Msc Project Draft Report

8/6/2019 Msc Project Draft Report

1/70


2/70

Evaluation of Corrosion Control Methods forChloride contaminated Reinforced concrete structure MSc Project Draft Report

1

Table of Content

Acknowledgment..........................................................................................................2

1. Introduction..............................................................................................................41.1 Background.............................................................................................. 4

1.2 Aims and Objectives ................................................................................ 41.3 Outline of Study....................................................................................... 5

2. Literature Review ....................................................................................................62.1 Background of Corrosion......................................................................... 6

2.2 The nature of reinforcing steel corrosion in concrete .............................. 7

2.2.1 Loss of Passivity by Carbonation......................................... 8

2.2.2 Loss of Passivity by Chloride .............................................. 9

2.2.3 Influence of cracking on concrete surface ............................ 11

2.3 Mitigation measure for contaminated reinforced concrete structures.... 12

2.3.1 Conventional concrete patch repair...................................... 12

2.3.2 Protective Coating/Barrier on Concrete Surface.................. 14

2.3.3 Electrochemical Treatment Cathodic Protection .............. 15

3. Implementation of Cathodic Protection System .................................................193.1 Design of Cathodic Protection System .................................................. 19

3.1.1 Structural assessment and field tests.................................... 19

3.1.2 Design consideration of cathodic protection system............ 20

3.1.3 Acceptance Criteria for Cathodic Protection System .......... 21

3.2 Technical Difficulties in implementation and operation........................ 21

3.2.1 Hydrogen Embrittlement of Prestressing steel .................... 21

3.2.2 Corrosion Interaction to surrounding structure.................... 23

3.2.3 Other potential side effect of cathodic protection system.... 24

3.2.4 Electrical Continuity of reinforcement ................................ 244. Case study of refurbishment of chloride contaminated Bridge.........................264.1 Background............................................................................................ 26

4.2 Structural Investigation.......................................................................... 26

4.3 Overview of Refurbishment scheme...................................................... 29

4.4 Site Constraints ...................................................................................... 31

4.5 Selection of Repair Methods.................................................................. 32

4.6 Cathodic Protection................................................................................ 34

4.7 Monitoring of CP systems ..................................................................... 35

4.8 Conclusion ............................................................................................. 36

5. Evaluation of Cathodic Protection System Performance...................................37

5.1 Background............................................................................................ 375.2 Investigation Data .................................................................................. 37

5.2.1 Survey Methodology.......................................................... 37

5.2.2 Testing Result .................................................................... 39

5.3 Discussions ............................................................................................ 42

5.4 Conclusion ............................................................................................. 44

6. Conclusion ..............................................................................................................45

7. Reference ................................................................................................................46

Appendix A Plan and Photo Record for Nettlehill Railway Bridge

Appendix B Breakout Window Record and Photo Record for HIT CP Investigation


3/70


2

Acknowledgment

I would like to express my highest gratitude to my supervisor, Dr. X.Y Li. He

provided me continuous encouragement, support and guidance during the process this

dissertation. He is so generous that to offer her time to give me her advice. Without

his advice, encouragement and support, this research would not be able to complete.

It is a difficult but invaluable experience during the process of data finding. I would

also like to thank to my employer company Maunsell Consultant Asia Ltd, in

providing me some useful information of the previous project. Being a member of the

construction industry, I am impressed by power of sharing experience and information

between different professionals within our industry. This experience taught me of how

importance of a teamwork and support from others of construction industry is. After

the dissertation, I think I am more equipped and confident to make further

advancement in my career in the construction industry.


4/70


3

Abstract

Reinforced concrete has become the most common materials for civil or building

construction over the last hundred years; the reason for reinforced concrete being so

popular are due to its mouldability / versatile, cheapness, fire-resistance and usuallydurable and strong, performing well throughout its service life. But sometimes it does

not perform adequately as a result of poor design, poor construction, inadequate

materials selection, or in a severe environment.

Many types of concrete structures are exposed to aggressive environments, especially

those with depassivating agents (e.g. Carbon Dioxide and Chloride), such as car parks,

marine facilities and bridges, it allows the corrosion process to proceed quickly. Most

of the problems with corrosion of steel in concrete are not due to loss of steel but the

growth of oxide. This leads to cracking and spalling of concrete cover. Concrete

damage would usually have to be well advanced before a reinforced concrete structure

is at risk.

In this study, a few commonly used corrosion control methods have been assessed on

its technical competence, financial implication and difficulties of implementation, and

aimed to find out the suitable solution for the chloride contaminated structure and thus

it could maintain / extend the service life of contaminated structures.

A case study on the refurbishment project of bridge structure at Scotland is assessed

base on the technical competence, financial implication and difficulties of

implementation for the cathodic protection system as a solution to corrosion problemin reinforced concrete structure.

In addition, performance evaluation for the existing cathodic protection system

installed at Hong Kong International Terminals berth 4 and 6 is presented, which

included some results and discussion from the investigation tests.


5/70


4

1. Introduction

1.1 Background

Rusting became a fact of life after human being started digging ores out of the ground

and refining them to produce iron or steel that we use so widely in the manufacturing

and construction industries. Nature sets about reversing the process of refining, the

refined iron/steel will react with non-metallic substances to form oxides, sulphates,

sulphides, chlorides, etc.

Reinforced concrete has become the most common materials for civil or building

construction over the last hundred years; the reason for reinforced concrete being so

popular are due to its mouldability / versatile, cheapness, fire-resistance and usually

durable and strong, performing well throughout its service life. But sometimes it does

not perform adequately as a result of poor design, poor construction, inadequate

materials selection, or in a severe environment.

Billions of dollars are spent every year in protecting, repairing and replacing corrosion

damages. Occasionally lives are lost when steel pipes, pressure vessels or structural

elements on bridges fail such as the Interstate 35W Bridge in United State collapsed

August 2007. The economic loss and damage caused by the corrosion of steel makes

it arguably the largest single infrastructure problem facing industrialized countries.

Those infrastructures are ageing, some can be replaced, others would cause great cost

and inconvenience if they were taken out of commission. It becomes crucial that the

existing structures perform to their design lives and limits and are maintained

effectively. The biggest causes of corrosion of steel are deicing salt on highways and

the chloride ingression at splash zone of marine environment.

There are two most commonly used repair strategies conventional patch concrete

repair and electrochemical treatment such as cathodic protection. In choosing the

repair options a number of factors including availability of budget, accessibility,

degree of disruption and loss of facility, anticipated remaining life of structure, future

intended use of structure etc. should be considered. In some cases, if the structure

which reached its design life, has been found in a fair condition on its structural

integrity through a detailed investigation, installation of cathodic protection system

could be a choice to extend its service life such as the South Hook LNG Terminal, the

cathodic protection system has been installed to extend the service life of the old

reinforced concrete jetty built in 1960s, and the jetty has been refurbished to be the

new approach way for LNG terminal. If the electrochemical treatment is furtherproved to be a cost effective option for refurbishment of old existing reinforced

concrete structure, it could significantly reduce the production of construction waste,

which is the most urgent problem faced by the government during the last two

decades, and minimize the impacts to the environments.

1.2 Aims and Objectives

Most of the Marine structures or other structures which are attacked by chloride

induced corrosion, the service life has been significantly shortened or extensive repair

work was required to maintain its structural integrity. These will produce huge amount

of construction wastes and burden the capacity of current landfill. Also due to the

frequency repair works, the service interruption was inevitable; citizen would requirepaying more for these indirect costs.


6/70


5

In this study, a few commonly used corrosion control methods have been assessed on

its technical competence, financial implication and difficulties of implementation, and

aimed to find out the suitable solution for the chloride contaminated structure and thus

it could maintain / extend the service life of contaminated structures.

1.3 Outline of Study

In this report, the literature review of corrosion mechanism and influence factors for

the initiation of corrosion in reinforced concrete and the introduction of most

commonly adopted mitigation methods is mentioned in section 2.

In section 3, the general procedures and consideration of cathodic protection system,

also its difficulties and problem during implementation is discussed.

A case study on the refurbishment project of bridge structure at Scotland is included

in section 4 to have a overview on the cathodic protection system as a solution to

corrosion problem in reinforced concrete structure.

In section 5, performance evaluation for the existing cathodic protection system

installed at Hong Kong International Terminals berth 4 and 6 is presented, which

included some results and discussion from the investigation tests. Finally the overall

conclusion to summarized the whole study is contained in section 6


7/70


8/70


7

O2 + 2H2O + 4e- 4OH

-

Oxygen Water Electrons Hydroxyl ions

2H+

+ 2e- H2

Hydrogen ions Electrons Hydrogen gas

2H2O + 2e-

H2 + 2OH-

The first of these reactions occurs in the presence of dissolved oxygen and

near-neutral conditions. The second is favored by acidity (excess of hydrogen ions)

while the third is dominant at pH values greater than neutral.

In aerated near neutral condition (e.g in tidal zone of marine condition), the iron ions

produced at the anode react with the hydroxyl ions formed at the cathodic sites to

produce ferrous hydroxide:

Fe2+

+ 2OH- Fe(OH)2

The ferrous hydroxide is readily oxidized by dissolved oxygen to form hydrated ferric

oxide Fe2O3 H2O:

4 Fe(OH)2 + O2 H2O + Fe2O3 H2O

Thus the overall reaction which proceeds through a series of intermediate steps may

be written as:

4 Fe + 3O2 + 2 H2O Fe2O3 H2O (Hydrated ferric oxide -- rust)In practice the rate of corrosion is often determined by the rate at which the cathodic

reaction can be sustained. In near neutral anaerobic waterlogged environment sulfate

reducing bacteria may give rise to a further type of cathodic reaction in the corrosion

of iron and steel. These microbes reduce dissolved sulfates to sulfides possibly

through the reaction:

SO42-

+ 8 H+

+ 8 e- S

2-+ 4 H2O

And the corrosion is characterized by the fact that it occurs in the absence of air andsulfides are present in the corrosion products. From the composition of the actual

products formed it is probable that the corrosion mechanism involves cathodic

depolarization which may be represented by the simplified equation:

4 Fe + 4 H2O + SO42 3Fe(OH)2 + FeS + 2OH-

Stimulation of the cathodic reaction depends on the bacteria possessing an enzyme

(hydrogenase) to enable them to oxidize hydrogen found at the cathodic sites. The

sulfide ions produced by the reduction of sulfate can sometimes stimulate the anodic

process of iron dissolution.

2.2 The nature of reinforcing steel corrosion in concrete

Concrete normally provides embedded steel with a high degree of protection against

corrosion. It is because the pore solution in a hydrated Portland cement system

contain high concentrations of soluble calcium, sodium and potassium oxides, these

oxides form hydroxides, which are strong alkaline with a pH normally exceeding 12.

This means that the concrete surrounding the steel provides an alkaline environment

for the steel. This leads to a passive layer forming on the steel surface by stabilizes

the oxide or hydroxide (Assume that Passivation by films of Fe2O3 and Fe3O4) film. A

passive layer is very dense, thin layer of oxide that leads to a very slow rate of

oxidation (corrosion). Besides, surrounding concrete restrict the ingression of outside

elements which are aggressive to the steel. Consequently good quality, well placed

concrete with adequate cover to the steel provides a high degree of protection to thesteel reinforcement. However, the duration of this protection depends on number of


9/70


8

factors including the retention of high pH and physical integrity of the cover concrete,

and the efficiency of concrete acts as a barrier to aggressive species, the passivating

environment is not always maintained. Two processes can deteriorate the passivity of

steel in concrete. One is carbonation of concrete and other is chloride attack.

2.2.1 Loss of Passivity by

Carbonation

According to the pourbaix diagram for

iron, the metal is passive when pH is

above 9.5. Carbon dioxide, however,

will diffuse into the concrete and, in

the presence of moisture, react with the

hydrated cement to form calcium

carbonate. This will remove hydroxyl

ions from the pore solution and reduce

the pH of the concrete.

CO2 + H2O H2CO3

Gas Water Carbonic acid

H2CO3 + Ca(OH) 2 CaCO3 + 2H2O

Carbonic acid Pore solution Calcium carbonate

This carbonation process will

start at the surface, and then

slowly move deeper and

deeper into the concrete. It is

not detrimental to the integrity

of a concrete component until

it penetrates to the embedded

steel. If the object is cracked,

the carbon dioxide of the air

will be better able to penetrate

into the concrete. The

alkalinity of the concrete

surrounding the embedded steel is reduced over sufficient time to point below pH 9.5where passivation is lost and the steel will be vulnerable to corrosion if other

necessary components, water and oxygen, are present. Carbonation is common in old

structures, badly built structures with low cement content and very porous.

The rate of carbonation depends on the permeability of the concrete to ingress of

carbon dioxide, which is strongly influenced by the water/cement ratio, the moisture

level of the concrete and the total alkali content of the hydration products. A

carbonation front proceeds into the concrete roughly following the laws of diffusion.

These are most easily defined by the statement that the rate is inversely proportional

to the thickness: xD

dt

dx/=

Where x is distance, t is time and D is diffusion constant. The diffusion constant D is


10/70


9

determined by the concrete quality. The resistance of carbonation of reinforced

concrete is influenced by many factors. The carbonation rate / the time to carbonation

induced corrosion, is a function of cover thickness, so good cover is essential to resist

carbonation. As the process is one of neutralizing the alkalinity of concrete, high

cement content is needed. The diffusion process is made easier at concrete with open

pore structure, both good compaction and well curing concrete with small pores andlower connectivity of pores could provide good resistance to carbonation. In addition,

the wet/dry cycling on the concrete surface will accelerate carbonation by allowing

carbon dioxide gas in during the dry cycle and then supplying the water to dissolve it

in the wet cycle that is what occurring everyday at marine concrete structures.

It is generally accepted that Ficks first law of diffusion describes the depth of

carbonation front as a function of time. This states that the rate of movement is

inversely proportional to the distance from the surface, the result as shown in

equation:

dtx

C

ADdQcb

cb

=

Where Q is amount of diffusing carbon dioxide, A is penetrated area, Ccb isdifference between carbon dioxide concentration in the atmosphere and at the

carbonation front, x is distance between carbonation front and concrete surface, t is

time, Dcb is carbon dioxide diffusion coefficient.

The diffusing carbon dioxide reacts with calcium, potassium and sodium hydroxides

at the carbonation front. And the process is modeled by this equation: dxAadQ = ,

where a is carbon dioxide binding capacity for concrete. Then after the integration of

the combination of above two equation, the following equation is obtained :

ta

CDx cbcbcb

=

2

The binding capacity of carbon dioxide of concrete, which is determined by

following:

CaO

CO

hcCaOm

mCCa 275.0 =

Where CCaO is content of calcium oxide in cement, Cc is cement content in concrete,

h is degree of hydration, mco2 is molar mass of carbon dioxide, mCaO is molar mass of

calcium oxide. The typical value for the rate of movement of carbonation front isaround 1mm per year for poor concrete and 0.2mm per year for good quality concrete.

Once the carbonation front has penetrated to the steel, and moisture present, there is

risk of corrosion. The corrosion products formed in these circumstances will have a

volume several times greater than the volume of steel from which they derived. The

built up of corrosion product on the surface of the steel creates tensile forces in the

concrete, as the result, cracks were built up on the concrete surface and these would

further increase the carbonation rate and accelerate the deterioration of structures.

2.2.2 Loss of Passivity by Chloride

Chlorides in concrete can come from several sources. They can be either cast into the

concrete (fixed chloride or free chloride) or diffuse in from the outside (mainly freechloride). The sources of chloride in fresh concrete include some admixtures, some


11/70


10

sources of aggregates (both inland and marine), and the cement. It is essential to

identify the source for introduction of chlorides during construction, such as the use of

calcium chloride as an admixture for reinforced concrete. With the current restriction

on chloride contaminants constituents in concrete mix, the risk of corrosion from

chlorides cast into current concrete construction is very low. However, chlorides from

the environment (e.g. sea water spray or direct wetting at marine environment anddeicing salts for highway) can also penetrate hardened concrete. A large portion of

these will stay as free chlorides in the pore water and are particularly aggressive to

embedded steel. Concrete exposed to such sources of chloride may need to be

especially designed and possibly given additional protection.

The depassivation mechanism for chloride attack is different from carbonation. The

chloride ions attack the passive layer by acting as the catalysts to corrosion. It is

generally accepted that passivity is maintained by dynamic balance between

breakdown by chloride ion and repair by hydroxyl ion of the film. Passive oxide layer

would be broken down when sufficient concentration of chloride reach the surface of

reinforcing steel and the broken down process is dominant. During the process,chloride is not consumed. The chloride threshold for corrosion given in terms of

chloride(Cl-) /hydroxyl(OH

-) concentration is 0.6. This ratio is approximately

equivalent to 0.4% of chloride by weight of cement content in concrete. If the ratio

exceeds the threshold level, corrosion would be observed. The localized deterioration

of passive layer would lead to the pitting corrosion. The pitting of steel was often

observed for those damaged reinforcing steel of marine concrete structure. At some

suitable site on the steel surface such as the location of insufficient cement paste, the

chloride ions was attached by electrochemical potential difference, and corrosion is

initiated and acids are formed (Hydrogen sulphide from the sulphide inclusion in steel

and HCl from the chloride ions). The Iron dissolves and reacts with water. The

process of chloride induced corrosion is illustrated in the following diagram.

As stated in previous, corrosion is proceeded by the formation of anodes and cathodes.

In the case of chloride attack, anodes and cathodes are always well separated, this is

known as marco-cell formation. Since the chloride attack is always accompanied with

high level of water in pores which facilitate the movement of free chloride and other

ions hence increased the conductivity of concrete. This high conductivity allows the

separation of anodic and cathodic reactions with large cathodic areas supporting small

concentrated anodic areas, thus a pit forms, rust may form over the pit, concentrating


12/70


11

the acid and excluding oxygen so that the iron stay in solution preventing the

formation of a protective oxide layer and accelerating corrosion. As a result, the

structural integrity of concrete structure would be significantly reduced without any

apparent sign of damage in such case.

Similar to carbonation, the chloride ingress rate into concrete is often approximated as

a diffusion process. However, the initial mechanism appears to be capillary flow/force,

the partially saturated surface zone is prone to accelerated chloride ingress due to

capillary forces. Besides, cracks and heterogeneity of concrete and the condition of

exposure would also affect the chloride ingress at surface zone, but it is generally

accepted that diffusion is the leading transport mechanism in concrete once the

chloride has passed the surface zone.

Chloride ingress is more complicated as it is dealing with structures exposed to

variable chloride concentrations. Also the chloride diffusion produces a concentration

gradient instead of a front for carbonation case. The usual form of the diffusion

equation used is Ficks second law:

2

2 ),(),(

x

txCD

t

txC

=

Where C(x,t) is the chloride concentration at depth x, at time t and D is the diffusion

coefficient, in atmospherically exposed concrete, it is difficult to obtain the surface

concentration as the chloride at the surface can vary greatly by wetting, drying,

evaporation and wash off etc. it is common to discard the first 5 mm. and in diffusion

calculations must use the depth from sampling depth, not the surface.

In some research, it pointed out that within defined mathematical limits the error

function expression for the diffusion coefficient can be approximated to the simple

parabolic function, and the solution of above equation becomes:

)4

()(),(Dt

xerfcCCCC isitx +=

Where Ci is the initial chloride concentration, Cs is the surface chloride concentration,

erfc() is the error function complement. By using approximation erf(z)=(1 - Z/3)2 ,the solution would become:

2

),( ]12

1)[(Dt

xCCCC

isitx +=

2.2.3 Influence of cracking on concrete surfaceReinforced concrete is

designed to allow several

degree of cracking on surface,

depending on the structure and

the exposure class, the design

limits the width of cracks,

under normal situation, up to

0.3mm. But crack width seems

to be less important to

corrosion risk than crack

frequency, cover depth and


13/70


12

concrete quality. Cracking will often initiate localized early corrosion under the effect

of carbonation, but the corrosion is unlikely to progress significantly. Cracks

coincident with a reinforcing bar will tend to expose a greater portion of reinforcing

bar to moisture and oxygen; the anodic and cathodic site areas are generally equal in

size, thus typical general corrosion occurs. In the case of cracks intersecting

reinforcing bars, the anodic areas are likely to be considerably small, thus this leads tolocalized corrosion.

When chloride penetrate the concrete through this crack from external source,

corrosion can be very significant because of the relatively small size of the anodic

zone. In this case, intensive localized corrosion of affected reinforcing bars can cause

very large losses of cross-sectional area. Clearly, the loss of a large proportion of the

cross-section area of adjacent reinforcing bars could seriously reduce the load

carrying capacity of a structure.

Many types of concrete structures are exposed to aggressive environments, especially

those with depassivating agents, such as car parks, marine facilities and bridges. Most

of the problems with corrosion of steel in concrete are not due to loss of steel but thegrowth of oxide. This leads to cracking and spalling of concrete cover. Concrete

damage would usually have to be well advanced before a reinforced concrete structure

is at risk.

2.3 Mitigation measure for contaminated reinforced concrete structures

The detailed knowledge of the condition of structural elements and a clear

understanding of the likely effectiveness of the rehabilitation options are essential for

any maintenance or rehabilitation projects. A repair strategy should be developed to

enable cost-effective repairs and maintenance.

2.3.1 Conventional concrete patch repairDetailed Procedures of Convention Patch Repair

Conventional concrete patch repair was the most common mitigation method for the

existing old structures in the past, but it was considered as a short term rehabilitation

method especially for those marine or chloride contaminated structures, as repeat

defects are likely to occur within a 3-4 year period. The conventional concrete patch

repair includes three main processes concrete removal, steel and substrate

preparation and concrete reinstatement. The common practice of concrete repair is

shown as follows

a) Pre-repair survey by visual inspection, delamination hammer taping; and mark outareas identified as defective;

b) Break out defective concrete to expose the reinforcing steel and a sound concretesubstrate and extending at least 25mm behind the exposed reinforcement;

c) Augment deteriorated steel if section loss of steel more than 10%d) Prepare all concrete surfaces that are to receive concrete repairs by cleaning to

remove loose or weak concrete, surface laitance and other contaminants;

e) Prepare all exposed reinforcement to remove rust productf) Apply primer to the cleaned reinforcementg) Apply bond coat to the substrate before reinstatement;h) Apply the repair mortar to reinstate concrete when the bond was still tacky;

i) Cure the repair mortar immediately following placement and surface finishing ofrepair


14/70


13

There are a number of methods for removing concrete; the most common techniques

are by hand held pneumatic hammers, hydro-jetting and milling machines. Depending

on the specification, budget and site constraints, suitable technique would be used for

repairing works.

Where spalling as a result of corrosion damage has occurred, preparation of the steel

will be required to remove corrosion products and remaining chlorides before making

good. Following surface preparation a primer or coating is applied to the bar, usually a

cement:polymer slurry, zinc rich epoxy or epoxy barrier coating. In addition,

supplementary steel may need to be added where there has been significant loss of

section in the damaged bar. In general terms the following application guide applies:

a) Corrosion arising from carbonation: this is more general over the surface of thebar and the worst of the corrosion products may be removed by mechanical means

to Swedish Standard ST3. Any remaining corrosion will be re-passivated by the

application of a sound, good quality, cementitious repair material.

b) Corrosion damage arising from chloride contamination leads to pitting type

corrosion. Chlorides can remain trapped in the pits and are difficult to remove bymechanical means as the brush or grinder cleans the surface of the steel only.

Abrasive techniques using particles fired at the steel surface are more appropriate

to give a finish consistent with Swedish Standard SA 2.5. These are generally

limited to grit blasting or high pressure water jetting with an abrasive material

(typically silica sand) drawn into the water jet. If grit blasting is used it is

preferable to water wash the steel and blow dry before coating to remove any

chloride salts that may be remaining. The most appropriate technique will depend

on the coating to be applied.

c) The use of epoxies, whether zinc rich or barrier type, requires a high standard ofsurface preparation. As mentioned at (b) above this is not always practical in

repairs to marine structures. The preferred primer is therefore a cement:polymerslurry which re-passivates and allows residual chlorides to diffuse into the general

repair material thus reducing concentrations to a level which does not lead to

corrosion.

Reinstatement of Breakout damaged areas may be achieved using various methods

depending on the extent, orientation and geometry of the areas to be replaced. These

include:

Hand application of polymer modified mortars applicable to small, shallow(preferably < 50mm, maximum 75mm) repairs.

Recasting using proprietary polymer modified micro-concretes applicable tolarger, deep (> 50mm) repairs.

Grouting using pre-packed aggregates and proprietary non-shrink grout applicable to larger, deep (> 50mm) repairs, but geometry likely to restrict the

flow.

Recasting using appropriate concrete applicable to complete or partialreplacement of whole elements.

Limitations of Convention Patch Repair

The key limitations to conventional patch repair methods are as follows:

a) Surface preparation is limited the space available behind the bar. Breakout has tobe limited to minimise disruption to the structure so the most that can be expected

is 20-25mm. This restricts the standards that can be achieved and it is likely that


15/70


14

the back of the bar will not, in some areas at least, meet the specified

requirements.

b) It is likely that areas adjacentto existing corrosion and

spalling have the potential for

corrosion (e.g. high chlorideconcentrations) but are being

protected by the corrosion

activity at the spall site. By

patching the spall, the steel at

that point is re-passivated and

made cathodic, thus leading

to corrosion commencing at

an adjacent area. This is

known as the incipient

anode effect. Coating the

whole structure to excludeoxygen and moisture will

retard this process but some

corrosion activity is likely to continue.

c) In order to remove corrosion products from behind the bar, breakout around itscircumference is required. For large areas of repair, propping or sequencing of the

breakout, perhaps combined with closure of the structure, will be required.

Depending on assessment of individual structures it may be necessary to unload

the element by jacking etc. before breakout commences. Any significant concrete

removal or corrosion damage must be assessed by structural engineer, since the

removal of concrete cover or concrete surrounding the steel redistributes the load

within the structure.

d) Chlorides can enter the patch area over time, both from sea water and backmigration that is diffusion of chlorides from the contaminated parent concrete into

the repair material and cause the recurrence of spalling.

2.3.2 Protective Coating/Barrier on Concrete Surface

Protective coating is not usually required for reinforced concrete structures, however

it can be beneficial in excluding undesirable species such as chlorides and carbon

dioxide. There is a huge range of coatings and sealers can be applied to concrete such

as anti-carbonation coating which should be applied after carbonation repairs to stop

further carbon dioxide ingress. But it is very unlikely that the chloride induced

corrosion could be stopped by coating. Protective coatings are applicable in a number

of circumstances to reinforced concrete in a marine environment:

a) The prevention or restriction of additional chlorides entering the concrete wouldbe the principal use of protective coatings. Chloride resistant coatings may be

applied to halt or retard the progress of the chloride ingress provided chloride

concentrations have not already reached critical levels (the corrosion threshold)

and/or the reservoir of chloride existing in the concrete is not predicted to cause

critical concentrations at reinforcement depth during the required life of the

structure (by diffusion) and any expected damage is not tolerable. These are

generally film forming barrier coatings or hydrophobic impregnation materials.

b) Anti-carbonation: where carbonation has not reached the majority of reinforcingsteel in a structure but is predicted to do so within the required life and the


16/70


15

expected damage is not tolerable, anti-carbonation coatings may be applied to halt

or retard the progress of the carbonation front. Clearly the application of such

coatings would probably be confined to drier areas of a pier where moisture

blockage of pores has not prevented penetration of the carbonation front.

c) Exclusion of oxygen and moisture: Where carbonation and/or chlorides have

reached the reinforcing steel and areas of the structure have yet to show visiblesigns of distress it may be appropriate to apply a coating which excludes oxygen

and moisture as a means of stifling the corrosion process as part of a medium term

patch, recast or sprayed concrete repair system.

Limitations of various types of coating

Generally coatings have a life less than that of the structure (typically 5-10 years) and

are likely to require overcoating at some stage as an ongoing maintenance

commitment. In relation to marine structures this is usually inconvenient and

expensive because of the need for special access. Specifically related to the types of

coating mentioned above, limitations include:

a) Film forming chloride barriers can trap moisture in the concrete and thereforerequire high bond strength to the concrete in order to resist the consequent build

up of vapour pressure during the temperature changes. This tends to limit the

generic type of coating to epoxies which are expensive, not environmentally

friendly and frequently have a limited colour range. These materials also require a

high standard of surface preparation to achieve the necessary bond strengths.

Hydrophobic impregnation materials require the substrate to be relatively dry in

order that they may penetrate the concrete to a depth sufficient to achieve the

desired function of excluding chlorides. This may be difficult to achieve in

practice on a marine structure.

b) Anti-carbonation coatings are formulated to exclude carbon dioxide but still allowwater vapour to escape from the concrete, they will not usually act as a barrier tochloride ingress.

c) Coatings to exclude oxygen and moisture are similar to the film forming chloridebarriers and the same comments apply. In addition they are unlikely to exclude all

oxygen and moisture and some corrosion may continue.

2.3.3 Electrochemical Treatment Cathodic Protection

For the reinforced concrete structure is planned to proceed with electrochemical

treatment, existing spalls and loose material have to be removed, reinforcement is

cleaned and damaged areas made good to original profile. There is no need to break

out behind the bar. In addition to concrete repair, electrical connections are made tothe reinforcement, following electrical continuity checks, and to an external anode

(inert metal mesh and cementitious overlay, conductive coating or flame spray metal)

applied to the concrete surface. A DC current is passed between the bar and the anode

protecting the bar from further corrosion. The details of principle and limitation would

be discussed in following section.

Principles of Impressed Current Cathodic Protection

The electrochemical treatment works by applying external anode and passing current

from it to the steel so that all of the steel is made into a cathode. The cathodic

protection could further divide into impressed current type and sacrificial type


17/70


16

Impressed current Cathodic

protection (ICCP) is the system

commonly used for

atmospherically exposed

reinforced concrete structures.

ICCP depends on the passage ofcurrent from an anode, through

an electrolyte onto the surface of

the metal to be protected. If the

magnitude of the current is

sufficient such that there is a net

current flow to all areas of the

metal surface, corrosion will not

occur. The passage of current onto the metal surface causes a change in the

electrochemical potential of the metal. A CP system is designed to convert the whole

of the reinforcing steel surface into a cathodic area, hence the name. The anodic

reaction is moved to a more durable material i.e. a specially manufactured anode. TheCP system limits further corrosion of the steel by the following actions:

a) A net positive ionic current flows through the concrete towards the steel

suppressing the flow of positive iron ions away from the surface.

b) Hydroxyl ions, which are formed at the steel surface, stimulate the formation of

a passive film that protects the steel by acting as a barrier to corrosion.

H2O + 1/2O2 + 2e- 2OH

-

c) Aggressive negative ions such as chlorides are transported away from the steel

surface as the result of the flow of ionic current in the concrete.

d) The potential of the steel is polarised towards more negative values which

inhibits the dissolution of positive iron ions.

e) To support the reactions occurring at the CP anode, oxygen is consumed at the

steel surface and is therefore not available to support anodic reactions occurring

on the steel.

The existence of effective cathodic protection can be validated by measuring the

change in electrochemical potential of the metal being protected and recording the

magnitude of the current required to effect this change.

Cathodic protection will halt or reduce corrosion on a metal surface by restore the

alkalinity and enhancing the passivity of the steel, but it cannot rehabilitate the steel or

return it to its original condition.

Typical components for ICCP systemAnodes

Anode is a critical part of the ICCP. It is usually the most expensive item. Available

Anodes systems used for reinforced concrete are as follows:

1 coating applied to the concrete surface;2 mixed metal-oxide coated titanium mesh anode embedded in a render overlayer

applied to the concrete surface;

3 mixed metal-oxide coated titanium ribbon anode placed in slots cut into theconcrete surface and filled with a cementitious filler;

4 zinc plates bonded to the concrete surface;5 Nickelized carbon cementitious anode overlay on the concrete;6 Flame sprayed anode systems; and7 Thin, conductive, sprayed coating with primary anodes.


18/70


17

8 Discrete anode with cementitious or graphite backfill.

Reinforcement continuity & bonding

A basic requirement of any system passing electrical current is that the conductor, in

this case the steel reinforcement, is electrically continuous. It has been found by

experience that only physical contact of reinforcement may produce a high resistanceor discontinuous joints. Reinforcement continuity shall be achieved by welding an

additional rebar to each reinforcement in the continuity breakout chases around the

structure.

Permanent reference electrodes

To determine the performance of the CP system, the potential of the reinforcing steel

should be measured relative to a reference electrode. For ease of testing and for

reproducibility of test data the potentials should be measured relative to permanent

reference electrodes embedded in the beams as close as possible to the reinforcing

steel. The reference electrode cables will be terminated inside a water-resistant

junction box. The associated cables would then run from these boxes to themonitoring facilities in Transformer Rectifier units.

Transformer Rectifier

It is the DC power supply that transforms mains AC to a lower voltage and rectifies it

to DC. The positive terminal is connected to anode and the negative to the cathode.

Negative return connections

To enable the current to return to the transformer rectifier and thus complete the

electrical circuit, a number of connections in the form of reinforcement connectors

will be connected to the electrically continuous reinforcement. Negative return cables

would connect these connectors to the negative point of the transformer rectifier.

Anode feed and negative return cables

Anode feed cables from the transformer rectifier to an anode, and negative return

cables will be installed.

Impressed current Cathodic protection is particularly applicable to where chloride

contamination is widespread on the structure or chlorides have penetrated well beyond

the cover to the reinforcing bar. Besides, at the situation that it is difficult or expensive

to erect access to the structure for repeat repair operations or it is not possible for

structural and/or operational reasons to have significant break out of concrete behind

the bar. Also cathodic protection system is usually applied to the existing structurewith remaining life greater than 10 years:

Limitation of Impressed Current Cathodic protection

Cathodic protection has the following limitations:

a) Cost restricts its use to applications requiring a remaining life greater than 10years.

b) It requires regular monitoring and adjustment to ensure optimum application ofcurrent (too high a current density can lead to problems, just as much as too low

allowing corrosion).

c) Permanent Power supply is required


19/70


18

d) It can not be used on structures with epoxy coating, injection or with poorelectrical continuity.

Sacrificial Cathodic

Protection is to directly

connect the steel to asacrificial or galvanic

anode such as zinc without

using any power supply.

This anode corrodes

preferentially, liberating

electrons with the same

effect as the impressed

current system.

For example: Zn Zn2+

+

2e-

The system is illustrated schematically in above figure. There are a number of

elements and their alloys which are more active than steel in the electrochemical

series. The main practical metals are zinc, aluminum and magnesium. All of these

metals are used, in alloy form, as sacrificial anodes for submerged steel structures. As

the power supply is not required, and this makes the system much cheaper and easier

in both implementation and operation. However the main restriction on this system is

that the zinc has only a small driving voltage when coupled to steel, therefore, the

resistance of the electrolyte is crucial to the performance of the system. The principal

advantage is the lack of power supply and this makes both implementation and

maintenance cost cheaper.


20/70


19

3. Implementation of Cathodic Protection System

3.1 Design of Cathodic Protection System

3.1.1 Structural assessment and field tests

Apart from the general structural assessment such as its material condition, itsstructural integrity, some additional investigation shall be undertaken in order to

confirm the suitability of cathodic protection and provide system design parameters.

The investigation shall generally include are shown in the following:

a) Records: All the available records during the construction and alternation shallbe retrieved and reviewed to assess the location, quantity, nature and continuity

of the reinforcement, and the constituents and quality of the concrete. These

information shall be further confirmed and supplemented by the site survey and

laboratory tests.

b) Visual inspection: Visual survey data shall be collected to ensure the type and

causes of defect. Besides, the features of the structure or its surroundingenvironment, which could influence the application and effectiveness of the

cathodic protection. Also all area of the structure which require to be protected

shall be examined especially those area have been previously repaired and the

repair methods and materials shall be identified.

c) Chloride content analysis: Dust/Core samples shall be removed from theconcrete elements at incremental levels and the chloride content can be

determined by acid extraction of powdered concrete, followed by a chemical

determination. The chloride concentration presents in concrete is particularly

important with respect to reinforcement corrosion. The presence of chloride

ions can depassivate steel reinforcement in concrete and promote corrosion.

d) Carbonation Depth: The depth and extend of carbonation should be assessedeither on site or in the laboratory. Phenolphthalein is used as an indicator to

determine the depth of carbonation on freshly fractured or drilled concrete

surface. Similar to the chloride content, it is used to determine the present

situation of concrete.

e) Concrete cover and rebar location: Concrete cover and reinforcement size andposition measurements shall be carried out to verify the information from the

construction records, or if the record was missed, to assess the anode/cathode

spacing will be adequate for the particular anode system and to identify dense

regions of reinforcement which may require high current density. Besides, it is

used to identify any shielding to the reinforcement to be protected, such as

embedded metal meshes, metal fibers or plastic sheets, also any possible shortcircuits site with extremely low concrete cover, which could impair the

efficiency of cathodic protection.

f) Reinforcement electrical continuity: the construction drawing s of reinforcementand other attached steel elements shall be checked for continuity. During site

investigation the continuity shall then be verified on site by measuring the

electrical resistance or potential difference between bars in locations across the

structure. The purpose of this test is to confirm the feasibility and provide design

information for cathodic protection system. The assessment shall include the

electrical continuity between elements of structure, reinforcement within element

and other ancillary steel items fixed on the structure. At the subsequent repair

stage, reinforcement electrical continuity shall be further checked at all exposed

reinforcement of concrete repair breakout.


21/70


20

g) Half-cell potential survey: electro-potential measurement is used to indicate theprobability of corrosion being active in reinforcement embedded in concrete. it is

found that there was 95% probability of corrosion in regions where the potential

was more negative than -350mV with respect to copper/copper sulphate half-cell

while only 5%where potential was less negative than -200mV. It is not necessary

to carry out the potential survey on entire structure, only representative areas ofboth damaged and apparently undamaged shall be surveyed for corrosion

activity.

h) Concrete electrical resistivity: since the corrosion is an electrochemical process,the electrical resistivity of the concrete will have a bearing on the corrosion rate

of the concrete as an ionic current must pass from the anodes to the cathodes.

The four probe resistivity meter was generally be used to determine the

resistivity of concrete. The measurement can be used to indicate the possible

corrosion activity if steel is depassivated. In general when the measured

resistivity above 20 K-ohm, it is considered with low corrosion risk, whiles the

measured value below 5 K-ohm, it is considered with very high corrosion risk.

The impact of variation in concrete resistivity shall be considered during thecathodic protection system design.

3.1.2 Design consideration of cathodic protection system

In the design of a cathodic protection system, the most import factors for the designer

is to consider the level of electrical current density on the steel and the current

distribution path. Beyond these requirements, there are several secondary concerns

such as cost, durability of system, life expectancy and the maintainability. Before the

design of cathodic protection system, the present situation of the structure and some

design parameters have to be obtained by some structural assessment and field tests.

Current Density requirementThe selection of a suitable current density output is critical for the cathodic protection

system design. The current density requirement is depending on the steel corrosion

state before cathodic protection is applied. According to the guidelines from British

standard, a typical current density of 2 20 mA/m2

for steel reinforcement is

sufficient for general aggressive environment such as the exposure to marine. For

some extreme cases, such as the industrial effluent, some trial shall be performed to

determine the current density requirement. In addition, for the case of cathodic

prevention that is the system installed in new structure, the typical value of current

density could be down to 0.2 2 mA/m2, it is because the electrochemical potential

for steel is less negative, also the passive steel is more easy to be polarized.

Choosing of suitable anode system

After determined the protective current density, then the current requirement for unit

area of element could be worked out. Based on the required output and on site

situation of the structure, the suitable anode system could be figure out. In the marine

exposure, the commonly used systems are titanium anode in overlay and titanium

ribbons in slots. The slot system is used on decks or beams where increase in load is

not acceptable or the required current density is too high for overlay system but

provided that the concrete cover is sufficient. The slot system will only be used for

site with these constraints, as it is an expensive and high difficulty option.

The main restrictions on the overlay type systems are caused by the cementitiousoverlay. The drawbacks are the increasing of dead load on the structure, the difficulty


22/70


21

for applying in complicated geometries and the limitation on the output current of

anode mesh on the concrete surface. It is essential that good quality control is

maintained to get a good quality, adherent overlay. For the structure would not be

exposed to wearing, conductive coating on concrete surface would be a good choice,

the advantages of this system are its negligible increase in dead load, it can be applied

too any geometry and it is cheap and simple to repair or replace.

Zone design of Cathodic protection system

In order to provide a efficiency cathodic protection system, individual areas where

there is a significant change in the environment of the steel reinforcement should be

protected by separate control circuits, that is so called zoning of cathodic protection

system. These changes are normally identified during the investigation survey by

large variation in the resistance of concrete and potential of steel. These can be

changes in moisture content, chloride content, concrete cover or geometry of the

component in a structure. Typically, zones of the order of 50-100m2

are recommended

to avoid excessive power loss in the cabling. For example in the marine structure, it is

common to split the structure into separate zones relative to the water level, such assplash zone, semi-submerged zone and submerged zone.

3.1.3 Acceptance Criteria for Cathodic Protection System

According to the British Standard, for any atmospherically exposed structure, any

representative point shall meet any one of the following criteria

a) An instant off potential (measured between 0.1s and 1s after switching the powersupply off) more negative than -720 mV with respect to Silver/Silver

Chloride/0.5M KCl reference electrode.

b) A potential decay over a maximum of 24 hours of at least 100mV from instant off.c) A potential decay over an extended period (typically 24 hour or longer) of at least

150 mV from the instant off subject to a continuing decay and the use of referenceelectrodes for the measurement extended beyond 24 hours.

The instant off potential are generally be used for assessment of cathodic protection,

as after switching off for instantaneous off (IR free) potential measurements,

sufficient time shall be allowed before measurement to avoid any transient voltage

arising from switching surges, capacitance or resistance effects that would affect the

measured values but this waiting period shall be sufficiently short to avoid significant

depolarization.

In practice, the best control criterion is based on a potential shift, which of 100 to 150

mV will reduce the corrosion rate by at least ten times. Field evaluation also had

shown that there is no further signs of corrosion damage in cathodically protected

structure achieved this criterion.

Besides, no instant off steel/concrete potential more negative than -1100 mV with

respect to Silver/Silver Chloride/0.5M KCl reference electrode shall be permitted for

plain reinforcing steel or -900 mV for prestressing steel to avoid hydrogen

embrittlement.

3.2 Technical Difficulties in implementation and operation

3.2.1 Hydrogen Embrittlement of Prestressing steel

The improper operation of cathodic protection to the steel in concrete may cause

hydrogen generation on the steel surface resulting in hydrogen charging of the steel if

the potential achieved is sufficiently negative. It is well known that hydrogen chargingcan cause embrittlement of high-strength steels.


23/70


24/70


23

(b) If corrosion related cracking and spalling is evident then the tendon should beexposed and inspected for uniform and localized corrosion. The structure is

qualified for cathodic protection if:

- the remaining cross-section is at least 85% in an area of uniform corrosion;- the remaining cross-section is at least 90% in areas of localized attack

(c) Measures to encourage uniform current distribution such as zoning of the anodesystem so that the IR drop within the anode system is less than 100mV and

matching the anode system layout to the density of the steel surface area in the

concrete.

(d) Installing reliable and stable reference electrodes near prestressed steel cables.(e) Installing a remote control and continuous monitoring system capable of adjusting

the system output and storing operating data. The system should be fitted with

failsafe current limiting devices.

3.2.2 Corrosion Interaction to surrounding structure

Cathodic protection of a structure may cause accelerated corrosion of neighboring

structures if that structure is in the same electrolyte. This is particularly observed forburied or immersed steel structures where the flow of cathodic protection current from

the anode to the structure through the earth or water can traverse other structures in

the vicinity. The corrosion rate on these neighboring structures increases where the

current leaves the structure to return to the cathodically protected structure.

The amount of damage likely to occur from stray current corrosion of a steel structure

can be calculated using Faradays law, and for 1 A passing for one year some 9kg of

steel will be corroded. In practice it is not possible to measure the amount of current

being discharged from a structure and so when testing for interaction a potential shift

criterion is adopted.

This is done by measuring the change of potential of the unprotected structure as the

cathodic protection system is energized. A change in the positive direction indicatescurrent leaving the structure at that point gives a maximum value of +20mV for all

structures apart from steel in concrete, before mitigation measures are required.

The position is more complex if the secondary structure is steel in concrete because

steel when immersed in a sufficiently caustic solution (around pH 11 and higher) can

be made to discharge current without any apparent metal loss. This is because the

current discharge leads to a loss of alkalinity in preference to the oxidation of steel

and the alkalinity from the bulk of the electrolyte. In practical terms however it is

often better to adopt a cautious approach and utilize the 20 mV criterion. This is

because the behaviour of steel may be affected by the presence of chlorides in the

concrete.

In general conditions, it is unlikely that above ground cathodically protected concrete

will create interaction problems with secondary structures. This is because of the dose

proximity of anode and cathode and the relatively high resistivity of the concrete. The

cathodic protection current will tend to flow between anode and cathode, i.e. between

the anode and the rebar, and is unlikely to flow into secondary structures.

However, if the anode system is some distance from the structure as could be the case

with buried reinforced structures, e.g. pile caps, prestressed concrete pipe etc., then

interaction is far more likely.

In all cases the risk of interaction should be considered at the design stage and the

design should cater for any high-risk items. Low-risk items can be tested when the

cathodic protection system is commissioned. If interaction does occur, then possibleremedial measures include making a resistive bond to the secondary structure to


25/70


26/70


25

film anode systems the resultant rust staining can be unsightly. Metallic anchors for

junction boxes and cable ducts also face a similar problem.


27/70


26

4. Case study of refurbishment of chloride contaminated Bridge

4.1 Background

The repair of corrosion damaged reinforced concrete structures is a problem facing

many owners of infrastructure in the United Kingdom.The Nettlehill Railway Bridge is situated in the Knightsridge area of Livingston and

carries the A899 Livingston Road over the Bathgate to Edinburgh railway line and a

pedestrian footpath. This is the main route into Livingston from the M8 Motorway

and is one of the busiest roads in West Lothian.

The bridge was built in 1967. It has two spans and consists of 2 decks, one for each

traffic direction. The decks are constructed with pre-cast, prestressed concrete

inverted T-beams with an in-situ reinforced concrete filling. Each deck is simply

supported on reinforced concrete abutments and central reinforced concrete piers. The

overall span of the bridge is 24.84m and its width 35.13m.Routine inspections noted

damage to the structure and detailed concrete investigations found extensive chloridecontamination.

In order to ensure the continued safe operation of the structure for and beyond the

design life repair and protection measures were necessary. The procurement and

delivery of such a project involves many parties; and in this instance required specific

attention to the constraints of a live railway, including carrying out work during night

time railway possessions.

4.2 Structural Investigation

Routine Inspections of the structure alerted the Council to the problems with the

structure and in 1999 an extensive concrete investigation was undertaken. Theinvestigation comprised:

- Visual examination

- Hammer tapping survey

- Covermeter survey

- Half cell testing

- Depth of carbonation

- Chloride content

- Reinforcement inspection

- Cement /Alkali

The investigation showed damage to the concrete abutments and piers mainly relatingto water ingress through the deck joints. As a result, in 2001 refurbishment works to

the deck were undertaken. The deck was waterproofed and the joints replaced. This

work was the minimum required to ensure that the deterioration of the sub-structure

was halted. Funding constraints did not permit sub-structure repairs to follow on

immediately.

In 2004 West Lothian Council provided an additional 7.5M to be spent on clearing

the backlog of repairs to council bridges. This increase in funds allowed the

refurbishment of Nettlehill Railway Bridge to be completed.

A principal inspection undertaken by the councils consultant, URS Corporation,

identified the extent of the work required. It found that the main composite deck wasin fair condition and confirmed that the abutments and central piers were in poor


28/70


27

condition with significant areas of delamination, cracking, spalling concrete and rust

staining. A further concrete investigation was undertaken to accurately identify the

extent of the repair work prior to any repair contract being undertaken.

Problems Identified by 1999 Concrete Investigation

The visual and hammer tapping survey found large areas of delamination, spalling andexposed reinforcement on both abutments and the piers. Some of the areas extended

from the abutment shelf down to ground level. It was later found that these areas

extended to the top of the foundations and double delamination was found in some

areas. When plotted out the shape of the delaminated areas appeared to coincide with

the pattern of water ingress from the joints.

LocationApproximate surface area

spalling and delaminated (%)

North Abutments 20

East Pier (south face) 2

West Pier (south face) 5East Pier (north face) 1

West Pier (north face) 20

South Abutments 20

This survey accurately identified the extent of the repair work prior to any contract

being undertaken. However, a full concrete investigation was required to accurately

identify the cause of the deterioration and any other potential problems.

The covermeter survey recorded the readings below and in general showed that thecover is adequate:

LocationMm. average cover in test

area (mm)

Max average cover in test

area (mm)

North Abutment 26 56

East Pier 52 61

West Pier 53 78

South Abutments 36 55

The half-cell potential survey found a large number of areas where reading indicated

an increased risk of corrosion. Readings more negative than -350mV with respect to

copper sulphate electrode (CSE) suggest a 95% risk of corrosion. These areas mostly

coincided with the areas where deterioration was already visible. Higher thanexpected results were also recorded due to leakage at some locations. The following

results are typical of what was found:

Location Test Area

% of readings more negative

than -350mV.CSE

Or Comments on corrosion risk

North West Abutment 5 / 6 81/20

North East Abutment 1 / 3 / (4) 33 / 67 / (ok)

East Pier 17 Increased risk but majority ok

West Pier 19/20/21 Increased risk on north face

South West Abutment 40 / 42 84 / 12South East Abutments 37 / 39 / 38 High / high / low risk


29/70


28

Depth of Carbonation measurements were carried out at 25 locations and the majority

of the test results were not greater than 5mm. Carbonation was limited to the surface

only and was not contributing to the deterioration of the structure.

Chloride content testing found that there was extensive contamination across the

whole structure. 0.4% weight of chloride to weight of cement is considered torepresent a high risk of corrosion. The figures below are often well above this

particularly in areas where water had been leaking through the deck joints. On all

parts of the structure samples taken from higher levels had higher concentrations. The

depth range clearly shows that reinforcement is well inside the contaminated concrete.

Reinforcement inspections were carried out at 16 locations. Moderate to severe pitting

corrosion was found at nine of the inspection locations with the remainder showing

slight to moderate general corrosion. All the pitting corrosion locations were situated

within areas of leakage and/or high chloride content on the abutments and the north

face of the pier.

Location Test Area Chloride Content

(%)

Depth Range

(mm)

North West Abutment General 1.82 2.05

0.27 0.49

5-30

120 -150

North East Abutment 3 (high level)

Low level

0.72

0.16

0.45

0.56

0.1

5 30

90 120

120 150

5 30

90 120

East Pier General 0.83 0.08

0.4 0.05

Surface

Reinforcement

West Pier 0.84 4.58

0.2

Reinforcement

120 150

South West Abutment

South East Abutments 1.01 -3.1

0.58 - 3.1

Surface

Reinforcement

Cement and alkali content testing concluded that the alkali contents of the eight

concrete samples are within normal limits.

In summary it was clear from the above testing that the substructure of NettlehillRailway Bridge needed urgent action to correct the defects before the structure

became unstable.

The cause of the defects could clearly be traced to the leakage through the movement

joints. The defects were worse at the top of the abutments and where the most water

ingress was noted. Frost damage, high half cell potentials and high chloride content

are all indicative of damage due to the ingress of water containing de-icing salts.

The other important factor revealed by the testing was that the problems are more than

superficial. There was deep penetration of chlorides, which if not treated would causefurther damage to the structure after repair.


30/70


29

4.3 Overview of Refurbishment scheme

Resource constraints, meant that the project could not be handled in-house and URS

Corporation was appointed as the councils consultants for the Refurbishment. Right

from the start this proved valuable as URS had recent experience of dealing with

Network Rail and First Engineering. First Engineering were brought in early in the

design process to advise on what requirements and restrictions Network Rail would

put on the works. Careful consideration was given to work required to be undertaken

during railway possessions.

Their first task was to carefully consider and cost out all the available repair options.

Various options were considered:

Option A Do nothing. Carry out regular inspections and allow deterioration to

continue. With the structure a long way from reaching its design life and serious

deterioration already evident this option was taken no further.

Option B Patch repairs. Breakout and repair the damaged and delaminated concrete

identified. This is no more than a cosmetic solution and would only provide a shortterm solution. Known areas of chloride contaminated concrete would be left in the

structures and would cause further deterioration to the structure.

Option C Cathodic protection. The installation of CP allows the chloride

contaminated concrete to remain in place and will halt the corrosion of steel

reinforcement. Only the damaged and delaminated concrete would be broken out and

replaced.

Option D Concrete replacement. Breakout and repair all areas of defective concrete

including those areas identified as chloride contaminated. Up to 150mm of concrete

would have to be removed in many locations and ongoing testing would be required

to identify all the contaminated concrete. Accurate costing would be impossible attender stage.

Option E Demolish and Rebuild. This option is extremely expensive and

unacceptably disruptive.

Estimated Repair Costs

The identified options were costed. This showed that breakout and repair in

conjunction with CP provided best value, the estimates of each option are shown as

follows

Option Estimated Cost

A N/AB 699 606

C 879 606

D 1 230 590

E 3.5M - 4.5M

Corrosion Protection

A drawback with patch repair (option B) is that if any chloride contaminated concrete

is left in place it will continue to cause corrosion. In some cases it can even accelerate

corrosion in the steel adjacent to the repairs. This process is known as the incipient

anode effect. The only guarantee that corrosion will be stopped is to remove all the

contaminated concrete. This could be difficult to achieve and expensive.Cathodic protection can prevent the above and has a number of benefits:


31/70


30

- One off treatment with a design life of up to 60 years.

- Well varified technique

- Cost effective

- Potential cost saving

- Only damaged concrete is removed

- Contaminated concrete can remain- Avoids extensive temporary propping / closures

- Reduce the time taken for the repairs

- Prevent corrosion from reoccurring even if deck joints fail in future

In West Lothian four bridges had already been given cathodic protection. All of them

had suffered from similar problems to Nettlehill Railway Bridge. In each case CP was

adopted for its economic benefits.

Chosen Repair Scheme

The cathodic protection option was selected for Nettlehill Railway Bridge. It is theoption that provided Best Value, was the most cost effective solution and provided the

best long term solution for the structure. More importantly all the work could be done

without disrupting traffic on the A899 and the trains. As a Local Authority this final

issue is often as important as the engineering issues that are considered.

Proiect Procurement and Tendering

Having undertaken additional site investigation we were confident that the quantity of

concrete repair required was relatively accurate so an NEC contract with an activity

schedule was used. Tenderers were provided with a specification, drawings of the

structure including reinforced concrete details and drawings showing the extent of thework required. The CP was to be a contractor designed element along with any

necessary propping and temporary access. Also included in the tender were a set of

Railway Possessions that were pre-booked. The tenderers were permitted to use all of

these possessions and book further possession at their own expense.

The activity schedule form of contract allows the contractor to select the repair

techniques, materials and construction programme that they are most comfortable

working with. They are best placed to consider this having been involved with work

like this before. From the clients perspective there is some assurance that tenderers are

giving proper consideration to the work required when they compile their tender

return.

The local authority cannot specify specific products and sub-contractors. Tenderers

are free to use any product or supplier provided they fully comply with the

specification.

The interface with the railway was always going to be critical for the success of the

refurbishment scheme. The tenderers selected to bid for the work all had experience

working on railways and were registered on Constructionline.

The tendering process followed West Lothian Councils standard procedures and was

won by Freyssinet Ltd. They in turn appointed Corrosion Control Services as theirspecialist sub-contractor for the CP. The tender price was within budget and the


32/70


31

contract was awarded in July 2005. The work started on site on 25 July 2005 and was

complete by Christmas.

4.4 Site Constraints

Railway (Possessions)

The Bathgate to Edinburgh branch line passes below the Nettlehill Railway Bridgeand it is an important commuter route into the city. This means that the bulk of the

land below the bridge belongs to Network Rail and any work undertaken is subject to

approval and oversight by Network Rail. The north abutment and the piers are all in

this area.

The railway was the main constraint to the scheme that would govern how the works

would be undertaken and programmed so the consultant made early contract with

Network Rail and their contractor First Engineering. A site meeting was held, all the

practical issues were discussed and suitable working practises were agreed at the

design stage.

No disruption to train services on the line was permitted by Network Rail and

possessions were only permitted for short periods during the night or longer durations

at the weekend. The line could be closed from 00:15 to 09:Oohrs on Saturday night

and 00:15 to 06:15 on Sunday night. These times include the time it takes Network

Rail to close the track and can be subject to late cancellation.

West Lothian Council had to sign and agreement with Network Rail before the works

could commence. It is a standard document that indemnifies Network Rail against any

accidents or disruption caused by works being undertaken on their land by outside

parties.

A couple of factors worked in our favour, there is only a single track and it is not

electrified. In consultation it was agreed at the design stage which areas of the

structure could be worked on during the day and which could only be done during

possession working. These were as follows:

As there was enough space between the track and the north abutment, a safe working

zone was permitted. This area had to be fenced in to prevent operatives straying onto

the live railway and was established during the first overnight possession. Once in

place all the work was able to proceed within the zone at any time of day.

The south face of the piers was also designated a safe working zone. Fences were

built at either side of the bridge and full height containment was provided between the

two piers. Network Rail also permitted the creation of a temporary access through

their boundary fence for site access.

The north face of the piers was too close to the track and all work had to be

undertaken during possessions. It was agreed that props could remain in place, as it

was impractical to remove them after each possession.

In addition to the working zones Network Rail also required all the scaffolding to be

fully contained and all debris to be removed from the site. Their ballast was not to be

contaminated with any waste from the site.

Network Rail required their contractor to be on site whenever work was being

undertaken. Their supervisor gave every operative and visitor a briefing before

entering the site and inspected the works at regular intervals. Unfortunately the cost ofthis supervision had to be met by West Lothian Council.


33/70


32

The good working relationship established by the consultant with Network Rail and

First Engineering carried on with the contractor, Freyssinet. They were able to

reorganise possessions to better suit their programme and were able to source

specialist plant from First Engineering. It was fortunate that none of the possessions

were cancelled.

Network Rails involvement with the scheme was to approve the CP installation. They

were concerned at the possibility of stray currents affecting their signaling equipment.

It was agreed that some inter-action testing should be carried out prior to the system

being switched on and fully energized.

Public Access

Pedestrians had to be given free access along the footpath beside the south abutment.

Closing the footpath was not considered practical, as the only safe alternative route

was a long diversion to a footbridge further down the A899. Inevitably people would

be tempted to try crossing the busy dual carriageway instead. The contractor was

required to undertake the repair work and guarantee the safety of pedestrians.

With properties close to the site, people were always going to have an interest in what

was going on. To comply with West Lothians Considerate Contractor Scheme,

adjacent houses were letter dropped and advance warning signs were posted giving

relevant information.

Unauthorised public access was also an issue that needed careful consideration, as

vandalism is now a widespread a problem. The site compound needed to be secure, as

did the working areas, particularly scaffolding when operatives were not on site.

Traffic & Traffic Loadings

The A899 is one of West Lothians busiest roads and is classed as traffic sensitive.

This means that no traffic Management is permitted on the road during the rush hours,

between 07:00 and 09:30hrs and 15:30 and 19:30.

No closure of the bridge was permitted as the knock on effect on adjacent roads and

given its proximity to the M8 any closure would have had severe consequences and

was not acceptable.

With the bulk of the repair work being on the substructure this was not a big issue on

this contract. However, there is always a risk when breaking out concrete close to the

bearing shelves that areas of the bridge could become unstable, In this instance a

bridge closure would have had a serious impact on traffic and trains and was an

unacceptable risk.The contractor was therefore required to provide props with enough capacity to

support the deck whenever work was to be undertaken in the area of the bearing

shelves.

4.5 Selection of Repair Methods

Concrete Repair

The areas of the piers and abutments that required repair were relatively extensive, in

particular on the abutments. However, generally the corrosion was limited to the outer

layer of reinforcement and the amount of reinf

Date post:	07-Apr-2018
Category:	Documents
Upload:	david-lee
View:	222 times
Download:	0 times

Msc Project Draft Report

Documents