Axios bridge final-english

Axios Bridge – Section Athens to Thessaloniki

Dr. Chris A Rodopoulos, Dr. P. Panetsos and Mr. I. Koropoulis

Application of galvanic cathodic protection using Zink Sheet Anodes according to ΕΝ 12696

Axios bridge consists of two sections. The section Athens to Thessaloniki was given to traffic in

1973. The section Thessaloniki to Athens was inaugurated in 1988. The bridge is part of E75

motorway.

Axios Bridge – Historical Data


The bridge is made of 29 spans of 30 m each resulting in overall length of 870m. Due to river

crossing the bridge is founded on R/C piles.


The superstructure is made of longitudinal and transverse prestressed beams.


Prestressed cross beam detailing


Typical reinforcement detailing. Concrete has been classified as C30/37 after coring.


A CuSO4 reference electrode has been used. All values in mV.

Measurements and Pathology – Half Cell Measurements of External Beam


– Projection of values on beam

The high negative values at area A indicate waterproofing failure of the expansion joint. The high

corrosion propensity at areas B and C, indicate potential local failure of the deck waterproofing

layer.

Area ΑArea Β Area C


– Projection of values on tendon tracks

Note that the anchors of tendons 3-8, exhibit high corrosion potential. Similar problem is

identified at the location of tendons 9, 11 and 12.

Measurements and Pathology – Concrete Electrical Resistivity Measurements

All values in KOhm cm.

Measurements and Pathology – Concrete Electrical Resistivity Measurements– Projection of

values on beam

Electrical resistivity is an indirect way of evaluating concrete’s build in moisture and hence the

susceptibility to ionic current movement (controls the cathodic / oxygen reaction). To

acknowledge the importance of the measurements, it is worth noting that the electrical

resistivity of C30/37 concrete, having build in (hygroscopic) moisture of 2% w. t., is around 70

KOhm cm.

Measurements and Pathology– Linear Polarisation Measurements (LPR)

All values in μΑ/cm2. The measured values classified corrosion belonging into the low to

medium rate according to the above Table. Certain areas in purple are classified as passive.

Measurements and Pathology– Linear Polarisation Measurements (LPR) – Projection of values

on beam

It is important to consider that low values of corrosion current density are most likely to

increase with time compared to high values which are bounded by the nature of concrete’s

pathology. It is also worth noting that values above 0.5 μΑ/cm2 can lead to complete loss of

bond strength within 2-10 years.

Measurements and Pathology– Linear Polarisation Measurements (LPR) – Projection of values

on tendon tracks

Projection of the measured values over the design and especially over tendon tracks is perhaps

the most crucial action when evaluating corrosion in prestressed elements. The generated

image help us identify critical locations while at the same time provides information regarding

potential causes. Herein, we can easily identify that tendons No. 8, 9, 11, 12, are within the area

demonstrating the highest corrosion rate of the sample. It is important to note that the

transverse beam is also critically corroding. The surface tendon 11 appears to experience

significant corrosion rate due to overhead failure of waterproofing.

Measurements and Pathology – Concrete Pathology

Axios Bridge is located in a semi-urban environment experiencing high humidity due to river. The distance

from the sea is over 5 Km and therefore the potential of airborne chlorides diffusing into concrete is

considered as negligible. The bridge is rarely subjected to de-icing salts. Even though vehicular traffic is

substantial, the location experiences CO2 concentration below 300 ppm. In conjunction with the high

humidity of the area, concrete experiences a rather low carbonation rate.

The bridge is suffering from poor storm water drainage, non sealed expansion joints and damaged deck

waterproofing layer.

Accessibility is the most crucial issue in the assessment of concrete's pathology in large structures and

especially prestressed bridges. Herein, it is imperative to collect samples from locations, a) being critical to

the load bearing capacity, b) being indicative to the actual problem both in terms of chemistry and

concentration and c) providing a sound basis for damage classification. The later, is mostly governed by

sample population and the type of pathology being initially identified. The level of carbonation and chloride

attack, coming from airborne chlorides, is perhaps the easiest of all since they rarely demonstrate significant

position variations.

Tendon anchors and tendons are critical items which require particular attention. Surface tendons are

notoriously difficult to locate and approach even though is widely known of being liable to corrosion.

Similarly, tendon anchors in old bridges are usually located in tight spaces with poor access.

Due to accessibility issues, samples were collected from areas being close to the expansion joints and

experiencing spalling. Indicative results are shown in the Table below.

Samples Concentration of Total Chlorides

Cement (% w.t) - ASTM C1152

Concrete Alkalinity

Sample 1 0.19 8.2

Sample 2 0.25 6.9

Sample 3 0.27 7.7

Sample 4 0.21 7.8

Sample 5 0.27 7.6

According to EN 8110, a total chloride concentration of 0.1% is considered as threshold value for

critical corrosion in prestressed elements. The above limit however refers to concrete alkalinity,

pH, being >11. In the samples, concrete alkalinity has dropped as low as 7.6 indicating that

critical chloride levels are several orders below 0.1%. In addition, alkalinity values are significant

below the lower limit value indicating carbonation, i.e. pH=8.4. The phenomenon of low

alkalinity is attributed to water soluble chemicals from tyre wear, brake wear particle emission,

and exhaust discharges (traffic dust).


All samples have been collected at depth equal to concrete cover thickness (30mm).

Traffic dust’s most common chemicals and their typical concentration


Most of these chemical are known to disintegrate both concrete and steel (ACI 515.1R).


Axios Bridge experiences the simultaneous effect of chlorides due to de-icing salts along with chemical

attack due to traffic dust. Both, in their soluble form shall be considered as the main cause of corrosion.

Herein, once again accessibility is vital in order to collect samples that can be analysed in order to provide

secure information regarding concentration of the hazardous chemicals and their reactions with cement

constituents. Indirectly we can overcome the problem by comparing our LPR measurements with the limits

of certain exposure conditions as depicted in the Table below.

Exposure Class per EN 206-1 LPR Values ( μΑ/cm2)

Average Standard Deviation

XC1 0 -

XC2 0.35 0.26

XC3 0.17 0.08

XC4 0.43 0.26

XD1 0.35 0.26

XD2 2.60 1.70

XS1 2.60 1.70

XS2 - -

XS3 6.00 3.50

Axios bridge belongs into classes XD2 and XD3 (de-icing salts). The reader can easily compared the

previously reported values and acknowledge their deviation form the above limits. Such difference indicates

the supplementary effect provide by traffic dust residues.

Basic Principles of Galvanic Protection

Corrosion is based on two reactions

1st Half Cell Reaction Anodic reaction – Oxidation of iron, Iron is oxidized from Fe (oxidation state

0) to Fe 2+ (oxidation state +2).

2nd Half Cell Reaction Cathodic reaction– Reduction of oxygen, the liberated electrons from the

oxidation of iron are consumed by oxygen in the presence of water to form hydroxyl (OH−).

In this reaction oxygen is electrochemically reduced from O2 (oxidation state 0) to OH− (oxidation

state −2).


If there is no external electric source of electrons, the anodic reaction must generate electrons

at exactly the same rate as the cathodic reaction consumes them.

If electrons were withdrawn from the metal surface, it might be anticipated that the anodic

reaction would speed up (to replace the lost electrons) and the cathodic reaction would slow

down, because of the existing shortfall of electrons. It follows that the rate of metal

consumption would increase.

If however additional electrons were introduced at the metal surface, the cathodic reaction

would speed up (to consume the electrons) and the anodic reaction would be inhibited; metal

dissolution would be slowed down.


Inhibiting corrosion via external source of electrons is the basic principle of cathodic protection.

In the case of electrons being provide by an electric source, cathodic protection refers to

Induced Current Cathodic Protection (ICCP). If the source of electrons is provided by a sacrificial

metal, being more electronegative, cathodic protection refers to Galvanic Cathodic Protection.

This fundamental principle can be appreciated via the Poubraix diagram for Iron.

The grey area in the diagram on the left

encapsulates the limits of cathodic

protection.


Schematically, the excess electrons provide by galvanic cathodic protection (leading to corrosion

inhibition) is shown below.


The availability and the potential of metals able to provide excess electrons is governed by the

Standard Reduction Potential Table. We can easily identify that, Zn, Al and Li are more

electronegative than Fe and therefore can provide excess electrons.

Since the excess electrons are generated by the corrosion of the more electronegative metal,

the term sacrificial metal is widely used.


In Galvanic Cathodic Protection, the potential difference alone is not enough as to proceed with the

selection of the anode metal. Generated anode (sacrificial metal) current density and electrochemical

capacity (Amp-h/Kg) are also important parameters .

The “Potential volts” refer also referred as polarisation potential. For steel in concrete, a polarized

potential more negative than –800 mV measured with respect to silver/silver chloride reference electrode is

required by EN 12696. The reader shall not confused potential values given by different reference

electrodes. In this case, the -1100 mV of the Cu/CuSO4 are equal to -1044 mV of Ag/AgCl 0.05M KCl.

Simplistically, electrochemical capacity is the result of Faraday's law. For example, pure zinc has a theoretical

maximum capacity of 820 Ah per kilogram. This means that if a zinc anode were to discharge one ampere

continuously, one kilogram would be consumed in 820 hours. If this kilogram was discharging one tenth of

an ampere, it would be totally consumed in 8200 hours or 48 weeks. Actually, zinc anodes operate, typically,

at about 95 % efficiency. This means that the energy content available for useful current output would be

820 x 0.95, or 779 Ah per kg.


Cathodic protection current density is currently defined by regulation. Even though differences can be easily

identified, the minimum requirements remain approximately fixed.


Variations in the range of cathodic protection current density are mostly acknowledged by the diagram

below.

Here the reader can study, different cathodic protection current density requirements as per chloride ions

concentration in concrete. SCE refers to Saturated Calomel Electrode.


Variations in cathodic protection current densities throughout the years have been classified in order to

provide a sound basis over which repair cost can be realistically estimated.

Current density requirements along with the surface area of steel to be protected (demand), define among

others parameters, whether ICCP or galvanic protection is to be used. It is clear that high current demand

for a large period of time consumes faster the sacrificial anode while at the same time might not be able to

generate the required potential.

The reader can easily relate the above table to that of slide no. 19 .

Axios Bridge Galvanic Protection

Tendon tracks, tendon anchors and non sealed joints create a rather complex scenario for uniform

polarisation potential distribution. In the case of existing bridges offering limited accessibility, the problem

exponentially increases leading to over-polarisation issues and potential hydrogen gas generation . The use

of control sensors and complex programming of the cathodic protection control unit can sometimes only

provide limited solution. In addition to the above, a cathodic protection control unit is usually unprotected

from actions of vandalism and theft.

For the above reasons, the use of Galvanic protection was chosen. Due to the large surface area and the

difficulty in boring to encase embedded anodes, the use of Zinc / Hydrogel Anode or known as Zinc layer

Adhesive Anode or ZLA. This anode consisted of a zinc foil, which measured 0.25 mm thick by 0.25 m wide ,

a conductive adhesive gel (3M Company’s Hydrogel™) bonded to one side of the foil, and a release paper

sticking to the other side of the adhesive gel. In this application, ZLA provided by Mapei under the

commercial name of Mapeshield E25 was used.

Mapeshield E25 provides 455 grams of Zinc per running

meter and is certified according to EN 12696.

Axios Bridge Galvanic Protection

Calculations and the subsequent plans were based on an initial protection current density of 5mΑ/m2 for

the first 12 months followed by a value of 1.5mΑ/m2 for the remaining protection period. A 10 year overall

protection period was considered. A safety factor of 1.1 was implemented. Two reference electrodes were

positioned to monitor the tendons. The overall anode performance level was set at 80%.

Application of Galvanic Protection - Step A - Hydro blasting 500 Bar

Rigorous cleaning of the surface is imperative for Hydrogel to maintain a uniform and maximum

ion current flow. The pressure refers to nozzle output.

It is important to note that the nozzle to surface distance was kept between 5-10 cm. It is a

common mistake of contractors performing hydro blasting to operate at longer distances. A

practical tip identifying correct distance (depends on concrete strength, pressure and flow rate

of the unit) is to achieve concrete skin (2-4mm) removal at a rate of > 0.1 m2/min.


Hydro-blasting at 500 bars, is usually enough to remove loose concrete due to spalling.


Prior to any patch repair it is important to check for electrical continuity. In the case where

electrical continuity (a value <0.1V at DC setting indicatives continuity) is not obtained, the

contractor shall proceed with artificial connection using the fixings and a piece of the

recommended cathodic protection cable. The photograph shows a “bridge” used to achieve

electrical continuity between two stirrups. Connections and fixings are protected using a

conductive sealant.

Application of Galvanic Protection – Step B – Local Patch Repairs

Fixing to steel reinforcement is made using a low voltage cable. Usually type FG7R-0.6/1KV 1 Χ

8. In this particular application connection was made using stainless steel rivets and ring eye

terminals.


Patch repair grout should be according to ΕΝ 1504 parts 2,3 while is mandatory to have an

electrical resistivity < 10 KOhm cm (ΕΝ 12696). This is because the grout should allow the

unrestricted row of ions generated by the Zinc / Hydrogel Anode.


Sufficient time shall be allowed prior to the application of the ZLA over the patched area. In such

applications, grout strength is not an indicative parameter. The contractor shall measure the

electrical resistivity using a Wenner Probe. Once the reading is within its declared value by the

manufacturer, the contractor can proceed to the next step.


After removing the releasing sheet protecting Hydrogel from air, ZLA can be applied on concrete

surface. A rubber mallet is usually used to secure one edge of the anode sheet. A rubber roller is

finally used for final placement and straightening. The stiffness of ZLA and the usually uneven

surface of concrete, usually prevent air being trapped underneath.

Application of Galvanic Protection – Step C – Placing the ZLA

A covermeter is used to identify the location and cover thickness of reinforcement intending to

receive connection to the anode. The action should be performed prior to the placement of ZLA

since measurement over zinc cancels the operation of the covermeter.


A wedge anchor is used to connect reinforcement to ZLA. The location has been previously

identified by covermeter (reinforcement free zone) and indicated on the ZLA using a marker.

Correct connection once again requires electrical continuity measurement.


The process is repeated until all ZLA sheets have been placed. Note that all required connections to

reinforcement are established. That allows the contractor to continuously check and recheck electrical

continuity. A valid order is to start the check between the first ZLA and the last reinforcement connection

and continue the process inwards, i.e. the second ZLA or reinforcement connection with the N-1

reinforcement connection (N is the total number of connections), the third ZLA or reinforcement connection

with the N-2 reinforcement connection, etc.

Connection to steel was made using the following steps,

a) Detection of location and depth of steel using covermeter,

b) Drilling of concrete using a Ø40 bit until reaching cover thickness,

c) Drilling steel reinforcement using a Ø4 Cobalt bit.

d) Placement of eye terminal on rivet and fixing using a pop rivet gun

e) Checking of electrical continuity with a multimeter

f) Placement of conductive sealant over the connection.


Once all ZLAs have been placed and checked for electrical continuity, their edges shall be

protected by moisture attacking the Hydrogel. In this application Mapeflex PU40 was used. A

layer of width around 30mm and thickness 2-3mm is enough.


Sealing of ZLAs with Mapeflex PU40.


Application of Galvanic Protection – Step D – Placement of Reference Anodes

Placement of two reference electrodes type Ag/AgCl/KCl (Castle Electrodes Ltd, LD10) according

to plans. Reference electrodes allows us to perform depolarisation testing according to EN

12696.

It is imperative to do an initial potential measurement to safeguard initialisation and uniform

voltage distribution of the circuit. In this case, values of -417 mV και -380 mV (Ag/AgCl) were

measured from the two reference electrodes.

Εφαρμογή Καθοδικής Προστασίας – Στάδιο Ε – Τοποθέτηση Κυτίων ΕλέγχουApplication of Galvanic Protection – Step E – Junction Boxes and Cable Conduits

Complete sealing of ZLA using Mapelastic Smart. Bond strength on ZLA has a declared value of

>1,6 MPa. Sealing protects the anode from consumption due to air oxidation.

Application of Galvanic Protection – Step F – Sealing the Circuit

Final form of ZLA application

Application of Galvanic Protection

Depolarisation values 7 days after initialisation.

All three EN 12696 requirements

are met.

Instant Off Value R1=-793 mV


Application of Galvanic Protection – Depolarisation Values - 7 days



Application of Galvanic Protection – Depolarisation Values - 30 days

Depolarisation values 30 days after initialisation.

All three EN 12696 requirements

are met.

Advantages and Disadvantages of ZLAs compared to ICCP

Advantages

Low initial investment cost,

The application does not require specialised equipment and personnel,

Significantly reduces damage due to concrete boring required for ICCP anodes,

Protection can last over 25 years as in the case of carbonation,

Relatively easy calculations based Faraday’s equation,

The anode can be easily replaced,

The potential for hydrogen gas generation is negligible,

It does not require control unit,

Certified according to ΕΝ 12696, BS 7361 and AS 2382.5.

Disadvantages

High cost per year when protection over 40 years is needed.

Limited performance and protection when high current density is required,

It is highly unlikely to cause chloride ions extraction,

It is sensitive to temperature and moisture changes.

Characteristic Applications of ZLAs

Bridges

Balconies

Local repairs

Facades

Slabs

Beams

Characteristic Applications of ZLAs

Final sealing can be painted or plastered. In the

case of plastering, surface treatment of the

sealant using quartz sand 0.85-1.2 mm is

recommended.

Characteristic Applications of ZLAs on bridges from around the World

List of key materials used in the application

More information can be found in

http://www.mapei.com/GR-EL/

and

http://www.mapei.com/public/GB/linedocument/Cathodic_Protection_GB.pdf

Application of Galvanic Protection – Project Partners

Date post:	13-Apr-2017
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