|
An important aspect in the use of cathodic protection of reinforced concrete is the need for regular monitoring of performance and maintenance of the anode and electrical systems. This is to ensure protection and to prolong the life of the anode system. The power supply should be checked regularly to ensure that ac power is being supplied to the transformer-rectifier, and that dc current is being passed to each anode zone.
Performance Monitoring
Performance monitoring is typically undertaken at intervals of 3 to 6 months, to include the recording of zone current and voltage and potential steel/concrete. Where necessary, adjustments are made to ensure that protection is provided at the optimum level. Usually every 12 months, this monitoring is extended to a detailed visual inspection of the electrical system and the conductive coating anode to confirm their integrity and to identify any maintenance requirements.
The Conductive Coating Anode
The conductive coating anode is one of the most widely used and established anode systems for cathodic protection to reinforced concrete structures. This type of anode system has been successfully applied world-wide on a variety of structures including motorway viaducts, carparks, commercial and industrial buildings. It is cost effective and relatively easy to maintain. It is estimated that in the UK alone in excess of 150,000 square metres of reinforced concrete have been protected using conductive coating anode based cathodic protection systems.
The rate of deterioration of this type of anode is directly related to the current density. Acidic products are evolved at the interface between the coating and the concrete surface and will attack either the cement paste or the coating, leading to flaking and loss of bond. However, these effects can be significantly reduced by a proper design and zoning of the system and operation at current density levels sufficient to control corrosion, but no more, i.e. not over-protected.
Operating Criteria
It is recommended that commissioning of CP systems should start at a low level of current density for the initial part of the commissioning period to avoid any adverse effects on the anode durability. Intermediate tests can be then carried out, typically 14 days after energising, to determine what adjustments are needed to satisfy the operating criteria. Various criteria for ensuring satisfactory protection of the reinforcement have been considered for reinforced structures. The draft European standard for the cathodic protection of steel in atmospherically exposed concrete (pr EN 12961-1) proposes the following:
“For any atmospherically exposed structure, any representative point shall meet any one of the criteria given in items a) to (c.
a. An instant-off potential (measured between 0.1s and 1s after switching the dc circuit open) more negative than - 720mV with respect to Ag/AgCl/0.5MKCl.
b. A potential decay over a maximum of 24h of at least 100mV from instant-off.
c. A potential decay over an extended period (typically 24h or longer) of at least 150mV from the instant-off subject to a continuing decay and the use of reference electrodes (not potential decay sensors) for the measurement extended beyond 24 hours.”
A virtue of b) or c) is that the decay is independent of the nature of the reference electrode used or any variation in its absolute potential in the long term.
Historically it has been common to use a decay period of 4 hours although it should be noted that other intervals (such as 24 hours) might be suitable depending on the nature of the structure and surrounding environment.
When current is applied to the anode system there will be an associated error in potential measured between the reference electrode and the steel termed the IR drop. The current must therefore first be switched off to obtain the true fully polarised potential of the steel, termed the instant-off potential. However, the steel will depolarise shortly after with a characteristic exponential decay curve as illustrated in Figure 1.
|
|
|
Figure 1. Potential decay curve.
|
In order to obtain an accurate measurement of decay, the instant-off potential must be measured within a very short time window, typically 0.1 to 0.4 seconds after the current is switched off. The reinforcement will continue to depolarise and the potential decay is measured after a period of 4 hours (or other suitable intervals) from switch off. The accurate measurement of the instant-off potential is essential to determine the value of potential decay.
A further requirement of the operating criteria is that no instant-off potential more negative than -1100mV with respect to Ag/AgCl/0.5MKCl is permitted, to avoid the possibility of hydrogen evolution which may cause embrittlement of sensitive steels.
However, it is most unlikely that these levels of potential will be reached in a properly operated system.
The performance monitoring should thus be designed to ensure that the steel is polarised to the correct level of potential to achieve protection, but not excessively so.
Monitoring of Condition and Performance
The following routine investigations and monitoring are normally carried out:
• Visual check of each transformer-rectifier (or power supply)
• Measurement of current and voltage for each anode zone
• Visual check of the system from accessible locations (typically on a monthly basis)
• Potential decay at embedded reference electrodes (typically on three or six monthly basis depending on the stability of the system)
• Potential decay at embedded reference electrodes and paint free reference windows (if used) and a detailed close visual inspection (typically annually)
The surveys and results are recorded on proforma suitably designed for each task. Each task can either be associated with a transformer-rectifier, a particular structure or an anode zone. Each proforma acts as a checklist for the inspector so that important aspects of the repetitive task concerned are not overlooked.
Regular Visual Inspections
For installations that are exposed to harsh environments and/or vandalism such as those fitted to urban motorway structures it may be necessary, typically at monthly intervals, to perform the following inspections and tests and record their data on the proforma as mentioned above:
a. Visual examination and condition assessment of the transformer-rectifier (internal and external) including maintenance checks of: heater, thermostat, lighting, (where applicable) supply, timers, circuit breakers, supply points, general cabinet condition and security. Measurement of output current and voltage of each zone to confirm that each unit is operating correctly. The temperature and general weather conditions are recorded using keywords such as sunny, overcast, cloudy.
b. Visual examination and condition assessment from a point of easy access (i.e without special access provision) of the CP installation including the cable trays, conduit, junction boxes (where applicable), the primary anode conductor and the conductive coating comprising the anode system for evidence of deterioration, corrosion or damage. Any leakage affecting the structure is also recorded.
The condition assessment is normally made by means of a coding system. For example:
|
|
|
0
|
Not relevant
|
|
1
|
Good
|
|
2
|
Fair (but functional)
|
|
3
|
Poor (but functional)
|
|
4
|
Requires maintenance (dysfunctional)
|
There should be provision on the proforma sheets to indicate where urgent maintenance action is required.
Occasional Monitoring
Typically at 3 or 6 monthly intervals (in some cases at 12 monthly intervals) depending on the stability of the system, the following tasks are performed in addition to the regular monitoring (above).
• Potential measurements at embedded reference electrodes (namely "current on", "instant off" and potential decay to 4 or 24 hours and longer periods if required);
• Adjustments to the applied currents, where necessary, are performed to ensure that a minimum current, consistent with ensuring adequate protection, is applied. The decision process for the current adjustments is as follows:
• A reduction in current where "instant off" potentials are more negative than -1000 mV with respect to Ag/AgCl/0.5MKCl reference electrode;
• A reduction in current where the current density is greater than 20mA/sq.m of concrete surface; (the maximum for long term operation of conductive coating anode systems);
• A reduction in current where the potential decay is significantly greater than 100mV (see note below);
• Where compatible with item i) and ii) above, an increase in current where the potential decay is less than 100mV (see note below).
• The alterations to the current at any particular occasion are normally limited to a maximum of 20% of the value before adjustment.
• NOTE: Items (iii) and (iv) only give a general guidance on the level of adjustments required. Other factors should be also considered such as potential readings at window locations, history and future anticipated performance of the system, present and future forecast weather and temperature.
• Zone output currents and voltages and “current on” steel/concrete potentials are re-measured and recorded following adjustments.
Major Inspection and Monitoring
In addition to the detailed inspection and monitoring undertaken, outlined above, the following tasks are performed typically on an annual basis:
• Detailed close visual inspection of the structure for evidence of deterioration or damage on the items listed in “Regular Visual Inspection” above (normally requiring access provision). Particular attention is given to areas of delamination, spalling, cracking, paint loss/flaking, rust spots/staining/streaking and other visual defects. Any areas of delamination and/or spalling are marked, measured and recorded;
• Potential measurements (namely “current on”, “instant off” and “decay” at all paint free reference windows in the paint anode system);
• The data from the monitoring is analysed and the performance of the CP system (for the whole year) is evaluated to identify required actions;
• The inspection record forms are examined and any maintenance requirements identified.
Maintenance
Any faults or defects identified in the cathodic protection or monitoring system as a result of the monitoring and/or inspections described above are repaired within a time scale consistent with the degree of urgency/condition assessment recorded in “Regular Visual Inspection” (mostly within the routine maintenance requirements for the structure).
A typical conductive coating anode system comprises primary anode conductors (wires or strips of coated titanium or niobium) and a conductive coating often overcoated with a compatible protective/cosmetic top coat.
Evidence from extensive site trials and full scale installations have demonstrated that major maintenance to the anode system which would involve large repair areas is not expected until at least 10 years operation. Minor repairs have been carried out earlier. However, in most cases to date, the defects have been attributable to outside factors (e.g. intense local wetting and/or ponding) or to isolated corroded metallic objects (tie wires or similar “rogue” steel from the construction phase) in the concrete surface, rather than a lack of performance or failure of the anode system.
Briefly the maintenance repair to the damaged concrete (from the effects of “rogue” steel) and anode system can be carried out as follows:
• Concrete and conductive coating anode damage due to "tie wires"
• Any remaining metal and existing damaged concrete are removed and repaired using appropriate repair mortar. In most cases the depth of concrete breakout necessary for this type of repair is less than 10 mm. The repairs are then allowed to cure and dry before the conductive anode paint is applied.
• Conductive anode coating and top coat deterioration
• Prior to coating application the new concrete areas and other areas where there is paint deterioration are normally prepared by local abrasion of the concrete surface using hand or power wire brush or grit blasting as appropriate to the area being repaired. Sufficient surface preparation is necessary to achieve a sound, clean, dust free surface to ensure a suitable base for a good physical bond for the conductive paint. In order to achieve sufficient electrical continuity between the existing and new conductive coating anode, the abrasive cleaning must be extended to create a sufficient overlap with the adjacent sound coating by removing a minimum of 50 mm of any decorative top coat around each repair area. Alternatively, the repaired area should be in sufficient electrical contact with existing or additional primary anode conductors.
• Following the curing of the conductive coating anode, the conductivity between the new and the surrounding existing conductive paint anode is tested at appropriate locations. Following the confirmation of sufficient electrical conductivity the decorative / protective topcoat is then applied.
• Primary anode conductors
• Usually replacement of the primary anode conductor wire or strips is necessitated as a result of mechanical damage rather than a failure of the anode conductor itself. Where replacement lengths of anode conductor are required to replace or bridge broken conductors they should be overlapped by a length of at least 500mm each side of the break. Each overlap is normally crimped or spot welded in 3 places at centres between 100mm and 150 mm. Each crimp connection is covered by suitable adhesive lined heat shrink sleeving.
Remote Monitoring and Control
It is a common practice to utilise remote electronic monitoring and control systems which are now available at an economic cost. Data acquired by an automated monitoring system, which is permanently installed on the structure, may be accessed remotely using an office-based computer. Remote monitoring offers significant benefits over manual monitoring including improved quality of data and reduced operating costs. In particular the reduced need for site access to the transformer-rectifier control cabinet allows a high elevation installation, which minimises the risk of vandalism or unauthorised interference.
Automated Monitoring
Introducing automated monitoring increases the opportunity to extend the scope of performance testing. For example, the full characteristic decay curve can be obtained for any time scale required, (4-24-72 hours) at all reference electrodes and can be carried out by the corrosion specialist from a remote office.
This superior data can be assessed and the cathodic protection system adjusted remotely, as necessary, from the specialist’s office.
System Requirements
There is currently no formally agreed document for the specification of remote monitoring and control systems although the draft European Standard pr EN 12961-2 does outline their requirements. The advice of the CP specialist should be sought to provide a detailed specification to ensure that the system fits the client’s and the structure’s requirements. As a general guidance the following should be complied with:
• The system should be capable of interrupting the power supply to each anode zone and acquiring instant-off data for all reference electrodes associated with the anode zone. Where a particular anode zone is likely to influence an adjacent anode zone, then the power supply to both zones must be simultaneously interrupted to acquire instant-off data.
• The system should be capable of monitoring the full potential depolarisation curve for each reference electrode and acquire data for up to 72 hours at operator specified intervals.
• The system should comprise modular data acquisition units suitable for permanent installation local to the structure and capable of independent operation.
• The data acquisition units should be capable of monitoring the required range of sensors at an appropriate level of accuracy.
• The system should enable the time and frequency of data acquisition to be operator specified and have sufficient temporary data storage capacity such that no data would be overwritten or lost.
• The system should have a degree of sophistication which will allow local area network communications to be established. Remote access from an office based computer to the LAN should be achieved via the telecommunications network using an industry standard modem.
• The system should be capable of fault diagnostics and reporting of equipment and communications failures.
• The system should enable the operator to set operational limits for all monitored sensors and be capable of alarm reporting of all monitored channels operating outside the set limits.
• The system should provide privileged access to all levels of system operation and allow access on entry of a valid operator name and associated password. The system should be capable of reporting unauthorised attempted access.
• The operation of the system should be “operator friendly”.
• The system software should provide graphical representation of data obtained locally or remotely at the time of retrieval and also of historical data.
• The system should be capable of storing acquired data as standardized ASCII text in an operator defined format to enable data transfer to a spreadsheet programme.
• The system should be capable of adjusting the dc constant current power output to the cathodic protection zones and to provide a limiting voltage or limiting current control.
Bibliography
CEN Standard, Final Draft pr EN 12696- 1 , Cathodic Protection of Steel in Concrete, Part 1: Atmospherically Exposed Concrete
Gower, MR and Beamish, SW, “Cathodic Protection on the Midland Links Viaducts”. Construction Repair, July/August 1995.
Roberts, MB, “Remote Monitoring and Control for the Maintenance and Repair Practitioner”. Construction Repair, March/April 1996.
Gower, MR and El-Belbol SMT, “Cathodic Protection of Reinforced Concrete - Which Anode?” International Congress : Concrete in the Service of Mankind, Dundee 1996.
Gower and El-Belbol SMT, Cathodic Protection on the Midland Links Motorways, Monitoring and Maintenance, “Cathodic Protection of Reinforced Concrete: The Present and the Future”. One-day Seminar, Aston University, 1997.
|