Impressed current cathodic protection (ICCP) is a well-established technique of reinforcing bars(rebar) protection in concrete against corrosion. Application of ICCP needs provision of an external anode while rebar becomes a cathode. It has been already reported that, in carbon fiber-reinforced polymer (CFRP) strengthened reinforced concrete (RC) structures, CFRP can act as an external anode due to its conductive properties and can successfully protect rebar from corrosion. However, higher protection current density reduces bond strength of reinforced concrete. In the present study, optimum protection current density is investigated that will protect the rebar from corrosion as well as restitute the bond strength. In addition, effect of flow of current through CFRP and adhesive is also studied. Large-scale flexural specimens (beams) are used for experimentation. Conductive adhesive was used to strengthen corroded beams with CFRP. ICCP was used to protect steel reinforcing bar corroded to three corrosion levels being exposed to corrosive environment using various protection current densities. Efficacy of cathodic protection was assessed using half-cell potential, flexural test, and mass loss. The effect of ICCP on strengthening system is also investigated.

The environmental degradation of reinforced concrete (RC) is one of the major issues in the construction industry now days. This is caused by several factors such as sulphate attack, carbonation, chloride attacks, etc. that disintegrate concrete. Carbonation and chloride attacks on concrete induces corrosion in the reinforcing bars (rebar). The corrosion products produced are nearly seven to eight times that of the original metal in volume which apply pull to the concrete around, there by developing tensile cracks in the concrete.1  Because of this, the bond between rebar and concrete is reduced; at a time it is completely lost and the material ceases to deliver as reinforced concrete. Corrosion of rebar also affects the mechanical properties of rebar.2-3 

In coastal regions of tropical countries like India, where ample amounts of moisture and chlorides are available in the environment, corrosion of rebar is the major threat to the construction industry. Several corrosion protection methods are available which are being used. Such methods are generally based on the principle of providing a barrier to the penetration of corrosive elements toward the rebar.

Use of fiber-reinforced polymer (FRP) in strengthening of corrosion-damaged RC structural members is one of the well-established practices in the construction industry. Due to high strength to weight ratio, long-term durability, and resistance to corrosion, FRP materials have proven themselves a good alternative to steel in construction (embedded as well as surface bonded) and strengthening of RC.4 

The structural performance of RC members is found to be enhanced by the use of FRP. At the same time, in addition to strengthening, FRP wrap acts as a resistive layer to the entry of moist air, thereby providing passive protection against corrosion.5-7  The studies show that FRP wrapping not only restores strength and ductility but also reduces the corrosion in RC members. It is seen that FRP wrapping does not stop but slows down the corrosion process and the extent of reduction in corrosion rate depends on the extent of FRP wraps, number of wrapping layers, fiber orientation, and type of epoxy which ultimately slows down the diffusion of oxygen and moisture and increases confinement.8-10 

Corrosion is an electrochemical process in which a galvanic corrosion cell forms which require an anode, cathode, and electrolyte and enables flow of electric current in addition to some chemical reactions. Cathodic protection (CP) is also an electrochemical technique to prevent corrosion in which the corrosion of rebar can be controlled by establishing an electrical connection between the rebar to act as a cathode and other metal, which act as an anode. CP of the metal to be protected can be attained by the sacrificial anode method and impressed current cathodic protection (ICCP). In ICCP, the rebar is made a cathode by impressing reverse current with the help of an external DC source, and the other metal, which is generally less reactive, is used as an anode (external anode). CP is achieved by attaching the rebar and external anode to a negative and positive terminal of DC source, respectively. In this CP system, the concrete surrounding rebar acts as an electrolyte. As per the study conducted by Federal Highway Administration USA, CP is the only technique which can stop chloride-induced corrosion completely in RC.11 

Although ICCP has been recognized as the most efficient corrosion protection method, its application needs to be accurate and precise in terms of the amount of protection current. Applying a higher cathodic protection current density definitely results in lowering mass loss due to corrosion, however, at the cost of reduction in bond between concrete and rebar due to hydrogen embrittlement which will be higher for a higher current.12-14  Obviously, it is not desirable to save on mass loss due to corrosion at the cost of bond loss in the case of RC structures. Therefore, to avoid such circumstances, appropriate current densities should be provided or should be kept at such a level, where bond loss is well within permissible limits.14-15 

CFRP is used for strengthening purposes, as CFRP contains graphite which is conductive; ICCP can be used to stop the corrosion of rebar by using CFRP as an anode. So, in addition to strengthening, further corrosion of rebar can be stopped.

The present study investigates the viability of ICCP in CFRP strengthened RC flexural members making use of the electrical conductivity of CFRP. Thus, besides strengthening of RC structure, corrosion of rebar can be stopped.15-17 An attempt has been also made to obtain the optimum ICCP current densities to protect rebar corroded at different levels. For this, the RC beam specimens were corroded at different corrosion levels by the impressed current method. After corroding, the beams were flexurally strengthened using CFRP laminates which were further used as an anode. Laminates were attached to corroded beams using conductive epoxy. Conductive pigments are added to epoxy adhesive to make it electrically conductive. ICCP was applied to the beam specimens that were simultaneously placed in saline container for 60 d to investigate performance of ICCP in a corrosive environment.15  The corrosion progression of rebar was monitored by observing half-cell potential, everyday throughout ICCP. After ICCP, the flexural strength of the specimens was determined. Mass loss of every beam was also discovered. Appropriate ICCP current density is proposed. The effect of ICCP on properties of CFRP and adhesives also has been investigated.

To resemble the conditions of corrosion-affected structures and to simulate actual site conditions, it is important to perform experimentation on large-scale or real-life specimens. After studying the effect of ICCP on small-scale samples treating CFRP as an anode,15  it was decided by the authors to extend the study on real-life specimens.

The experimentation was done in the following steps.

Casting Reinforced Concrete Beam Specimens

The RC beam specimens of size 150 mm × 200 mm × 2,000 mm (b × d × l) were casted. The concrete mix design was performed to obtain M45 grade of concrete in accordance to IS 10262: 2009. The mix was prepared using IS Grade 53 OPC of nominal strength 53 MPa,18  fine river sand, crushed stone as aggregates, and 0.45 water cement ratio. IS Fe 500 Grade reinforcing bars of 10 mm and 8 mm diameter were used as longitudinal and transverse reinforcement, respectively.19 

Reinforcement details in beam specimens is shown in Figure 1. Four number of bars, two at top and two at bottom were provided in the beam. The transverse reinforcement stirrups were provided at the distance of 150 mm in the middle one-third, while 100 mm spacing was maintained in the end spans. The continuity in connecting all rebar was ensured for uninterrupted application of ICCP. The reinforcing cage made of longitudinal bars (tension/compression reinforcement), stirrups/links (shear reinforcement), was weighed on a weighing machine to the accuracy of one gram before casting the concrete. The cover concrete of 20 mm was maintained from all borders. Copper wires connected to reinforcing cage at three locations were taken out of concrete for electrical connections. Moist curing of the specimens was done for 28 d.

FIGURE 1.

Detailing and casting of RC beam.

FIGURE 1.

Detailing and casting of RC beam.

Close modal

Corroding Rebar by Impressed Current Method

After curing the beam specimens for a specified period, the rebar in concrete were corroded by accelerating the corrosion process, to simulate the condition of corrosion that affected the RC flexural member. Various methods of an accelerated corrosion process are available, e.g., exposure to salt mist or chloride diffusion,20-23  directly adding salt to concrete while mixing, wetting, and drying in salt solution, and impressed current by keeping reinforcement as an anode.15,17,24-26 

In the present study, the impressed anodic current method was used for accelerating corrosion in RC beam specimens. Beams were placed in a 3.5% salt solution for 2 d to saturate completely. The steel mesh was rolled in hollow cylinder around the specimen, to act as cathode. The DC regulated power supplier of 2 A–30 V capacity was used for the study. The positive terminal was connected to rebar through the copper wire taken out of the RC specimen and steel mesh was made a cathode by connecting to a negative terminal. Direct current of 2 A (current density = 0.4 mA/cm2) was impressed to the rebar cage for a time span that is required to attain prescribed levels of corrosion/mass loss. Faraday’s Law was used to find the time span of impressing anodic current corresponding to a particular mass loss. The experimental setup for inducing corrosion is shown in Figure 2. The specimens were corroded to different corrosion levels with respect to the percentage of mass loss, as presented in Table 1. It was observed that the voltage required for maintaining the constant current flow kept on dropping.

Table 1.

Levels of Corrosion Depending on the Amount of Mass Loss Induced

Levels of Corrosion Depending on the Amount of Mass Loss Induced
Levels of Corrosion Depending on the Amount of Mass Loss Induced
FIGURE 2.

Inducing corrosion to rebar.

FIGURE 2.

Inducing corrosion to rebar.

Close modal

After inducing corrosion into the specimen as explained in Table 1, all specimen showed signs of corrosion in the form of brown colored rust appearing on the surface of the concrete, as well as the formation of cracks. The cracks appeared on the surface in the direction of longitudinal reinforcement from all sides. The width and extent of the cracks were found to be increasing with the level of corrosion, as shown in Figure 3.

FIGURE 3.

Beam specimen after corrosion.

FIGURE 3.

Beam specimen after corrosion.

Close modal

Treating Specimens by Carbon Fiber Reinforced Polymer Laminates

Precorroded, RC beam specimens subjected to different corrosion levels as mentioned earlier were then strengthened by CFRP laminates, to simulate the condition of CFRP strengthened RC flexural members. CFRP laminate 100 mm wide, 1.7 mm thick, and 1,700 mm long were attached to the bottom face by conductive epoxy adhesive. Generally, the adhesives used for adhering laminates to the surface of RC are nonconductive polymers, however, in the present investigation, conductive pigments (graphite) were added make it conductive. In the present composite system, except the epoxy adhesive, CFRP and concrete were electrically conductive. Therefore, to confirm the flow of electrons through the present composite system, adhesive was made conductive. A pilot study was conducted to find the appropriate percentage of adhesive to be replaced with conductive pigments so as to get better consistency for its application on the surface as well as to confirm the required amount of current. Beam specimens were air dried for two days prior to applying CFRP. The concrete surface was made plane using a grinder to ensure proper bonding between the concrete surface and CFRP laminate using modified adhesive. Immediately before applying adhesive to the concrete surface, it was cleaned with acetone to make it dust free. While adhering CFRP laminate to the concrete surface, it was ensured that no air bubbles remained between concrete and laminates that may create hindrance to flow of current. For electrical connections, copper wire was connected to CFRP laminate.

Applying Active Protection

Two days after applying CFRP, the beam specimens were fully saturated with water. After that, they were immersed in 3.5% Salt solution. For applying ICCP to rebar, CFRP laminate was made an external anode, rebar as a cathode, and NaCl solution acted as an electrolyte. To induce protection current, an external (30 V and 2 A) was used. Positive and negative terminal of DC regulated power supplier was connected to CFRP and rebar, respectively as shown in Figure 4.

FIGURE 4.

Active protection of CFRP strengthened beam.

FIGURE 4.

Active protection of CFRP strengthened beam.

Close modal

Every specimen was corroded to the prescribed corrosion levels, as reported in Table 1. While applying ICCP, each of the specimens corroded to specific corrosion level was protected using varying protection current densities. The specimens were protected using: 5 mA/m2, 10 mA/m2, and 20 mA/m2 current densities, as presented in Table 2. One specimen from the corrosion each level was kept unprotected, which was considered as a control.

Table 2.

Experimental Matrix for ICCP of Rebar for the Specimens

Experimental Matrix for ICCP of Rebar for the Specimens
Experimental Matrix for ICCP of Rebar for the Specimens

According to the codal provisions, the current densities for applying the ICCP to rebar should range from 2 mA/m2 to 20 mA/m2.27-28  From a previous study conducted, the authors have found that protection current density of 2 mA/m2 is insufficient to counter the corrosion process in the rebar in a highly corrosive environment.15  Therefore, in the present study, 2 mA/m2 protection current density was not considered. In addition, one of the specimens precorroded to high level was protected by applying current density as high as 100 mA/m2 to investigate the effect of very high protection density. In the case of one of the specimens, CFRP and rebar were directly kept in contact through copper wire, to study progression of corrosion when direct electrical contact is established between rebar and CFRP. One specimen from each corrosion level was kept unstrengthened. The specimens were exposed to alternate wetting and drying to a saline solution (3.5% NaCl solution), to simulate RC structures protected against corrosion by applying ICCP that are subjected to severe corrosive environment. Alternate wetting and drying cycle was set for 12 h, such that the salt solution was removed and poured in the container alternately. Care was taken to maintain the salt solution level 2 cm below the upper concrete surface instead of complete immersion to enhance severity of exposure. The duration of exposure was for 60 d.27-28 

Corrosion Monitoring

The corrosion activity in each of the protected and unprotected specimens was monitored by observing half-cell potentials as specified by ASTM C876 every day throughout the exposure period.29 Ag-AgCl electrode was used as a reference electrode for half-cell potential measurements. As the specimens remained polarized due to a continuous impressed current, suddenly after shutting the protection current instant-off potentials were taken. Half-cell potentials were observed once again after 4 h of shutting down such that the complete depolarization is achieved.

Performing Flexure Test on Specimens

After completion of 60 d, the ICCP was terminated and beams were taken out of salt solution to dry. Flexure test was then performed on the specimens by applying single-point loading at the center of a beam span as specified in IS: 516-1959 reaffirmed 1999.30  The load is applied with a hydraulic jack of capacity 45 ton that was measured on a proving ring. The proving ring of 25 ton capacity was used. Midspan deflection of the beam was discovered by two linear variable differential transformers with a least count of ±1 micron. The setup for flexure test is shown in Figure 5.

FIGURE 5.

Experimental setup for the load deformation testing.

FIGURE 5.

Experimental setup for the load deformation testing.

Close modal

Determining Mass Loss in Rebar

A load deformation curve for each of the specimen was obtained in the flexure test. After this, a reinforcing cage embedded in the beam was removed from the beam specimens to find the actual corrosion in the rebar. ASTM G1-90 was used to prepare the solution to clean off corrosion products from corroded reinforcing bars. The solution was prepared using 1,000 mL of reagent water, 500 mL of hydrochloric acid (1.19 specific gravity), and 3.5 gm of hexamethylene tetraamine as stipulated in ASTM G1-90. Rebar were kept immersed in the solution for 60 min.31  By this period, all corrosion products were spalled off the bars. Every cleaned rebar was weighed on the weighing machine having a least count of 1 gm. Then considering initial weight before corrosion induction and weight after cleaning, percentage mass loss in every specimen was found out. Figure 6 shows the cleaned reinforcing bars.

FIGURE 6.

Rebar cleaned as per ASTM G1-90.

FIGURE 6.

Rebar cleaned as per ASTM G1-90.

Close modal

Performing Tests on Carbon Fiber-Reinforced Polymer System

In the present investigation, CFRP laminates are continuously impressed with the direct electric current. Therefore, it is worth studying the effect of ICCP on the CFRP system, too. For this, a mechanical test and microscopic test were performed on a used CFRP anode.

Tensile Test

After the protection duration, a uniaxial tensile test was performed on CFRP laminates that were used as an anode for ICCP application. CFRP laminates used to strengthen the corroded beam specimens were exposed to anodic polarization. The CFRP strip was peeled off from the concrete surface. The concrete adhered to the CFRP strip was cleaned off. The strip was then cut into a dumbbell shape for a tensile test in accordance to ASTM D3039-D3039M. The specimen size was kept at 12 mm (width), 24 mm (tab width), and 170 mm (overall length). Three specimens of equal length were tested for tensile strength per CFRP. The specimen ends were prepared by applying tab material oriented at 45° to the longitudinal direction of the strip. Tensile test was performed on a universal testing machine (Instron 4467) at the loading rate of 0.2 mm/min. The applied loads and corresponding displacements were recorded.

Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy

Microstructure observation and element identification of the CFRP system, which included CFRP laminate and adhesive, was performed for various applied protection densities using SEM and EDS. SEM and EDS give the information of elemental composition and failure analysis. SEM is used for failure investigation as it reveals the location at which the fracture initiates, how is it propagated, and the fracture mode. The study was done on scanning electron microscope with EDS facility (JEOL-6380A) at 5 kV with a I-nA probe current. The samples were coated with an auto fine coater (JFC-1600) to form a conductive surface before putting them into SEM chamber.

In the study conducted, RC beams were corroded to different predefined corrosion levels to represent the structures corroded to a low, medium, and high level. Then they were repaired to enhance their flexural capacity CFRP laminates. The strengthened specimens were then applied with ICCP to prevent further progression of corrosion while they were exposed to severe corrosive environments. The specimens were induced with different cathodic protection current densities. Progression of corrosion during ICCP application was monitored by observing half-cell potentials twice a day. Instant-off potential, as well as decayed potential after 4 h was noted when the complete depolarization of the specimens was ensured. After the period of 60 d protection was stopped and flexural capacity of all the specimens was determined for single-point loading. In the end, the mass loss of each specimen was determined.

The instant off potential for the specimens was found to be more negative than 100 mV at every instance of measurement. The instant off potential increased with increasing protection current density, which is an expected observation. The decay of 100 mV after shutting off protection current was also observed in all the protected specimens, which is indicative of the specimens being protected against corrosion.

Figures 7(a), (b), (c) presents decayed half-cell readings of rebar for specimens corroded for all three levels plotted against exposure time in days. To get the trend of half-cell throughout the exposure period, a linear fit was obtained. From Figure 7 it can be seen that the specimens with no protection are showing higher or similar potentials in all three corrosion levels showing continuity of corrosion with respect to exposure time. In contrast, specimens applied with ICCP are showing lower potentials with respect to exposure time. Among the specimens applied with ICCP, the dropping rate of half-cell potentials with an increase in exposure period is higher for specimens protected with higher current density, clearly indicating that all the applied current densities were sufficient for corrosion protection. This shows the implementation of ICCP to rebar in concrete irrespective of protection current density.

FIGURE 7.

Half-cell potentials against exposure for (a) low, (b) medium, and (c) high corrosion level.

FIGURE 7.

Half-cell potentials against exposure for (a) low, (b) medium, and (c) high corrosion level.

Close modal

In one specimen corroded to a low corrosion level, direct contact was made between rebar and laminate. This specimen showed higher half-cell potentials than the control specimen which states that the progression of corrosion in this specimen is faster than the unprotected control specimen. It is worth noting here that direct electrical contact between rebar and CFRP laminate used for structural strengthening forms galvanic coupling between rebar and CFRP and the corrosion process is enhanced as compared to an unprotected structure. In the present study, the direct contact between rebar and CFRP was made intentionally. However, in the field, it can happen accidentally. This is alarming and, therefore, repair/strengthening experts need to be alert as CFRP strengthened structural member may exhibit rapid corrosion rate than the natural corrosion rate it may be subjected to otherwise, under specific exposure condition. This demands careful and critical application and execution of CFRP as strengthening material as it might prove to be more detrimental.

One of the specimens, corroded to the medium level, was applied with very high protection current density at 100 mA/m2. This specimen showed a steep decrease in the potential of rebar during the exposure period, so much so that toward the end of the exposure duration the potential reached to nearly −124 mV, indicating passivation. Thus, it indicates that a higher protection current may completely stop the corrosion process.

After shutting ICCP, a load deformation relationship was plotted for all the specimens in different corrosion levels by performing a single-point bending test, as shown in Figure 8. At the end of flexure test all the beam specimens had brittle failure with large sound. After the test, the specimens were smashed and the rebar cage was separated from the concrete. Rebar were properly cleaned off as per ASTM G1-90 and the percentage of loss of rebar was calculated. Table 3 shows maximum load, central deflection with respective percent mass loss.

Table 3.

Load and Deflection with Respective Percentage Mass Loss

Load and Deflection with Respective Percentage Mass Loss
Load and Deflection with Respective Percentage Mass Loss
FIGURE 8.

Load deflection curve for (a) low, (b) medium, and (c) high corrosion level.

FIGURE 8.

Load deflection curve for (a) low, (b) medium, and (c) high corrosion level.

Close modal

Among the specimens which are unprotected, sample with low corroded level (LC) has shown the highest load carrying capacity and the control specimen corroded to a high level (HC) has shown to have the lowest load carrying capacity. The load carrying capacity of medium corroded control specimen (MC) lies between low and high corrosion level control specimens, which is quite expected. The control specimens in all levels of corrosion are exhibiting the highest loss of mass as compared to the protected specimens. MC control specimen has shown mass loss between low and high corrosion level control specimens.

All specimens, which were applied with active protection, showed less mass loss than control specimens in same corrosion level. Among the protected specimens in all corrosion levels, it is discovered that mass loss is falling with higher protection current densities, indicating higher rebar corrosion protection.

It is also found that, in every corrosion level, the load carrying capacity is lowest and the corresponding deflection is highest in the specimens that were applied with higher protection current density.

It can be seen here that, although increasing corrosion protection in the current density reduces percentage mass loss in the rebar, it lowers the load carrying capacity of the beam as compared to unprotected control specimens. However, it is worth noting here that the load carrying capacity of unstrengthened specimens in all corrosion levels is much lower.

The main aim of applying ICCP is to stop the corrosion process of rebar, so as to increase the service life of an RC structure. While realizing this, the CP design engineer would expect to have lower mass loss in rebar in combination with higher load carrying capacity of a flexural member. The authors have proposed a term, ratio of mass loss to bond strength in the previous study,15  such that lower ratio values would ensure lowest mass loss and highest bond strength.

By the same token, in this case also, the authors propose a ratio of percentage mass loss in rebar to the load carrying capacity of the beam. The efficient cathodic protection will result in a lower value of ratio of a percentage mass loss to the load carrying capacity of the beam. The lower ratio values will be obtained with low mass loss and high load carrying capacity. Thus, the ratio of percentage mass loss to respective load carrying capacity is calculated for all cases which are presented in Table 4. From the values obtained it is observed that specimens protected with density of 5 mA/m2 are exhibiting a lower value of percentage mass loss to load carrying capacity ratio of beam. Thus, 5 mA/m2 can be taken as an optimum cathodic protection current density that may be used to protect CFRP strengthened RC structures present in a corrosive environment which will result in a maximum flexural strength with minimum mass loss.

Table 4.

Ratio of Percentage Mass Loss to Corresponding Load Carrying Capacity of Respective Specimens

Ratio of Percentage Mass Loss to Corresponding Load Carrying Capacity of Respective Specimens
Ratio of Percentage Mass Loss to Corresponding Load Carrying Capacity of Respective Specimens

It is worth mentioning here that the specimens supplied with highest protection densities failed due to the delamination of surface-bonded CFRP laminates. This is indicative that adhesive ceases to resist the applied loads even though CFRP laminates remain intact. It may be believed here that the oxidation of epoxy adhesive (anode) is responsible for degrading its bonding properties. However, weakening in the bonding property of adhesive, for any reason, is not desirable for a strengthened RC member or structure.

Between the two special cases, LR is the one in which rebar is directly connected to CFRP through a conductive wire. Due to potential difference, rebar becomes an anode and CFRP act as a cathode. The results of the bending test shows that the load carrying capacity of this specimen (LR) is less than the similar control specimen (LC) and mass loss is higher than the control specimen. Thus, the deliberately induced current flow from rebar to CFRP enhances corrosion in rebar as compared to control specimens.

Another special specimen, M100, has been supplied with very high protection density (100 mA/m2). The bending test result of this specimen shows that the load carrying capacity is substantially reduced as compared to the specimen that has been protected by impressing 20 mA/m2 and the mass loss in this specimen is least among all MC specimens. Also, the ratio of load carrying capacity to corresponding percentage mass loss for this specimen is highest among all specimens. The delamination of the CFRP laminate in this specimen was observed at lower load as compared to other specimens. This establishes that with an increase in corrosion protection density the bonding properties of adhesive seems to be altered adversely. This finding encouraged the authors to study microscopic structure of anode system (adhesive and CFRP) subjected to ICCP which is discussed as follows.

As mentioned earlier, a tensile test was performed on CFRP laminate that has been used as an anode, in the present CP system for all corrosion protection densities. It was observed that the thickness of the CFRP strip was increased from its original thickness and it varies directly with protection current density. It is observed that the tensile strength of CFRP (anode) is inversely proportional to the corrosion protection density it is exposed to. The one that is exposed to the lowest current density (5 mA/m2) exhibited the highest tensile strength and vice a versa. The variation of tensile strength of the CFRP laminate as against protection current density is presented in Table 5.

Table 5.

Results of Tensile Test on CFRP for Respective Protection Density

Results of Tensile Test on CFRP for Respective Protection Density
Results of Tensile Test on CFRP for Respective Protection Density

To investigate the effect of impressing direct current to CFRP system while it acts as an anode, microstructure observation and element identification of the CFRP laminate as well as adhesive was performed using SEM and EDS. Figure 9 shows the morphology evolution of adhesive and Figure 10 shows the morphology evolution of CFRP laminate by SEM for all protection densities up to 60-micron scale. It can be seen that there is no significant change in the morphology of adhesive exposed to 5 mA/m2 current density as compared to control adhesive which is not exposed to any current density. However, with an increase in protection current density, the morphology is showing significant disturbances in its composition. This may be correlated to degraded bonding properties of the adhesive.

FIGURE 9.

Morphology evolution of adhesives for given protection density by SEM.

FIGURE 9.

Morphology evolution of adhesives for given protection density by SEM.

Close modal
FIGURE 10.

Morphology evolution of laminates for given protection density by SEM.

FIGURE 10.

Morphology evolution of laminates for given protection density by SEM.

Close modal

In the same manner, from Figure 10, it can be observed that the morphology of CFRP laminate exposed to 5 mA/m2 protection current density is more or less similar to the control laminate which is not exposed to current density, except for accumulation of white deposits. With an increase in protection current density, the carbon fibers in CFRP laminate are observed to move apart. The spacing between the carbon fibers in the case of 20 mA/m2 current density can be seen to be maximum whereas the fibers are not clearly seen in the case of 100 mA/m2 current density. Increasing thickness of laminate can be attributed to increased spacing between the fibers due to increasing current density. It can also be observed that the accumulation of white deposits is also growing with an increase in current density.

Further, the deposit composition was analyzed using EDS, the results of which are presented in Table 6. The EDS results indicate that the white deposits are chloride rich crystals absorbed on the surface of carbon fibers. Increase in the chloride deposit in CFRP laminates with current density is due to the rapid extraction of chloride ions from rebar (cathode) although the exposure period remains same for all protection current densities. Withdrawal of chloride ions from the surface of rebar and deposition on laminates reconfirms the corrosion protection of rebar by ICCP.

Table 6.

Results of EDS Analysis

Results of EDS Analysis
Results of EDS Analysis

Carbon percentage in CFRP laminate and conductive adhesive is reducing with an increase in protection current density, as seen from Table 6. This observation is alarming, where CFRP is made an anode of ICCP as it will adversely affect the strengthening system. The reduction in the tensile strength of CFRP laminates is also due to the distortion of carbon fibers. The distortion of carbon in adhesives and laminates with increase in protection current densities justifies early failure of strengthened beam specimen regardless of corrosion level. In view of the above results and discussions, it can be reaffirmed that the reduction in load carrying capacity of the beam with an increase in protection current density is due to the degradation in the anode system, even if there is reduction in mass loss clearly indicating effective protection of rebar against corrosion.

Based on the results, the following conclusions are drawn.

  • Concrete surface bonded CFRP laminates used for strengthening RC flexure members are found to be suitable for use as anode in ICCP, thereby eliminating the need of any external anode.

  • In all corrosion levels, that is low, medium, and high all the protection current densities (5 mA/m2, 10 mA/m2, 20 mA/m2) fit in preventing rebar corrosion as compared to unprotected members.

  • Rate of corrosion and corresponding mass loss in rebar decrease with an increase in protection current density irrespective of corrosion level, which ensures successful implementation of cathodic protection. However, the load carrying capacity of flexural members lowers with higher protection current density, at the same time the load carrying capacity of strengthened members remained higher than the un-strengthened members regardless of corrosion level.

  • To work out the optimum protection current density which will induce lower corrosion in rebar giving higher flexural strength, a parameter called ratio of percentage mass loss in rebar to load carrying capacity of beam is proposed. Lower ratio is indicative of lowest corrosion and highest flexural strength for given corrosion level. From the present study, in all corrosion levels, it can be seen that the current density of 5 mA/m2 exhibited the lowest ratio. Therefore, it is concluded that 5 mA/m2 can be taken as optimum cathodic protection current density that may be used to protect RC structures in corrosive environment so as to ensure minimum corrosion of rebar giving higher flexural strength. However, the proposed cathodic protection current density is applicable to the ICCP of the CFRP strengthened flexural members in which carbon FRP acts as an anode. The same current density may or may not provide necessary protection to the flexural member with other types of anode systems.

  • If CFRP laminate used for strengthening of RC arises in straight contact with rebar, although unintentionally or accidentally, rebar corrosion expedites.

  • The reduction in flexural strength of the beam with increase in protection current density is due to the degradation in the anode system, even if there is reduction in mass loss.

  • The anode system (surface bonded CFRP laminate) is found to be adversely affected due to flow of protection current through the system.

Trade name.

The authors wish to express their gratitude towards Dr. A. P. Patil, in charge corrosion laboratory, MME Department, VNIT, Nagpur, for making laboratory facility available for conducting experiments. We also wish to express our gratitude and sincere appreciation to the Ministry of Human Resource Department, Government of India for financing the research work under TEQIP-II scheme. Our sincere thanks also goes to Dr. Madan Kamat, Krishna Conchem Products Pvt. Ltd. and Dr. Mangesh Joshi, Speciality Reinforced Matrix Pvt. Ltd., who provided the material required for the study.

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