Current Distribution In Reinforced Concrete Cathodic Protection Systems Construction Essay

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Cathodic protection is an electrochemical technique which minimizes the corrosion of metals in contact with any ionic conducting medium. Current distribution from a surface mounted anode to steel reinforcement in atmospherically exposed concrete is modeled and is governed by the condition of the steel, the resistivity of the concrete and anode-steel geometry. The boundary conditions at the steel have a remarkable effect on current distribution with more uniform distribution arising at low steel corrosion rates. In a typical circumstance the surface of a steel bar facing the anode may receive 50% more current than the opposite surface. As cathodic protection has proved to be useful in these cases, a basis for many design decisions that influence current distribution is that their effect is negligible by comparison. When more than one layer of reinforcement is present the current distribution is remarkably worse. In this review paper protection current distribution in reinforced concrete cathodic protection systems in regard to effectiveness of sacrificial anodes have been probed. Also three-layer reinforced concrete cathodic protection (CP) system, with carbon fibre reinforced cement (CFRC) will be probed.

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Keywords: Reinforced concrete; Cathodic protection; Cathodic prevention; zinc overlay ; Chloride removal; Electric field; Electrochemical repair; Hydroxyl generation; Modeling

1. Introduction

One of the objectives in cathodic protection (CP) design is to deliver a fairly uniform current density to the protected steel bars. This will minimize the current required to get the protection criterion, thus reducing the cost and improving the life of the system components. The achievement of uniform current distribution in an atmospherically exposed reinforced concrete CP system is, however, hindered by the location of the steel in a resistive environment near to a large planar anode.

Many previous tests has examined current distribution with regard to the design of CP mechanism applied to steel elements/structures in sea water and soils using experimental methods and mathematical models.

In mathematical models the boundary conditions at the polarizing interfaces have always presented some problems. In early tests the resistance to polarization presented by the interface was often ignored. Empirical formulas approximating the polarization behaviour have also been used and recent advances have allowed the time dependence of the polarizability of the cathodic interface resulting from the precipitation of deposits there to be modeled.

Corrosion of steel induced by chlorides is the major

cover and reach to the onset of pitting corrosion when their concentration near the steel surface reaches a critical threshold. This threshold is difficult to evaluate, since it depends on several factors related both to the concrete and the environment.

Since reinforced concrete is one of the common construction materials in civil engineering nowadays, the durability problems have been obsessing people (Cramer SD et al, 2002).

The worst of these problems was caused by corrosion of steel in concrete, inducing the early deterioration of concrete infrastructures. In marine structures and road or bridges sprayed with deicing salt, the passivity of the embedded steel bars are affected mainly by the presence of chlorides, by a decrease of pH in pore solution at the reinforcement depth, or a combination (Oladis TR, Yolanda HL., 2008).

The influence of steel bars initial corrosion state, concrete resistivity and magnitude of impressed current density on the current distribution will discussed. Testing results show that the initial corrosion rate of steel has a great effect on the protection current distribution (Jing Xu, Wu Yao, 2008).

2. Utilizing conductive polymer overlays

2.1 Experimental part

In 2007 A.S.S.Sekar and V.Saraswathy tested reinforced concrete slab of size 1m x 1m x 0.1m were cast with 3% sodium chloride by weight of cement by varying the parameters as follows:

Slab - 1: Cast without chloride and without cathodic protection

Slab - 2: Cast with chloride and without cathodic protection

Slab - 3: With chloride and with cathodic protection

Slab - 4: With chloride with cathodic protection and with zinc overlay.

Slab - 5: With chloride, with cathodic protection and with conductive coating.

After curing the slabs for 28days, the Magnesium anode was placed centrally utilizing the backfill of 75% gypsum, 20% bentonite, 5% Na2SO4 by weight of the anode and the ratio of anode to backfill being 1:2. The top surface of the anode is plastered by cement mortar having the lead wire projecting from anode. The anode is electrically connected to the steel reinforcement (cathode) assembly at the two diagonal opposite points.

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2.2 Discussion

Corrosion of the embedded steel was monitored by stimating the potential of steel, resistivity of concrete and cathodic protection current. The Figure 5 relates potential and resistivity with time. In the slab 1, cast without chloride and without cathodic protection, the potential of steel is around -200 mV. Comparing that with the slab 2, which is cast with chloride and without cathodic protection, the potential of steel has become more ve, in the order of -500mV. This is attributed to the effect of chloride, which is negatively charged and has promoted the corrosion current distribution. Further in slab 2, rust stains were noticed after a period of 1806 hrs (75days).

In the slab which is cast with chloride, with cathodic protection and with zinc overlay, the potential value of steel at various distances from the anode follow the same profile. From Figure 6 it can be seen that, near the anode the potential shift is found to be around 200mV and if the distance increases the shift is found to be negligible near the edge of the slab. This implies that the addition of zinc does not have any effect on shift in potential or uniform distribution of current.

In the slab 5, which is cast with chloride, with cathodic protection and with a conductive coating, an important observation is noted. Figure 7 relating potential and resistivity has shown the same observation as the previous system. The concrete cover laid over the conductive coating started to separate to after a period of 1507 hrs,(I,e, 65 days), Subsequently, minute cracks originating from the area of anode assembly and propagating outwards were noticed.

Figure 1. Rebar skeleton arrangement before placing concrete (James H.Meyer et al., 2000)

Figure 2. Slab with chloride and without cathodic protection (James H.Meyer et al., 2000)

Figure 3. Slab with chloride and zinc overlay with cathodic protection (James H.Meyer et al., 2000)

Figure 4. Slab with chloride and conductive polymer overlay with cathodic protection (James H.Meyer et al., 2000)

Figure 5. Effect of time and resistivity on the potential of embedded steel in chloride free concrete

Figure 6. Effect of cathodic protection on the potential of steel in slab coated with mortar containing zinc in chloride contaminated concrete

Figure 7. Effect of cathodic protection on the potential of steel in slab coated with conductive paint in the middle of the cover in chloride contaminated concrete

Figure 8. Effect of time on the current flowing in slabs with cathodic protection

Figure 8 shows the variation of cathodic protection current in regard to time in slabs 3, 4 and 5. From the graph it can be seen that the cathodic protection current in the slabs 4 & 5 is active an earlier time and has reached noticeable and stable values also after 1900 hours. Cathodic protection applied to steel in concrete is considered effective if the 100 mV decay criterion is fulfilled, i.e. if a decay of at least 100 mV is achieved within a certain period (usually 4 or 24 h). This criterion, which was developed empirically has shown to be effective in practical applications, and is recommended by standards. Recently, it has been suggested that the achievement of a decay of 100 mV should imply that a close passive state has been induced on the protected steel.

The 100 mV decay standard has shown to be applicable to cathodic prevention as well. Tests with very low cathodic current densities used to passive steel in chloride contaminated concrete showed that this criterion is reliable in evaluating the effectiveness of this technique. In fact, current densities is able to get a 4-h decay higher than 100 mV were sufficient to maintain passivity on steel bars even when a chloride content up to 3% by weight of cement was reached very close the steel surface .

So, the 100 mV decay criterion can be used to both cathodic protection (i.e. application of a cathodic current in order to control corrosion rate of already corroding steel) and to cathodic prevention (i.e. application of a cathodic current to passive steel in chloride contaminated concrete because of preventing the onset of pitting corrosion). Therefore, it can be assumed that submerged sacrificial anodes are able to control corrosion only on the reinforcement in the emerged part of a pile that experiences values of 4-h decay steadily more than 100 mV. In chloride contaminated concrete such decay can guarantee a cathodic polarization sufficient to obtain protection to corroding steel, while in chloride free concrete it can guarantee a cathodic polarization sufficient to induce a significant increase in the critical chloride content (and thus to contrast pitting corrosion initiation even when chlorides penetrate the concrete in contact with the steel surface).

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In this investigation potential shift of 200mV is observed in conducting polymer overlay and zinc overlay used systems. Another interesting observation is that the points very close to the anode have the potential shift of 200mV and the farther points has very low shift in potential even in chloride contaminated concrete. This may be because of the poor throwing capacity of the anode.

3. CP system using carbon fibre reinforced cement (CFRC) composite material as the conductive overlay anode

According to the literature review (Jing Xu, Wu Yao, 2008) by experimental investigation, the influence of initial corrosion state of steel, concrete resistivity and magnitude of impressed current density on the protection current distribution was probed.

Figure 9. The schematic of specimen which was used- All dimensions are in mm- (Jing Xu, Wu Yao, 2008)

3.1- For a reinforced concrete structure with complex mesh reinforcement system, using of half cell potential measurement to determine the corrosion state of steel bars is not sufficient. It is important to accompany this measurement with complementary information, such as the corrosion rate and/or the resistivity of the concrete.

3.2- The initial corrosion state of steel has a good effect on the protection current distribution.

3.3- Current distribution in reinforced concrete with more than one layer of reinforcement placed at different cover depths is markedly effected by concrete resistivity.

3.4 Magnitude of impressed current density has a bit effect on the current distribution when the corrosion rate of steel is relatively low.

4. Electrochemical Engineering Approach

The CP technique is based on the principles of electrode kinetics, which are briefly explained as follows:

Without any polarization, a metal in contact with concrete or an electrolyte will remain at its corrosion potential (Ecor). At this potential, the metal surface sustains at least two reactions happening at equal rates: a metal dissolution (or anodic metal oxidation) reaction, and a cathodic conjugate reaction, such as oxygen reduction or hydrogen evolution. If the metal is electrically polarized to potentials positive to Ecor, the metal dissolution reaction will be accelerated, whereas the cathodic conjugate reaction will be get slowly..

The converse is true when the metal is polarized negative to Ecor. Thus, when the metal is polarized away from Ecor to a positive or negative value, a net anodic or a net cathodic current, respectively, will flow across the metal/electrolyte conection. A metal is under cathodic protection when it is polarized effectivly negative to Ecor to reduce the metal dissolution rate by 3 orders of magnitude or more. Under most conditions, a polarization of about 2200 to 2300 mV is sufficient to achieve cathodic protection.

Excessive cathodic polarization should, however, be avoided to prevent onset of the hydrogen evolution reaction and to decrease the possibility of hydrogen embrittlement of the metal. Also, cathodic polarization,like corrosion, is a surface process. So, to achieve uniform protection at all locations on a given surface, it is imperative that the cathodic current density is uniform at all locations. Any nonuniformity in the current flow, especially with values less than some critical minimum, can make localized variations in the metal dissolution rate. These variations can result in the structure corroding more severely in some places than in others. In a bridge, for example, if the CP current is nonuniformly distributed, those area of the bridge that do not receive the current will continue to corrode, whereas those that do receive the current will be well protected from corrosion. Typical CP systems used in protecting metal concrete structures are explained as follows: In these structures, the metal is usually steel, and the cement and water from the electrolytic medium. Normally, the CP system has a rectifier as the voltage source. The return electrode for the current is either a palladium coated titanium mesh, 12 a thin layer of zinc, 7 or a conducting polymer mixed with concrete. They are inert electrodes, not consumed or destroyed by the reactions associated with the cathodic protection, and

are named ground beds. Normally, enerally, the ground bed is two-dimensional, is spread over the entire structure, and is covered with concrete and asphalt. All the rebars are electrically connected to one another, and the electrical connections between the rebars and the rectifier are made at one or two remote locations on the bridge. Similarly, the electrical connections between the ground bed and the rectifier are also made at one or two remote positions. Thus, in most cases, the ground bed is distributed evenly with respect to the rebars, whereas the electrical contact points are more localized. Since the bridges are located in various geographical positions from Washington to Maryland, and from Florida to New York they are exposed to a wide variety of environmental and climatic conditions.

5. Current and voltage distribution

The current distribution near the rebar/concrete conection for the top layer of rebars, obtained by the FEM analysis for the model structure is shown in Fig. 2. The rebar surface where the electrical contacts were made is also showed in the figure. The magnitude of the current is maximum at the end of the rebar surface where the electrical voltage was specified and is minimum at the far end. The potential distribution at a location near to one of the rebars is shown in Fig. 3; the spacing of the grids in this figure is on the order of centimeters. Near to the rebar/concrete interface, the drop in the potential is relatively negligible; majority of the drop occurs over a distance of a few centimeters within the concrete. At all other locations of the interface, the potential distribution was nearly identical to the one shown in Fig. 3. The relatively larger drop in the concrete is commensurate with its higher resistivity (0.5 105 V~cm) in comparison with steel (0.18 1024 V~cm). This observation is incomplete agreement with those made by other corrosion researchers on in-service concrete structures.

An important conclusion drawn from the FEM analysis is as follows: For asymmetrical geometric configurations (of the rebars and the ground bed), with asymmetrical electrical configurations, the rebar/concrete interface that is farther away from the electrical contacts gets very little current. As explained in the section Principles of Corrosion and Cathodic Protection, the degree of protection from corrosion decreased as the current across the interface decreased.

Actually, CP designs similar to the one described previously, if adapted for a real concrete structure, may not protect the rebars from corrosion over their entire length. Many of the 350 bridges mentioned previously, and many other structures that are presently under cathodic protection, are protected using asymmetrical electrical connections.

6. CP current mapping on the concrete bridge

Figure 4 shows a steel-reinforced concrete bridge which is located in Maryland. The bridge is 93 ft long in the east west direction and 133 ft wide in the north south direction. The bridge deck is cathodically protected by a single rectifier. A defining of the bridge is shown in Fig. 5.

The deck has two layers of uncoated rebars, one on the top and the other on the bottom, with concrete which is located in between. All rebars are shorted to one another. A palladium-coated titanium mesh, spread over the entire bridge, is located over the top layer of the rebars and acts as the ground return. A latex concrete mix covers the titanium mesh. A 133-ft-long conducting bar, placed along the north south axis at about 46 ft from the west end of the bridge, is joined to the titanium mesh. A point contact made to the conducting bar at about 60 ft from the north end of the bridge is joined to the positive terminal of the rectifier. The negative terminal of the rectifier is connected to the rebars at two positions along the west end of the bridge. Therefore, the bridge is a textbook combination of a uniformly distributed ground bed laid over uniformly distributed rebars, with the non-textbook condition of remote electrical connections. CP currents were mapped from the top of the bridge deck. For this target, the deck was divided into a matrix of many parallel and perpendicular lines at intervals of 10 ft. At each intersection point, the current flow along the east west axis and the north south axis was estimated using magnetometer sensors. The resulting current distribution is shown in Fig. 6. This figure shows only an 80 80-ft part of the 93 133 ft area of the bridge deck; the amplitude of the CP current in the rest of the deck is less than 1 mA. .The CP currents are concentrated only in the northwest area of the bridge, where they reach a peak value of about 5 mA. The location where the maximum current happened matches well with one of the points where the rebars are joined to the rectifier. This current distribution confirms that while the distributed geometry of the ground bed, namely, a mesh spread over the entire deck, appears intuitively correct, it did not help to get uniform current distribution.

The CP current, mapped with magnetometer sensors

over the entire deck of the bridge, shows that more than 60% of the area did not get any current. Visual observation made on the top and the bottom of the deck revealed a important amount of cracks in the structure in the east and the southeast position (Fig. 7). Figure 6 shows that these regions get little or no CP current. The northwest locations that received remarkably larger currents showed no evidence of cracking. It is possible that in the east and the southeast locations, the rebars are corroding due to a lack of cathodic current. Direct confirmation of the corrosion of the rebars through visual inspection is yet to be obtained. If the rebars are indeed corroding, that could be causing spalling and cracking of the concrete.

Figure10. Diagram of concrete bock used in the FEM model to determine the current and potential distribution.

Figure 11. Current distribution near the rebar/concrete interface as obtained from the FEM analysis for the model shown in Fig. 1. Note that the currents are higher near the locations where the electrical contacts are made to the rebars. .

Figure 12. Potential distribution near a small section of a rebar/concrete interface as obtained from the FEM analysis for the model shown in Fig. 1. Note that the drop in the potential is higher in the concrete as compared to steel or the steel/concrete interface.

Figure 13. Picture of the 93 133 ft concrete bridge, which is CP protected. The CP current on this bridge was mapped using magnetic sensors over a period of 4 h.

Figure 14. A diagram of the bridge in Fig. 4. The ground bed is a titanium mesh, is two-dimensional, is spread over the entire structure, and is covered with concrete and asphalt. Electrical connections between the rectifier and the rebars (negative) and the ground bed (positive) are made at remote locations as shown.

Figure 15. Cracks found on the bridge in Fig. 4. Regions of low CP current in Fig. 6 can be matched with locations of cracks on the bridge deck.

6. Conclusion

In this paper Protection current distribution in reinforced concrete cathodic protection systems has reviewed.So the following conclusion can be drawn:

1. The boundary conditions at the steel have a significant effect on current distribution with factors that increase the potential drop across the steel concrete interface relative to the potential drop through the concrete improving the uniformity of the current distribution.

2. An increase in the concrete resistivity and concrete cover and a decrease in the cathode to anode area ratio at a constant anode current density will increase the voltage drop in the concrete. These factors are important to consider in the selection of the design current density when the principal protective effect is to generate an improvement in the environment at the steel promoting passivity. Thus the current is more uniformly distributed when the corrosion rate is low as the high resistance to polarisation of interface is a controlling factor.

3. The chloride ions significantly contribute to the corrosion of steel rebars in concrete structures.

4. Zinc overlay is found to have an initial cathodic protection current density distribution effect.

5. Sealed conductive coatings as adopted in slab 5 have to be modified to suit concrete structures.

6. The sacrificial anode system is found to protect the steel rebars against corrosion. The shift in potential is found to be significant near the anode.