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CORROSION PROTECTION OF REINFORCED

CONCRETE STRUCTURES IN REMOTE

LOCATIONS: CAPE DON LIGHTHOUSE, NT

A Bird1, M McLean2 & I Godson2

Marine & Civil Maintenance Pty Ltd1 Infracorr Consulting Pty Ltd2

Cape Don Lighthouse is located at the tip of the Cobourg Peninsula in the Garig Gunak Barlu National

Park, Northern Territory. It is 170km north-east of Darwin and accessible only by sea or by air. The 28m

high, reinforced concrete tower was built in 1916 and has suffered from corrosion of the reinforcing steel

for many years. Restoration works were called for in 2012 to extend the working life of the lighthouse

for a further 50 years under a design-and-construct contract that included extensive concrete repairs,

corrosion protection of the reinforcing steel and protective coatings.

The remote location required the amount of future maintenance to be minimised. A Hybrid corrosion

protection system was therefore selected for the reinforced concrete elements. This technology offers

many of the advantages of traditional impressed-current cathodic protection (ICCP), including corrosion

control and reduced concrete removal, without the high cost and maintenance of power supplies, cables

and electronic control systems. It utilizes zinc alloy anodes grouted into drilled holes, with the anodes

initially powered with direct current and subsequently operating in galvanic mode.

This paper is a case study of the design philosophy, installation and operation of a hybrid corrosion

protection system, including concrete repairs and coatings, to protect the reinforced concrete elements of

a heritage structure in a remote location. The site works, including a novel access system, are described,

with reference to the technical and practical aspects of working in a remote and exposed tropical location.

1. INTRODUCTION

1.1 History

The Cape Don Lighthouse has been in operation since 15th September 1917. It has served the northern Australian coastline

well and stands as a monument to that marvellous period referred to as the “Golden Age of Australian Lighthouses.”

Construction of the original structure was a great challenge, not least because of its remote, tropical location and the difficulties

of transporting supplies and materials to the site. Work could only be conducted in the dry season and many of the

construction materials were shipped from Melbourne to a purpose-built jetty and then hauled along a 3 mile rail line by men

and horses. It took three years to build the lighthouse, which remained the tallest solid structure in the Northern Territory until

1973 [1].

The lighthouse was de-manned and automated in 1983 but continues to operate to this day.

It has withstood nearly a century of assault by extreme local climatic conditions, including cyclones and at least one severe

earthquake (at time of construction). It is now registered as a heritage structure.

During its long life, however, the concrete structure has lost durability as a result of corrosion of its reinforcing steel. This has

caused delamination and spalling of isolated patches of concrete on the surface of the barrel and of extensive areas on the

octagonal base. In late 2012, after a number of condition surveys, the Australian Maritime Safety Authority (AMSA) awarded

Marine & Civil Maintenance Pty Ltd (MCM) a design-and construct contract to repair and protect the above-ground sections

of the reinforced concrete base, barrel and balcony. MCM appointed Infracorr Consulting Pty Ltd to design and monitor the

system.

Details of the original construction are in the archives of The Institution of Civil Engineers (UK) [1], or from AMSA.

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1.2 The Structure

The lighthouse consists of a reinforced concrete tower 28m high, including an octagonal base, an18m cylindrical barrel and a

reinforced concrete balcony just below the lantern. The barrel has a wall thickness of 380mm, and the base is approximately

6m wide and 8.5m tall with a tapered upper section.

The reinforcing steel consists of round bar, generally12 and 16mm in diameter. A single (outer) layer of structural reinforcing

extends from the base to the parapet. In the octagonal base and balcony walls, a layer of light expanded steel mesh was used to

resist cracking in the outer cover zone. This mesh was transferred to the inside face in the barrel. Refer Fig 1.

Fig 1: Original Drawing of the Lighthouse

The concrete barrel was built in lifts of approximately 1.0m in height. Historical data [1] states the water used for concreting

was sourced from a local well stretching 46 feet into the ground, and was a “very good sample of potable water with no traces

of magnesium or sulphur compounds present”. The cement used was imported Portland, tested to British Standards at the

time. The mix design was 1cement: 1.5sand: 3aggregate; the latter included what were referred to as large “plums.”

During investigations commissioned by AMSA, carbonation was found to be between 20 and 60mm; cover to the reinforcing

steel varied from 50 to 100mm. Chloride testing indicated an increasing concentration of chlorides with depth, suggesting

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contamination in the original construction. Corrosion was thought likely to have been initiated in several locations, notably in

the mesh of the octagonal base, which averaged 75mm in cover. Testing for alkali-aggregate reaction found moderately active

aggregates but little evidence of this deterioration mechanism was identified on site. The reinforcing steel was found to be

discontinuous in places, mainly associated with the panels of expanded mesh on the base and inner face of the barrel.

On establishment on site, the concrete generally appeared to be porous and of low strength, and spalling and delamination were

found on almost 100% of the octagonal base surfaces and on 25% of the barrel.

2. DESIGN CONCEPTS

2.1 Repairs

The octagonal base was deteriorated to the extent that its surface was delaminated almost completely. It was therefore decided

that it would be repaired by removal of 100mm of concrete (and the underlying mesh) over the entire surface (Fig. 2).

The barrel and parapet was in better condition and defective areas would be repaired as individual patches.

Given all repairs fell within areas subsequently receiving a corrosion protection system, it was only necessary to remove the

delaminated concrete, meaning that it was generally not necessary to break out sound concrete behind the reinforcement, or to

extend the repair area until non-rusting steel was exposed, as would be required in normal patch repairs. Other standard

requirements, such as prevention of feather edges to repairs, were maintained, and no mortar placement was to take place

during ambient temperatures in excess of 35°C.

Reinforcement was to be cleaned of any loose scale or rust product, and existing reinforcement exhibiting loss in excess of

15% of its original diameter was to be repaired by welding supplementary bars to the existing bar with a single-lap splice weld.

A curing compound, compatible with both cathodic protection and the subsequent coating, was required for all repairs.

Fig 2: Cross section of staged concrete repairs to octagonal base

2.2 Hybrid Cathodic Protection

For reasons of low maintenance, the tropical climate and the remote location of the site, it was decided that a hybrid anode

cathodic protection system would be the most appropriate method of corrosion protection for this lighthouse.

Hybrid anode cathodic protection [2] utilizes zinc alloy anodes installed into drilled holes within the concrete. The anodes are

embedded in a proprietary calcium hydroxide based mortar that is formulated to ensure sufficient porosity to allow the zinc

anode corrosion products to be contained without exerting bursting forces. The anodes are connected via titanium wire to a

small junction box on the structure. A portable DC transformer rectifier provides current to the anodes for a period generally

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in the order of 1-2 weeks, polarizing the steel and inhibiting the corrosion process. The impressed current and all main cables

are then removed and the anodes take over in galvanic (sacrificial) mode, producing a small current to maintain the protection

of the reinforcement for the design life of the system; in this case 50 years [3].

Factors affecting the life of the zinc anodes that were considered in the design included:

Impressed current consumption of the anode

Average temperature of the anode system

Galvanic current consumption of the anode

Reinforcement surface area

Concrete resistivity, chloride content

The total concrete surface area to be protected was approximately 275m2, including the outer faces of the base, the barrel and

the balcony.

The lighthouse was divided into 8 “zones”, with locally installed junction boxes placed periodically inside the lighthouse to

allow the anodes’ titanium wires, the negative (reinforcement) connections and the reference electrode connections to be

terminated. The general anode arrangement for the hybrid protection design is summarised as follows.

For the octagonal base and barrel walls, 110mm long anodes were selected. Spacings of 350mm horizontally and 350mm

vertically were designed for this section, with the anodes placed in 180mm deep holes, providing 30mm cover to the anode on

the outside surface.

In the parapet floor 50mm anodes were used in two circular panels of 38 anodes, equally spaced as shown in Figs. 3 and 4.

Fig 3: Anode layout of the parapet floor. Fig 4: Detail of anode layout of the parapet floor showing

radial spacing.

The parapet walls required 50mm long anodes, at 320mm centres both laterally and vertically.

Panels of up to 40 anodes were grouped into sub-zones for the impressed current phase. The anodes required 9V temporary

power for a predetermined amount of time, before being switched to galvanic mode once the performance criteria were met.

The main criteria for the successful installation of a hybrid system vary from those of ICCP systems and accordingly the

performance criteria from the concrete CP standard (AS 2832.5) are not applicable. The criteria applied to this system were:

a) Minimum 7 day impressed current (varied subject to applied voltage) with minimum charge passed of 50KC/m2

steel (≈14 Amp.hour/m2 steel) This minimum charge to produce steel passivity in chloride contents up to 4% by wt

cement has been established by research. [4]

b) Corrosion rate of less than 2 mA/m2 steel (equivalent to a section loss of 2mm every 1000 years), measured in

accordance with ISO 12696:2012 [5]

Other possible criteria include:

c) Absolute passive criterion of a depolarized potential of > -150mV (AgAgCl 0.5M).

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Cabling from the anode (positive) and the steel (negative) connections were to be routed to junction boxes at easily accessible

locations, placed inside the barrel wall. A total of 8 negatives were required in equal distances up the barrel, and 6 were

required in the base structure.

2.2.1 Hybrid CP Design

The design process of the Hybrid CP system requires the calculation of the surface area of the reinforcement to be protected

and the estimation of the consumption of the zinc anodes in both the short term impressed current phase and the long term

galvanic phase. The calculations below illustrate the life of the hybrid anodes for the barrel section of the lighthouse.

The barrel section of the lighthouse is relatively lightly reinforced, with the outer reinforcing bars and the inner expanded mesh

totalling less than 0.8m2 per m2 concrete surface area. Allowing a safety factor of 15%, the applicable reinforcement density

totals 0.92 m2 per m2 concrete surface area.

Each 110mm long x 17mm diameter zinc anode has a total electric charge capacity of 125 Amp.hour. With a 350mm anode

spacing in both directions, there are over 8 anodes per m2 of concrete surface area, providing a total anode electric charge

capacity of 1000 Amp.hour. The impressed current phase consumes zinc proportional to the current passed and while the

minimum charge is 14 Amp.hour, a typical maximum is 70 Amp.hour. Accordingly, 930 Amp.hour are available after the

impressed current phase for the long term galvanic phase.

The average maximum galvanic current is less than 1mA/m2 steel for passive steel. Over a 50 year design life, this equates to

approximately 400 Amp.hour of the 930 Amp.hour available, allowing a significant contingency.

2.3 Coatings

A surface coating system was required for the external tower walls in order to unify the tower appearance and limit the ingress

of aggressive species. The choice of colour and texture required site trials in order to find the best match with the existing.

For surface preparation all existing coatings needed to be removed prior to application. The substrate was to be free of dirt,

dust, grease, oil, mould, release agents, bond breakers, laitance and any other contaminants that may interfere with adhesion.

Application was to be performed in two coats using airless spray, roller or brush. Coatings could not applied under the

following conditions:

Within 14 days of placement of repair mortar

When the relative humidity was higher than 85%.

When air or surface temperature was below 5°C.

When surface temperature was 35°C or greater.

When there was the likelihood of an unfavourable change in weather conditions within two hours after coating.

When there was a deposition of moisture in the form of rain, dew, condensation, frost etc. on the surface.

During fog, wind, dust, air pollution or other conditions unfavourable for application of coating.

3. SITE WORKS

3.1 Planning

Although the lighthouse is located in a National Park, the facility included disused accommodation for fishing tour groups, a

rock jetty for barges and an airstrip some 8km from the site. These facilities proved to be very helpful for accommodating the

site team and assisting in the storage and transport of plant and equipment (Fig 5).

It was determined at the outset that the work would need to be carried out in the months of May to October, in order to avoid

the cyclone season. The contract was awarded in December 2012, so there was an opportunity to use the next four months

prior to establishing the site by planning every operation in detail. Due to the remote location, anything overlooked had an

increased potential to cause difficulties. The project team spent considerable time in identifying every item of plant, materials,

spare parts and consumables that would be needed to carry out the work and support the site team, working on the basis that

only one delivery by sea would be made. Thereafter, deliveries would be by air and could be small items only.

During this preliminary phase, the design was completed and the site utilities and living quarters were refurbished to a suitable

standard to accommodate the site workers, including an upgrade of the electrical, plumbing and catering services.

At the end of April 2013, all the site plant and materials, including some 15,000 litres of fuel for the site generators and crane,

were brought to site by barge in a 12-hour trip out of Darwin (refer Fig 6).

The running of the remote site was also an exercise in logistics. The fifteen site staff were rostered on a FIFO basis, with

weekly shuttle flights operated from Darwin to rotate the crews every three weeks and bring in food and sundry materials. A

full-time cook was employed, and for leisure activities satellite televisions, a fishing boat and other amenities were provided.

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Fig 5: General view of site

Fig 6: Landing materials at the Jetty

3.2 Access to the Structure

As it was necessary to gain access to all the external surfaces of the lighthouse, its height presented some challenges. Rather

than enclosing the full structure in bulky scaffolding, it was decided to use rope access to work on the barrel, and at the same

time to use a 7.2m high mobile scaffolding to work on the octagonal base.

Specialist abseiling subcontractor Townview Australia Pty Ltd was engaged to develop a method of carrying out the repair,

anode installation and painting works on the barrel. A novel rope-supported platform, which was raised and lowered from the

tower balcony, was developed and purpose-built. This provided the independently-roped technicians with a stable and

contained access to the work, as well as a catch-screen to protect the workers below.

For additional safety, the rope technicians worked on one side of the barrel while the base crew worked on the other.

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Fig 7: Access systems including rope supported platform and scaffold tower

3.3 Concrete Repairs

The octagonal base required 120m2 of concrete repair; the barrel required 65m2 and the balcony 2m2.

To minimise potential stress on the structure the base was repaired in two vertical non-adjacent faces at a time. The base was

completely stripped of all its cover concrete to a depth of around 80-120mm, permitting the removal of all the corroding light-

gauge mesh that lay near the surface. After installation and charging of the anodes (refer 3.4 below), the entire octagonal base

was formed up and poured to the original profile in lifts of about 1.2m using prefabricated steel place formwork bolted into the

substrate with removable bolts (refer Fig 8). A telehandler was used to lift the steel formwork into position.

A drummy survey was carried out on the barrel and balcony elements, and it was found that about 30% of the surface was

delaminated. The concrete was broken out using electric jackhammers and the breakout locations were formed and poured

with the aid of ratchet straps which circumvented the barrel, securing the form ply to the existing curve.

A high-strength, shrinkage-compensated, bagged microconcrete was used as the repair mortar.

3.4 Cathodic Protection Installation

Electrical continuity was found to be insufficient in the reinforcing steel generally and continuity establishment was required in

the base, barrel and parapet before the anode system could be installed. In the base, this required deep chases to expose the

rebar, in parts as much as 200mm deep. Once each panel had been broken out, a horizontal chase was cut in the concrete with

angle grinders and broken out to expose the steel, which was made continuous with a 6mm rod welded across all the exposed

bars. The chase was backfilled during the pouring of the panel repair.

On the outside of the barrel a vertical chase 120mm deep was cut the full height of the tower to pick up the outer horizontal

rebar. Three equally-spaced horizontal chases, 120mm deep, picked up the vertically oriented rebar. After welding the 6mm

continuity rods in place, the chases were backfilled with a polymer-modified, shrinkage compensated repair mortar.

On the inside of the barrel, the embedded reinforcement consisted of light-gauge steel mesh. In order to establish continuity to

this mesh a vertical chase was broken out on the inside of the barrel, using the internal stairway for access. The exposed laps

in the mesh were welded together.

Once continuity was established, 30mm diameter holes were drilled into the surface of the structure. A minimum clearance of

20mm was required between the anode and the reinforcing steel to ensure that no electrical shorts could occur. A 400mm

down-hole electronic probe was used in the anode holes to check the cover to the nearest reinforcement, and the holes were

approved on this basis or re-drilled.

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Negative connections were established on the exposed rebars in the required locations and the negative return cables were fed

back via the vertical continuity chases and a core hole to the tributary junction box, which was located inside the barrel to

allow for monitoring from the staircase. Reference electrodes were then installed and the reference return cables were also run

within these chases.

Anodes were installed in a section of each panel of the base structure after it had been broken out (refer Fig. 9). They were

installed in strings of approximately 40 at a time. A proprietary mortar was hand-pumped into the holes and the anodes were

partially inserted into the mortar. The anodes were then electrically connected with titanium feeder wire using strippers to

locally remove the sheath, and plastic screws completed the connection. After a check for electrical continuity, each anode

was then pushed further into the hole and additional mortar was pumped over the top. It was then capped with repair mortar to

prevent drying out of the mortar during the energizing phase. The impressed current phase commenced as soon as each anode

was installed, with the 9Volt DC maintained for an average of 12 days.

Four panels of forty anodes were connected together, and a 2.5mm2 positive cable was routed to the junction box in the same

continuity chase as the negative cables.

Once the anodes in a section of a panel had commenced their polarization phase, the forming and pouring of that section was

completed.

The same general process was carried out on the barrel and balcony, except that anodes were installed into the surface of the

structure after the concrete patch repairs were completed. This required 4mm wide and 15mm deep slots to be cut into the

surface to allow for the reticulation of the titanium feeder wires, and 30mm chases were required for the negative, positive and

reference returns. (refer Fig 10.)

Fig 8: Forming up a lift of one panel of the base Fig 9: Anode installation in base structure

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Fig 10: Installing anodes in the tower barrel

3.5 Coatings

Following completion of the hybrid anode installation, the tower, balcony and base were water blasted in preparation for the

coating. The local water supplies included a 14m deep well that provided an irregular feed to an above-ground water tank, and

the buildings’ roof catchments supplied rainwater to other storage tanks. While the latter was deemed initially to be a

contingency backup, it was found during the works that the well was unable to keep up with demand, which included drinking,

washing and laundry requirements as well as construction use.

After four months of occasional usage and no rainfall, the rainwater tanks were emptied. This became significant during the

water blasting phase at the end of the project given the large volumes of water required; pumping was required three times a

day and into the evening, and shower times were restricted.

The water supply pressure was insufficient to operate the water-blasting unit, even at ground level. To combat this, an existing

water cart was filled with water, and this fed the waterblaster. In addition, a water pump was needed to get sufficient water to

the top of the parapet, 30m above ground.

On completion of the water blasting, two coats of an engineered protective coating, designed to be breathable and compliant

with cathodic protection, was applied to the structure by rope access. The colour was selected to best represent the original

appearance.

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Fig 11: The completed structure

4. MONITORING AND MAINTENANCE

4.1 Monitoring

For monitoring and maintenance purposes, after 14 days from conversion to galvanic phase the initial performance assessment

required the following data:

The current and voltage of each zone.

The subzone current from junction boxes for the purpose of deciding whether balancing resistors were required.

The “ON” potential of all references.

The instant-off potential of all references (off period between 0.1 and 0.5 seconds).

The system was to be disconnected and the reference potentials recorded at 4, 24, 48 and 72 hours off.

The monitoring system consisted of five small junction boxes inside the lighthouse for all system component terminations,

with one master junction box (see Fig 12) beside the ground floor door for centralized monitoring of the whole system. All

junction boxes were positioned to be accessed from the periodic stair landings to allow local zone monitoring and

troubleshooting, if required at a later date.

Monitoring is available through permanently embedded manganese/manganese dioxide reference electrodes, installed adjacent

to steel reinforcement at suitable locations. A total of 10 references were installed in equal distances up the barrel, 4 in the

base and 4 in the parapet.

The master junction box was connected to the local junction boxes through a combination of instrumentation and insulated

copper cabling and allows the monitoring of the system to be conducted at the one central location. Measurements able to be

taken from the master junction box include:

Galvanic zone currents.

Reference potentials.

Interrupting the system.

Instant-off potentials.

Depolarized potentials.

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Fig 12: Installation of the master junction box which allows for centralised monitoring of the whole system

4.2 Maintenance

As this system was designed with the remote locality in mind, the system requires minimum maintenance to the system

components. A five yearly onsite check will be performed by a Corrosion Engineer on top of the regular Performance

Assessment monitoring to ensure the integrity of the system.

5. SYSTEM RESULTS & PERFORMANCE

The installation of the system was completed by late October 2013, with the system allowed to settle into galvanic mode for

several weeks before final monitoring was completed.

Monitoring consisted of recording the zone “On” currents and reference readings for “On” and “Instant-off”. Depolarization

testing was completed for 4hr, 24 hr and 72 hr off. The table below details results from the installed reference electrodes. This

data is used in the Butler Volmer equation [3] as per Notes 9 &10 of ISO 12696 [5] to calculate the corrosion rate. A corrosion

rate less than 2 mA/m2 steel is the major criterion of the hybrid system and this is reassessed at all future system monitoring.

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Fig 13: Results from the lighthouse after being placed into galvanic mode. Corrosion rates below 2 mA/m2 were the criterion

of the system in galvanic mode.

The expected life of each anode panel was calculated by subtracting the impressed charge passed from its initial charge

capacity, and calculating the time until the anode material is exhausted at the galvanic currents measured within the

commissioning tests. The average calculated life expectancy of the anode panels in the subject structure is in excess of 78

years, with all zones reaching the design life criterion.

6. CONCLUSION

The use of the hybrid anode system as part of the repair strategy for the lighthouse allowed the extent of concrete repair to be

greatly reduced when compared with traditional patch repairs. The project included a mixture of formed and trowelled

concrete repairs.

The hybrid anode system includes some 2,500 anodes, with 18 reference electrodes for future monitoring. The system is

expected to provide long-term protection to beyond the 50-year design life.

A protective coating was also applied to all external concrete surfaces in order to reduce moisture ingress, future carbonation

and to reinstate its appearance.

With low monitoring and maintenance requirements, the galvanic protection system is well suited to the remote location and

tropical conditions.

7. REFERENCES

[1] Jackson HA, “Cape Don Lighthouse, Northern Territory, Australia” - part 2nd Paper No 4298, (Institution of Civil

Engineers U.K.) 1919-20

[2] Glass GK, Roberts AC and Davison N, Hybrid corrosion protection of chloride-contaminated concrete, (Construction

Materials) 161 (4) (2008)

[3] Burstein GT, A Hundred Years of Tafel’s Equation: 1905-2005. Corrosion Science, 2005, 47, No12, 2858-2870

[4] Polder, Peelen, Stoop & Neft, Early Stage Beneficial Effects of Cathodic Protection in Concrete. Eurocorr 2009 paper

8408

[5] ISO 12696:2012 Cathodic Protection of Steel in Concrete