Current Distribution In Reinforced Concrete Cathodic Protection Systems Construction Essay

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 support in atmospherically open concrete is modeled and is governed by the status of the steel, the electric resistance of the concrete and anode-steel geometry. The boundary conditions at the steel have a singular consequence on current distribution with more unvarying distribution originating at low steel corrosion rates. In a typical circumstance the surface of a steel saloon confronting the anode may have 50 % more current than the opposite surface. As cathodic protection has proved to be utile in these instances, a footing for many design determinations that influence current distribution is that their consequence is negligible by comparing. When more than one bed of support is present the current distribution is unusually worse. In this reappraisal paper protection current distribution in strengthened concrete cathodic protection systems in respect to effectivity of sacrificial anodes have been probed. Besides three-layer reinforced concrete cathodic protection ( CP ) system, with C fiber reinforced cement ( CFRC ) will be probed.

Keywords: Reinforced concrete ; Cathodic protection ; Cathodic bar ; zinc overlay ; Chloride remotion ; Electric field ; Electrochemical fix ; Hydroxyl coevals ; Modeling

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1. Introduction

One of the aims in cathodic protection ( CP ) design is to present a reasonably unvarying current denseness to the protected steel bars. This will minimise the current required to acquire the protection standard, therefore cut downing the cost and bettering the life of the system constituents. The accomplishment of unvarying current distribution in an atmospherically exposed reinforced concrete CP system is, nevertheless, hindered by the location of the steel in a resistive environment near to a big planar anode.

Many old trials has examined current distribution with respect to the design of CP mechanism applied to steel elements/structures in sea H2O and dirts utilizing experimental methods and mathematical theoretical accounts.

In mathematical theoretical accounts the boundary conditions at the polarizing interfaces have ever presented some jobs. In early trials the opposition to polarisation presented by the interface was frequently ignored. Empirical expression come closing the polarisation behavior have besides been used and recent progresss have allowed the clip dependance of the polarizability of the cathodic interface ensuing from the precipitation of sedimentations there to be modeled.

Corrosion of steel induced by chlorides is the major

screen and range to the oncoming of opposing corrosion when their concentration near the steel surface reaches a critical threshold. This threshold is hard to measure, since it depends on several factors related both to the concrete and the environment.

Since reinforced concrete is one of the common building stuffs in civil technology presents, the lastingness jobs have been haunting people ( Cramer SD et Al, 2002 ) .

The worst of these jobs was caused by corrosion of steel in concrete, bring oning the early impairment of concrete substructures. In marine constructions and route or Bridgess sprayed with defrosting salt, the passiveness of the embedded steel bars are affected chiefly by the presence of chlorides, by a lessening of pH in pore solution at the support deepness, or a combination ( Oladis TR, Yolanda HL. , 2008 ) .

The influence of steel bars initial corrosion province, concrete electric resistance and magnitude of impressed current denseness on the current distribution will discussed. Testing consequences show that the initial corrosion rate of steel has a great consequence on the protection current distribution ( Jing Xu, Wu Yao, 2008 ) .

2. Using conductive polymer sheathings

2.1 Experimental portion

In 2007 A.S.S.Sekar and V.Saraswathy tested reinforced concrete slab of size 1m ten 1m ten 0.1m were cast with 3 % Na chloride by weight of cement by changing the parametric quantities 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 sheathing.

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

After bring arounding the slabs for 28days, the Magnesium anode was placed centrally using 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 howitzer holding the lead wire projecting from anode. The anode is electrically connected to the steel support ( cathode ) assembly at the two diagonal opposite points.

2.2 Discussion

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

In the slab which is cast with chloride, with cathodic protection and with zinc sheathing, the possible value of steel at assorted distances from the anode follow the same profile. From Figure 6 it can be seen that, near the anode the possible displacement is found to be around 200mV and if the distance increases the displacement is found to be negligible near the border of the slab. This implies that the add-on of Zn does non hold any consequence on displacement in possible or unvarying distribution of current.

In the slab 5, which is cast with chloride, with cathodic protection and with a conductive coating, an of import observation is noted. Figure 7 associating possible and electric resistance has shown the same observation as the old system. The concrete screen laid over the conductive coating started to divide to after a period of 1507 hour, ( I, e, 65 yearss ) , Subsequently, minute clefts arising from the country of anode assembly and propagating outwards were noticed.

Figure 1. Rebar skeleton agreement before puting 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 Zn sheathing with cathodic protection ( James H.Meyer et al. , 2000 )

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

Figure 5. Consequence of clip and electric resistance on the potency of embedded steel in chloride free concrete

Figure 6. Consequence of cathodic protection on the potency of steel in slab coated with howitzer incorporating Zn in chloride contaminated concrete

Figure 7. Consequence of cathodic protection on the potency of steel in slab coated with conductive pigment in the center of the screen in chloride contaminated concrete

Figure 8. Consequence of clip on the current flowing in slabs with cathodic protection

Figure 8 shows the fluctuation of cathodic protection current in respect to clip in slabs 3, 4 and 5. From the graph it can be seen that the cathodic protection current in the slabs 4 & A ; 5 is active an earlier clip and has reached noticeable and stable values besides after 1900 hours. Cathodic protection applied to steel in concrete is considered effectual if the 100 millivolt decay standard is fulfilled, i.e. if a decay of at least 100 millivolt is achieved within a certain period ( normally 4 or 24 H ) . This standard, which was developed through empirical observation has shown to be effectual in practical applications, and is recommended by criterions. Recently, it has been suggested that the accomplishment of a decay of 100 millivolt should connote that a close inactive province has been induced on the protected steel.

The 100 millivolt decay criterion has shown to be applicable to cathodic bar as good. Trials with really low cathodic current densenesss used to passive steel in chloride contaminated concrete showed that this standard is dependable in measuring the effectivity of this technique. In fact, current densenesss is able to acquire a 4-h decay higher than 100 millivolts were sufficient to keep passiveness on steel bars even when a chloride content up to 3 % by weight of cement was reached really near the steel surface.

So, the 100 millivolt decay standard can be used to both cathodic protection ( i.e. application of a cathodic current in order to command corrosion rate of already eating steel ) and to cathodic bar ( i.e. application of a cathodic current to passive steel in chloride contaminated concrete because of forestalling the oncoming of opposing corrosion ) . Therefore, it can be assumed that submerged sacrificial anodes are able to command corrosion merely on the support in the emerged portion of a heap that experiences values of 4-h decay steadily more than 100 millivolt. In chloride contaminated concrete such decay can vouch a cathodic polarisation sufficient to obtain protection to eating steel, while in chloride free concrete it can vouch a cathodic polarisation sufficient to bring on a important addition in the critical chloride content ( and therefore to contrast pitting corrosion induction even when chlorides penetrate the concrete in contact with the steel surface ) .

In this probe possible displacement of 200mV is observed in carry oning polymer sheathing and Zn sheathing used systems. Another interesting observation is that the points really near to the anode have the possible displacement of 200mV and the farther points has really low displacement in possible even in chloride contaminated concrete. This may be because of the hapless throwing capacity of the anode.

3. CP system utilizing C fiber reinforced cement ( CFRC ) composite stuff as the conductive sheathing anode

Harmonizing to the literature reappraisal ( Jing Xu, Wu Yao, 2008 ) by experimental probe, the influence of initial corrosion province of steel, concrete electric resistance and magnitude of impressed current denseness 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 strengthened concrete construction with complex mesh support system, utilizing of half cell possible measuring to find the corrosion province of steel bars is non sufficient. It is of import to attach to this measuring with complementary information, such as the corrosion rate and/or the electric resistance of the concrete.

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

3.3- Current distribution in strengthened concrete with more than one bed of support placed at different screen deepnesss is markedly effected by concrete electric resistance.

3.4 Magnitude of impressed current denseness has a spot consequence on the current distribution when the corrosion rate of steel is comparatively low.

4. Electrochemical Engineering Approach

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

Without any polarisation, a metal in contact with concrete or an electrolyte will stay at its corrosion potency ( Ecor ) . At this possible, the metal surface sustains at least two reactions go oning at equal rates: a metal disintegration ( or anodic metal oxidization ) reaction, and a cathodic conjugate reaction, such as O decrease or H development. If the metal is electrically polarized to potencies positive to Ecor, the metal disintegration reaction will be accelerated, whereas the cathodic conjugate reaction will be acquire slowly..

The converse is true when the metal is polarized negative to Ecor. Thus, when the metal is polarized off from Ecor to a positive or negative value, a net anodic or a net cathodic current, severally, will flux across the metal/electrolyte conection. A metal is under cathodic protection when it is polarized effectivly negative to Ecor to cut down the metal disintegration rate by 3 orders of magnitude or more. Under most conditions, a polarisation of about 2200 to 2300 millivolt is sufficient to accomplish cathodic protection.

Excessive cathodic polarisation should, nevertheless, be avoided to forestall oncoming of the H development reaction and to diminish the possibility of H embrittlement of the metal. Besides, cathodic polarisation, like corrosion, is a surface procedure. So, to accomplish unvarying protection at all locations on a given surface, it is imperative that the cathodic current denseness is unvarying at all locations. Any nonuniformity in the current flow, particularly with values less than some critical lower limit, can do localised fluctuations in the metal disintegration rate. These fluctuations can ensue in the construction eating more badly in some topographic points than in others. In a span, for illustration, if the CP current is nonuniformly distributed, those country of the span that do non have the current will go on to eat, whereas those that do have the current will be good protected from corrosion. Typical CP systems used in protecting metal concrete constructions are explained as follows: In these constructions, the metal is normally steel, and the cement and H2O from the electrolytic medium. Normally, the CP system has a rectifier as the electromotive force beginning. The return electrode for the current is either a Pd coated Ti mesh, 12 a thin bed of Zn, 7 or a conducting polymer mixed with concrete. They are inert electrodes, non consumed or destroyed by the reactions associated with the cathodic protection, and

are named land beds. Normally, enerally, the land bed is planar, is spread over the full construction, and is covered with concrete and asphalt. All the rebars are electrically connected to one another, and the electrical connexions between the rebars and the rectifier are made at one or two distant locations on the span. Similarly, the electrical connexions between the land bed and the rectifier are besides made at one or two distant places. Therefore, in most instances, the land bed is distributed equally with regard to the rebars, whereas the electrical contact points are more localised. Since the Bridgess are located in assorted geographical places from Washington to Maryland, and from Florida to New York they are exposed to a broad assortment of environmental and climatic conditions.

5. Current and electromotive force distribution

The current distribution near the rebar/concrete conection for the top bed of rebars, obtained by the FEM analysis for the theoretical account construction is shown in Fig. 2. The rebar surface where the electrical contacts were made is besides showed in the figure. The magnitude of the current is maximal at the terminal of the rebar surface where the electrical electromotive force was specified and is minimal at the far terminal. The possible 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 centimetres. Near to the rebar/concrete interface, the bead in the potency is comparatively negligible ; bulk of the bead occurs over a distance of a few centimetres within the concrete. At all other locations of the interface, the possible distribution was about indistinguishable to the one shown in Fig. 3. The comparatively larger bead in the concrete is commensurate with its higher electric resistance ( 0.5 105 V~cm ) in comparing with steel ( 0.18 1024 V~cm ) . This observation is uncomplete understanding with those made by other corrosion research workers on in-service concrete constructions.

An of import decision drawn from the FEM analysis is as follows: For asymmetrical geometric constellations ( of the rebars and the land bed ) , with asymmetrical electrical constellations, the rebar/concrete interface that is farther off from the electrical contacts gets really small current. As explained in the subdivision Principles of Corrosion and Cathodic Protection, the grade of protection from corrosion decreased as the current across the interface decreased.

Actually, CP designs similar to the one described antecedently, if adapted for a existent concrete construction, may non protect the rebars from corrosion over their full length. Many of the 350 Bridgess mentioned antecedently, and many other constructions that are soon under cathodic protection, are protected utilizing asymmetrical electrical connexions.

6. CP current function on the concrete span

Figure 4 shows a steel-reinforced concrete span which is located in Maryland. The span is 93 foot long in the east west way and 133 foots broad in the north south way. The span deck is cathodically protected by a individual rectifier. A shaping of the span is shown in Fig. 5.

The deck has two beds of uncoated rebars, one on the top and the other on the underside, with concrete which is located in between. All rebars are shorted to one another. A palladium-coated Ti mesh, spread over the full span, is located over the top bed of the rebars and Acts of the Apostless as the land return. A latex concrete mix covers the Ti mesh. A 133-ft-long conducting saloon, placed along the north south axis at about 46 foot from the west terminal of the span, is joined to the Ti mesh. A point contact made to the conducting saloon at about 60 foot from the north terminal of the span is joined to the positive terminus of the rectifier. The negative terminus of the rectifier is connected to the rebars at two places along the west terminal of the span. Therefore, the span is a casebook combination of a uniformly distributed land bed laid over uniformly distributed rebars, with the non-textbook status of distant electrical connexions. CP currents were mapped from the top of the span deck. For this mark, the deck was divided into a matrix of many analogues 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 utilizing gaussmeter detectors. The ensuing current distribution is shown in Fig. 6. This figure shows merely an 80 80-ft portion of the 93 133 ft country of the span deck ; the amplitude of the CP current in the remainder of the deck is less than 1 ma. .The CP currents are concentrated merely in the northwest country of the span, where they reach a peak value of about 5 ma. The location where the maximal current happened lucifers good with one of the points where the rebars are joined to the rectifier. This current distribution confirms that while the distributed geometry of the land bed, viz. , a mesh spread over the full deck, appears intuitively right, it did non assist to acquire unvarying current distribution.

The CP current, mapped with gaussmeter detectors

over the full deck of the span, shows that more than 60 % of the country did non acquire any current. Ocular observation made on the top and the underside of the deck revealed a of import sum of clefts in the construction in the E and the southeast place ( Fig. 7 ) . Figure 6 shows that these parts get small or no CP current. The northwest locations that received unusually larger currents showed no grounds of checking. It is possible that in the E and the southeast locations, the rebars are eating due to a deficiency of cathodic current. Direct verification of the corrosion of the rebars through ocular review is yet to be obtained. If the rebars are so eating, that could be doing spalling and snap of the concrete.

Figure10. Diagram of concrete bock used in the FEM theoretical account to find the current and possible distribution.

Figure 11. Current distribution near the rebar/concrete interface as obtained from the FEM analysis for the theoretical account 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 little subdivision of a rebar/concrete interface as obtained from the FEM analysis for the theoretical account shown in Fig. 1. Note that the bead in the potency is higher in the concrete as compared to steel or the steel/concrete interface.

Figure 13. Picture of the 93 133 ft concrete span, which is CP protected. The CP current on this span was mapped utilizing magnetic detectors over a period of 4 H.

Figure 14. A diagram of the span in Fig. 4. The land bed is a Ti mesh, is planar, is spread over the full construction, and is covered with concrete and asphalt. Electrical connexions between the rectifier and the rebars ( negative ) and the land bed ( positive ) are made at distant locations as shown.

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

6. Decision

In this paper Protection current distribution in strengthened concrete cathodic protection systems has reviewed.So the undermentioned decision can be drawn:

1. The boundary conditions at the steel have a important consequence on current distribution with factors that increase the possible bead across the steel concrete interface relation to the possible bead through the concrete bettering the uniformity of the current distribution.

2. An addition in the concrete electric resistance and concrete screen and a lessening in the cathode to anode country ratio at a changeless anode current denseness will increase the electromotive force bead in the concrete. These factors are of import to see in the choice of the design current denseness when the chief protective consequence is to bring forth an betterment in the environment at the steel advancing passiveness. Thus the current is more uniformly distributed when the corrosion rate is low as the high opposition to polarization of interface is a controlling factor.

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

4. Zinc sheathing is found to hold an initial cathodic protection current denseness distribution consequence.

5. Sealed conductive coatings as adopted in slab 5 have to be modified to accommodate concrete constructions.

6. The sacrificial anode system is found to protect the steel rebars against corrosion. The displacement in possible is found to be important near the anode.


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