Use of Corrosion Monitoring Sensors to Monitor the In-Situ Performance and Intervention needs in Reinforced Concrete Structures

Fred Andrews-Phaedonos

1, Dr Ahmad Shayan

2, Dr Aimin Xu

3

1Principal Engineer – Concrete Technology, VicRoads, Australia

2Chief Research Scientist, ARRB Group Pty Ltd, Australia

3Senior Engineer, ARRB Group Pty Ltd, Australia

Abstract: It is well recognised that visual inspections alone are not sufficient to establish the in-situ condition of a concrete structure, or to ensure that appropriate maintenance and rehabilitation works are undertaken. Such inspections only identify damage or deterioration which has manifested itself with no indication as to the internal condition of otherwise visibly sound concrete. In addition, such inspections can only provide a superficial view of the condition of the structure at any given time and do not give a clear or comprehensive picture of corrosion risk and future maintenance needs. As such diagnostic assessments consisting of a number of physical, electrical and chemical techniques are generally used in combination with visual inspection to obtain a better assessment of the condition of reinforced concrete structures already affected by corrosion induced deterioration. The information from these techniques is used collectively to ensure correct interpretation of results and subsequent diagnosis and prognosis. However, as an alternative to the combination of visual inspection and diagnostic assessment which are generally used to establish the cause, extent and degree of deterioration already apparent, corrosion monitoring sensors may be cast into new structures or installed in existing structures alone or as part of repairs, to provide the asset manager with real time information as to the current state and performance of the structure or remedial works. Monitoring sensors can provide early detection of initiation and/or propagation of corrosion and if required allow for an early diagnostic assessment, as well as, facilitate timely and cost effective preventative maintenance intervention as opposed to more disruptive and expensive rehabilitation. Such early detection can also allow the asset manager to pre-plan and invest in a timely manner and thereby minimise the number of costly interventions, mobilisations and disruptions to the travelling public. This paper presents a number of case studies where monitoring sensors have been built into new structures, as well as where sensors have been installed as part of concrete rehabilitation works with a view to monitoring their effectiveness and ongoing performance. Keywords: corrosion, monitoring, sensors, detection, intervention

1. Introduction Monitoring sensors in the form of reference electrodes were first utilised by VicRoads for the monitoring of a number of impressed current and sacrificial Cathodic Protection systems applied to reinforced concrete bridges since the late 1980s. A number of Silver/Silver Chloride (Ag/AgCl) reference electrodes were installed for monitoring the half-cell potential of the steel reinforcement, as part of remedial works undertaken in 1991/92 at Sawtells Inlet Bridge, located in an aggressive marine environment south east of Melbourne. Stainless steel pins were also installed for measuring the resistivity of concrete. The internal Ag/AgCl reference electrodes were connected by cables leading to monitoring boxes placed at the upper edge of the crossheads. Monitoring of potentials and resistivities commenced in July 1991 (1, 2, 3). In the mid 1990s six macro-cell/galvanic current corrosion monitoring sensors (ladders) were installed in the downstream column of Pier No.2, as part of a multi-level durability strategy adopted for the new Patterson River Bridge (Nepean Highway over Patterson River) constructed in 1994/95 in a very aggressive marine environment. The monitoring sensors consist of an array of steel pieces acting as anodes and a carbon cathode. The macro-cell or galvanic corrosion current flowing between the steel anodes and the carbon cathode was measured periodically. As the chloride ions penetrate the cover concrete the steel anodes in-turn become active and corrosion currents are measured (4, 5).

Project A, located in a severe marine environment, comprising a number of major reinforced concrete structures including bridges and retaining walls was completed in 2005. These structures are potentially subject to chloride-induced corrosion, resulting from exposure to salt water. For this reason some seventy five silver-silver chloride reference electrodes and seven macro-cell/galvanic current corrosion monitoring sensors (ladders) were installed at different locations within major bridge components and exposed retaining wall piles. The sensors were installed as a proactive measure to be able to identify the onset of corrosion activity and put in place preventive intervention to avoid corrosion-induced damage to the structures (6). A corrosion monitoring system using a number of embedded Silver/Silver Chloride (Ag/AgCl) reference electrodes was installed during concrete repair works at both Church Street Bridge and Racecourse Road Bridge as part of major rehabilitation works undertaken at these bridges in 2010 and 2011 respectively. These bridges are located in the inner metropolitan area and they were both suffering from corrosion induced deterioration due to carbonation effects of the low quality concrete and further exacerbated by moisture ingress from the deck above (7, 8). A galvanic anode cathodic protection (CP) system was installed in the piers of Queen Street Bridge in 2011 as part of major concrete jacketing of the piers to mitigate the chloride induced corrosion of the steel reinforcement. The new galvanic anodes were connected onto the steel reinforcement and encased with self compacting concrete (SCC) to form the new reinforced concrete jacketing for the deteriorated concrete piers. The effectiveness of the galvanic anode CP system has been monitored by testing the potentials of the steel relative to reference electrodes embedded in Pier 1 of the bridge (9). In 2009 a number of half-cell reference electrodes were also installed adjacent to the steel reinforcement in two landscape retaining walls at a bridge over the Yarra River to monitor the ability of geopolymer concrete to provide long term to the steel reinforcement (10, 11). It is important that seasonal temperature variations are taken into account and allowed for when monitoring and assessing corrosion monitoring sensors, when cast in, to monitor the corrosion state of the steel in concrete structures or the performance of concrete repair works. Temperature variations may result in seasonal fluctuations of corrosion rates and electro potentials of the steel reinforcement.

2. Monitoring of patch repairs and coating systems at Sawtells Inlet Bridge – Marine Exposure Sawtells Inlet Bridge at Tooradin on the South Gippsland Highway was constructed in 1968 and underwent major remediation work in 1991 and 1992 due to chloride induced corrosion related deterioration of the reinforced concrete elements (1, 2, 3). The repairs to the columns and crossheads included patch repairs (Fig. 1) and the application of a polymer modified cementitious coating to Pier 1 (on the Melbourne side) and a three part epoxy coating to Pier 2 (on the Tooradin side), in both atmospheric and tidal/splash zone microclimates. The repair work also included the installation of a permanent corrosion monitoring system comprising eight permanently embedded Ag/AgCl reference electrodes (Fig. 1) and thirty sets of four stainless steel pins embedded in the concrete surface to allow measurement of the concrete resistivity. This was to enable the ongoing monitoring of the effectiveness of the various concrete repair methods and protective coatings in limiting chloride induced corrosion of concrete coastal

bridges. An area of 0.4 m × 0.9 m was left untreated, as a control area, immediately below the crosshead on each downstream column for subsequent comparison. A dual silane impregnation plus acrylic anti-carbonation coating system was applied to the superstructure elements. Three reference electrodes were embedded within the major patch repairs on each pier with similar locations on both piers (Fig. 1). Monitoring of all electrodes was done initially on a monthly basis and progressively at regular time intervals (Fig. 2), at junction boxes which are attached to the side of crossheads to eliminate the possibility of vandalism. A high impedance voltmeter was been used for the monitoring. All electropotential measurements were considered relative to the original readings of the reference electrodes. One reference electrode was installed close to the steel reinforcement in the major patch repairs of the two downstream crosshead overhangs during the application of the repair material (i.e. D1, T1). A second reference electrode was installed in the two untreated control areas of the two downstream columns, about 400 mm from the underside of the crosshead (i.e. D2, T2).

Figure 1. Location of Ag/AgCl reference electrodes and patch repairs (1).

A third Ag/Ag Cl reference electrode was installed in the crossheads adjacent to the interface of the major patch repairs and the parent concrete (i.e. D3, T3). The purpose of this was to monitor the effects, if any, of the concrete repair on the adjacent unrepaired areas (Fig. 1). This is to check the general expectation that repaired areas normally act as large cathodes with the adjacent unrepaired areas becoming anodic and thus prone to corrosion. If this is the case this monitoring will give some indication as to the timing of this. Reference electrodes D2, T2, D3, and T3 were embedded in mortar of similar resistivity to the original concrete. The locations of the resistivity measurements were also numbered accordingly and monitored at regular time intervals (Fig. 3). Monitoring of potentials and resistivities commenced in July 1991.

Figure 2. Half-cell potential of the internal sensors. “D” = Melbourne Pier (Pier 1); “T”= Tooradin Pier (Pier 2).

Figure 3. Resistivity of concrete for Tooradin Pier columns.

0

5

10

15

20

25

30

35

40

45

50

Jun-94 Oct-95 Mar-97 Jul-98 Dec-99 Apr-01 Sep-02

Resistivity (kΩΩ ΩΩ·cm)

1-Up

1-Mid

1-Low

2-Up

2-Mid

2-Low

3-Up

3-Mid

3-Low

-600

-480

-360

-240

-120

0

120

Jul-91 Nov-92 Mar-94 Aug-95 Dec-96 May-98 Sep-99 Jan-01 Jun-02 Oct-03

D1

D2

D3

T1

T2

T3

Additional testing over the years included chloride and pH testing, external half-cell potential and resistivity measurements and more recently corrosion rate measurements, as well as visual inspections and delaminations survey. Recent investigations and assessments revealed that although after almost 20 years of service a significant proportion of the concrete repairs and protective coatings have approached the need for re-intervention in this very aggressive marine environment, overall the combination of some repairs and associated coatings has proven to have performed in a reasonably satisfactory manner. In effect it has been established that good quality conventional patch repairs in combination with good protective coatings can perform reasonably well and provide a service life of almost 20 years in such harsh marine environments compared to a service life of some 5 years achieved in earlier years using lower quality procedures and materials being adopted. As a result of this ongoing monitoring over the near 20 year period, it may be concluded that both a polymer modified cementitious coating and an epoxy coating have been relatively effective in limiting the ingress of chlorides into the concrete piers particularly early on, despite their shortcomings which may be attributed to mixing, application, curing and other operational practices. The application of polymer modified cementitious repair materials in combination with the protective coatings has resulted in delaying the onset of macro-cell corrosion (incipient anode effects) in the neighbouring coated areas for a significant period of time although such effects appeared to have manifested themselves in the past 3 to 5 years in a moderately accelerating manner.

3. Monitoring of Corrosion Monitoring Sensors at Patterson River Bridge since Construction – Marine Exposure Six macro-cell/galvanic current corrosion-monitoring sensors were installed within the cover concrete in the downstream column of Pier 2 during construction in 1994/95 (4), with three installed in the upstream face and three in the downstream face (Fig. 4), to monitor the performance of the new bridge and the various durability provisions over time. The macro-cell or galvanic corrosion current flowing between the steel anodes and the carbon cathode is measured periodically. As the chloride ions penetrate the cover concrete the steel anodes in turn become active and corrosion currents are measured. The outer steel anodes were located within the first 15 mm of the cover concrete. They were placed at 0.2, 0.7 and 2.2 metres above the top of the pile cap to basically measure the effects within the tidal, splash and atmospheric zones respectively. A dummy sensor was also cast in a more permeable concrete so that corrosion could be induced at a faster rate in order to compliment the system (Fig. 5). Monitoring is done on a frequent basis via a remote monitoring system. Readings were found to be very low and averaging less than 0.006 micro Amps (Fig. 5), with some very minor seasonal variations, which are very similar to the data presented in a previous paper on this bridge some 4 years ago (4), further highlighting the effectiveness of the high concrete quality (low VPV – 10.5% - 12.5% < 14% of maximum allowable in concrete cores as per Section 610 (5)) and protective measures adopted for this bridge from the outset. The very low corrosion current results of the monitoring sensors are quite clearly supported by other data including the external corrosion rate measurements, the negligible chloride penetration into the concrete and the subsequent inability to calculate chloride diffusion coefficients and hence time to corrosion initiation.

Figure 4. Galvanic Monitoring sensors; monitoring box-later converted to remote monitoring; dummy probe placed in more permeable concrete.

Figure 5. Typical results of corrosion monitoring sensors (T), Dummy sensor results (B). The same western column of the middle pier (Pier 2 D/S Column) was tested externally for the corrosion state of the steel reinforcement. The corrosion rate and half-cell potential measurements (which are obtained simultaneously) are summarised in Table 1. Comparing with the results of 2001, the corrosion rate has marginally increased, however it is still below 0.1 µA/cm

2 which has been considered as the

criterion for “no corrosion expected” as indicated in Table 2. On both occasions at all three locations the corrosion rate is negligible. This conclusion is confirmed by the half-cell potential, which is more positive than –200 mV CSE (copper-copper sulphate electrode potential). There is essentially no difference in the corrosion activity at the three locations measured. An example of the output of the embedded sensors is given in Figure 5 together with results for the dummy sensor placed in more permeable concrete. The sensors also show negligible or no corrosion activity, which is in agreement with the results of the linear polarisation measurements. Based on these results no corrosion activity is expected in the concrete in the foreseeable future, for at least 20 years and beyond. This is the result of the high quality of the concrete and the surface coating applied soon after construction.

Table 1. Corrosion state of the reinforcing steel in Pier 2 D/S Column.

Location Corrosion current density,

µA/cm2

Half-cell potential, mV CSE

Resistance,(kΩ) (Resistivity, kΩ·cm)

2001 2008 2001 2008 2001 2008

1.05 m above pile-cap 0.015 0.028

-136 -83 2.39 (33.7) 3.42 (48)

0.45 m above pile-cap 0.020 0.039 -107 -88 2.30 (32.4) 3.05 (43)

0.16 m above pile-cap 0.016 0.044 -117 -93 2.31 (32.6) 3.93 (55)

Table 2. Corrosion rate criteria for reinforcing steel.

Corrosion current density, icorr (µA/cm2)1 Corrosion rate category

< 0.1 no corrosion expected

0.1 to 0.5 low to moderate rate

0.5 to 1.0 moderate to high rate

> 1.0 high rate

Note 1: For steel, a corrosion current density of 1.0 µA/cm2 is equivalent to a corrosion rate of 11.6 µm/year

4. Monitoring of Corrosion Monitoring Sensors at Project A (Various Structures) Since Construction – Marine Exposure

The group of structures constructed during 2004/2005, are located in close proximity to seawater, and are prone to chloride induced corrosion of the steel reinforcement. Various parts of these structures have been fitted with reference electrodes and ladder type macro-cell electrodes (Fig. 6) to enable monitoring of the half-cell potential and corrosion activity of the steel reinforcement (6). These sensors have been installed as a proactive measure to be able to identify the onset of corrosion activity and thus afford the asset manager the opportunity to undertake a more detailed diagnostic assessment if required, and put into place timely preventive intervention to avoid corrosion-induced damage to the structures. This requires regular inspection and monitoring to prevent development of corrosion-induced defects in the structure by addressing minor problems before they become unmanageable. Some twenty five monitoring points were installed in each of the two long walls, fifteen monitoring points in base slabs, ten monitoring points in other slabs and superstructure components and seven macro-cell ladders were installed in two major structural slabs. The initial monitoring measurements were made in March 2007 which comprised half-cell potential measurements. Later corrosion monitoring measurements were conducted between 2009 and 2012 to provide a comparison of results with previous measurement and checking differences in the output of the sensors. The measurements conducted in 2009 established that the half-cell potentials were influenced by temperature (Fig. 7) and it was determined then that the monitoring measurement should be conducted at similar months of the year to obtain comparable results. Since 2010 the monitoring measurements have been made in the month of February each year. The monitoring consists of measuring the potentials of the installed reference electrodes against the reinforcement, and the macro-cell ladders installed in the walls, slabs, and small bridges which are components of the overall complex of structures. With the exception of a small number, most reference electrodes were found to be functioning normally during monitoring.

Figure 6. Galvanic ladder type macro-cell sensor positioned within thickness of cover concrete.

Figure 7. Monthly mean temperatures at Project A (Source: www.bom.go v.au). As indicated in Figs 8 to 11 compared with the results obtained in 2007, the half-cell potentials measured in the 2012 investigation were almost the same or slightly more negative. Particularly, compared with the data obtained in the previous two years, the half-cell potentials in walls have become slightly more negative. This indicates that corrosion activity may be commencing. The small scale of potential changes over the past few years indicates that the corrosion state of the reinforcement in the structures has been stable.

Figure 8. Half-cell potentials of retaining wall on east side of the road. The ongoing monitoring has established that for the retaining walls, steel corrosion is likely at 36% of the monitored locations and uncertain at 36% to 41% of locations for the two walls. However, the east wall showed more likely corrosion activity than the west wall, and other parts of the overall complex. Seawater seeping through a number of locations at the east wall has been observed. For the base slabs, steel corrosion is likely at 25% of the monitored locations; and uncertain at 67% of the locations. For the main

structural slabs, steel corrosion has not yet started, but it is uncertain at 13% of locations. The percentage of corrosion active areas has increased compared to the data of 2011.

Figure 9. Half-cell potentials of retaining wall on west side of the road.

Figure 10. Half-cell potentials of base slabs. The half-cell potential data for one of the macro-cell ladders, installed in the road base slab, indicates high risk of corrosion at the bottom of the slab. The data of macro-cell ladders installed within the main structural slab indicate that there is no corrosion risk at present. Comparatively, the corrosion activity in the overall complex of structures has not significantly changed over the past three years of monitoring, and at the present time is still considered very mild. However, the ladder sensor and the potentials measured in the vicinity of the road base showed a high possibility of chloride penetration and corrosion activity.

Figure 11. Potentials of anodes in ladders installed in base slab of E90. In order to establish an initial base level of overall condition of the various structural components the monitoring measurements conducted on the embedded electrodes and macro-cell ladders, was further supplemented with a diagnostic assessment which also included determination of concrete properties and corrosion status of the reinforcement steel. The diagnostic assessment involved cover thickness determination, delamination testing, measurements of the half-cell potentials, and corrosion current density of reinforcement at selected areas of slabs and walls. Concrete cores were drilled from slabs and walls and laboratory testing was conducted on the cores for compressive strength, volume of permeable voids (VPV) and chloride profile. Chloride profiles determined from the cores showed that the chloride content is high at the surface and lower in the interior of the concrete. Comparatively, the chloride content in the retaining walls was found to be much higher than that in the base slabs. The chloride ingress profiles show that the threshold chloride content (0.4% by cement mass) has reached only 5 mm depth in the base slabs, and about 35 mm in the retaining walls (Fig. 12). Neither of these has reached the depth of reinforcing steels.

Figure 12. Chloride profiles in structural components.

Generally the results of the diagnostic assessment were reflecting the results of the various monitoring sensors and further diagnostic assessment was considered necessary over the next two years, including determination of the depth of chloride ingress, which is the trigger for corrosion initiation. A maintenance strategy would then be developed on the basis of the updated measurements, although observed leakage, dampness, shrinkage cracks and corrosion of some structural steel work (base plates, bolts etc) was considered to require intervention much earlier.

5. Monitoring of Patch Repairs and Coating Systems at Church Street Bridge – Carbonation and Moisture Effects

For Church Street Bridge, a total of ten embedded reference electrodes were installed in 2010 in cell walls and deck soffit at Cell 21 and Cell 27 within major patch repairs located in Span 3 (the southern span of the bridge), due to carbonation induced corrosion of the steel reinforcement (7). Some reference electrodes were also installed within the patch repairs adjacent to the interface with the parent concrete. At each location, an electrode was horizontally placed adjacent to a main steel bar at the same cover depth. It was required that the concrete over the steel not be disturbed by the installation. Calibration and Initial monitoring measurements were carried out upon installation, followed by a new set of measurements on two cells of the bridge deck, which was conducted in mid December 2011. The monitoring of the corrosion of reinforcing steel is conducted through measurement of the potential of steel relative to the reference electrode adjacent to it. According to ASTM C876-99 a potential of more negative than -350mV CSE may indicate active corrosion in steel in concrete. It should be noted that ASTM C876-99 has been withdrawn in 2008 and the new version of the ASTM C876-09 does not contain any definite value to evaluate the possibility of corrosion based on the potentials. Nevertheless, changes in the potential indicate changes in the environment and/or electro-chemical condition of the steel. In each deck cell, there is a junction box containing terminals for the cables leading to the reference electrodes and reinforcing steel bars (Fig. 13). All test results are presented in Table 3 and Table 4. The analysis of the measurements (if the value of -350mV CSE potentials is used) indicates that the half-cell potentials of reinforcement in cell wall (Cell 21) and spandrel column (Cell 27) have become more positive than this value and indicate that the possibility of corrosion activity is less than 10%. It should be noted that the patch repairs are characterised by a trend of increase in resistivity with time as the repair material continues to cure and dry out. On the other hand some areas of the deck showed potentials progressively more negative than -350mV CSE since the installation. The difference in these trends may be related to the differences in the moisture condition of concrete in the two locations. The deck concrete is likely to be wetter, leading to lower levels of oxygen and more negative potentials. This may not necessarily indicate corrosion activity. It should be noted that although a waterproofing membrane was applied to the deck during rehabilitation of the bridge, subsequent ground penetrating radar (GPR) investigation confirmed that the waterproofing system was being slightly undermined by moisture ingress and movement particularly at the southern area of the bridge where the monitoring systems are located. A more detailed monitoring program in a drier period is needed to verify the likelihood of corrosion in the monitored areas of the deck, and to generalise the results to the whole bridge deck.

Figure 13. Location and details of monitoring box at Church Street Bridge including testing.

Table 3. Steel reinforcement potentials of Cell 21, CSE (mV)

Date Age (d) 21A Wall-lower

21D Wall-upper

21E Deck-north

21F Deck-mid

21P Deck-South

16/06/10 0 - 406 - 415 - 409 - 391 - 375

21/07/10 35 - 288 -303 - 260 - 252 - 249

14/12/11 546 - 152 - 195 - 339 - 201 - 432

14/06/12 729 -154 -211 -320 -209 -472

Table 4. Steel reinforcement potentials of Cell 27, CSE (mV)

Date Age (d) 27A Wall-lower

27D Wall-upper

27E Deck-north

27F Deck-mid

21P Deck-South

16/06/10 0 - 530 -536 -383 -490 -334

21/07/10 35 -192 -94 -245 -343 -398

14/12/11 546 -111 -111 -372 -577 -535

14/06/12 729 -109 -112 -402 -574 -512

6. Monitoring of Patch Repairs and Coating Systems at Racecourse Road Bridge – Carbonation and Moisture Effects

A total of eight embedded reference electrodes were installed in a column and deck soffit within and at the periphery of major patch repairs of Racecourse Road Bridge in 2011 (Fig. 14), due to carbonation induced corrosion of the steel reinforcement (8). The results of all the measurements are presented in Table 5 and Figure 15. All reference electrodes with the exception of R5 and R6 (columns) (refer Table 5) are embedded in the deck soffit which is susceptible to water seepage from the top, as influenced by the footpath and tram tracks. At present, the potentials of all the pairs are more negative than -350mV (CSE), which indicate that the reinforcing steel in these locations is corrosion active by the criteria of ASTM C876-99. It should be noted that this standard was withdrawn in 2008, and that the revised new standard (ASTM C876-09) does not have such criteria. R3 and R4 (in the damp area of soffit) appear to have become more negative in the past 200 days; R5 and R6 (both in repaired area of column) have become less negative, and the other locations have been more or less stable, although they are also susceptible to water ingress from the top. It is considered that the observed potential variations could be related to the different humidity conditions to which the electrodes are subjected to and at this stage the lack of maturity of the material embedding the electrodes in the concrete. More realistic results would be expected to be generated, after the repair mortar has reached equilibrium conditions with the repaired element and the surrounding environment. These early measurements should only be taken as base readings, as they cannot be interpreted at this stage with respect to the corrosion status of steel reinforcement. It is expected that the potentials will become more positive with time. Several monitoring measurements are required over a period of time, including warmer periods to clarify the trend of potential changes in the concrete, after which a more reliable assessment of the corrosion status of the reinforcement could be made.

Figure 14. Racecourse Rd Bridge- junction box; embedded reference; terminals in junction box.

Table 5. Steel reinforcement potentials at Racecourse Road Bridge, CSE (mV)

Date Age (d) R1 Soffit

R2 Soffit

R3 Soffit

R4 Column

R5 Column

R6 Soffit

R7 Soffit

R8 Soffit

15/11/2011 -455 -473 -655 -625 -387 -315 -601 -366

14/12/2011 29 -501 -489 -696 -653 -451 -424 -639 -464

14/06/2012 212 -592 -532 -900 -790 -439 -395 -522 -524

9/10/2012 329 -620 -537 -878 -781 -463 -430 -459 -494

Figure 15. Racecourse Road Bridge half-cell potentials of rebar since the installation.

7. Monitoring of Combined Repair System of Reinforced Concrete Jackets and encased galvanic anodes at Queen Street Bridge – Marine Exposure A combined repair system of reinforced concrete jackets and encased galvanic anodes was used in the four piers of Queen Street Bridge to mitigate the deterioration caused by chloride induced corrosion of the steel reinforcement (9). The reinforced concrete jacket was constructed using a self compacting concrete (SCC) of grade VR450/50 (50 MPa) with a minimum of 25% fly ash of the total cementitious content and a maximum coarse aggregate size of 14 mm. In order to further enhance the protective capability of the reinforced concrete jacket, a zinc-based galvanic corrosion protection system (Galvanode DAS with self regulating voltage and current output – first such known installation in Australia) was encapsulated within the reinforced concrete jacket (Fig. 16). The effectiveness of the cathodic protection (CP) system has been monitored by testing the potentials of the steel relative to four Ag-AgCl half-cell reference electrodes embedded in Pier 1 of the bridge. The terminals of cables to anodes, steel reinforcement and the reference electrodes are grouped in a junction box fixed on western face of Pier 1 of the bridge. No test facilities are provided for Piers 2, 3 and 4. The monitoring measurements on the CP system are checked against the CP criteria outlined in Australian Standard AS 2832.5-2002: Cathodic protection of metals Part 5: Steel in concrete structures as follows: (a) 100mV polarisation decay criterion - The criterion for the protection of a buried structure shall be to maintain an instantaneous off-potential, which is at least 100mV more negative than the depolarised potential. (b) Extended potential decay criterion - A potential decay over a maximum of 72 hours of at least 100mV

from the instant off potential subject to a continuing decay and the use of reference electrodes (not potential decay sensors) for the measurement extended beyond 24 hours.

(c) Absolute potential criterion - An instant off potential (measured between 0.1 s and 1 s after switching the DC circuit open) more negative than -720 mV with respect to Ag/AgCl/0.5 KCl. (d) Absolute passive criterion - A fully depolarised potential or a potential which is continuing to

depolarise over a maximum of 72 hour after the cathodic protection system has been switched off which is consistently less negative than -150 mV with respect to Ag/AgCl/0.5 KCl.

Figure 16. Repair system of RC jackets and encased galvanic anode and Junction box at Queen St The initial monitoring results (Table 6) following energisation showed that the CP system satisfied Criterion (a) and (b); that is “the 100 mV polarisation decay” a short period after its encapsulation within the reinforced concrete jacket, while the SCC was developing its strength and reducing its permeability. However, the results obtained in subsequent monitoring, namely some six months (Table 7) and twelve months (Table 8) after energisation did not comply with the CP criteria given above, although the S4-R4 electrode pair approached the ‘Absolute passive criterion’. It must be emphasised however, that the DAS galvanic anodes are encapsulated within high quality 25% fly ash concrete which will be characterised by high strength and low permeability and therefore a higher resistivity as the concrete continues to cure and dry out, thus preventing any moisture ingress. The fly ash component of the concrete continues to refine the microstructure of the concrete thereby reducing the amount of free water in the concrete. A good quality acrylic anticarbonation coating with some waterproofing capability has also been applied on the concrete jackets thereby further inhibiting the ingress of moisture into the concrete. The sacrificial anodes cathodic protection for steel is based on the fact that the metal used as the anode has a more negative electrode potential, the corrosion of which is thermodynamically favoured and provides electrons to satisfy the demands of oxygen reduction of the seel (corrosion reaction). The output of galvanic anodes depends on the corrosion of the anodes in the concrete. The reduction in the current output with age of re-instated concrete after installation indicates that the anode corrosion is not rapid. However, this level of current may be what the reinforcing steel needs as this stage, as the high quality fly ash concrete in combination with the protective coating are providing adequate protection to the steel reinforcement. It is expected that as the combined effectiveness of the good quality concrete and coating diminishes over time the contribution of the DAS galvanic anodes in protecting the steel reinforcement will be increased.

Table 6. Initial potential of structure against reference electrode (mV)

Location NativeStructure 5/07/11

Energised 5/07/11

OnPotential 18/07/11

InstantOff 18/07/2011

After 24hr Off 19/7/11

Polarisation Decay

S1 vs. R1 -612 -635 -667 -664 -555 109

S2 vs. R2 -478 -524 -598 -594 -395 199

S3 vs. R3 -416 -509 -603 -599 -443 156

S4 vs. R4 -322 -418 -431 -428 -284 144

Table 7. Potential of structure after six months against reference electrode (mV)

Location On Potential*

13/02/12 Instant Off (see note)

After 72-hr Off 16/02/12

After 7-day Off 20/02/12

Polarisation decay

S1 vs. R1 -448 -439 -393 -371 65

S2 vs. R2 -292 -283 -247 -245 61

S3 vs. R3 -352 -337 -288 -261 79

S4 vs. R4 -248 -241 -198 -191 57

Table 8. Potential of structure after twelve months against reference electrode (mV)

Note: Potential drop was determined individually and the value was added to ‘On Potential’ to result in ‘Instant Off’ potential.

8. Monitoring of Geopolymer Concrete Retaining Walls In order to obtain a greater understanding of the practical potential of geopolymer concrete, in 2009 VicRoads undertook a small number of trials including the in-situ construction of two landscape retaining walls at a bridge over the Yarra River (Fig. 17) (10, 11, 12). Construction of the in-situ geopolymer concrete landscape retaining walls was undertaken utilising conventional techniques for formwork construction, concrete placement by pumping, compaction with a poker vibrator, finishing and curing with polyethylene plastic. In order to monitor the long term performance of the geopolymer concrete and enable monitoring of the corrosion state of the reinforcing steel, three MnO2 half-cell reference electrodes were also installed at the centre of each of the in-situ walls adjacent to the steel reinforcement at three different levels along the height of the wall (Fig. 17). Initial measurement of the potentials of the steel reinforcement against the reference electrodes commenced a few weeks after construction in 2009, and subsequently monitored on a regular basis. The initial half-cell potentials readings after the hardening of the geopolymer concrete were very negative, namely in the order of -600 to -800 mV for upstream wall and about -1000 mV for the downstream wall, reflecting the initial quality of the two walls. The half-cell potential of the steel in concrete, however, appeared to be stabilising over the following six months after construction with the potentials having shifted to more positive values by about 200 mV, as shown by the results of the monitoring system incorporated in the walls (Fig. 18). Further measurements on the embedded reference electrodes in 2011/2012 showed that the half-cell potentials of both wing walls have become more positive since the 2010 measurements (Fig. 18), and are stabilising between -350mV and -250mV (CSE- copper sulphate electrode) which based on conventional criteria, it is unlikely that corrosion of the steel is taking place. These values may become even more positive, at least in some areas of wing walls, indicating that the corrosion risk is not significant at present. This is in agreement with results of very low penetrability to chloride ions (ASTM C1202) and very low chloride diffusion coefficient determent using the NT Build 443 test method. It should be noted however, that the VPV (volume of permeable voids) values to AS 1012.21 did not comply with the criterion of a maximum value of 16%, for structural concrete of VR400/40 grade as set out in Section 610 (5). Nevertheless, it is argued that the higher VPV is not due to larger interconnected pore volume, but due to additional loss of water from the gel-like materials included in the geopolymer. It is likely that an excess amount of sodium silicate (which releases water as part of the chemical reaction) was used in the geopolymer formulation, which was not fully assimilated into the geopolymer binder and caused the high VPV. It is considered that further refinement of the geopolymer concrete mix design with the use of compatible water reducers and superplasticisers to reduce the amount of water in the mix will significantly reduce the VPV of the geopolymer concrete.

Location On Potential* 14/06/2012

Instant Off (see note)

After 4-day Off 18/06/12

Polarisation decay

S1 vs. R1 -448 -439 -393 46

S2 vs. R2 -292 -283 -247 36

S3 vs. R3 -352 -337 -288 49

S4 vs. R4 -248 -241 -198 43

Figure 17. Finished painted geopolymer concrete wall and installed reference electrodes.

Figure 18. Monitoring of reinforcement potentials – Bridge over Yarra River west retaining walls.

9. Conclusion Corrosion monitoring sensors may be cast into new structures or installed in existing structures alone or as part of repairs, to provide the asset manager with real time information as to the current state and performance of the structure or remedial works. Monitoring sensors can provide early detection of initiation and/or propagation of corrosion and therefore facilitate early diagnostic assessment and timely and cost effective preventative maintenance intervention as opposed to expensive rehabilitation. Such early detection can also allow the asset manager to pre-plan and invest in a timely manner and thereby minimise the number of costly interventions, mobilisations and disruptions to the travelling public. The case studies presented in this paper demonstrate the value of utilising the various types of corrosion monitoring sensors to monitor the effectiveness and ongoing performance of various durability provisions incorporated in new bridge construction, as well as, the effectiveness and ongoing performance of various rehabilitation and repair techniques. It is important when undertaking measurements of monitoring sensors cast in concrete or embedded in concrete repairs to take into account any potential seasonal variations and moisture conditions and the age of the concrete when assessing and interpreting test results. In addition, it is important to keep in mind that generally the resistivity of concrete and patch repair materials increases as they continue to cure and dry out, thus resulting in more positive potentials and lower corrosion rates of the steel reinforcement.

10. Acknowledgement The authors wish to thank VicRoads for permission to publish this paper. The views expressed in this paper are those of the authors and do not necessarily reflect the views of VicRoads.

11. References

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