Cathodic protection has been proven to be one of the most widely applicable and cost-effective solutions for tackling steel corrosion in reinforced concrete. In this study, the possible use of carbon fibre composites, which are primarily used to strengthen concrete members, has been investigated as impressed current cathodic protection anodes. Carbon fibre anodes have been assessed in both concrete and calcium hydroxide solution. Two bonding mediums incorporating epoxy and geopolymer have also been investigated. The results demonstrate that epoxy resin can be used for bonding carbon fibre fabric anodes to reinforced concrete structures while geopolymer is more effective for bonding carbon fibre reinforced polymer (CFRP) rod into preformed grooves in the concrete surface. The dissolution of carbon fibre anode appears to stablise after a period of time, dependent upon the size and shape of the anode and applied voltage and current. Based on the present results, a maximum current density of 128 mA/m2 of reinforcing steel area is recommended for the operation of CFRP fabric anode and 64 mA/m2 of reinforcing steel area for that of CFRP rod anode.
Cathodic protection (CP) is a proven method of controlling corrosion in reinforced concrete through the application of a small DC current [
Cathodic protection systems require a level of monitoring, generally by reference electrodes, to assess how well the system is controlling corrosion. For ICCP systems in reinforced concrete, the current can be adjusted to the appropriate value to protect the steel reinforcement [
Carbon fibre composites as civil engineering materials are a relatively recent development. They have previously been used in a wide range of mostly “hi-tech” industries [
The epoxy adhesive was supplied by Sika Corporation (US). In this test, Sikadur300 adhesive was used, which is a two-component 100% solids, moisture-tolerant, high strength, high modulus epoxy widely used in CFRP strengthening applications. It is documented by the manufacturer that Sikadur300 is used as a seal coat and impregnating resin for horizontal and vertical applications.
A commercially available geopolymer developed at Sheffield Hallam University was used to bond the CFRP rods to the grooved reinforced concrete beams. Chopped carbon fibres were added to the geopolymer at 1.5% of volume in order to control shrinkage and increase the electrical conductivity of the mix. The compressive strengths of different compositions of geopolymer which were tested ranged from 44.4 MPa to 52.0 MPa at 28 days age.
A high strength, unidirectional carbon fibre fabric (type Sika Wrap Hex 103C, supplied in the UK by C-Probe Systems Ltd.) was used. In normal applications, the material is field laminated using epoxy resin to form a carbon fibre reinforced polymer for the strengthening of concrete structures.
Carbon fibre rod (type Sika CarboDur, supplied in the UK by C-Probe Systems Ltd.) was also used which is designed for strengthening concrete, timber, and masonry structures. The rods have very high strength, light weight, high modulus of elasticity, are non-corrosive, and display excellent fatigue resistance.
A series of tests was established in which carbon fibre was employed as an ICCP anode with a range of DC voltages applied. The aim was to identify the optimum range of currents that could provide adequate protection to the steel reinforcement without causing significant damage to the carbon fibre anode. Prior to conducting the tests, both the carbon fibre anode and steel bars were weighed and then subsequently reweighed at the end of the test. The ICCP was established with the DC positive terminal connected to the carbon fibre anode while the DC negative terminal was connected to the steel bars.
The DC voltage was maintained at 5 V, 10 V, or 20 V. The tests were run in the ambient laboratory environment (temperature 20°C, RH =
Test programme of carbon fibre anode in saturated Ca(OH)2 and concrete electrolyte.
Specimen | Electrolyte | ID | Total surface area (cm2) | Voltage applied (V) |
---|---|---|---|---|
1.1 | Ca(OH)2 | Steel 1 | 37.7 | 10 |
CFRP rod 1 | 37.7 | |||
1.2 | Ca(OH)2 | Steel 2 | 37.7 | 10 |
CFRP fabric 2 |
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2.1 | Ca(OH)2 | Steel 3 | 37.7 | 20 |
CFRP rod 3 | 37.7 | |||
2.2 | Ca(OH)2 | Steel 4 | 37.7 | 5 |
CFRP fabric 4 |
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|||
3.1 | Concrete | Steel 5 | 43.96 | 10 |
CFRP rod 5 | 43.96 | |||
3.2 | Concrete | Steel 6 | 43.96 | 10 |
CFRP fabric 6 |
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Test arrangements for the carbon fibre anodes (rod and fabric) in saturated Ca(OH)2 and concrete electrolytes.
A variant of the carbon fibre anode test was further developed by the use of small scale specimens in which carbon fibre rod or fabric was bonded to concrete by epoxy (Specimens 4.1, 4.2a and 4.2b) or by a non-resinous geopolymer (Specimens 4.2c), see Figure
Test programme for carbon fibre anodes with respect to the electrical conductivity of the bonding medium.
Specimens | ID | Bonding medium | Total surface area (cm2) | Voltage applied (V) |
---|---|---|---|---|
4.1 | Steel 7 | Epoxy | 157.0 | 5 |
Fabric |
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4.2a | Steel 8 | Epoxy | 157.0 | 10 |
Rod | 94.2 | |||
4.2b | Steel 9 | Epoxy + chopped carbon fibres | 157.0 | 10 |
Rod | 94.2 | |||
4.2c | Steel 10 | Geopolymer + chopped carbon fibres | 157.0 | 10 |
Rod | 94.2 |
Schematic test of electrical conductivity of carbon fibre fabric or rod bonded to concrete by different media.
The continuous electrical conductivity of the carbon fibre anodes used in combination with the epoxy adhesive or geopolymer was assessed by visual monitoring, applied current, and circuit resistance of the electrolyte with time. The electrochemical performance of the CF rod or fabric anode was assessed in terms of the current or corresponding CP circuit resistance required to maintain a constant voltage between the steel cathode and carbon fibre anode. During operation of the ICCP, the voltage was maintained at 5 V or 10 V (see Table
Two pre-corroded reinforced concrete beams (
Application of ICCP CF anode to corroded reinforced concrete beams.
The performance of both the carbon fibre fabric and rod was observed daily during the tests. In each test with the saturated calcium hydroxide solution electrolyte, hydrogen evolution was observed at the cathode (steel bar). Simultaneously, it was observed that the carbon fibre anodes (fabric and rod) suffered from dissolution (Figure
Performance of steel and carbon fibre anodes in saturated Ca(OH)2 electrolyte.
The steel bars and carbon fibre rod and fabric were subsequently washed, dried and reweighed to establish their weight losses, as shown in Table
Mass loss of carbon fibre anodes and steel bar cathodes—test in Ca(OH)2 and concrete electrolyte.
Specimen | Electrolyte | ID | Weight |
Weight loss |
Test duration | |
---|---|---|---|---|---|---|
Before test |
After test | |||||
1.1 | Ca(OH)2 | Steel 1 | 66.80 | 66.80 | 0.00 | 165.5 |
CFRP rod 1 | 17.52 | 17.43 | 0.51 | |||
1.2 | Ca(OH)2 | Steel 2 | 67.51 | 67.50 | 0.01 | 165.5 |
CFRP fabric 2 | 4.83 | 4.70 | 2.69 | |||
2.1 | Ca(OH)2 | Steel 3 | 67.21 | 67.19 | 0.03 | 249.0 |
CFRP rod 3 | 17.78 | 17.68 | 0.56 | |||
2.2 | Ca(OH)2 | Steel 4 | 66.70 | 66.66 | 0.06 | 249.0 |
CFRP fabric 4 | 6.13 | 6.04 | 1.47 | |||
3.1 | Concrete | Steel 5 | 82.73 | 82.73 | 0.00 | 888.0 |
CFRP rod 5 | 21.12 | 21.05 | 0.33 | |||
3.2 | Concrete | Steel 6 | 82.34 | 82.34 | 0.00 | 888.0 |
CFRP fabric 6 | 9.50 | 9.44 | 0.63 |
At the time corresponding to a cumulative current of 13 mA applied to the carbon fibre rod anode (Specimens 3.1 and 3.2), a gaseous and yellow liquid deposit appeared around the rod. The pH of this material was in the range of 1 to 2, confirming it is highly acidic. Concrete Society Technical Report number 73 states that acid and oxygen is generated during operation at the surface of anodic materials commonly used for the cathodic protection of reinforced concrete due to the electrochemical reactions. It has been suggested that some anodes may also generate chloride depending on the environment [
Performance of carbon fibre (CF) anodes in concrete.
The performance of the CF rod and fabric anodes was monitored and reported in Specimens 4.1, 4.2a, 4.2b, and 4.2c. In the case of the CF fabric, a gaseous and yellow liquid deposit again appeared on the surface of the fabric, which may in turn be reducing the bond at the concrete-fabric interface. In comparison, there was no sign of debonding at the CF rod/geopolymer interface due to the electrochemical reaction. However, a fine crack was observed at the concrete-geopolymer interface.
After 985 hours of operation of the ICCP, a small gaseous and yellow liquid deposit appeared on the surface of the CF fabric anode (Specimens CFF1, CFF2). This could, if more widespread, result in the debonding of the CFRP fabric from the concrete interface. In the case of the CF rod (Specimens CFR1, CFR2), there was no sign of damage or debonding problems [
The performance of an ICCP anode is represented by a number of parameters including consumption rate (the rate in mass of anode consumed per ampere of current per year), efficiency (a comparison of actual anode mass consumed to the theoretical mass required), steel polarization and CP circuit resistance (CR) [
CP circuit resistance at time
It is a DC measurement. While interfacial polarization effects contribute to CR, the changes that occur in CR with time are expected to be largely related to resistive elements in the CP system and these elements would be the same whether measured by DC or AC techniques.
The equivalent circuit resistance between carbon fibre anodes and steel cathodes in concrete consist of the anode resistance at the anode-concrete interface, the electrolyte at the steel/concrete interface, and the steel bar resistance. The circuit resistance can be calculated as follows.
The current from the carbon fibre fabric anode passes through the electrolyte to the cathode. The area of contact between the carbon fibre fabric anode and the electrolyte provides the passage for the current. This area is not simply the plan surface area
From (
The applied current versus exposure time and circuit resistance versus exposure time relationships for different specimens are plotted in Figures
Applied current versus exposure time (Ca(OH)2 electrolyte).
Applied current versus exposure time (concrete electrolyte).
The circuit resistance versus exposure time (Ca(OH)2 electrolyte).
The circuit resistance versus exposure time (concrete electrolyte).
It can be seen from Figure
Figure
Figure
Figure
Specimens 4.1 consisted of CF fabric bonded to the surface and 4.2a of a CF rod bonded into pre-formed grooves of the beams using epoxy. CP was applied to the reinforced concrete prism with the CF fabric or rod employed as the anode. The CP was applied 7 days after the application of the CF anodes to the concrete prisms.
The observed current passing to the CFRP fabric demonstrated that the CF fabric with epoxy had sufficient electrical conductivity to operate as an anode (Figure
Bonding CF rod to grooved concrete prism using (a) epoxy (4.2a) and (b) epoxy plus chopped CF fabric (4.2b).
The performance of CF fabric anode bonded to concrete surface by epoxy (4.1) and CF rod anode bonded to grooved concrete by geopolymer plus chopped CF (4.2c).
The test was repeated with geopolymer and chopped CF fabric. This aimed to both increase the electrical conductivity and control the shrinkage of the geopolymer during curing. The CP current was then applied to the small scale CF anode specimen (Figure
Applied current versus exposure time (4.1 and 4.2c).
Circuit resistance versus exposure time (4.1 and 4.2c).
Figure
Figure
During the 1100 hours of operation of the ICCP, the on and instant-off potential values of all the steel bars of small beams (CFF1, CFF2, CFR1, and CFR3) were recorded. The potential decays were monitored and are plotted in Figure
Potential decays of reinforcing steel specimens [
This preliminary study into the electrical conductivity and potential use of carbon fibre fabric and rod as an anode in impressed current cathodic protection has produced the following conclusions. The anode current decreased with increasing exposure time. This is attributed to dissolution of the CF anode which results in an increase in the CP circuit resistance. The rate of consumption of CF anodes is relatively low. However, CF may be susceptible to damage after employment as an anode of CP. Acidic deposits can be generated at the CF anode-concrete interface. Epoxy can be used to bond CF fabric anode to concrete surfaces. Epoxy is not suitable for bonding CF rod anode into concrete because of the high resistivity of the epoxy. As a result there is no current passing through the CF anode. Geopolymer with chopped carbon fibre modification can be used to replace epoxy to bond CF rod anodes to concrete. CF can be employed effectively as an anode for ICCP. CF rod anode is capable of operating at more than 64 mA/m2 of reinforcing steel area without significant signs of damage or debonding. CF fabric anode can operate at 128 mA/m2 of steel area with small liquid deposit generated on the surface.