selecting reinforced concrete cathodic protection

SELECTING REINFORCED CONCRETE CATHODIC PROTECTION SYSTEMS J. Dyson1, F. Papworth2 and M. Marosszeky1 1 BCRC, Sydney, 2BCRC(WA), Perth SUMMARY: Although cathodic protection (CP) systems for above ground structures have been used for over 30 years in Australia many designers and contractors still consider it a complex and expensive method and do not properly consider the life cycle cost savings, better structural outcome or improved performance and repair reliability that cathodic protection can provide. This paper provides some insight into these benefits. Current codes dealing with cathodic protection of concrete above ground provide little guidance on how to select anode systems. This paper discusses an options assessment approach that has been developed to systematically compare available CP systems and identify which system will best suit a client’s overall needs. The assessment considers multiple factors in line with two key themes: cost and suitability. Cost considered initial and maintenance cost. Suitability included reliability, installation speed, customer friendliness, flexibility and aesthetics. A weighting system was given to each component. A project case study where CP systems were compared using the developed approach is included. On a recent project where large areas of reinforced concrete structural elements where prematurely corroding and required cathodic protection to achieve a future service life of 40+ years, the authors compared impressed current CP systems available for each type of structural element (slabs, columns, walls, beams) using the developed approach. Sacrificial anode CP systems were excluded from the comparison due to the long service life requirement and large extent of cathodic protection coverage. ICCP systems reviewed were all mixed metal oxide coated titanium and included ribbon mesh, net, sawtooth ribbon and discrete anodes.

Keywords: Concrete, Cathodic protection, Systems, Compare, Select, Repair. 1. INTRODUCTION Although cathodic protection (CP) systems for above ground structures have been used for over 30 years in Australia many designers and contractors involved in small to medium sized concrete repair projects consider it a complex and expensive method and give it no consideration for projects they manage. Owners of these project are often even less informed and rely on the advice of the designer or contractor who they consider the expert. Hence, on many projects no consideration is given to the life cycle cost savings, better structural outcome or improved performance and repair reliability that cathodic protection can provide. This paper provides some insight into these benefits and explains how parties not experienced in cathodic protection might consider its application. The term cathodic protection is sometimes misused and applied to any system where an anode is installed to polarise the reinforcement, regardless of the extent of that polarisation. Cathodic protection is defined in ISO 12696 (International Standards 2011) and specific requirements are listed for achievement of cathodic protection. Other terms such as cathodic prevention and corrosion control are sometimes used to describe anode systems that provide low levels of polarisation. The meanings of these terms are discussed. Current codes dealing with cathodic protection of reinforced concrete above ground provide little guidance on how to select anode systems. ISO 12696 Appendix C (International Standards 2011) and AS 2832.5 Appendix D (Standards Australia 2002) give a little information on anode types and Concrete Society TR 73 Appendix A and B (Concrete Society 2011) provides significantly more information, but none provides guidance on how to compare systems or select the most appropriate anode. Similarly, text books on corrosion of steel in concrete (Broomfield 2007; Chess & Broomfield 2014) outline some advantages of different anode systems but do not provide a systematic approach to selecting the most appropriate anode for a project. Corrosion & Prevention 2015 Paper ### - Page 1

Given this lack of available guidance, CP contractors often offer the cheapest or most profitable solution available to them, which may not be the most appropriate solution available that best meets the client’s needs. In this paper an approach is outlined which systematically identifies what will best suit the client’s overall needs. The approach is relatively simple and can be managed by a non-CP expert. However, the effectiveness of any comparison using this approach will depend on the breadth and depth of the reviewer’s knowledge of available CP systems. The approach can incorporate any anode system objectively so that the client will be sure of getting the best CP system for his project. 2. SELECTING THE REPAIR APPROACH When reinforcement corrodes and causes concrete to crack, spall or delaminate the repair designer has to determine the approach to adopt for inspection and repair. Typically the first step is to determine if the issue will only ever be localised or if it is a much wider future problem. This can be determined using visual inspection, drummy testing, electrical potential measurement and cover testing. Commonly chloride ingress, carbonation and cover are relatively uniform and hence initial distress will be followed by widespread distress. On a bridge deck in Western Australia delaminations increased from 20% to 80% in the two years between first inspections and letting a repair contract. Conversely on a LPG tank in Western Australia a few lower cover bars caused distress but cover distribution assessment identified the probability of wider future corrosion was low. Hence, testing is generally required to identify the extent of repair required. One of the most common techniques for assessing the extent of corroding reinforcement is to test the reinforcement’s electrical potential. Although taking measurements is simple, interpretation of the results requires great care. Various misinterpretations have been identified on Australian projects, for example: a)

Highly negative potentials interpreted as being due to corrosion. Concrete had been cast in permanent PVC forms before exposure for testing. No causes of corrosion were apparent so this, and the formwork type, should have led to the conclusion that low potentials were due to oxygen starvation. b) Potentials in the range +50 to -150mv CuSO4 interpreted as indicating corrosion due to high potential differentials. The results were for passive steel in uncontaminated concrete. c) Potentials in the range -300 to -350mv CuSO4 interpreted as indicating no corrosion as the potential differentials were low. The potentials were active corrosion in carbonated concrete. d) Potentials of -150mv CuSO4 on a new water tank interpreted as the tank requiring CP because potentials were approaching -200mv which would indicate a heightened corrosion risk. Potentials were in the passive range. In each case those taking the potentials took the measurements correctly but did not consider the exposures effect on anodic and cathodic polarization curves when assessing the results. Once testing has been undertaken the first step in the repair decision process can be made. Repairs to only evidently distressed concrete is only likely to be an acceptable approach if:   

Corrosion initiation is localised (e.g. low cover areas). The owner accepts the risk that additional repairs may become necessary in a relatively short time frame. This may occur if the immediate maintenance budget is limited. The structure has a short design life requirement, hence short-term holding repairs are adequate.

Where a long term repair is required and chloride ingress or carbonation is widespread then the decision becomes whether to use cathodic protection to eliminate the break out of sound but chloride contaminated/carbonated concrete or to break out around actively corroding reinforcement to remove sound but chloride contaminated/carbonated concrete (Figure 1). Breakout of sound concrete can be a noisy process disturbing tenants and lead to expensive structural support requirements. If impressed current (ICCP) or sacrificial anode (SACP) cathodic protection are used, only loose concrete needs to be removed. ICCP gives a 50+ year design life but has high cost for design and maintenance while SACP has a shorter 15-20 year life and lower design and maintenance requirements. 3. TYPES OF REPAIR SYSTEMS 3.1 Patch Repair Typically patch repairs require breakout behind the reinforcement to remove chloride contaminated concrete. With no testing for the extent of chloride activated reinforcement beyond the distressed area conventional practice is to breakout 150mm beyond the evident area of corrosion. The repair would typically include an isolation coating or zinc rich coating on the reinforcement.

Corrosion & Prevention 2015 Paper ### - Page 2

Figure 1 Typical area of concrete breakout where no cathodic protection is employed. Breakout of sound concrete behind reinforcement is highly expensive. 3.2 Patch Repair plus Incipient Anode Protection. Similar to above (3.1) but low powered sacrificial anodes are incorporated around the edge of the patch to prevent corrosion of incipient anode areas (refer Section 4). Because the anodes do not provide cathodic protection breakout and patching around the bar is still required with all the associated issues. 3.3 Patch Repair plus Local Cathodic Protection In this technique higher powered cathodic protection sacrificial anodes are embedded within the repair patches to provide local cathodic protection. This means that breaking out behind the bars is not required. They also provide incipient anode protection. The repair is still localized as the anodes only protect for a limited area around the patch. Protection can be extended by using discrete sacrificial anodes with high output. 3.4 Sacrificial Anode Cathodic Protection There are various sacrificial anodes systems (surface applied, discrete and embedded) that provide high enough polarization to give cathodic protection in accordance with CP code requirements. The zinc of such anodes is consumed much faster than the lower power output anodes used for cathodic prevention. Hence a significantly higher zinc mass is required for the same life. These systems are very simple to install as they require little monitoring. On small projects a risk analysis can be used to determine whether any monitoring is appropriate. Generally it is not as the systems are used to minimize cost and monitoring provides no great benefit. On larger projects it is likely that monitoring of a limited number of areas would be appropriate to validate the system performance. 3.5 Impressed Current Cathodic Protection. The typical repair approach where there is widespread corrosion activation is to patch the corrosion damaged areas (no breakout of sound contaminated concrete required) and apply impressed current cathodic protection (ICCP) to all actively corroding areas. The systems are expensive to design, install and maintain but because they have a long life and monitored performance they are frequently the repair system of choice for major projects. 4. CATHODIC PREVENTION Over 30 years ago it was identified that when just visibly distressed areas were patch repaired the areas around the patch failed a short time later. This is called the ‘incipient anode’ or ‘halo’ effect. To provide protection to the incipient anodes low powered sacrificial anodes were introduced. These replaced the once anodic but now passivated reinforcement as the polarizing agent and gave a degree of protection to the incipient anode. Figure 2 diagrammatically explains this process. There is some debate about the incipient anode process as another theory suggests potential incipient anodes, i.e. cathodic zones adjacent to anode area prior to patching, would generate hydroxyl ions and chloride ions would be drawn to the anode. In theory this could make the incipient anode areas quite passive but in the authors practical experience the process outlined in Figure 3 generally applies. Incipient anodes have also been shown to exist by Barkey (2004). He was able to measure the corrosion quite precisely because his bar was made up as a number of separate sections so that he could measure the current flow (Figure 2). He also developed a theoretical model that matched the experimental data. The results showed that:  The corrosion rate is controlled by the resistance between the anode and cathode and hence potential difference is not a key control mechanism. Corrosion & Prevention 2015 Paper ### - Page 3

 

The incipient anode area was quite small. Just 5-10cm long and it was quite close to the patch, 1-2cm away. This is significant because it confirms that sacrificial anodes within the patch are able to control the whole incipient anode. The majority of current flowed towards the repair. Again significant as this shows that epoxy coatings on the bar in the patch area will largely restrict the size of the cathodic area and hence corrosion rate at the incipient anode.

Figure 2 Schematic of samples made by Barkey showing the current flow from the incipient anode area.

Figure 3 The top sketch shows the potential field for an anode area surrounded by an area of slightly more positive potentials. At this stage the anode cathodically protects the surrounding area that would otherwise be anodic. The lower sketch shows the same bar after repair has passivated the previously anodic area such that the surrounding area becomes the new anode.

Cathodic prevention and cathodic protection are two very different processes used in different circumstances (see Section 3). The processes are described in Figure 4. For cathodic prevention a small shift in the potential of uncorroded reinforcement (incipient anodes) leads to an increase in the chloride activation level (A  B  C). If the anodes have sufficient power to polarize the reinforcement a little more they will provide a little slowing of the corrosion rate, sometime referred to as corrosion control, but this is not cathodic protection. The same small shift in potential for corroded rebar would not fully protect the active steel (D  E). A much higher shift (D  F) is required to give cathodic protection to corroding reinforcement.

Figure 4 Sergi’s (1999) simplification of Bertollini’s (1993) two domains of electrochemical behaviour of steel in concrete, explaining the significance of reinforcement polarisation on corrosion control/cathodic prevention and cathodic protection. 5. CATHODIC PREVENTION Cathodic protection systems supply negatively charged electrons to the reinforcement that cause a shift in electrical potential to a more negative potential. This potential shift can be measured using reference cells embedded in the concrete.


ISO 12696 (International Standards 2011) provides general information on what inspection measures should be employed and lists general repair considerations, i.e.: 

Designed, tested and installed to meet its intended life.



Monitoring using reference electrodes to show the system complies with standard potential shift criteria.



Anode current densities not to exceed design values.

The two main types of cathodic protection systems are Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection (SACP). Major differences between ICCP and SACP systems are shown in Table 1. Table 1. Major differences between ICCP and SACP SACP

ICCP

Current availability

Anode consumption rate limited

Virtually limitless.

Reinforcement polarisation

Limited (no hydrogen embrittlement risk)

Variable. Should be limited to mitigate hydrogen embrittlement, particular in prestressed concrete.

Monitoring cost

Low. Often no monitoring.

High, hence not as suitable for remote areas unless a remote monitoring system is included.

6. COMMON CP SYSTEMS SACP or galvanic anode systems typically cost more for the anodes relative to ICCP systems and have a shorter design life of 15-20 years (hence may need replacing). However, they have significantly lower design, installation and monitoring costs compared to ICCP systems. Typically SACP systems are more cost effective for shorter (≤20yr) design lives or smaller repair areas where the cost of design, wiring, control system and maintenance make ICCP system uneconomic. ICCP systems are more cost effective for longer design lives or larger repair areas. To give some perspective Roads and Maritime Services in NSW state they have many issues with bridges in NSW and ideally they would roll out ICCP systems. However, due to budgetary constraints they are instead developing SACP systems with a 15-20 year life. There are many different ICCP and SACP systems available. Types of cathodic protection systems include: 

Surface applied sacrificial anodes



Internal sacrificial anode systems.



Mixed Metal Oxide (MMO) ribbon mesh



Net MMO anode systems.



MMO Sawtooth ribbon anode systems.



Discrete/tube anodes.



Lightweight surface mounted MMO anodes.



Conductive coating ICCP systems.

7. CP SYSTEM COMPARISON The authors have developed a procedure for system comparison to assist the designer in selecting the most appropriate system for a particular project depending on the owner’s requirements. 7.1 Evaluation Process The evaluation process has four steps: 1.

Establish owner preferences. The designer determines the owners’ values in regards to reliability, speed of installation, intrusiveness during installation, flexibility and aesthetics.

2.

Review CP systems. A five point scale is used to assess each of the acceptance criteria and a weighting applied that is specific to the project (Table 2). Corrosion & Prevention 2015 Paper ### - Page 5

3.

Indicative pricing. The systems that best meet the client preferences are then priced to give a dual (suitability and cost) assessment (Table 3).

4.

Specification. The systems that are both economic and meet the client needs are incorporated into the CP specification so that they can be priced by the market.

A ranking and weighting approach is applied to the acceptance criteria. This is designed to determine how well the system meets the building owner’s requirements for the structure. These criteria fall into two broad categories – suitability and cost. 7.2 Suitability Factors Suitability factors are assessed first and any anodes not meeting the owner’s requirements are discarded. Remaining systems are then assessed based on both suitability and cost factors. Reliability - Systems that have inbuilt resistance to acid generation get a high score. Mesh and Advanced Discrete Anode score reasonably high due to their robustness and proven performance. Ribbon anode reliability would be improved if acid proof grout were used. Reliability has a high weighting as owners will be opposed to systems that are disruptive and costly in the future. Install Speed - Owners prefer to have work completed quickly. However this has low weighting compared to reliability. Customer Friendly - Noise and dust are the primary consideration. This has low weighting compared to reliability. Flexibility - Flexibility is of principle interest in determining the ease to which a system can be adapted to a situation. For walls with global application this has a low weighting. For soffits where systems may need to follow cracks it has a higher ranking. Aesthetics – Weighting depends on the type of structure, context and owners preference. 7.3 Cost Factors It can be tempting for owners to focus predominantly on initial cost particularly when available budget is limited, but the approach also considers ongoing maintenance cost to enable whole of life (life cycle) cost to be assessed. Initial Cost - The cheapness of each system is calculated. Because the ranking is based on broad cost bands effects of minor inaccuracies are removed and, if there were no significant difference in cost between anode systems, they would all rank the same, i.e. the ranking is based on a cost scale rather than a relative scale. This is given a high weighting as initial cost has higher significance than maintenance costs that are sometime in the future. Maintenance Cost - This is a broad classification of the estimate of maintenance cost. Hence there are only minor variations associated with possibility of needing to replace anodes. If galvanic anodes need replacing at short intervals the maintenance cost might be higher. As ICCP systems require frequent inspection none of the systems has a low maintenance cost. The aim of this CP system comparison method is not to enable a final CP system decision based solely on the weighted cost and suitability scores but rather to act as an informative guide to assist owners/stakeholders in making a decision. The approach is flexible and adaptable in that the weightings of each assessment criterion can be altered according to the unique preferences of each project owner. Also, the weighted score of each assessed factor can be reviewed individually as well as the combined weighted scores for suitability and cost. This can be seen in the project case study (Section 8). Where anode systems are shown to be positive (high score) or negative (low score) on the weighted cost or suitability scores, the reason behind that score should be reviewed to ensure the scores give a reasonable impression of the anode acceptance. 8. PROJECT CASE STUDY On a recent project where large areas of reinforced concrete structural elements required cathodic protection to achieve a future service life of 40-plus years, the authors compared ICCP systems available for each type of structural element (slabs, columns, walls, beams) using the system described in Section 7. SACP systems were excluded from the comparison due to the long service life requirement and large extent of cathodic protection coverage. 8.1 Weighting Factor To combine the various acceptance criteria a weighting system is used to describe the relative importance of each item. The weighting is developed for each project, and possibly each element, from the discussions with the owners. The weighting for the case study is given in Table 2.


8.2 ICCP Anode Systems There are a multitude of anode systems available but the number readily available in Australia is limited. Principle systems being marketed were compared for the project. All anodes were Mix Metal Oxide (MMO) coated titanium. This is the standard material used in CP systems but there are limited manufacturers that produce high quality products. The anodes are described below. Sawtooth ribbon - This solid ribbon is bent to create ridges and troughs at close centres. It can be installed in small saw cut slots. It could have been suitable for all locations on the project as it is reliable and has low cost per linear meter due to material and installation methods. The drawback is the number of slots and hence high lineal meterage required. Table 2. Acceptance criteria weighting system Item

Acceptance Criteria

Weighting

A

Initial Cheapness

10

B

Maintenance Cheapness

5

C

Reliability

7

D

Initial Speed Application

3

E

Customer Friendliness

4

F

Flexibility

3

G

Aesthetics

3

Ribbon mesh - This is a fine mesh to create a high surface area. Like saw tooth ribbon it is grouted into slots in the concrete. The slot can be a saw cut but sometimes the depth would be too great and it is cast in a reamed slot. The ribbon is highly reliable in atmospheric exposures. Where installed in splash or tidal zones grout resistivity variations along the anode line can lead to localized current dumping. This can lead to acidification locally and failure. Expanded mesh or net – Mesh can be applied over the entire concrete surface. The surface area is less critical in this case and so the net grillage can be more open. This more open nature is important as after pining the net to the concrete surface it is covered with a mortar to make the ionic connection to the concrete substrate. The systems are highly reliable except that application of the mortar layer can be difficult in splash and tidal zones and always requires a high level of quality control. Discrete / Tube anodes – These anodes are generally lengths of ribbon mesh shaped to go into a hole drilled in the concrete. Tubes and star shapes provide high surface areas and this minimizes the number of anodes required to achieve a set level of current output. As the anodes are buried in the concrete they are reliable but in wet areas methods to prevent current dumping still need to be considered. Surface Anode 1 - This is essentially a surface mounted ribbon mesh. The mesh is enclosed in a totally corrosion resistant tray with concrete contact via a felt pad – called a “cassette”. It has a high speed of installation so can be highly cost effective. Being acid resistant it can be reliably used in wet areas. Its drawback is that it is cannot be made to look attractive and hence is generally only used in basements, bridges, tunnels, wharves and industrial structures Surface Anode 2 - This is a surface mounted mesh anode in an acid resistant precast mortar disc. It is suited to all exposure zones as its output is high, it is simply installed and is reliable. Aesthetically it is rated lower than fully embedded systems but the anodes can be applied in a pattern to make the overall appearance suitable for most structures. Advanced Discrete Anode - This is a small diameter tube anode system that is simple to install due to the small diameter hole required. It is also a well-developed system in terms of control (inbuilt resistor), installation (grouting system) and reliability (proprietary) connection system. 8.3 Analysis for Elements to be Protected The results and conclusions from the comparison for each element are described below. Costs are ranked for an element type based on broad classifications of cheapness. To give a scoring system where a high number is a positive, low cost is considered as highly cheap and would get a ranking of 5. Table 3a shows the ranking system as developed for the columns. The cost bands are not fixed but are developed to cover the range of costs expected for each element type. The ranking method in Table 3b is applied to cost (cheapness) and suitability factors. A high number for both factors is positive.


Table 3. Ranking system a)

Cheapness Class

b)

Ranking Method

Limit ($/m)

Classification

Classification

Ranking