Development of a new impressed current anode

for cathodic protection of rebars in concrete Xavier HALLOPEAU a, Christian Tourneur b

, Olivier Lesieutre c

a,Freyssinet International & Cie, 280, avenue Napoléon Bonaparte CS 60002, 92506 Rueil Malmaison Cedex, France, xavier.hallopeau@freyssinet.com b Freyssinet International & Cie, 280, avenue Napoléon Bonaparte CS 60002, 92506 Rueil Malmaison Cedex, France, christian.tourneur@freyssinet.com c Freyssinet France, 280, avenue Napoléon Bonaparte CS 60002, 92506 Rueil Malmaison Cedex, France, olivier.lesieutre@freyssinet.com

ABSTRACT This presentation reports the development of a new impressed current anode for the cathodic protection of concrete reinforcements. The application of a DC current on a carbon fiber reinforced polymer, used both as a structural reinforcement and as an anode surface, allows to polarize the concrete reinforcements.

Keywords: corrosion, anode, carbon fiber, cathodic protection

There is a number of scientific challenges to be overcome to be able to couple the mechanical

reinforcement and the protection against corrosion. It is first necessary to make electrically conductive

the polymer matrix which bounded the carbon fibers and bond the FRP laminates on concrete.

The last two decades have seen increasing applications of fiber reinforced polymer (FRP) in civil

engineering structures due to their many benefits in comparison with traditional materials used for

strengthening and reinforcing concrete structures. As an example, the external epoxy polymer bonding

of FRP composite products (sheets or strips) on Reinforced Concrete (RC) structures allows to increase

the load-bearing capacity and ultimate deformation.

The FRP materials have a modulus of elasticity ranging from 100 to 180 GPa and a tensile strength

ranging from 600 to 2500 MPa. Light and highly resistant to chemical attack, such composite materials

are particularly used in the case of strengthening of existing structures as additional reinforcements by

external bonding. It is also possible to use them as repair materials to allow damage structures to restore

their initial mechanical performance (service overload, steel corrosion, physico-chemical damage of the

concrete).

FRP composites have found their place as reinforcing materials for concrete elements (beams, slabs,

columns, etc.) in thousands of applications worldwide, where conventional reinforcement techniques

could be problematic.

FRP composites are more and more used as reinforcing materials for the following reasons:

• Insensitivity to corrosion;

• Low weight;

• Easier application in confined spaces;

• Avoid the need for scaffolding or stop traffic on bridge and reduction of labor costs;

• Very high tensile strength;

• Large deformation capacity; and unlimited availability of FRP in terms of size, geometry.

Due to these advantages, FRP reinforcement system in buildings and civil engineering works are now

used currently.

Nevertheless, little improvement of the technique was achieved these last 20 years and despite the

progress made in the field of composites, the construction engineers do not fully use all the features of

this reinforcement. In fact this material is conductive, waterproof and can include fiber optic sensor (or

other type of sensor) to give physical, chemical or mechanical data. If conductivity of carbon fiber

reinforced polymer (CFRP) is already used in others industrial field, it is not the case in construction and

it could be useful for cathodic protection. It is then possible to combine the strengthening function with

the development of a multifunctional composite allowing protecting RC structure from steel

reinforcement corrosion and also allowing the monitoring of repairing function versus time.

Indeed, the cathodic protection is an electrochemical technique that minimizes the corrosion of steel in

contact with an ionically conductive medium. In this technique, the steel/electrolyte interface is

electrically polarized using external impressed current. For optimum protection, the polarization should

be carried out uniformly throughout the interface. Otherwise, the steel continues to corrode. The key to

success in the cathodic protection (CP) is the possibility of obtaining uniform distribution of current

through the steel/electrolyte interface. Generally, it is easy to achieve these conditions in controlled

laboratory conditions.

However, cathodic protection under in-situ conditions is sometimes difficult to achieve and maintain

over time. The reasons for these defections should be critically evaluated in terms of the uniformity of

the current distribution of cathodic protection, geometry rebar, locations of electrical contacts, coupled

effects of the environment, and the temperature on the distribution of cathodic protection current.

The goal is to design a composite reinforcement material acting as a cathodic protection which enables a

better electrical distribution on concrete surface in a thermo-hydric environment better controlled by a

theoretical / experimental approach.

The study on the durability of concrete structures in aggressive environments faces the following

challenges:

- The reinforcements based on carbon fibers are bonded using insulating resins, the qualification and

implementation of a conductive resin keeping the mechanical properties of the bonding is a major issue;

- The durability of the conductive resin under the coupled actions of mechanical loading,

electrochemical reactions that occur and the aggressive environment should be characterized. It is

possible that the anodic reactions linked to the intrinsic principle of cathodic protection, acidify and

reduce both the electrical and mechanical properties (in particular bonding) of the resin. These effects

could be exacerbated by the continued loading (creep of the resin) and the aggressive environment

(presence of chlorides in the atmosphere);

Only some researchers have studied the effect of CFRP repair on further corrosion process. The main

protective effect is “The barrier protection‟ which is sometimes called “passive corrosion protection‟,

comparable to some techniques like anticorrosion coatings of concrete structures (EN 1504). Indeed,

CFRP materials, applied as external reinforcement material on RC structures, form a protective barrier

against chloride and moist penetration. This passive protection has been studied in some experimental

researches (1),(2),(3),(4),(5). Besides the impenetrability of the wraps, it was found in these studies that the

confinement of the concrete due to CFRP wrapping has a positive influence on corrosion initiation and

rate.

The idea of using the CFRP in externally bonded reinforcement as an anode for cathodic protection has

been experimentally investigated (5). In this experiment, heavily corroded test specimens were

cathodically protected. A positive effect on total amount of corrosion products was found through mass

loss and pull-out measurements. Less attention is paid to the actual corrosion rates during the CP

program, and conductivity of the FRP sheets. In 2011, P. De Schoesitter (6) has studied the feasibility of

the use of externally bonded CFRP for Cathodic Protection. He added different carbon black powder to

the epoxy resin which is used to bond the CFRP sheets on concrete to obtain an electrical conductivity.

He also used some commercial conductive epoxy resin from different companies. The conclusions of the

study were that externally bonded CFRP appear to be effective for impressed current cathodic protection

(ICCP) (for short term experiments) with carbon black powder probably not the best solution for surface

conductivity, nevertheless the influence of fillers on epoxy strength characteristics should be

investigated and optimized. No mechanical tests are performed in terms of bond between CFRP

laminates and concrete or global mechanical behavior.

Nguyen et al. (7) have shown that CFRP acts as an electrically conductive material and can be used as an

ICCP anode for RC structures. Nevertheless, the bonding must be modified since epoxy resin is not

suitable for bonding CFRP used as anode on the concrete because of the high resistivity of the epoxy.

These authors then used geopolymer with chopped carbon fiber modification to replace epoxy to bond

CFRP anodes to concrete.

Very few researches have studied the coupling between mechanical strengthening and ICCP system.

Lambert et al. (8) have done some ICCP tests using CRFP sheets used as anode. The beams were pre-

corroded using the impressed current technique but it must be highlighted that accelerated technique

do not lead to similar corrosion as natural one.

In their study, there is no coupling between the sustained load and the ICCP system. The cathodic

protection was applied during a short time (max 2000 hours – 83 days) and the efficiency of cathodic

protection was assessing by the depolarisation decay at 4 hours.

It is interesting to note that Lambert et al. found a 13.5 % difference in load-bearing capacity between

sample with dual function (strengthening and CP) comparing with only strengthening function. Moreover

they found that the beams with CRFP having operated as an ICCP anode show a more brittle behaviour

than the one using only as strengthening.

As a result, it seems important to check the aging of the CFRP laminates during the ICCP process.

Zhu et al. (9) have performed a comprehensive experimental program to study the electrical and

mechanical behaviors of a CFRP plate in simulated ICCP systems with varied solutions. CFRP strips

behavior appears to be stable as electrical and mechanical point of view. Nevertheless, the applicability

of coupling between ICCP and strengthening is not studied.

Numerous studies have been achieved about the use of carbon black (CB) powder to obtain electrical

coating which could be used as secondary anode in conductive coating ICCP system, Poltavtseva (10). It

appears that coating loaded with carbon black powder could work as secondary anode but it is necessary

to achieved high percentage of carbon powder (typically -35% to 40% by mass).

Helm and Raupach (11) have performed numerical simulation of the current and potential distribution of

surface applied CP-Anode Systems based on the electrochemical behavior of corroded steel.

Preliminary results achieved by Freyssinet

Freyssinet Company is able to build maintaining solution for concrete structures in both aspects:

structural repairing using CFRP solution, namely Foreva®TFC, and cathodic protection using by

comparison ribbon ICCP solution, namely Foreva®CP Ribbon, but a coupled solution is not proposed at

this moment.

Foreva®TFC is a strengthening technique using Carbon Fiber sheets which are impregnated and bonded

on the concrete structures through an epoxy polymer.

Foreva®CP Ribbon is an activated titanium ribbon which is installed in concrete groove using a mortar

and acting as anode in ICCP system.

Nevertheless, since 2010 Freyssinet has begun some research work on the possibility to use CFRP sheet

as anode in ICCP system and then offer the dual function of structural reinforcement and ICCP anode.

The preliminary tests consisted in adding graphite powder in the epoxy polymer to assure an electrical

conductivity between the Carbon Fibers and the concrete surface to allow a current discharge and to

obtain a comparable polarization of steel bars as classical ribbon. Previous results were presented during

the Cefracor PCRA colloque at Antibes in 2014.

No tests have been really performed to evaluate the durability of the modified FRP and the possible

evolution of the resin due to current injection.

Another important point is to study the optimal repartition of the strips on the concrete structure

depending on the electrochemical performance of the anodic system. As a result there is still some

research work to do to formulate a satisfactory bonding polymer and to be able to correctly design the

strip repartition, the level of current to be applied and the durability of the system in different aggressive

environments.

For that reasons, a site application has been done since April 2014 on the soffit of a reinforced concrete

swimming pool.

The aim of this work was to protect the first layer of rebar of a bottom slab polluted by chloride on a

joint leakage. The further away from the joint, more the chloride concentration decreases

Three CFRP sheets were applied parallel to the joint, length 4 meters, width 15 cm, unit surface 0,6 m²

per sheet. The ratio of steel surface vs concrete surface is equal to 0,7. Figures 1 and 2.

The system is monitored by seven Ti/MMO pseudo electrodes distributed both between sheets and

under sheets to allow potential measurement and depolarization. Figure 3.

The first conclusions of this trial are:

- The current distribution depends on concrete resistivity and chloride concentration,

- 100 mV depolarization at 24h is achieved between the joint and the second CFRP sheet. Figures 4

a) b)

- In case of humidification, the ICCP system reacts very fast, also in non-polluted zone.

- Anodic current density vary from 0,5 to 4 mA/m² of CFRP depending on CFRP location, so on

chloride concentration

- Under constant voltage, the current demand has decreased in the first 6 months to be stabilized.

- No disbonding were observed since one year.

Fig. 1 – Joint leakage Fig.2 – CFRP sheet view

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

A

B

C Positive connectionCFRP 8 10

D n° 3

E9

CFRP 5 7

F n° 2

G

CFRP 3 4

H n° 1

I Negative connection Ti/MMO electrode

Joint - Leakage

T/R unit

Fig. 3 – Electrical scheme

Fig. 4 a)-b) – 24h depolarization test

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