In another example, four sacrificial anodes were located in a line along the side of a reinforced concrete beam suffering from chloride induced corrosion. A relatively weak anode was used and the anodes were spaced at a spacing of 500mm. Potential differences between reference electrodes held on the concrete surface were recorded as the spacing increased from 0mm at the first anode in steps of 125 mm along the line of the anodes to just past the last anode in the line. The potential of the moving electrode is recorded relative to the potential of the stationery electrode. At the starting point of the line the potentials are the same and the difference is zero.

[0054] The data is plotted in Fig.9. In this case the location of the anodes are indicated by weak positive peaks in the potential of the moving reference electrode [41]. However another anode [42] was also detected between the installed anodes suggesting that the steel was at risk of corrosion at this location. This suggests that the discrete sacrificial anode system was not providing adequate protection to the steel. A remedy to this risk would be to install additional sacrificial anodes at the location of these intermediate sites at a high risk of corrosion.

Claims

[0001] Use of measured potentials to identify the presence or absence of a risk of steel corrosion as a performance assessment criterion for a discrete sacrificial anode cathodic protection system comprising a plurality of individually distinct sacrificial anodes adapted to protect steel in reinforced concrete construction which use comprises measuring potentials while protection is delivered at a minimum of three potential measurement points located within an electric field surrounding steel between neighbouring sacrificial anodes designed to protect all the steel between the neighbouring sacrificial anodes wherein the three potential measurement points are located between but away from the neighbouring sacrificial anodes and at least one of the three measurement points is located closer to the other two measurement points than to any of the neighbouring sacrificial anodes.

[0002] Use as claimed in claim 1 wherein the measured potentials are interpreted using at least one criterion from the list comprising; the absence of an anodic area between the discrete sacrificial anodes indicates that the steel is protected, the presence of an anodic area between the discrete sacrificial anodes indicates that the steel is not protected.

[0003] Use as claimed in any of claims 1 or 2 wherein the three potential measurement points span a distance of no more than 200mm.

[0004] Use as claimed in claim 3 wherein the three potential measurement points span a distance of no more than 100mm.

[0005] Use as claimed in any of claims 1 to 4 wherein there are at least 5 closely spaced measurement points between but away from the same neighbouring sacrificial anodes.

[0006] Use as claimed in any of claims 1 to 5 wherein a non-destructive method is used to locate the steel in the concrete and the potential measurement points are located close to the steel.

[0007] Use as claimed in any of claims 1 to 6 wherein an area of the structure surrounded by neighbouring sacrificial anodes is selected for performance assessment of the discrete sacrificial anode cathodic protection system and no more than 8 potential measurement points are located within the selected area.

[0008] Use as claimed in claim 7 wherein the area of the structure selected for performance assessment of the discrete sacrificial anode cathodic protection system contains no more than 5 potential measurement points. [0009] Use as claimed in any of claims 1 or 2 wherein an area of the structure containing neighbouring sacrificial anodes is selected for performance assessment of the discrete sacrificial anode cathodic protection system and potentials are measured on a grid of potential measurement points wherein the average distance between adjacent potential measurement points between neighbouring sacrificial anodes is no more than 100mm.

[0010] Use as claimed in claim 9 wherein the average distance between adjacent potential measurement points is no more than 50mm.

[0011] Use as claimed in any of claims 1 to 10 wherein the measured potentials are potential differences between two electrodes and at least one electrode is a removable electrode located on the concrete.

[0012] Use as claimed in claim 11 wherein the measured potentials are potential differences between two removable electrodes on the concrete surface that are removed from the concrete surface after taking the measurements.

[0013] Use as claimed in claims 11 or 12 wherein the surface of the concrete is wetted prior to measuring the potentials.

[0014] Use as claimed in any of claims 1 to 8 where at least three electrodes are embedded within the concrete at the potential measurement points.

[0015] A combination of a discrete sacrificial anode cathodic protection system applied to protect steel in reinforced concrete construction and a method of assessing the protection delivered for use in any of claims 1 to 14 wherein the discrete sacrificial anode system comprises a plurality of discrete sacrificial anodes embedded within a concrete structure and the method of assessing the protection delivered comprises measuring potentials at potential measurement points to identify the presence or absence of a risk of steel corrosion within a section of a discrete sacrificial anode cathodic protection system wherein potentials are measured at a minimum of three potential measurement points that are located between but away from neighbouring sacrificial anodes designed to protect all the steel between the neighbouring sacrificial anodes and one of the three measurement points is located closer to the other two measurement points than to any of the neighbouring sacrificial anodes.

[0016] A combination as claimed in claim 15 wherein the discrete sacrificial anodes are located in a spaced relationship in the concrete outside concrete patch repairs.

[0017] A method of assessing the protection delivered to steel in reinforced concrete construction by a discrete sacrificial anode cathodic protection system that comprises applying any use as claimed in claims 1 tol4.

[0018] A method of assessing the protection delivered to steel in reinforced concrete construction by a discrete sacrificial anode cathodic protection system substantially as herein described above and illustrated in the accompanying drawings.