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Measurement of loss of steel from reinforcing bars in concrete using linear polarisation resistance measurements
NDT&E International 37 (2004) 381–388 www.elsevier.com/locate/ndteint
Measurement of loss of steel from reinforcing bars in concrete using linear polarisation resistance measurements D.W. Lawa,*, J. Cairnsa, S.G. Millardb, J.H. Bungeyb a
Department of Civil and Offshore Engineering, School of the Built Environment, Riccarton Campus, Heriot-Watt University, Edinburgh EH14 4AS, UK b Department of Civil Engineering, Liverpool University, Liverpool, UK Received 21 July 2003; accepted 13 November 2003
Abstract The measured weight loss data for a number of mild steel bars contained in Portland Cement concrete, together with predicted weight loss values monitored using potentiostatically controlled linear polarisation resistance (LPR) measurements are reported. Three sets of reinforced concrete specimens, each containing electrically isolated mild steel bars, were subjected to either † chloride-induced corrosion, † carbonation-induced corrosion † a control nitrogen rich environment with minimal corrosion. Each set of specimens was initially exposed to a 22-hour dry, 2-hour wet controlled environment for a duration of between 1026 and 1085 days. This was later changed to a 6-day dry, 1 day wet cycle for the carbonation exposure specimens after the initial set of gravimetric testing. The weight loss for each bar due to corrosion was recorded. Instantaneous LPR measurements were also taken on each bar at regular intervals throughout the exposure period. These resistance measurements were then integrated over the exposure period to estimate total weight loss. The results show that weight loss evaluated from experimental LPR measurements gives a significant over-estimate of the weight losses measured gravimetrically. q 2004 Elsevier Ltd. All rights reserved. Keywords: Corrosion; Reinforced concrete; Linear polarisation resistance; Durability; Weight loss measurements; Non-destructive testing
1. Introduction The problem of accurately and rapidly assessing the rate of corrosion of steel in reinforced concrete structures has long been a problem for the civil engineering industry. Due to widespread corrosion problems of reinforcing steel in concrete structures, there has been a concerted demand for the development of non-destructive techniques to enable accurate assessment of the condition of reinforced concrete structures. A number of electrochemical techniques have been developed to assess in situ the corrosion equilibrium and corrosion rate of the reinforcing steel and to enable an estimate to be made of the service life remaining to a reinforced concrete structure [1]. However, while * Corresponding author. Tel.: þ44-131-451-4411; fax: þ 44-131-4514617. E-mail address: [email protected] (D.W. Law). 0963-8695/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2003.11.003
the instantaneous corrosion rate may be used as a guide to estimate the probable loss of steel, concerns remain over the accuracy of these monitoring techniques and the reliability of such estimates. The most established of the electrochemical techniques to assess corrosion activity is half-cell potential mapping. Steel reinforcement potentials relative to a stable reference half-cell, which can be measured from the surface of the concrete, can be related to the probabilities of corrosion using the guidelines in ASTM C876-91 [2]. However, these are only broad guidelines developed from studies of reinforced concrete bridge decks contaminated with deicing salts. The different environmental conditions in individual structures can significantly affect the likelihood of finding corrosion. The half-cell method provides no information on the rate of corrosion occurring and hence no estimate of the actual amount of reinforcing steel lost due to corrosion can be made.
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Other techniques have been developed to give a direct measure of the instantaneous corrosion rate, notably AC Impedance and Linear Polarisation Resistance (LPR) methods.
The AC impedance technique is a laboratory method that enables information to be obtained about the mechanisms occurring within the system by applying a sinusoidal potential perturbation to the reinforcing steel and measuring both the current flow and the shift in phase of the resulting current [3]. A series of measurements can be taken at frequencies typically from 1 kHz to 10 mHz. However, this technique is particularly time-consuming, and data is often difficult to interpret when evaluating reinforced concrete. It is generally considered to be unsuitable for field application on reinforced concrete structures.
3. Linear polarisation resistance The LPR technique has become a well-established method of determining the instantaneous corrosion rate measurement of reinforcing steel in concrete [4 – 6]. LPR monitoring has been developed to address this need. The technique is rapid and non-intrusive, requiring only localised damage to the concrete cover to enable an electrical connection to be made to the reinforcing steel. LPR has been used in the UK, Europe, the USA and elsewhere to assess ongoing corrosion in distressed reinforced concrete structures for a number of years. The technique was originally developed based on Stern– Geary theory [7] where the corrosion current, Icorr ; is given by
ba ; bc B Rct
b a bc 1 B ¼ Rct 2:3ðba þ bc Þ Rct
icorr ¼ Icorr =A
ð2Þ
Where A is the surface are of the steel polarised. The present residual strength and, by extrapolation, the remaining service life of the structure can then be estimated.
2. AC impedance
Icorr ¼
assuming the corrosion is spread uniformly over the area of reinforcement being assessed.
ð1Þ
Tafel constants, Stern –Geary constant, charge transfer resistance.
To calculate Rct the reinforcing steel is polarised from its equilibrium potential by a small overpotential, DE: This is usually in the range 10 – 30 mV, to ensure that for active corrosion the potential shift lies within the linear Stern– Geary region. The resulting current is then monitored at the end of a selected time period, usually between 30 s and 5 min. A value for Rct is calculated by dividing the applied overpotential by the induced current. For greater accuracy an average value for Rct is taken by applying both a positive and negative overpotential to the steel. By measuring the corrosion current, Icorr at regular intervals over a period of time, it is possible to calculate the total mass of steel lost. It is also possible to estimate the loss of section of the bar from the corrosion current density, icorr ;
4. Experimental studies In order to correlate the measured weight loss with the predicted weight loss from LPR measurements a series of reinforced concrete specimens have been manufactured containing electrically isolated mild steel bars. The weight of the cleaned bars pre- and post-exposure have been measured and LPR measurements taken at regular intervals throughout the exposure period. The weight loss predicted from an integration of the LPR measurements was then related to the gravimetric weight loss. A total of 27 laboratory specimens were manufactured, comprising three sets of nine specimens. Each set of specimens was subjected to one of three different environmental regimes. † chloride exposure, † carbonation exposure, † nitrogen rich environment, to inhibit corrosion (control). The chloride and carbonation regimes were selected to represent the two most common causes of reinforcement corrosion. In addition the chloride regime was expected to produce localised pitting-type corrosion and the carbonation regime was expected to result in a more uniform corrosion. Thus the LPR measurements could be used to assess the accuracy of the technique when applied to structures with either type of corrosion mechanism. A low-grade 20 N/mm2 Portland Cement concrete mix was used to promote rapid corrosion. Each of specimens was placed in one of three separate environmental control cabinets. All specimens were initially subjected to a 22-hour dry/2-hour wet daily cycle. This was changed to a 6-day dry/1-day wet cycle for the carbonation exposure specimens after the initial testing period of 1085 days. The exposure regime remained the same for the specimens contained in the chloride cabinet. An electric fan was used during the dry cycle to increase the rate of drying in the initial stages of the exposure, for all specimens (Fig. 1). Sprayed wetting of the chloride specimens took place using a 1 M sodium chloride solution and a normal air atmosphere was maintained. Carbonation was achieved by exposing the carbonation specimens to a 100% carbon dioxide atmosphere for 2 weeks. At this point the carbonation depth of a concrete test cube was measured, using phenophlalein, to be greater than
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Each specimen contained two electrically isolated 6 mm diameter mild steel bars, two 10 mm diameter mild steel hanger bars and four bond bars to be used in parallel bond tests. The bond bars in each group of specimens comprised either † four 16 mm diameter plain mild steel bars, † four 16 mm diameter ribbed mild steel bars or † two 16 mm diameter hooked plain steel bars and two 16 mm diameter plain steel bars.
Fig. 1. Environmental control cabinet.
the depth of the bars. Following this carbonation procedure the specimens were then subjected to the wet/dry cycling regime using tap water in a normal air atmosphere. For the control specimens a nitrogen-rich atmosphere, of over 90% nitrogen, was maintained, other than during monitoring. This ensured that the oxygen available to support corrosion was reduced to a minimal level. However, these specimens were still subjected to the same wet/dry cycle using tap water as the corrosion specimens, to ensure that any differences measured were solely due to corrosion. A 22-hour dry/2-hour wet cycle was maintained throughout the exposure period for the control specimens. All three environmental cabinets were located in an indoor laboratory. Atmospheric temperature and humidity within each cabinet were monitored on an hourly basis. In addition, one specimen in each cabinet was instrumented with an internal temperature and relative humidity sensor that was also monitored on an hourly basis. The nine specimens in each cabinet were split into three groups. Design of the specimens was determined by the requirements of the study described here and also by a bond test study conducted in parallel. Details of the individual specimen design are given in Figs. 2 and 3.
Two bond bars were confined by links to the hanger bars, giving electrical continuity between all bars. The other two bond bars were electrically isolated from all other bars. The specimen dimensions were 200 £ 300 £ 300 mm3 for the specimens containing ribbed and plain bars and 200 £ 300 £ 380 mm3 for the specimens containing hooked bars. The length of the 6 mm bars exposed within the concrete was 260 mm for all specimens. Plastic sheathing was fixed over a portion of the 16 mm diameter bars as part of the design for the bond tests. The length of the 16 mm bars exposed within the concrete was 225 mm for the plain and ribbed bar specimens. For the hooked bar specimens the length of the plain bar exposed within the concrete was 145 mm and for the hooked bar 250 mm. All bars were cleaned using an inhibited acid and were then individually weighed prior to casting. The surface of each steel bar projecting from the specimen was coated with a bitumen paint to prevent atmospheric corrosion of the exposed steel or crevice corrosion just below the concrete surface. This was to ensure that any loss of steel would only be due to reinforcement corrosion within the concrete. Corrosion rate measurements using LPR were conducted on all bars but weight loss measurements were only taken on those bars that were electrically isolated, when the corrosion rate measured should be a direct measurement of the loss of steel. LPR measurements were taken by first measuring the solution resistance, Rs ; of the concrete at an AC frequency
Fig. 2. Details of test specimens, plain and ribbed bar specimens.
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Fig. 3. Details of test specimens hooked bar specimens.
of 300 Hz. This was later subtracted from the polarisation resistance, Rp ; of the bar, measured at the surface of the concrete, to correct for the resistance of the zone of cover concrete. The compensated LPR measurement, or the charge transfer resistance, Rct ; represents the polarisation resistance at the surface of the reinforcing bar. The polarisation resistance, Rp ; was measured by perturbing the steel potentiostatically both positively and negatively from its equilibrium potential using a potential shift of 10 mV, DE; and measuring the resulting current, DI; after an equilibrium period of 30 s (Fig. 4). LPR readings on each test bar were taken at intervals varying between 2 and 8 weeks. The monitoring interval was increased once corrosion had been initiated. Initial weight loss data was measured after 1168 days for one chloride exposure specimen, 1085 days for three carbonation exposure specimens and 1026 days for four control specimens. A second set weight loss measurements were conducted after 1705 days for an additional three
chloride exposure specimens and after 1625 days for an additional three carbonation exposure specimens. A total of twenty-eight 6 mm ‘weight loss’ bars and twenty 16 mm ‘bond’ bars from 14 specimens were tested in total. Following removal from the concrete, all bars were first cleaned of all rust using inhibited acid and were then weighed. The mass lost was calculated by subtraction of the final weight from the initial weight. This in turn could be converted into a %loss and a mean corrosion penetration.
5. Calculations The polarisation resistance, Rp ; of the steel can be determined from the equation Rp ¼ DE=DI
The charge transfer resistance, Rct ; is then calculated by subtracting the solution resistance, Rs ; from the polarisation resistance. Rct ¼ Rp 2 Rs
Fig. 4. Typical current vs time decay curve.
ð3Þ
ð4Þ
The corrosion rate, Icorr ; can then be calculated from Eq. (1), and the corrosion current density, icorr ; from Eq. (2). The corrosion rate may be correlated to corrosion current density and mean corrosion penetration rate (Table 1). A value of 25 mV has been adopted for corroding steel and 50 mV for passive conditions. An active condition was taken as that where the Rct value was less than 10,000 V/cm and a passive condition where the Rct value was 10,000 V/cm or higher. For these experiments the length of the bar being measured was similar to that of the auxiliary electrode and hence it was assumed that the whole surface area of the bar was polarised. For reinforcing steel within real concrete
D.W. Law et al. / NDT&E International 37 (2004) 381–388 Table 1 Correlation between corrosion rate, corrosion current density and mean corrosion penetration rate
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Table 2 Predicted and measured weight loss for 6 mm steel bars, chloride specimens at 1168 days and carbonation specimens at 1085 days
Corrosion current density (mA/cm2)
Mean corrosion penetration rate (mm/year)
Corrosion classification
Bar code
Exposure regime
Measured loss (g)
LPR loss (g)
Ratio LPR/ measured
Up to 0.1–0.2 0.2–0.5 0.5–1.0 .1.0
Up to 1–2 2– 6 6– 12 .12
Very low or Passive Low to moderate Moderate to High High
9 10 11 12 13 14 15 16
Chloride Chloride Carbonation Carbonation Carbonation Carbonation Carbonation Carbonation
0.3 1.2 1.2 1.1 0.8 1.0 0.6 2.1
2.0 2.2 1.4 1.5 1.1 1.3 1.1 2.5
6.67 1.83 1.17 1.36 1.38 1.30 1.83 1.19
structures, where the steel reinforcement is linked, uncertainty of the area of steel polarised during a LPR corrosion rate measurement is a major potential source of error. To determine the total corrosion current over the duration of the exposure period the area beneath the corrosion rate against time graph is integrated. Typical plots for chloride and carbonation exposure bar specimens are given in Fig. 5a and b. Once the total corrosion current has been integrated the total weight of steel lost can be calculated. Steel lost ¼
ð
Icorr =C M
Ð
Icorr ¼ total current passed, C ¼ charge per mole of iron, M ¼ atomic mass iron
If a uniform rate of corrosion over the surface of the bar is assumed, the loss of cross-sectional area can also be calculated from the total weight of steel lost. For pitting corrosion this clearly cannot be calculated due to the localised nature of this type of corrosion. However, it has been estimated that pitting corrosion may potentially give up to five times the corrosion penetration of uniform corrosion [4].
ð5Þ 6. Results The control specimens, Bars 1– 8 (6 mm diameter), exhibited minimal corrosion, as expected. A mean total weight loss for all eight bars of 0.138 g was measured. The integrated LPR readings gave a mean corrosion loss of 0.263 g. The measured and predicted weight loss for the individual 6 mm diameter ‘weight loss’ bars are given in Tables 2 and 3. The initial weight of each bar was between 65 and 75 g. The weight loss data for the electrically isolated 16 mm diameter bond bars contained within each specimen are given in Tables 4 and 5. The initial weights of these bars were 1400– 1410 g for plain bars, Table 3 Predicted and measured weight loss for 6 mm steel bars, chloride specimens at 1705 days, carbonation specimens at 1625 days
Fig. 5. Typical LPR corrosion measurements.
Bar code
Exposure regime
Measured loss (g)
LPR loss (g)
Ratio LPR/ measured
17 18 19 20 21 22 23 24 25 26 27 28
Chloride Chloride Chloride Chloride Chloride Chloride Carbonation Carbonation Carbonation Carbonation Carbonation Carbonation
1.3 0.6 1.5 0.6 1.4 1.1 1.5 1.4 1.5 1.2 1.5 1.1
6.8 1.6 1.8 1.1 4.3 2.0 2.5 2.2 2.6 2.2 2.6 2.1
5.23 2.67 1.20 1.83 3.07 1.82 1.67 1.57 1.73 1.83 1.73 1.91
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1335 – 1345 g for ribbed bars, 1525 –1535 g for straight plain bars in ‘hooked’ specimens and 1455 – 1470 g for hooked plain bars.
Table 4 Predicted and measured weight loss for 16 mm steel bars, initial chloride at 1168 days and carbonation specimens at 1085 days Bar code
Exposure regime
Measured loss (g)
LPR loss (g)
Ratio LPR/ measured
29 30 31 32 33 34 35 36
Chloride Chloride Carbonation Carbonation Carbonation Carbonation Carbonation Carbonation
5.5 3.1 1.2 1.3 0.6 1.4 1.6 0.5
2.4 5.7 1.8 2.3 1.5 1.9 1.4 1.7
0.44 1.84 1.50 1.77 2.50 1.36 0.88 3.40
7. Discussion The control specimens, Bars 1 –8, indicate that the control environment has inhibited almost all corrosion. The mean weight losses recorded are minimal compared to the overall mass of the samples, less than 0.3%. The predicted values are slightly higher than the measured values but the difference is not thought to be significant given the accuracy of the weight measurements, 0.1 g. The LPR measurements showed all the steel bars to be passive. Corrosion rate measurements of steel in concrete normally fluctuate with time. The degree of fluctuation can depend upon the corrosion mechanism occurring. While corrosion is a dynamic process and subject to random variations even under notionally constant conditions, the local environment within the chloride-induced pits is very sensitive to the microclimate in which they exist, in particular the moisture content. Thus the point in the wet/dry cycle at which the corrosion rate measurements are taken can have a pronounced effect. The monitoring was conducted at random points during the dry cycle, rather than a fixed point. This method was adopted to produce an average corrosion rate over the duration of the experiment and to avoid a skewing of the results by monitoring the corrosion rate at the same point in the exposure cycle, when environmental conditions within the concrete were always similar. Variations in the measurements of corrosion rate for nominally identical bars exposed to chloride and carbonation attack are shown in Fig. 5a and b, respectively. These figures both show that corrosion was rapidly initiated in a 2 –4 weeks period. The corrosion rate stabilised at a mean value of between 1 and 2 mA/cm2 for both the chloride and carbonation specimens. The corrosion rate for the carbonation specimens has remained predominantly within this range for the duration of the trial period, never consistently exceeding a corrosion rate of 2 mA/cm2. Some cycling in the rate is observable which is consistent with seasonal variations in temperature and humidity [7]. The specimens in the chloride environment show a significant increase in corrosion rate after 400– 500 days, with a corrosion rate of up to 10 mA/cm2 being observed. This increase in the corrosion rate for the chloride exposure specimens can be attributed to an increase in the concentration of chloride ions at the steel concrete interface and a corresponding decrease in the pH of the pits. No cracking was observed on the surface of the specimens, but microcracking may have occurred in the vicinity of the bars due to a build up of corrosion products, which may in turn have allowed easier access to the chloride ions. Peak rates measured are consistent with the upper limit reported by Rodriguez et al. [8].
In more uniform corrosion, which is associated with carbonation attack, the moisture content is not such a significant factor. The very high localised corrosion rates that can be reached in chloride induced corrosion pits are not achieved, thus the scatter in measurement of the corrosion rate is not as marked during the wet/dry cycle. The results illustrate the sensitivity of the measurements to environmental conditions and reinforce the need to take a large number of measurements to determine an average annual rate. Analysis of the measured weight loss and LPR results indicates that the electrochemical assessment of corrosion loss (Tables 2– 5), gives a mean overestimate over the measured weight loss of 86%, which corresponds to a ratio of LPR:measured of 1.86, where a value of 1 indicates exact correlation. The data had a standard deviation of 1.19 and is shown in Fig. 6. All of the data points for the 6 mm bars, Fig. 6, lie below the line of equality, i.e. LPR measurements give an overestimate of the weight loss for 6 mm bars. For the 16 mm bars, six points out of the 20 lie above the line. These are evenly distributed between the chloride and carbonation exposure. Only one point, Bar 29, lies a significant distance above the line. The reason for this is unclear. Table 5 Predicted and measured weight loss for 16 mm steel bars, second set of chloride specimens at 1705 days, and carbonation specimens at 1625 days Bar code
Exposure regime
Measured loss (g)
LPR loss (g)
Ratio LPR/ measured
37 38 39 40 41 42 43 44 45 46 47 48
Chloride Chloride Chloride Chloride Chloride Chloride Carbonation Carbonation Carbonation Carbonation Carbonation Carbonation
11.7 7.0 6.3 3.3 7.2 3.2 6.1 5.2 2.0 2.4 1.1 4.5
10.6 10.6 7.3 5.5 6.2 12.2 4.2 4.7 2.9 2.9 2.7 4.7
0.91 1.51 1.38 1.67 0.86 3.81 0.69 0.90 1.45 1.21 2.45 1.04
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Fig. 6. Ratio of measured to LPR weight loss.
The mean values of the LPR/measured ratio for each set of bars, together with the Standard Deviation (SD) are reported in Tables 6 and 7. In addition the total LPR weight loss/total measured weight loss for each set of bars is presented. In a number of sets only a small number of data points are available and the mean ratio can be skewed to a large degree if one of these points gives an extreme value. Breaking the data down into exposure type, bar size and time of test shows reasonable correlation between both bars sizes subjected to carbonation attack, irrespective of testing time. This correlation is also observed between these bars and 16 mm bars subject to chloride attack. However, the 6 mm bars subjected to chloride attack show significantly higher overestimates of weight loss from LPR measurements. This variation is reduced when taking the ratio of total LPR/total measured for the 6 mm, chloride exposure bars tested at 1168 days but is slightly increased for the 6 mm, chloride exposure bars tested at 1705 days. It was noted that the SD values for the 6 mm chloride exposure
bars were considerably higher than all the other sets of bars tested. Indeed the SD values for the chloride exposure bars were all higher than those of the carbonation exposure bars. Combining all the carbonation exposure specimens gives a mean ratio of 1.57 and a SD of 0.60. Combining all the chloride exposure specimens gave a mean ratio of 2.28 and a SD of 1.68. This indicates that LPR monitoring of carbonation-induced corrosion gives both a more accurate and a more reliable measure of weight loss than for chloride induced corrosion. The greater reliability may be attributed to the different types of corrosion. Carbonation attack results in more uniform corrosion and lower mean corrosion rates, whilst chloride attack will result in localised pitting corrosion, as shown in Fig. 5a and b. Thus an error in determining the charge transfer resistance, Rct Eq. (1), which is inversely proportional to the corrosion current, will result in a larger error in the case of the chloride induced corrosion compared to carbonation induced corrosion.
Table 6 Data analysis of 6 mm mild steel bars
Table 7 Data analysis of 16 mm mild steel bars
Set
LPR:measured mean
SD
Total LPR weight/ total measured weight
Set
Ratio mean
SD
Total LPR weight/ total measured weight
Chloride, 1168 days Chloride, 1705 days Carbonation, 1085 days Carbonation, 1625 days
4.25 2.64 1.37 1.74
3.42 1.43 0.24 0.12
1.60 2.71 1.31 1.73
Chloride, 1168 days Chloride, 1705 days Carbonation, 1085 days Carbonation, 1625 days
1.14 1.65 1.90 1.29
0.99 1.11 0.91 0.63
1.07 1.37 1.61 1.09
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A possible explanation for any overestimate of the measured weight loss by LPR assessment is that the perturbation current that is applied to the steel to induce a 10 mV potential shift, DE; does not uniformly affect the whole bar. The potential of the steel varies across the bar surface. Where the corrosion is uniform this variation is small as the majority of the surface area of the bar is corroding at a similar rate. However, in chloride induced corrosion the potential in the pits can vary by several 100 mV from the passive steel and also from one pit to the next dependent upon a number of factors including pH, chloride concentration and condition of the steel. Due to these differences in potential along the bar it is possible that not all the steel will be polarised to the full 10 mV. As such the assumed DE value of 10 mV may in reality be higher than the DE value achieved in practice. This would result in the value for Rp ; Eq. (2), being lower, which would in turn result in the weight loss value calculated by LPR measurements being higher, giving an overestimate, as observed. This could account for the overestimate observed for the 6 mm diameter bars but not for the underestimate observed in some of the 16 mm diameter bars, Fig. 6. Previous research has indicated [9] that the auxiliary electrode may not polarise all the steel surface of reinforcing bars. The perturbation current applied from the auxiliary electrode, on the concrete surface, has to travel through the concrete to reach the most distant surface of the reinforcing steel. It has been suggested that this may not be fully achieved in some areas due to the increased distance through the concrete. The result of this effect could be an underestimate of the corrosion rate by a factor of two [10]. This could account for the observation of a limited number of 16 mm bars giving an underestimate of the corrosion rate, as the two different effects compete with each other. Overall, while the LPR technique is shown to give an overestimate of the weight loss of the bars, this still represents a reasonable assessment of the magnitude of the corrosion behaviour. An overestimate of corrosion rate corresponds to an early indication of potential damage and the need for a more detailed inspection. An underestimate of corrosion rate could result in significant damage occurring prior to the monitoring indicating the need for inspection of the reinforcing steel. It must be noted that the technique does not indicate the level of corrosion loss present in the bars prior to the commencement of monitoring. This must be ascertained on site by a detailed visual inspection, to determine what if any levels of damage have already occurred. Whilst it may be possible to use the LPR technique to assist with a service lifetime prediction, the errors inherent in the technique must be borne in mind. The technique may be used to give a qualitative measure of the condition of the structure but care must be taken when using the technique in a quantitative manner.
8. Conclusions 1. The LPR corrosion rate measurement technique can be used to estimate the mass of steel lost when either chloride or carbonation induced corrosion is occurring. 2. The corrosion rate measurements for chloride exposure and carbonation exposure specimens show a distinct variation. Chloride exposure specimens display a significantly higher maximum corrosion rate and a higher degree of scatter in the results. 3. The predicted mass of steel lost from LPR measurements usually gives an overestimate of the mass steel lost determined from weighing of the actual specimens. 4. The mean ratio measured:predicted weight loss for all specimens was 1.86 which corresponds to a mean overestimate of 86% for the total mass of steel lost. 5. The mean overestimate for chloride exposure specimens was 128% and for carbonation specimens 57%. 6. The reliability of the LPR measurements was higher for carbonation-induced corrosion than for chloride-induced corrosion. 7. The LPR measurements confirm that the external environment may influence the accuracy of the LPR measurements. 8. The technique can be used as a qualitative measure of the corrosion rate of reinforcing steel in concrete. If the results are found to be generally applicable then it may be possible to apply the technique to give a quantitative value using a correction factor for chloride or carbonation induced corrosion.
References [1] Gowers KR, Millard SG. Proc Inst Civ Engrs, Struct, Bldg 1999;134: 129 –37. [2] ASTM C886-91. Standard test method for half-cell potentials on uncoated reinforcing steel in concrete. ASTM; 1999. [3] MacDonald DD, EL-Tantawy YA, Rocha-Filho RC, UrquideMacdonald M. Evaluation of electrochemical techniques for detecting corrosion on rebar in reinforced concrete. National Research Council, Washington DC; 1991, SHARP-ID/UFR-91-524, vol 1, Summary Report. [4] Millard SG, Gowers KG, Gill RP. Br J NDT 1992;9:444. [5] Millard SG. Current Practice Sheet No. 132. Concrete 2003;36–8. [6] Broomfield JP, ASTM STP 1276, Techniques to assess the corrosion acitivity of steel reinforced concrete structures; 1996. p. 91– 106. [7] Stern M, Geary AL. J Electrochem Soc 1957;January:56–63. [8] Millard SG, Law DW, Bungey JH, Cairns JJ. In: Uomoto T, editor. Symposium on Non-Destructive Testing in Civil Engineering (NDTCE 2000), Tokyo; 2000. p. 625 –36. [9] Rodriguez J, Ortega LM, Garcia AM. Proceedings of the International Conference on Corrosion and Protection of Steel in Concrete; July 1994. p. 171– 85. [10] Sehgal A, Li D, Kho YT, Osseo-Asare K, Pickering HW. Corrosion 1995;48:706–12.
Measurement of loss of steel from reinforcing bars in concrete using linear polarisation resistance measurements D.W. Lawa,*, J. Cairnsa, S.G. Millardb, J.H. Bungeyb a
Department of Civil and Offshore Engineering, School of the Built Environment, Riccarton Campus, Heriot-Watt University, Edinburgh EH14 4AS, UK b Department of Civil Engineering, Liverpool University, Liverpool, UK Received 21 July 2003; accepted 13 November 2003
Abstract The measured weight loss data for a number of mild steel bars contained in Portland Cement concrete, together with predicted weight loss values monitored using potentiostatically controlled linear polarisation resistance (LPR) measurements are reported. Three sets of reinforced concrete specimens, each containing electrically isolated mild steel bars, were subjected to either † chloride-induced corrosion, † carbonation-induced corrosion † a control nitrogen rich environment with minimal corrosion. Each set of specimens was initially exposed to a 22-hour dry, 2-hour wet controlled environment for a duration of between 1026 and 1085 days. This was later changed to a 6-day dry, 1 day wet cycle for the carbonation exposure specimens after the initial set of gravimetric testing. The weight loss for each bar due to corrosion was recorded. Instantaneous LPR measurements were also taken on each bar at regular intervals throughout the exposure period. These resistance measurements were then integrated over the exposure period to estimate total weight loss. The results show that weight loss evaluated from experimental LPR measurements gives a significant over-estimate of the weight losses measured gravimetrically. q 2004 Elsevier Ltd. All rights reserved. Keywords: Corrosion; Reinforced concrete; Linear polarisation resistance; Durability; Weight loss measurements; Non-destructive testing
1. Introduction The problem of accurately and rapidly assessing the rate of corrosion of steel in reinforced concrete structures has long been a problem for the civil engineering industry. Due to widespread corrosion problems of reinforcing steel in concrete structures, there has been a concerted demand for the development of non-destructive techniques to enable accurate assessment of the condition of reinforced concrete structures. A number of electrochemical techniques have been developed to assess in situ the corrosion equilibrium and corrosion rate of the reinforcing steel and to enable an estimate to be made of the service life remaining to a reinforced concrete structure [1]. However, while * Corresponding author. Tel.: þ44-131-451-4411; fax: þ 44-131-4514617. E-mail address: [email protected] (D.W. Law). 0963-8695/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2003.11.003
the instantaneous corrosion rate may be used as a guide to estimate the probable loss of steel, concerns remain over the accuracy of these monitoring techniques and the reliability of such estimates. The most established of the electrochemical techniques to assess corrosion activity is half-cell potential mapping. Steel reinforcement potentials relative to a stable reference half-cell, which can be measured from the surface of the concrete, can be related to the probabilities of corrosion using the guidelines in ASTM C876-91 [2]. However, these are only broad guidelines developed from studies of reinforced concrete bridge decks contaminated with deicing salts. The different environmental conditions in individual structures can significantly affect the likelihood of finding corrosion. The half-cell method provides no information on the rate of corrosion occurring and hence no estimate of the actual amount of reinforcing steel lost due to corrosion can be made.
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Other techniques have been developed to give a direct measure of the instantaneous corrosion rate, notably AC Impedance and Linear Polarisation Resistance (LPR) methods.
The AC impedance technique is a laboratory method that enables information to be obtained about the mechanisms occurring within the system by applying a sinusoidal potential perturbation to the reinforcing steel and measuring both the current flow and the shift in phase of the resulting current [3]. A series of measurements can be taken at frequencies typically from 1 kHz to 10 mHz. However, this technique is particularly time-consuming, and data is often difficult to interpret when evaluating reinforced concrete. It is generally considered to be unsuitable for field application on reinforced concrete structures.
3. Linear polarisation resistance The LPR technique has become a well-established method of determining the instantaneous corrosion rate measurement of reinforcing steel in concrete [4 – 6]. LPR monitoring has been developed to address this need. The technique is rapid and non-intrusive, requiring only localised damage to the concrete cover to enable an electrical connection to be made to the reinforcing steel. LPR has been used in the UK, Europe, the USA and elsewhere to assess ongoing corrosion in distressed reinforced concrete structures for a number of years. The technique was originally developed based on Stern– Geary theory [7] where the corrosion current, Icorr ; is given by
ba ; bc B Rct
b a bc 1 B ¼ Rct 2:3ðba þ bc Þ Rct
icorr ¼ Icorr =A
ð2Þ
Where A is the surface are of the steel polarised. The present residual strength and, by extrapolation, the remaining service life of the structure can then be estimated.
2. AC impedance
Icorr ¼
assuming the corrosion is spread uniformly over the area of reinforcement being assessed.
ð1Þ
Tafel constants, Stern –Geary constant, charge transfer resistance.
To calculate Rct the reinforcing steel is polarised from its equilibrium potential by a small overpotential, DE: This is usually in the range 10 – 30 mV, to ensure that for active corrosion the potential shift lies within the linear Stern– Geary region. The resulting current is then monitored at the end of a selected time period, usually between 30 s and 5 min. A value for Rct is calculated by dividing the applied overpotential by the induced current. For greater accuracy an average value for Rct is taken by applying both a positive and negative overpotential to the steel. By measuring the corrosion current, Icorr at regular intervals over a period of time, it is possible to calculate the total mass of steel lost. It is also possible to estimate the loss of section of the bar from the corrosion current density, icorr ;
4. Experimental studies In order to correlate the measured weight loss with the predicted weight loss from LPR measurements a series of reinforced concrete specimens have been manufactured containing electrically isolated mild steel bars. The weight of the cleaned bars pre- and post-exposure have been measured and LPR measurements taken at regular intervals throughout the exposure period. The weight loss predicted from an integration of the LPR measurements was then related to the gravimetric weight loss. A total of 27 laboratory specimens were manufactured, comprising three sets of nine specimens. Each set of specimens was subjected to one of three different environmental regimes. † chloride exposure, † carbonation exposure, † nitrogen rich environment, to inhibit corrosion (control). The chloride and carbonation regimes were selected to represent the two most common causes of reinforcement corrosion. In addition the chloride regime was expected to produce localised pitting-type corrosion and the carbonation regime was expected to result in a more uniform corrosion. Thus the LPR measurements could be used to assess the accuracy of the technique when applied to structures with either type of corrosion mechanism. A low-grade 20 N/mm2 Portland Cement concrete mix was used to promote rapid corrosion. Each of specimens was placed in one of three separate environmental control cabinets. All specimens were initially subjected to a 22-hour dry/2-hour wet daily cycle. This was changed to a 6-day dry/1-day wet cycle for the carbonation exposure specimens after the initial testing period of 1085 days. The exposure regime remained the same for the specimens contained in the chloride cabinet. An electric fan was used during the dry cycle to increase the rate of drying in the initial stages of the exposure, for all specimens (Fig. 1). Sprayed wetting of the chloride specimens took place using a 1 M sodium chloride solution and a normal air atmosphere was maintained. Carbonation was achieved by exposing the carbonation specimens to a 100% carbon dioxide atmosphere for 2 weeks. At this point the carbonation depth of a concrete test cube was measured, using phenophlalein, to be greater than
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Each specimen contained two electrically isolated 6 mm diameter mild steel bars, two 10 mm diameter mild steel hanger bars and four bond bars to be used in parallel bond tests. The bond bars in each group of specimens comprised either † four 16 mm diameter plain mild steel bars, † four 16 mm diameter ribbed mild steel bars or † two 16 mm diameter hooked plain steel bars and two 16 mm diameter plain steel bars.
Fig. 1. Environmental control cabinet.
the depth of the bars. Following this carbonation procedure the specimens were then subjected to the wet/dry cycling regime using tap water in a normal air atmosphere. For the control specimens a nitrogen-rich atmosphere, of over 90% nitrogen, was maintained, other than during monitoring. This ensured that the oxygen available to support corrosion was reduced to a minimal level. However, these specimens were still subjected to the same wet/dry cycle using tap water as the corrosion specimens, to ensure that any differences measured were solely due to corrosion. A 22-hour dry/2-hour wet cycle was maintained throughout the exposure period for the control specimens. All three environmental cabinets were located in an indoor laboratory. Atmospheric temperature and humidity within each cabinet were monitored on an hourly basis. In addition, one specimen in each cabinet was instrumented with an internal temperature and relative humidity sensor that was also monitored on an hourly basis. The nine specimens in each cabinet were split into three groups. Design of the specimens was determined by the requirements of the study described here and also by a bond test study conducted in parallel. Details of the individual specimen design are given in Figs. 2 and 3.
Two bond bars were confined by links to the hanger bars, giving electrical continuity between all bars. The other two bond bars were electrically isolated from all other bars. The specimen dimensions were 200 £ 300 £ 300 mm3 for the specimens containing ribbed and plain bars and 200 £ 300 £ 380 mm3 for the specimens containing hooked bars. The length of the 6 mm bars exposed within the concrete was 260 mm for all specimens. Plastic sheathing was fixed over a portion of the 16 mm diameter bars as part of the design for the bond tests. The length of the 16 mm bars exposed within the concrete was 225 mm for the plain and ribbed bar specimens. For the hooked bar specimens the length of the plain bar exposed within the concrete was 145 mm and for the hooked bar 250 mm. All bars were cleaned using an inhibited acid and were then individually weighed prior to casting. The surface of each steel bar projecting from the specimen was coated with a bitumen paint to prevent atmospheric corrosion of the exposed steel or crevice corrosion just below the concrete surface. This was to ensure that any loss of steel would only be due to reinforcement corrosion within the concrete. Corrosion rate measurements using LPR were conducted on all bars but weight loss measurements were only taken on those bars that were electrically isolated, when the corrosion rate measured should be a direct measurement of the loss of steel. LPR measurements were taken by first measuring the solution resistance, Rs ; of the concrete at an AC frequency
Fig. 2. Details of test specimens, plain and ribbed bar specimens.
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Fig. 3. Details of test specimens hooked bar specimens.
of 300 Hz. This was later subtracted from the polarisation resistance, Rp ; of the bar, measured at the surface of the concrete, to correct for the resistance of the zone of cover concrete. The compensated LPR measurement, or the charge transfer resistance, Rct ; represents the polarisation resistance at the surface of the reinforcing bar. The polarisation resistance, Rp ; was measured by perturbing the steel potentiostatically both positively and negatively from its equilibrium potential using a potential shift of 10 mV, DE; and measuring the resulting current, DI; after an equilibrium period of 30 s (Fig. 4). LPR readings on each test bar were taken at intervals varying between 2 and 8 weeks. The monitoring interval was increased once corrosion had been initiated. Initial weight loss data was measured after 1168 days for one chloride exposure specimen, 1085 days for three carbonation exposure specimens and 1026 days for four control specimens. A second set weight loss measurements were conducted after 1705 days for an additional three
chloride exposure specimens and after 1625 days for an additional three carbonation exposure specimens. A total of twenty-eight 6 mm ‘weight loss’ bars and twenty 16 mm ‘bond’ bars from 14 specimens were tested in total. Following removal from the concrete, all bars were first cleaned of all rust using inhibited acid and were then weighed. The mass lost was calculated by subtraction of the final weight from the initial weight. This in turn could be converted into a %loss and a mean corrosion penetration.
5. Calculations The polarisation resistance, Rp ; of the steel can be determined from the equation Rp ¼ DE=DI
The charge transfer resistance, Rct ; is then calculated by subtracting the solution resistance, Rs ; from the polarisation resistance. Rct ¼ Rp 2 Rs
Fig. 4. Typical current vs time decay curve.
ð3Þ
ð4Þ
The corrosion rate, Icorr ; can then be calculated from Eq. (1), and the corrosion current density, icorr ; from Eq. (2). The corrosion rate may be correlated to corrosion current density and mean corrosion penetration rate (Table 1). A value of 25 mV has been adopted for corroding steel and 50 mV for passive conditions. An active condition was taken as that where the Rct value was less than 10,000 V/cm and a passive condition where the Rct value was 10,000 V/cm or higher. For these experiments the length of the bar being measured was similar to that of the auxiliary electrode and hence it was assumed that the whole surface area of the bar was polarised. For reinforcing steel within real concrete
D.W. Law et al. / NDT&E International 37 (2004) 381–388 Table 1 Correlation between corrosion rate, corrosion current density and mean corrosion penetration rate
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Table 2 Predicted and measured weight loss for 6 mm steel bars, chloride specimens at 1168 days and carbonation specimens at 1085 days
Corrosion current density (mA/cm2)
Mean corrosion penetration rate (mm/year)
Corrosion classification
Bar code
Exposure regime
Measured loss (g)
LPR loss (g)
Ratio LPR/ measured
Up to 0.1–0.2 0.2–0.5 0.5–1.0 .1.0
Up to 1–2 2– 6 6– 12 .12
Very low or Passive Low to moderate Moderate to High High
9 10 11 12 13 14 15 16
Chloride Chloride Carbonation Carbonation Carbonation Carbonation Carbonation Carbonation
0.3 1.2 1.2 1.1 0.8 1.0 0.6 2.1
2.0 2.2 1.4 1.5 1.1 1.3 1.1 2.5
6.67 1.83 1.17 1.36 1.38 1.30 1.83 1.19
structures, where the steel reinforcement is linked, uncertainty of the area of steel polarised during a LPR corrosion rate measurement is a major potential source of error. To determine the total corrosion current over the duration of the exposure period the area beneath the corrosion rate against time graph is integrated. Typical plots for chloride and carbonation exposure bar specimens are given in Fig. 5a and b. Once the total corrosion current has been integrated the total weight of steel lost can be calculated. Steel lost ¼
ð
Icorr =C M
Ð
Icorr ¼ total current passed, C ¼ charge per mole of iron, M ¼ atomic mass iron
If a uniform rate of corrosion over the surface of the bar is assumed, the loss of cross-sectional area can also be calculated from the total weight of steel lost. For pitting corrosion this clearly cannot be calculated due to the localised nature of this type of corrosion. However, it has been estimated that pitting corrosion may potentially give up to five times the corrosion penetration of uniform corrosion [4].
ð5Þ 6. Results The control specimens, Bars 1– 8 (6 mm diameter), exhibited minimal corrosion, as expected. A mean total weight loss for all eight bars of 0.138 g was measured. The integrated LPR readings gave a mean corrosion loss of 0.263 g. The measured and predicted weight loss for the individual 6 mm diameter ‘weight loss’ bars are given in Tables 2 and 3. The initial weight of each bar was between 65 and 75 g. The weight loss data for the electrically isolated 16 mm diameter bond bars contained within each specimen are given in Tables 4 and 5. The initial weights of these bars were 1400– 1410 g for plain bars, Table 3 Predicted and measured weight loss for 6 mm steel bars, chloride specimens at 1705 days, carbonation specimens at 1625 days
Fig. 5. Typical LPR corrosion measurements.
Bar code
Exposure regime
Measured loss (g)
LPR loss (g)
Ratio LPR/ measured
17 18 19 20 21 22 23 24 25 26 27 28
Chloride Chloride Chloride Chloride Chloride Chloride Carbonation Carbonation Carbonation Carbonation Carbonation Carbonation
1.3 0.6 1.5 0.6 1.4 1.1 1.5 1.4 1.5 1.2 1.5 1.1
6.8 1.6 1.8 1.1 4.3 2.0 2.5 2.2 2.6 2.2 2.6 2.1
5.23 2.67 1.20 1.83 3.07 1.82 1.67 1.57 1.73 1.83 1.73 1.91
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1335 – 1345 g for ribbed bars, 1525 –1535 g for straight plain bars in ‘hooked’ specimens and 1455 – 1470 g for hooked plain bars.
Table 4 Predicted and measured weight loss for 16 mm steel bars, initial chloride at 1168 days and carbonation specimens at 1085 days Bar code
Exposure regime
Measured loss (g)
LPR loss (g)
Ratio LPR/ measured
29 30 31 32 33 34 35 36
Chloride Chloride Carbonation Carbonation Carbonation Carbonation Carbonation Carbonation
5.5 3.1 1.2 1.3 0.6 1.4 1.6 0.5
2.4 5.7 1.8 2.3 1.5 1.9 1.4 1.7
0.44 1.84 1.50 1.77 2.50 1.36 0.88 3.40
7. Discussion The control specimens, Bars 1 –8, indicate that the control environment has inhibited almost all corrosion. The mean weight losses recorded are minimal compared to the overall mass of the samples, less than 0.3%. The predicted values are slightly higher than the measured values but the difference is not thought to be significant given the accuracy of the weight measurements, 0.1 g. The LPR measurements showed all the steel bars to be passive. Corrosion rate measurements of steel in concrete normally fluctuate with time. The degree of fluctuation can depend upon the corrosion mechanism occurring. While corrosion is a dynamic process and subject to random variations even under notionally constant conditions, the local environment within the chloride-induced pits is very sensitive to the microclimate in which they exist, in particular the moisture content. Thus the point in the wet/dry cycle at which the corrosion rate measurements are taken can have a pronounced effect. The monitoring was conducted at random points during the dry cycle, rather than a fixed point. This method was adopted to produce an average corrosion rate over the duration of the experiment and to avoid a skewing of the results by monitoring the corrosion rate at the same point in the exposure cycle, when environmental conditions within the concrete were always similar. Variations in the measurements of corrosion rate for nominally identical bars exposed to chloride and carbonation attack are shown in Fig. 5a and b, respectively. These figures both show that corrosion was rapidly initiated in a 2 –4 weeks period. The corrosion rate stabilised at a mean value of between 1 and 2 mA/cm2 for both the chloride and carbonation specimens. The corrosion rate for the carbonation specimens has remained predominantly within this range for the duration of the trial period, never consistently exceeding a corrosion rate of 2 mA/cm2. Some cycling in the rate is observable which is consistent with seasonal variations in temperature and humidity [7]. The specimens in the chloride environment show a significant increase in corrosion rate after 400– 500 days, with a corrosion rate of up to 10 mA/cm2 being observed. This increase in the corrosion rate for the chloride exposure specimens can be attributed to an increase in the concentration of chloride ions at the steel concrete interface and a corresponding decrease in the pH of the pits. No cracking was observed on the surface of the specimens, but microcracking may have occurred in the vicinity of the bars due to a build up of corrosion products, which may in turn have allowed easier access to the chloride ions. Peak rates measured are consistent with the upper limit reported by Rodriguez et al. [8].
In more uniform corrosion, which is associated with carbonation attack, the moisture content is not such a significant factor. The very high localised corrosion rates that can be reached in chloride induced corrosion pits are not achieved, thus the scatter in measurement of the corrosion rate is not as marked during the wet/dry cycle. The results illustrate the sensitivity of the measurements to environmental conditions and reinforce the need to take a large number of measurements to determine an average annual rate. Analysis of the measured weight loss and LPR results indicates that the electrochemical assessment of corrosion loss (Tables 2– 5), gives a mean overestimate over the measured weight loss of 86%, which corresponds to a ratio of LPR:measured of 1.86, where a value of 1 indicates exact correlation. The data had a standard deviation of 1.19 and is shown in Fig. 6. All of the data points for the 6 mm bars, Fig. 6, lie below the line of equality, i.e. LPR measurements give an overestimate of the weight loss for 6 mm bars. For the 16 mm bars, six points out of the 20 lie above the line. These are evenly distributed between the chloride and carbonation exposure. Only one point, Bar 29, lies a significant distance above the line. The reason for this is unclear. Table 5 Predicted and measured weight loss for 16 mm steel bars, second set of chloride specimens at 1705 days, and carbonation specimens at 1625 days Bar code
Exposure regime
Measured loss (g)
LPR loss (g)
Ratio LPR/ measured
37 38 39 40 41 42 43 44 45 46 47 48
Chloride Chloride Chloride Chloride Chloride Chloride Carbonation Carbonation Carbonation Carbonation Carbonation Carbonation
11.7 7.0 6.3 3.3 7.2 3.2 6.1 5.2 2.0 2.4 1.1 4.5
10.6 10.6 7.3 5.5 6.2 12.2 4.2 4.7 2.9 2.9 2.7 4.7
0.91 1.51 1.38 1.67 0.86 3.81 0.69 0.90 1.45 1.21 2.45 1.04
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Fig. 6. Ratio of measured to LPR weight loss.
The mean values of the LPR/measured ratio for each set of bars, together with the Standard Deviation (SD) are reported in Tables 6 and 7. In addition the total LPR weight loss/total measured weight loss for each set of bars is presented. In a number of sets only a small number of data points are available and the mean ratio can be skewed to a large degree if one of these points gives an extreme value. Breaking the data down into exposure type, bar size and time of test shows reasonable correlation between both bars sizes subjected to carbonation attack, irrespective of testing time. This correlation is also observed between these bars and 16 mm bars subject to chloride attack. However, the 6 mm bars subjected to chloride attack show significantly higher overestimates of weight loss from LPR measurements. This variation is reduced when taking the ratio of total LPR/total measured for the 6 mm, chloride exposure bars tested at 1168 days but is slightly increased for the 6 mm, chloride exposure bars tested at 1705 days. It was noted that the SD values for the 6 mm chloride exposure
bars were considerably higher than all the other sets of bars tested. Indeed the SD values for the chloride exposure bars were all higher than those of the carbonation exposure bars. Combining all the carbonation exposure specimens gives a mean ratio of 1.57 and a SD of 0.60. Combining all the chloride exposure specimens gave a mean ratio of 2.28 and a SD of 1.68. This indicates that LPR monitoring of carbonation-induced corrosion gives both a more accurate and a more reliable measure of weight loss than for chloride induced corrosion. The greater reliability may be attributed to the different types of corrosion. Carbonation attack results in more uniform corrosion and lower mean corrosion rates, whilst chloride attack will result in localised pitting corrosion, as shown in Fig. 5a and b. Thus an error in determining the charge transfer resistance, Rct Eq. (1), which is inversely proportional to the corrosion current, will result in a larger error in the case of the chloride induced corrosion compared to carbonation induced corrosion.
Table 6 Data analysis of 6 mm mild steel bars
Table 7 Data analysis of 16 mm mild steel bars
Set
LPR:measured mean
SD
Total LPR weight/ total measured weight
Set
Ratio mean
SD
Total LPR weight/ total measured weight
Chloride, 1168 days Chloride, 1705 days Carbonation, 1085 days Carbonation, 1625 days
4.25 2.64 1.37 1.74
3.42 1.43 0.24 0.12
1.60 2.71 1.31 1.73
Chloride, 1168 days Chloride, 1705 days Carbonation, 1085 days Carbonation, 1625 days
1.14 1.65 1.90 1.29
0.99 1.11 0.91 0.63
1.07 1.37 1.61 1.09
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A possible explanation for any overestimate of the measured weight loss by LPR assessment is that the perturbation current that is applied to the steel to induce a 10 mV potential shift, DE; does not uniformly affect the whole bar. The potential of the steel varies across the bar surface. Where the corrosion is uniform this variation is small as the majority of the surface area of the bar is corroding at a similar rate. However, in chloride induced corrosion the potential in the pits can vary by several 100 mV from the passive steel and also from one pit to the next dependent upon a number of factors including pH, chloride concentration and condition of the steel. Due to these differences in potential along the bar it is possible that not all the steel will be polarised to the full 10 mV. As such the assumed DE value of 10 mV may in reality be higher than the DE value achieved in practice. This would result in the value for Rp ; Eq. (2), being lower, which would in turn result in the weight loss value calculated by LPR measurements being higher, giving an overestimate, as observed. This could account for the overestimate observed for the 6 mm diameter bars but not for the underestimate observed in some of the 16 mm diameter bars, Fig. 6. Previous research has indicated [9] that the auxiliary electrode may not polarise all the steel surface of reinforcing bars. The perturbation current applied from the auxiliary electrode, on the concrete surface, has to travel through the concrete to reach the most distant surface of the reinforcing steel. It has been suggested that this may not be fully achieved in some areas due to the increased distance through the concrete. The result of this effect could be an underestimate of the corrosion rate by a factor of two [10]. This could account for the observation of a limited number of 16 mm bars giving an underestimate of the corrosion rate, as the two different effects compete with each other. Overall, while the LPR technique is shown to give an overestimate of the weight loss of the bars, this still represents a reasonable assessment of the magnitude of the corrosion behaviour. An overestimate of corrosion rate corresponds to an early indication of potential damage and the need for a more detailed inspection. An underestimate of corrosion rate could result in significant damage occurring prior to the monitoring indicating the need for inspection of the reinforcing steel. It must be noted that the technique does not indicate the level of corrosion loss present in the bars prior to the commencement of monitoring. This must be ascertained on site by a detailed visual inspection, to determine what if any levels of damage have already occurred. Whilst it may be possible to use the LPR technique to assist with a service lifetime prediction, the errors inherent in the technique must be borne in mind. The technique may be used to give a qualitative measure of the condition of the structure but care must be taken when using the technique in a quantitative manner.
8. Conclusions 1. The LPR corrosion rate measurement technique can be used to estimate the mass of steel lost when either chloride or carbonation induced corrosion is occurring. 2. The corrosion rate measurements for chloride exposure and carbonation exposure specimens show a distinct variation. Chloride exposure specimens display a significantly higher maximum corrosion rate and a higher degree of scatter in the results. 3. The predicted mass of steel lost from LPR measurements usually gives an overestimate of the mass steel lost determined from weighing of the actual specimens. 4. The mean ratio measured:predicted weight loss for all specimens was 1.86 which corresponds to a mean overestimate of 86% for the total mass of steel lost. 5. The mean overestimate for chloride exposure specimens was 128% and for carbonation specimens 57%. 6. The reliability of the LPR measurements was higher for carbonation-induced corrosion than for chloride-induced corrosion. 7. The LPR measurements confirm that the external environment may influence the accuracy of the LPR measurements. 8. The technique can be used as a qualitative measure of the corrosion rate of reinforcing steel in concrete. If the results are found to be generally applicable then it may be possible to apply the technique to give a quantitative value using a correction factor for chloride or carbonation induced corrosion.
References [1] Gowers KR, Millard SG. Proc Inst Civ Engrs, Struct, Bldg 1999;134: 129 –37. [2] ASTM C886-91. Standard test method for half-cell potentials on uncoated reinforcing steel in concrete. ASTM; 1999. [3] MacDonald DD, EL-Tantawy YA, Rocha-Filho RC, UrquideMacdonald M. Evaluation of electrochemical techniques for detecting corrosion on rebar in reinforced concrete. National Research Council, Washington DC; 1991, SHARP-ID/UFR-91-524, vol 1, Summary Report. [4] Millard SG, Gowers KG, Gill RP. Br J NDT 1992;9:444. [5] Millard SG. Current Practice Sheet No. 132. Concrete 2003;36–8. [6] Broomfield JP, ASTM STP 1276, Techniques to assess the corrosion acitivity of steel reinforced concrete structures; 1996. p. 91– 106. [7] Stern M, Geary AL. J Electrochem Soc 1957;January:56–63. [8] Millard SG, Law DW, Bungey JH, Cairns JJ. In: Uomoto T, editor. Symposium on Non-Destructive Testing in Civil Engineering (NDTCE 2000), Tokyo; 2000. p. 625 –36. [9] Rodriguez J, Ortega LM, Garcia AM. Proceedings of the International Conference on Corrosion and Protection of Steel in Concrete; July 1994. p. 171– 85. [10] Sehgal A, Li D, Kho YT, Osseo-Asare K, Pickering HW. Corrosion 1995;48:706–12.