MEASURING THE CORROSION RATE OF REINFORCED CONCRETE USING THE LINEAR POLARISATION RESISTANCE METHOD GOOD PRACTICE GUIDE

Transcription

1 MEASURING THE CORROSION RATE OF REINFORCED CONCRETE USING THE LINEAR POLARISATION RESISTANCE METHOD GOOD PRACTICE GUIDE By S G Millard * and DTI DME 5.1 Consortium + Note * Senior Lecturer, Dept Civil Engineering, Liverpool University, Liverpool L69 3GQ, UK + The DTI DME consortium consists of BRE, John Broomfield Consultants, Risk Review Ltd., Trend 2000 and other Industrial Advisors A report from the Concrete Society / Institute of Corrosion liaison committee Contents Introduction Scientific/Technical background Measuring corrosion rate Equipment Limitations Training Procedure for surface measurements Interpretation Further reading References

2 Introduction Reinforcement corrosion is one of the main causes of premature deterioration of reinforced concrete structures. The progress of corrosion cannot be visually detected until cracking or rust stains appear at the concrete surface. The timing of repair and refurbishment work is dictated by economics and safety considerations. Information about the rate at which corrosion is proceeding can give early warning of future durability problems and helps structure owners to select appropriate remediation options. Measuring the polarisation resistance of reinforcing steel is one method of assessing corrosion rate. It is one of the few measurement techniques that can give an indication of the rate at which corrosion is progressing. Thus it could be used for planning of future maintenance strategies rather than simply giving information about the present location of corrosion activity. Instrumentation is relatively expensive (typically > 5k) and measurements can be quite time consuming (typically 5-10 minutes per measurement). Although the techniques are well tried & tested in the laboratory, LPR is not routinely used in the field in the way that half cell potential measurement (1) and concrete resistivity measurements (2) often are. Scientific/Technical background The amount of steel dissolving to form oxide (corrosion) can be calculated directly from measurements of the electric current generated by the anodic reaction: Fe Fe e - and consumed by the cathodic reaction: H 2 O + ½ O 2 + 2e - 2OH - Metal loss can be calculated from Faraday s law: MIt m =, zf where m = mass of steel consumed, I = current (A), t = time (s), F = 96,500 A.s, z = ionic charge (2 for Fe Fe e - ), M = molar mass of metal (56g for Fe). Thus a current of 1mA results in a loss of 9.2 grams of metal in one year. This can be converted into a volume of metal using the density of steel (7652 kg/m 3 ), which in turn can be converted into a loss of section or reduction in diameter if assumptions about the area that this loss is occurring over are made. Thus a current density of 10 ma/m 2 (1 μa/cm 2 ) is equivalent to a metal loss of 11.6μm/year, where the area being considered is the surface area of the reinforcing bar in the measurement region. A corroding metal in an environment will be characterised by the corrosion potential, E corr. This can be measured relative to a reference electrode (half-cell) using a voltmeter (1). The metal potential may be shifted ΔE by an external perturbation (e.g. an externally applied current, ΔI). This process is termed polarisation. The polarisation resistance (Rp) may be defined as the steady state resistance that the metal interface presents to a change in potential, ΔE when the perturbation ΔI is small. Typically ΔI is selected so that ΔE is no more than ±20 mv. Values of 5 to 10 mv are commonly used. The corrosion current, i corr, can then be calculated using the Stern-Geary equation.

3 B i corr =, Rp where B is a constant relating to the electrochemical characteristics of the metal under test, ΔE is the potential shift, and ΔI is the current flow (3). Polarisation resistance R p is the ratio of the potential shift ΔE to the current flow ΔI: ΔE Rp = ΔI B is empirically derived and a value of 26 mv is often used for reinforcing steel actively corroding in concrete and a value of 52 mv if the steel is considered to be passive (4). A more pragmatic value for the constant of proportionality (B) is to use 26 mv in all cases. This could result in an error of 2 being incorporated into the evaluation of the corrosion rate. A justification for applying the theoretical 26mV value for actively corroding steel to passive steel is because the time needed for a steady-state response to the applied perturbation for passive steel is usually longer. This is often not fully compensated by an increase in the measurement time. Indeed such compensation would require some prior knowledge of the corrosion rate. The perturbation may be applied in one of several ways: A galvanostatic perturbation involves applying a controlled current, ΔI. The change in the potential, ΔE is measured A potentiostatic perturbation involves applying a controlled potential shift, ΔE. The current, ΔI required to achieve this is measured. A potentiodynamic perturbation involves applying a controlled potential sweep ΔE/t. The resulting current, ΔI/t is measured. A coulostatic perturbation involves measuring the potential-time transient following a small charge perturbation. (The area under the potential-time transient divided by the charge gives the polarisation resistance.) The practical measurement of polarisation resistance is subject to various competing demands. The polarisation resistance is determined from the current-potential relationship after transient effects resulting from the perturbation have died away. However, the perturbation must not be applied for too long as this could change the condition of the steel interface. The perturbation must have a small magnitude to avoid errors resulting from non-linear effects, but it must also be large enough to be measurable in field conditions. The measurements may include a component of the resistance presented by the concrete cover and various methods may be used to compensate the results for this. The area of steel that is perturbed is needed to calculate the corrosion current density and various methods are used to estimate this. The manner in which the different instruments address these problems will result in some variation in the data produced.

4 Measuring corrosion rates Polarisation resistance and hence corrosion rate can be measured on existing reinforced concrete structures either with a movable sensor to the surface or by with embedded retrofitted probes. Both techniques are applied to selected key locations on a structure. Two or three electrode systems are available. In both systems the working electrode, W E, is the electrode whose corrosion activity is to be measured i.e. the steel reinforcement. For embedded probes in either new-build or laboratory trials this may be a length of steel that is embedded in concrete to simulate the properties and behaviour of the actual reinforcing steel. With a three-electrode system (Figure 1) a reference electrode, R E measures the potential changes induced in the steel bar by a current introduced into the ionically conductive system through an auxiliary electrode, A E, to complete the circuit. Current output to the auxiliary electrode is variable and is often controlled automatically via a potentiostat to maintain a pre-set potential at the reference electrode. The reference electrode is commonly a copper-copper sulphate half-cell, placed on the surface of the concrete for surface measurement methods. For embedded probes, a silver/silver chloride electrode is often used and is embedded close to the working electrode (i.e. the reinforcing bar) With a two-electrode system, potential changes are measured with respect to the surface potential of the auxiliary electrode. This electrode can be an inert metal electrode embedded beneath the concrete surface. Alternatively, a metal plate, connected to the concrete surface using a damp sponge interface could be used. It is assumed that the potential of the auxiliary electrode remains constant for the duration of the corrosion rate measurement. Figure 1. Typical three-electrode LPR measurement If corrosion is assumed to be uniform over the surface of the working electrode W E (i.e. the surface area of the reinforcing bar beneath the auxiliary electrode), then corrosion current density can be calculated by dividing the value of corrosion current by the surface area of the electrode. This can then be converted to an equivalent metallic loss of section in μm/year by multiplying by However where pitting corrosion is occurring, usually due to the presence of chloride contamination, then corrosion activity may be concentrated in small, localised anodic zones. In this case the localised loss of section may be significantly greater than that obtained from assuming a uniform rate of corrosion over the whole area of reinforcing bar being investigated. Underestimates of 5-10 times are quite possible. Thus care should be taken to determine if chloride contamination is present before placing reliance on evaluation of the loss of reinforcing bar section determined from LPR corrosion measurements

5 One of the difficulties with making an LPR assessment on reinforcing steel in concrete is that a measurement taken some distance away from the bar will measure not just the interfacial resistance R p, but will also include the resistance of the concrete between the reinforcing bar (working electrode) and the auxiliary electrode. This additional resistance is called the solution resistance, R s. To obtain an accurate evaluation of R p, it may be necessary to compensate for R s. Some instrumentation incorporates an automatic compensation for the solution resistance, measuring the transient response as the perturbation current is interrupted. However it may also be possible to independently measure R s and subtract it from the uncompensated LPR measurement. This can be done by applying a high frequency (typically Hz) current or voltage input signal between the auxiliary electrode and the reinforcing bar. This will short-circuit across the capacitive part of the corrosion interface at the surface of the bar and hence the resistance measured will be solely due to the concrete solution resistance. This is known as IR compensation, where I is the current passing through the concrete and R s is the solution resistance of the concrete. When using the potentiodynamic method, R s can be found from the slope of the potential-current response before the transient effects of the interfacial capacitance have died away. A second problem when measuring the corrosion rate of insitu reinforcing bars from the surface of the concrete is evaluating the area of steel being assessed. The potential shift applied will tend to stray outside the 'shadow area' directly beneath the auxiliary electrode. One solution is to use a large auxiliary electrode so that any additional area perturbed is small in comparison and hence errors remain small. An alternative is the guard-ring approach, where an additional electrode placed around the auxiliary electrode serves to confine the perturbing current to a prescribed area (5). The most successful guard-ring system employs two potential sensors between the auxiliary electrode and the guard-ring to control the applied guard-ring current. Equipment A simple LPR system consists of a potentiostat fitted with an IR compensation facility and a multimeter. For potentiodynamic measurement where a potential sweep is to be performed (rather than a potential step), a ramp generator, often contained within the same unit, is used to gradually polarise the steel. However, most instruments of this type are designed for laboratory use, require mains 240v supply and are not robust enough for general site application. As discussed above, field equipment for surface measurements can use a large auxiliary electrode to minimise unknown reinforcement area effects (6), see Figures 2 and 3. Alternatively, a guard-ring technique can be used to confine the area of measurement to a prescribed zone, see Figures 4 and 5. The guard-ring applies extra current to keep the electric field lines from the auxiliary electrode parallel and restricted to a known area of steel below. Most of the current (typically 70-90%) will flow into the steel nearest the surface and it is usual to discount the influence of deeper steel since, in most situations, only the top steel will be corroding. However full confinement of the current is questionable when macro-corrosion cells exist (7). For an LPR system without a guard-ring, the potential / current will flow outside the zone of the auxiliary electrode and perturb an unknown area of reinforcing bar. However, if the diameter of the auxiliary electrode is at least 4 times the concrete cover then the error in using the shadow area for the bar beneath the electrode is less than 10%. Any errors due to uncertainties in the area of reinforcement being measured are much less significant when corrosion rates are high.

6 Figure 2. BGB Bigfoot LPR system. Figure 3. CAPCIS RCC LPR system. Figure 4. Geocisa Gecor-6 LPR guard ring system. Figure 5. ACM LPR guard-ring system. As an alternative to taking measurements using a surface electrode system, LPR electrodes can be retrofitted to existing structures as part of a repair strategy. The electrodes are embedded adjacent to reinforcing bars and are generally one of three types: Probes containing a non-corroding auxiliary electrode and a reference electrode, placed close to existing bars to minimise solution resistance errors, Figure 6. Probes containing a working electrode of precisely known area designed to corrode at a similar rate to the existing steel. If the loss of a single steel bar is not considered structurally significant then a core can be cut through the existing steel, turned through 90 o and grouted back into the structure, Figure 7. This provides a working electrode of known surface area and in the same corrosion condition as the existing reinforcement.

7 (BGB Projects Ltd) Figure 6. Example of retrofit probe installation (Liverpool University) Figure 7. Re-grouting a core containing a steel bar Embedded electrodes can also be installed in new construction as part of a long-term corrosion assessment and maintenance strategy. If equipment is unused for extended periods, it is recommended that the reference electrode (halfcell) is maintained as follows; copper-copper sulphate (Cu/CuSO 4) half-cells - remove the copper sulphate solution silver-silver chloride (Ag/AgCl) half-cell - care should be taken that the half-cell does not dry out. For long-term storage they should be kept in a sealed container to prevent loss of moisture. Ag/AgCl cells that have become dry may be irreparably damaged. Any suspect half-cell should be calibrated against a half-cell of known reliability. Electrode surfaces should be routinely cleaned with a soft cloth and contact sponges replaced if signs of wear are seen. Limitations 1. In general, measurements in conditions of extremely high ambient temperature and relative humidity (above 50 o C and 80% RH) should be avoided, as is common with any instrument that incorporates electronic circuit boards. Where temperatures are below 5 o C representative corrosion rate results that are typical of those at normal ambient conditions (10-25 o C) are not given. Corrosion rate sensors should not be used during freezing conditions. 2. If pitting corrosion has occurred on the surface of the reinforcing bars, this will affect the accuracy of a corrosion rate measurement, based upon the assumption of uniform corrosion. Pitting has a two-fold effect: The corrosion current is not evenly distributed along the reinforcement surface area under test, as assumed; therefore the local section loss in the pit is therefore underestimated. The corrosion rate within the pit is very high, approximately five to ten times that found with general corrosion. 3. It has been observed that in the period between initiation of corrosion and the first signs of

8 cracking of the concrete the corrosion rate rises to a maximum and then subsides. This is attributed to the pitting density, i.e. the number of pits per unit area, increasing to the point where pits join up. This results in generalised corrosion occurring, which generates sufficient corrosion product to cause cracking. 4. Large errors may result if the steel has been subject to electrochemical treatment, e.g. cathodic protection or realkalisation, and the effects of this have not yet died away. (The oxidation of electrochemically-reduced products such as dissolved hydrogen may contribute to the oxidation rate measured.) 5. Large errors may result while oxygen starved conditions currently exist if the steel was previously allowed to corrode in conditions characterised by a readily available supply of oxygen. 6. Large errors may result while oxygen starved conditions currently exist if the steel was previously allowed to corrode in conditions characterised by a readily available supply of oxygen. (The corrosion potential of the steel may be very close to the redox potential of Fe 2+ /Fe 3+, which may now be an important cathodic reaction.) 7. Large errors may result when trying to determine the corrosion rate of steel that is corroding at a high rate close to the limiting current for oxygen reduction. (Neither the anodic nor the cathodic reactions may be under activation control) Training Personnel carrying out linear polarisation corrosion rate measurements must be adequately trained, usually through working with and assisting experienced staff. Anyone responsible for a corrosion rate survey must be familiar with: 1. Use and maintenance of all equipment 2. Care and maintenance of the electrodes 3. Achieving and demonstrating the required level of electrical continuity 4. Selecting and marking appropriate positions for the probes 5. Understanding the influence of moisture and temperature on the corrosion rate 6. Personnel performing the measurements need to be aware of the corrosion theory and measurement risks. Procedure for surface measurements on reinforced concrete structures 1. Details of the area to be assessed and the equipment and settings used should be recorded on a suitable form, e.g. Table 1 2. Locate the position of the reinforcing bars using a covermeter. 3. Make a sound electrical connection by one of the following methods: Expose the reinforcing steel, drill a hole in the reinforcement and insert a selftapping screw. Drill a hole in the concrete, until it just reaches the steel, then hammer a lead plug and copper wire connection into the hole using a cathode setting tool, a special tool

9 sometimes used in cathodic protection installations Drill two holes in the concrete exposing the reinforcement and spot-weld wire to the reinforcement. Prestressing steel should not be drilled or welded. Instead a clamping system should be employed. 4. Check the electrical continuity of the reinforcement system by making two electrical connections on opposite sides of the test area. A resistance of 5Ω or less between these points indicates acceptable continuity. 5. Select the measurement locations: These may be on a regular grid (e.g. 500mm grid), to allow corrosion rate contour plotting. Spot measurements at the most anodic locations indicated by a half-cell survey may be used to indicate the severity of the corrosion problem but will not be representative of the overall condition. Whilst the selection of measurement locations depends on the particular requirements of a project, representative sample areas that appear to be in good, intermediate and poor condition should be selected for monitoring. These areas may have been highlighted during a visual or half-cell potential survey. If it is possible to identify the key structural elements where deterioration is taking place then corrosion at these locations should also be monitored. Where cracked and/or delaminated reinforced concrete form part of the test area, measurements should be recorded together with notes on relevant points that could affect them. Measuring over or immediately adjacent to cracks, delaminations or voids will produce misleading readings. Corrosion rate measurements using surface applied monitoring equipment through coatings is difficult and not recommended. Where bare concrete cannot be exposed, fitting probes below the coating should be considered. Wherever possible, select areas of concrete surface that are free from any form of surface contamination. When this is not possible, clean the contaminated surface using the smallest possible amount of water/detergent. Cleaning agents on the concrete surface could affect the corrosion rate readings, as the presence of long term additional moisture may seal the surface pores, reducing the amount of oxygen that can reach the steel. Wetting the concrete surface may also change the junction and membrane potentials and the local resistivity of the concrete. Any cleaning procedures applied to the concrete should be recorded at the time of survey. Measurements should be taken over isolated lengths of reinforcing bar, Figure 7a, or over intersections of orthogonal bars, Figure 7b.

10 a) Isolated bar b) Intersection Figure 7. Location of auxiliary electrode If a sensor-controlled guard-ring system is used, the sensors should be aligned with the reinforcing bar. Figure 8. Alignment of sensors 6. A water retentive contact mat ("sponge") at the point of contact of the sensor and surface ensures good electrical contact. It is recommended that the contact solution should consist of water from a potable supply, with a small addition of alcohol (usually Iso-propyl alcohol) and some detergent. The detergent improves the wetting characteristics of the contact solution. When uneven or curved surfaces are being monitored the electrical connection between the sensor and the surface of the concrete can be achieved by using additional sponge layers. 7. The time taken for each reading depends to a certain extent on the condition of the structure under investigation. Typically, corrosion rate measurements take 2-5 minutes per location, excluding access time. 8. All measurements should be recorded, together with any other available information on a suitable form, e.g. Table 2 9. When taking repeat measurements it should be borne in mind that a few mm displacement of the probe may result in variations in readings. These can be particularly significant where pitting is present and careful notes on probe orientation and position should be made.

11 Interpretation Corrosion rate measurements can be used to estimate the loss of reinforcing steel section and the time until corrosion-induced concrete cracking is likely to occur. However, since an individual corrosion rate measurement is instantaneous, and will be affected by a variety of factors such as the concrete internal environment and the measurement procedure itself, it should only be assumed to be accurate within 2-5 times the underlying corrosion rate. It is preferable to take a series of measurements over a period of days/months/years in order to understand the trends in the data rather than to use individual values taken at one time as a basis for discussion about the structure. Corrosion current measured in micro-amps is a measure of the ongoing rate of corrosion. It is calculated by converting the corrosion current to a current density, based on the steel reinforcing bar surface area, then to a rate of section loss using Faraday s law, where 10mA/m 2 (1 μa/cm 2 ) is equivalent to 11.6 μm/year section loss. Corrosion current density criteria that have been suggested for surface applied measurements are given in Table 3. These are only valid if the corrosion is of a general form and is relatively uniform over the area considered i.e. no localised, chloride-induced pitting corrosion is suspected For retro-fitted embedded probes it is usual to consider a factor of ten variation in corrosion rate measurements between probes in active and passive zones as significant. For comparative evaluations, corrosion current iso-contour plots can be used to find the areas that have the highest likelihood of corrosion activity. These may not fully agree with those identified by half-cell iso-potential contour plots especially in situations where the concrete is saturated. Here the half-cell may record a large negative potential suggesting a high corrosion risk. However, this is due to lack of oxygen at the steel and therefore corrosion rates will be low. Table 2 Pro-forma LPR setup sheet

12 Table 3 Pro-forma LPR data recording sheet Corrosion current density (μa/cm 2 ) Corrosion rate * loss of reinforcement section (μm/year) Corrosion classification Up to 0.1 to 0.2 Up to 1-2 Very low or passive 0.2 to Low to moderate 0.5 to Moderate to high >1.0 >12 High * Corrosion rate calculated from Faraday s Law with the assumptions that the initial corrosion reaction at the anode is the oxidation of iron metal (Fe) to ferrous ions (Fe 2+ ) in solution and that the current density remains at a constant value for one year. Table 3 - Corrosion current criteria for surface applied corrosion rate measurements (8)

13 Further reading Bentur A, Diamond S & Berke NS, Steel corrosion in concrete Fundamentals & engineering practice, E&FN Spon, 1997, ISBN Berke NS, Chaker V & Whiting D, ASTM STP-1065: Corrosion rates of steel in concrete, ASTM, 1990, ISBN Broomfield J, Corrosion of steel in concrete Understanding, investigation & repair, E&FN Spon, 1997, ISBN Bungey JH & Millard SG, Testing of concrete in structures, Blackie Academic & Professional/ Chapman & Hall, 3 rd Ed., 1996, ISBN Page CL, Bamforth & Figg JW, Corrosion of reinforcement in concrete construction, SCI, 1996, ISBN X Page CL, Treadaway KWL & Bamforth PB, Corrosion of reinforcement in concrete, SCI/Elsevier, ISBN Pullar-Strecker P, CIRIA: Corrosion damaged concrete assessment & repair, Butterworths, 1987, ISBN Schiessl P, RILEM Report: Corrosion of steel in concrete, Chapman & Hall, 1988, ISBN References 1 Technical Liaison Committee of the Institute of Corrosion and the Concrete Society, Half-cell potential surveys of reinforced concrete structures, Current Practice Sheet No. 120, Concrete, July/August Broomfield J, Millard SG, Technical Liaison Committee of the Institute of Corrosion and the Concrete Society, Measuring concrete resistivity to assess corrosion rates, Current Practice Sheet No. 128, Concrete, Feb BRE, John Broomfield Consultants, Risk Review Ltd, Trend 2000 Ltd & Indust. Adv., Report 2/DME 5.1, Handbook for corrosion rate measurements, 2001, 26p. 4 BRE, John Broomfield Consultants, Risk Review Ltd, Trend 2000 Ltd & Indust. Adv., Report 3/DME 5.1, Corrosion rate measurements, 2001, 39p. 5 Broomfield JP, Rodriguez J, Ortega LM & Garcia AM, Field measurement of the corrosion rate of steel in concrete using a microprocessor controlled unit with a monitored guard ring for signal confinement, ASTM STP 1276, 1995, 15p. 6 John G, Hladky K & Dawson J, Recent developments in electrochemical inspection techniques for corrosion damaged concrete structures, Industrial Corrosion, Vo.11, No.2, Feb/Mar 1993, p Strategic Highway Research Program, Condition evaluation of concrete bridges relative to reinforcement corrosion: Volume 2 Method for measuring the corrosion rate of steel in concrete, SHPP-S-324, 1993, 105p 8 BRE Digest 434, Corrosion of reinforcement in concrete: Electrochemical monitoring, Nov.1998, 12p.