Concrete Repair 1

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ENHANCING THE LIFE OF REINFORCED CONCRETE STRUCTURES USING

CATHODIC PREVENTION:

ADAPTIVE REUSE OF THE FORMER HOLLYWOOD ROAD POLICE QUARTERS

C. T. Wong, M. K. Leung, C. Y. Kan, H. M. Chow and K. Y. LiArchitectural Services Department, HKSAR, China.

Email: [email protected]

ABSTRACT

In recent years, there has been increasingly awareness among the public towards adaptive re-use andrehabilitation of aged buildings rather than demolishing them for redevelopment. In such projects, concreterepair forms an inevitable part. Yet, for chloride-contaminated concrete, the conventional chip and patchmethod cannot arrest further deterioration of the defective concrete. The fresh concrete, though passivates thesteel in the repair region, elevates the steel potential in the surrounding still contaminated concrete (Sags et al,2005). Thus, the risk of corrosion increases in the surrounding region. Cathodic prevention and/or protection

have therefore been employed for such repair. This paper presents the design and installation of a system ofcathodic prevention using galvanic anode cells in repairing defects of the reinforced concrete structure in theproject of adaptive reuse of the former Police Married Quarters located at Hollywood Road, Hong Kong. Detailsof the set-up of the in-situ measurement and the results of the monitoring work of the anode cells embedded inconcrete in this project will be discussed.

KEYWORDS

Life Cycle Costing, Concrete Repairs, Cathodic Protection and Prevention, In-Situ Monitoring, FormerHollywood Road Police Married Quarters.

INTRODUCTION

In the last decade in Hong Kong, adaptive re-use and rehabilitation of aged buildings have drawn much attention

from the public. Though the design life of reinforced concrete structures is usually taken as 50 years, there havebeen so many reinforced concrete structures that have survived over such design life. Corrosion of the steelreinforcement has been commonly recognised as the dominant factor of deterioration of reinforced concretestructure. The steel reinforcement in concrete is, however, normally passivated as a consequence of the alkalinenature of the cement paste (pH ~12.5 to 13.9 (Duff and Farina, 2009)), which facilitates the formation andmaintenance of a passive iron oxide film. Consequently, the corrosion rate is low and minimal maintenancesuffices for decades. However, the corrosion rate will increase to an unacceptable level if the concrete at the steelreinforcement depth becomes either carbonated or chloride-contaminated (or both). Spalling of concretestructure therefore often occurs in aged buildings and is commonly repaired by conventional chip and patchmethod at the corroded area. However, further corrosion and spalling are found at the repaired location again in arelative short time. The repair life cycles are often shorter than expectation and the continuous repairing costconstitutes a major portion of the maintenance cost, and hence should not be neglected in the life cycle of astructure.

It has been established that the presence of a number of contaminants cause the deterioration of steel of concrete,e.g. seawater salts, airborne salts, carbon dioxide in the atmosphere, chloride containing substance and seadredged aggregates. In order to effectively solve the problem, cathodic prevention has been used to halt futurecorrosion of steel reinforcement, especially in chloride-contaminated concrete. Yet, limited research has beencarried out in Hong Kong and few publications have been published summarizing the application of such methodin actual repair work. This paper describes the usage of sacrificial anodes, including the installation andmonitoring work in the project of adaptive reuse of the former Police Married Quarters located at HollywoodRoad in Hong Kong, which was originally built in the 1950s.

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CORROSION MECHANISM

Corrosion of steel is subjected to an electrochemical reaction which involves four essential active components,namely, the anode, the cathode, electron flows and the electrolyte. The anode is the location where the steel losesits electrons and the electrons are gained at the cathode. The electrons flow through the corroding steel itselffrom the anode towards the cathode and thus a voltage difference is induced. The electrolyte is the solution or asubstance (such as water, soil and concrete) that can conduct electrical currents by containing free ions, both

positive and negative, with equal charge. In the corrosion process of steel, iron atoms (Fe) lose its electrons andbecome iron ions (Fe2+

) forming the anodic reaction. With the presence of oxygen (O2) and water (H2O),hydroxide ions will be produced by gaining the moving electrons forming the cathodic reaction. Rust, i.e. iron(III) hydroxide, is then the chemical by-product of the corrosion process and accumulates at the surface of steel.The increasing size of rust and its induced expansive stresses finally crack the concrete surface and spallingoccurs. This in turn triggers the progressive deterioration of concrete. The key reactions of steel corrosion are asfollows:

Fe Fe2+ + 2e- (Anodic reaction) (1a)

4e- +O2 + 2H2O 4(OH)- (Cathodic reaction) (1b)

Fe2+ + 2(OH)- Fe(OH)2 (Ferrous hydroxide) (1c)4Fe(OH)2 + 2H2O + O2 4Fe(OH)3 (Ferric hydroxide) (1d)

Once the alkaline passive environment around the steel reinforcement has been lost, the above equations will betriggered.

Corrosion in Chloride-Contaminated Concrete

Prior to the 1970s in Hong Kong, chloride may be present in some buildings due to the incorrect use of marinesand or sea water in the concrete mixes. Otherwise, concrete may contaminated by the spillage of seawaterwhich is commonly used as flushing water in toilets. Different mechanisms take place for chloride-contaminatedconcrete. Equations (2a) - (2e) show the corresponding reactions of steel corrosion in chloride-contaminatedconcrete. For chloride-contaminated concrete, the conventional chip and patch repair method is still widelyadopted. Loose or delaminated concrete is removed from the full circumference of the steel and continue alonguntil there are no visible signs of corrosion. However, after cleaning the steel reinforcement and repairing thearea with chloride-free patch, potential difference will be induced between the new repair patch and existingchloride-contaminated concrete. This repairing work actually accentuates corrosion in the reinforcing steeladjacent to the repair area as shown in Figure 1. This triggering action is to create incipient anodes, newcorrosion sites just outside the repaired area (Sags et al, 2005), and is often referred as incipient anode

induced corrosion or halo corrosion damage. As such, conventional repair method is not applicable tochloride-contaminated concrete, and alternatives (e.g. impressed current cathodic protection, sacrificial anodecathodic protection, coating, dechlorination) have already been explored and adopted in such repair. Amongthem, cathodic prevention provides a simple and inexpensive method (Bertonlini et al, 1998), and has beenadopted in Hong Kong since the 2000s. Yet, few publications have been published on the long-term performanceof such method.

Figure 1 Incipient anode induced corrosion(Source: modified from Ball and Whitmore, 2003)

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GALVANIC CATHODIC PROTECTION AND PREVENTION

To extend the service life of the repairing work of reinforced concrete, galvanic (sacrificial) cathodic protectionor cathodic prevention, which are described as Principle 10 in BS EN 1504:9 (BSI, 2008), could be aneffective solution. Both galvanic cathodic protection and prevention are to generate a small electrical current byconnecting two dissimilar metals together, i.e. embedding sacrificial anodes in repaired area. These anodes aresacrificial in nature and interfere with the corrosion reaction on the steel. In other words, the sacrificial anode

must be some other metals with a more negative electrode potential difference from the steel, e.g. zinc (Figure 2),aluminium or magnesium. Whitmore and Ball (2005) explained that zinc sacrificial anode has become the mostcommon material today due to its high corrosion efficiency and low rate of expansion.

Figure 2 Typical layout of zinc galvanic sacrificial anode(Source: Sergi, 2009)

Bertonlini et al (1998) and BS EN 12696(BSI, 2000) illustrate (Figure 3) that the basic difference of the twomethods is the application of different intensity of electrical potential to the concrete and hence changing itscorrosion potential (degree of passivity).

The amount of current used to the steel reinforcement in cathodic prevention is only to prevent the onset ofcorrosion but not enough to control existing corrosion. Pedeferri (1996) gave the current densities needed toprevent, reduce or stop corrosion as: between 0.5 and 2mA/m2 to get prevention conditions, up to 15 mA/m2 toreduce the corrosion rate; and up to 20mA/m

2to repassivate a corroding rebar. Such indicated current densities

have then been incorporated into European code, and BS EN 12696 distinguishes cathodic protection fromcathodic prevention by their current density. The current density of cathodic prevention ranges from 0.2mA/m

2

to 2mA/m2

while that of cathodic protection ranges from 2mA/m2

to 20mA/m2, which is approximately one

order of magnitude larger than the former. For cathodic protection, BS EN 12696 requires that a depolarizationlevel of the steel reinforcement after disconnection of the anodes for periods of either 4 hours or 24 hours shallexceed 100mV. There is, however, no such requirement for cathodic prevention.

Figure 3 Schematic illustration of changing paths of potentialand chloride content on steel reinforcement surface during its service life

(Source: Bertonlini et al, 1998 andBS EN 12696)

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APPLICATION OF CATHODIC PREVENTION IN THE FORMER POLICE MARRIED QUARTERS

The Site

The site is located in Mid-Levels, Hong Kong and the area of the site is about 6,000 m2. There are three existing

buildings (Figure 4) within the site including two residential blocks (Block A and Block B) and a 2-storeyancillary building (the JPC building), which were the former police quarters and have been vacant since 2000.The buildings were constructed in the 1950s and are now about 60 years old. These former police quarters are

being preserved, reinforced and refurbished for conversion into a creative industrial centre at a contract sum ofabout HK $360M, which is scheduled to open to the public in 2014.

Figure 4 Location plan of the site

The Structure

Block A and Block B are reinforced concrete structures of eight storeys and seven storeys respectively foundedon strips footings at a depth varying from 1.2m to 4.5m below ground. The plan area of each block is72.1m14.6m. They comprise of reinforced concrete beams and slabs structures supported on reinforcedconcrete walls spaced at about 4.1m c/c typically (Figure 5).

Figure 5 Typical framing plan of a residential block

Structural Condition

Structural condition survey was carried out by Chung & Ng Consulting Engineers Ltd. and ArchitecturalServices Department to assess the structural conditions of the existing buildings in 2009-2011. A number of in-situ tests (including comprehensive visual inspection, hammer tapping, carbonation, concrete cover, moisturelevel tests and corrosion potential measurement) and laboratory tests (including cement content, chloride content,concrete core tests and tensile test of rebar) had been carried out. The structural survey results are summarized inTable 1 as follows:

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The sacrificial anodes were secured as close as possible to the concrete patch edge by wrapping the tie wiresaround the cleaned reinforcement bars and twisting tightly. No free movement of anodes was allowed andsufficient clearance between anodes and concrete surface was provided to allow encasing the anodes with repairmaterial. Figure 7 and Photo 1 show the typical layout and a view of the installation of sacrificial anodes to ther.c. wall respectively. An ohm meter was applied to confirm the electrical connection between the tie wire and

reinforcement bars by measuring their DC resistance (Photo 2). The specified resistance was not greater than 1:,and the specified potential difference was to exceed 1mV. Otherwise, the whole system was malfunctioned due

to discontinuity, and steel tie wire would then be applied to the exposed reinforcing bars to enhance the electricalcontinuity.

Figure 7 Typical installation of sacrificial anodes

Photo 1 Sacrificial anodes installed at a chloride-contaminated wall

Photo 2 Measurement of resistivity of the system

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Upon completion of the installation of the sacrificial anodes, the concrete area was patch repaired with polymermodified cementitious material following the Principle 3.1 inBS EN 1504-9. A particular point to be noted isthat to ensure the proper function of cathodic prevention, the resistivity of the mortar should be less than 15,000ohms, and hence, epoxy mortar is inappropriate.

MONITORING

The designed corrosion prevention system was then monitored to check for its long-term performance. The mosteffective method is to measure the current output of the sacrificial anode cells. The current density requirementas perBS EN 12696ranges between 0.2mA/m

2and 2mA/m

2of the steel area for cathodic prevention. To set up a

monitoring system, a normal sacrificial anode cell was modified (Figure 8) by connecting a 2.5mm2

copper corewire with sheathing to one of the steel tie wire from a normal anode and cut the unused tie wires as short aspossible to ensure no contact between the modified anode and steel reinforcement. The modified anode wasplaced in the patched repair area and the lead wire was extended to the junction box. Besides, a cathodeconnection to rebar was installed and also extended by wire for monitoring. The junction box also providedswitches with the following functions:(1) Switch Up = for measuring current to anode(2) Switch Middle = for disconnecting from anode(3) Switch Down = for connecting anode to rebar

When the current to the anode was measured, only the anode being measured was switched up. All otherswitches were either in middle or down position.

Figure 8 Setup of monitoring system

For each monitoring station, the initial readings of energizing current were recorded. Monitoring readings werethen taken periodically including current, instant-off potential of the steel and 24 hour off potential of the steel.The current being produced by the monitoring anodes was then found out to calculate the current density.Depolarization of the steel could be found out from the 24 hours off potential and instant off potential value.

RESULTS AND DISCUSSION

The anodes were installed at 400mm spacing along the edge of repair area and the design life of anodes wasspecified to be at least 15 years. The total surface area of the reinforcement bars within a 400mm400mm area is

calculated as 0.105m2 (horizontal reinforcement: I at 6 c/c; vertical reinforcement: I at 8 c/c). 2numbers of patch repair areas were installed with 1 number of monitoring anode and junction box. Readings forthe first three months after the installation of sacrificial anodes have been taken, and Table 3 summarizes themeasurement results.

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Table 3 Readings of galvanic corrosion protection

Location: Wall area at 2/FToilet, Block A

Wall area at 4/FToilet, Block B

Repair area: Approx. 1400mm 800 mm

Approx. 1300mm 1100 mm

Current density: 3.3 - 14.3 mA/m 1.1 - 7.0 mA/mDepolarization potential 7 - 45 mV 11 - 87 mV

The cathodic prevention current density as required in BS EN 12696ranges between 0.2 mA/ m2 to 2 mA/ m2 ofthe steel surface area. The above are initial readings of current density and the levels of depolarisation takensince mid-May 2012. The initial results prove that the systems have been operating as designed to cathodicallyprevent further corrosion of the reinforcement. It is, however, noted that the initial current densities at bothmonitoring stations are much higher than that required for cathodic prevention. Sergi et al (2008) and Sergi(2009) reported that the current will drop with time when the steel is polarized and the repair material dries up.The anode systems for corrosion protection have therefore been designed to be monitored periodically. Thecontinuous monitoring was intended to verify the life-long performance of the designed system.

Figure 9 and Figure 10 show the changes of current density for the first three months of installation, and they aregenerally decreasing and agree with Sergi et al(2008) and Sergi (2009). Figure 11 and Figure 12 illustrate thedepolarization potentials for the first three months of installation. The depolraization potentials were found rarelyexceeding 50mV. This agrees with the report of Sergi (2009) that the depolarization criterion for cathodicprotection systems (i.e. 100mV) is unlikely to be met for cathodic prevention systems.

Figure 9 Current density against time (measured at 2/F, block A)

Figure 10 Current density against time (measured at 4/F, block B)

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Figure 11 Depolarization (measured at 2/F, block A)

Figure 12 Depolarization (measured at 4/F, block B)

CONCLUSIONS

Corrosion of steel rebars in chloride-contaminated concrete is a major problem of rehabilitation of agedbuildings. It would probably be induced soon after the repair at the adjacent area if only conventional chip andpatch method is used. New technology such as applying zinc sacrificial anode has been introduced in reinforcedconcrete for corrosion protection and corrosion prevention. Though it has been used in many rehabilitationprojects in Hong Kong, its long-term performance has not yet extensively been studied and published. For thesacrificial anodes installed in this project, initial reading shows the performance is satisfactory and meets thecathodic prevention requirement in BS EN 12696. Long-term monitoring is in progress, and the data will becollected and analysed.

ACKNOWLEDGEMENTS

The authors would like to record their thanks to the Director of Architectural Services of the Hong Kong SARGovernment for his kind permission of publishing the paper. The authors would also like to thank the staff inChung & Ng Consulting Engineers Ltd for their works of structural survey, and to the staff in ArchitecturalServices Department of the Hong Kong SAR Government for their help in preparing the manuscript.

REFERENCES

ASTM (2009),ASTM C 87609: Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steelin Concrete (West Conshohocken: ASTM International).

Ball, J.C. and Whitmore, D.W. (2003), Corrosion Mitigation Systems for Concrete Structures, ConcreteRepair Bulletin, July/August, pp. 6-11.

Bertolini, L., Bolzoni, F. and Pedeferri, P. (1998), Cathodic Protection and Cathodic Prevention in Concrete:Principles and Applications,Journal of Applied Electrochemistry, 28 , pp. 1321-1331.

BSI (2000),BS EN 12696:2000: Cathodic Protection of Steel in Concrete (London: BSI).BSI (2008), BS EN 1504-9:2008: Products and Systems for the Protection and Repair of Concrete StructuresDefinitions, Requirements, Quality Control and Evaluation of Conformity (London: BSI).Duff, G.S. and Farina, S.B. (2009), Development of an Embeddable Sensor to Monitor the Corrosion Process

of New and Existing Reinforced Concrete Structures, Construction and Building Materials, 23, pp. 27462751.

Pang, H.W., Chan, C.O. and Au, B.L.K. (2010), The Three Enablers of Building Sustainability BuildingPathology, Performance Monitoring and Estate Improvement, Presented at Joint Structural DivisionAnnual Seminar 2010, 15 June 2010, Hong Kong.

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Pedeferri, P. (1996), Cathodic Protection and Cathodic Prevention, Construction and Building Materials, 10(5)pp. 391-402.

Sags, A.A., Balakrishna, V. and Powers, R.G. (2005), An Approach for the Evaluation of Performance ofPoint Anodes for Corrosion Prevention of Reinforcing Steel in Concrete Repairs, in Di Maio, A. and Zega,C. (eds),Proceedings of fib Symposium: Structural Concrete and Time, pp. 35-41.

Sergi, G., Simpson, D. and Potter, J. (2008), Long-term Performance and Versatility of Zinc Sacrificial Anodesfor Control of Reinforcement Corrosion, Presented in European Corrosion Congress, 7-11 September

2008, Edinburgh, UK.Sergi, G. (2009), Ten Year Results of Galvanic Sacrificial Anodes in Steel Reinforced Concrete,Presented in

European Corrosion Congress, 6-10 September 2009, Nice, France.Whitmore, D.W. and Ball, J.C. (2005), Galvanic Protection for Reinforced Concrete Structures, Concrete

Repair Bulletin, Spetember/October, pp. 20-22.

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Concrete Repair 1