EP0186334A1

EP0186334A1 - Cathodic protection system for reinforcing bars in concrete, a method of carrying out such protection and an anode for use in the method and system

Info

Publication number: EP0186334A1
Application number: EP85308692A
Authority: EP
Inventors: Peter Charles Steele Hayfield; Ian Robert Scholes
Original assignee: MARSTON PALMER Ltd (FORMERLY KNOWN AS IMI MARSTON LTD); Marston Palmer Ltd; Ebonex Technologies Inc
Current assignee: Ebonex Technologies Inc
Priority date: 1984-12-15
Filing date: 1985-11-29
Publication date: 1986-07-02
Anticipated expiration: 2005-11-29
Also published as: GB8431714D0; DE3575953D1; JPS61186487A; CA1279606C; EP0186334B1; ATE50291T1

Abstract

A cathodic protection system to protect the reinforcement bars in concrete wherein there is provided as an anode a hydraulically porous material bonded to the concrete so that the anode can be irrigated by water to reduce chloride concentration. A preferred material for the anode is TiO_x in bulk coherent form where x is in the range 1.6 to 1.9 preferably 1.67 to 1.85.

Description

This invention relates to cathodic protection systems and has particular reference to cathodic protection systems used to protect iron or steel reinforcement bars in concrete structures.
Impressed current cathodic protection systems are well known in use to protect structures immersed in water, particularly sea water. In such a system the object to be protected is made a cathode whilst the counter electrode is made an anode. Negatively charged ion species are attracted towards the anode where they tend to concentrate, unless sufficient diffusion of ions occurs in the region of the anode to disperse them. In free sea water, the movement of the negatively charged ions occurs freely and readily, such that there is a minimal build-up of ions around the anode.
Reinforced concrete essentially comprises a series of steel reinforcing bars (commonly referred to as rebars) surrounded by a concrete mixture.
It is well known that steels are not corroded in alkaline media. Reinforcing bars are very frequently covered with an adherent "rust" layer when embedded in concrete, which experience has shown improves the adhesion between concrete and steel. With time, as may be shown by removing the concrete cover, the rust changes chemically allowing the formation of a dark, protective, film well adherent to concrete. This is the very satisfactory usual situation that exists.
Concrete, by various mechanisms, is porous to water, albeit very slowly, and even after so called full curing, will allow a slow uptake with some kind of equilibrium being established with the surrounding environment This again is a normal situation. However, if salt water is present on the surface, then the salt and its contained chloride ions may penetrate into the concrete. It is not immediately obvious why salt contamination is more of a problem to some concrete than others, because concrete is widely used as- a constructional material for use in seawater. The design of the concrete structure and thickness of the outer concrete layer may be particularly important
In the case of chloride contaminated concrete, a risk exists that chloride ions will enhance the corrosion of the steel. The resultant corrosion product formed by the enhanced reaction occupies a greater volume than the space occupied by components prior to chemical reaction, eventually creating intense local pressure that brings about cracking of the concrete and eventual spalling of the concrete cover to expose rebars directly to the atmosphere.
A great deal of reinforced concrete has been used in building and in road construction and particularly in the fabrication of support pillars, cross beams and road decks for bridges. Over the years increasing amounts of common salt, sodium chloride, has been used in winter to prevent ice formation on the road. The melted snow or ice and sodium chloride solution tends to seep down into the concrete, and it has been found that the presence of the chloride ion penetrating to the rebars can give rise to corrosion. In some cases, calcium chloride has been added to the concrete as a setting agent or the water used to make the concrete contained naturally high levels of chloride ions and this also increases the rate of corrosion of the rebars. Further, some structures are exposed to salt-laden atmospheres, particularly in marine locations.
Electrode potential mapping of the outside surface of concrete rebars is used as a means of assessing the state of corrosion of embedded rebars and by inference the depth of penetration and concentration of salt Around a concrete cross bar to a motorway bridge, the variation in electrode potential may be 0.5 volts or more. It might be logical to expect that salt contamination from the bridge roadway would leak onto the top surface of the cross beams and hence lead to more rapid penetration to rebars lying near the top surface than on the bottom surface, and this is exactly what is found.
The problem of corrosion of the rebars in bridges has become so significant that much effort is being expended in an attempt to slow down or haft the corrosion before the concrete structures in the bridges fail.
By cathodic protection is meant the application of an electrolytic system whereby the electrode potential of the steel is depressed to a cathodic (negative) potential to stop or significantly decrease corrosion.
The cathodic protection of steel in concrete represents an especially challenging problem for application of cathodic protection for a variety of reasons. An obvious difference between cathodic protection in concrete and seawater is the difference in ionic mobility of species within the electrolyte. Although there is an electrolytic path in concrete, otherwise application of cathodic protection would be impractical, nevertheless the level of diffusion between anode and cathode is many orders of magnitude lower than in the seawater case, and the distance between the anode and the cathode to be protected cannot usually exceed 15 to 30cms.
Another difference between the seawater and concrete example relates to the change in pH surrounding the electrodes. It is well. known. that media surrounding a cathode will tend to alkalinity, and around an anode will tend to acidity. Alkalinity around a rebar in concrete, which is already alkaline, is no problem. Indeed additional alkalinity could be helpful towards the stabilisation of the steel from corrosion.
- - The formation of-acidity around anodes in concrete is a major issue. Acidity cannot readily diffuse away from the anode either by diffusion under a concentration gradient, or by field transport to the cathode (ie H^* to the cathode) brought about by the applied cell voltage. Concrete is readily attacked by acid, even at very low levels of acidity. Attack is significant at pH 6, and while some concretes may be more acid resisting than others, attack at pH's down to 2 or 1 (common in some cathodic protection situations) is extremely rapid.
Hence in practical terms, the problem of cathodic protection of rebars in concrete is not the ability to arrest the corrosion of steel by depressing the electrode potential, but the problem of acidity surrounding anodes. Indeed the longevity of cathodic protection systems applied to concrete may well not be related to the durability of the electrode materials involved, but be related to the acid attack on concrete surrounding anodes. In this respect the cathodic protection of rebars in concrete is very different to cathodic protection of steel in seawater.
By the present invention there is provided a cathodic protection system for the protection of iron or steel reinforcement bars in concrete which includes a source of electrical current connected to the reinforcement bars and to an anode, so that, in use, the reinforcement bars are connected as a cathode, wherein the anode is a hydraulically porous material bonded to the concrete so as to make electrical contact therewith and being exposed to the environment over part of its surface area.
The present invention also provides a cathodic protection system for the protection of iron or steel reinforcement bars in concrete which includes a source of direct current connected to the reinforcement bars and to an anode, wherein the anode is a hydraulically porous material bonded to the concrete so as to make electrical contact therewith. The material may be a ceramic material.
The hydraulically porous material may be directly bonded to the concrete by cement. Aftematively the porous material may be embedded in a conducting backfill.
A preferred material for the anode is porous TiO_x where "x" is in the range 1.67 to 1.95. Preferably "x" is in the range of 1.75 to 1.8. The porous TiO_x material is preferably in the form of a tile grouted to the exterior of the concrete. A liquid mortar may be used as the grout The porous TiO_x material may have a thickness in the range 2-3 mm. The density of the material may be in the range 2.3 to 3.5. The porous TiO_x material may be in the form of a tube passing into a hole in the concrete structure.
The porous material may be graphite or porous magnetite, porous high silicon iron or porous sintered zinc, aluminium or magnesium sheet
The porous material may be in the form of a tile bonded to the concrete, the tile having a projecting ear to which electrical contact can be made. The ear may be provided with a hole or slot The ear may be connected to a power supply cable having an electrically conducting core and a lead metal exterior in electrical contact with the core, the lead being deformed by the action of being pushed into the slot to make electrical contact therewith.
The present invention further provides a cathodic protection system for the cathodic protection of steel reinforcement bars embedded in concrete, the system comprising a plurality of anodes embedded in spaced location in the concrete, the anodes being formed of hydraulically porous TiO_x where x is in the range 1.67 to 1.95, the anodes being electrically interconnected by titanium conductors, the anodes being anodically polarised relative- to the steel reinforcement bars by the titanium conductors. The titanium conductors may be in the form of strips.
The titanium may be anodically passivated and may be coloured. The anodically passivated layer may be removed where the titanium is in contact with the TiO_x material anodes.
The titanium may be in the form of strips having sections cut and bent out of the plane of the strips to form tabs to which the anodes are connected. The anodes may be connected to the titanium strips by nuts and bolts of titanium or by a titanium rivet The strips may mechanically locate the anodes on the concrete. The strips may be provided with slots.
There may be a plurality of strips with the strips being bolted, riveted, welded or otherwise electrically joined together.
By way of example embodiments of the present invention will now be described with reference to the accompanying drawings, of which:

Figure 1 is a schematic view in section of a road bridge;
Figure 2 is a schematic view of a support member of a bridge wired with a series of anodes;
Figure 3 is a cross-section of a support member showing anodes and reinforcement bars;
Figure 4 is a perspective view of one form of anode;
Figure 5 is a perspective view of an alternative form of anode;
Figure 6 is a schematic view of a concrete cross beam and pillars;
Figure 7 is a schematic perspective view of an anode connected to a portion of a strip;
Figures 8 and 9 are schematic views of tabs formed in titanium strip;
Figure 10 is a schematic view of a connection between two titanium strips incorporating an anode at the connection;
Figure ₁1 is a plan view of a strip with an anode embedded in concrete;
Figure ₁2 is a perspective view of an anode and strip; and
Figure 13 is a sectional view of an anode and strip.

It is necessary to understand the causes of acidity to understand the present invention. In any electrolytic system involving anodes and cathodes, the overall reaction is the summation of component parts, which includes specific electrochemical reactions at both electrodes. In the case of an anode in concrete, the predominating reactions, albeit at very slow rate compared with most cathodic protection systems, is either the oxidation of chloride ions to release chlorine gas, or the oxidation of. water ta release oxygen and leave behind H⁺ ions. This latter reaction is the particularly important one, 2H,0 - 4e - 0, + 4H⁺.
For every 96,540 coulombs of electricity passed involving this reaction, 1g H⁺ will be produced. Such H⁺ concentration (which is, of course, acidic) will react with concrete, in a volume depending upon the available calcium hydroxide accessibility within the concrete. Thus a simple model can be considered. Hydrogen ions generated by the reaction set out above may:

a) react with all. the calcium compounds in the volume of concrete immediately surrounding the anode;
b) depress the pH of the surrounding concrete to a low pH;
c) migrate towards the OH- near the cathode;
d) pass through the porous anode to be oxidised by the air; or
e) pass through the porous anode and be diluted by the moisture from the atmosphere or rainwater.

Short term (seven day) practical tests with a single anode passing 25mA suggests that the volume of concrete attacked will be 12cm³, and the pH depressed to a value of O. Assuming normal porosity, density and Ca(OH), content of concrete, the amount of H⁺ ion produced required to react with 12cm³ of concrete will be 0.036gH⁺.
At a pH of 0 the H⁺ concentration will be 1g/l H⁺ so that the H⁺ requirement to depress the pH of available moisture will be 0.012gH^+. Therefore the total H⁺ generated to the anode over seven days would approximate to 0.036 + 0.012 = 0.048gH^*.
The number,of coulombs of electricity passed in seven days is 15,120 coulombs. Assuming 30% current efficiency for oxygen evolution, the reaction 2H,O - 4e - 0, + 4H⁺ will result in 0.047gH⁺ ion generation. Hence it appears that a 25mA anode will produce between 0.05 to 0.1gH⁺ per week. At 1mA/anode the corresponding figure would be 0.002 to 0.004gH⁺ per week. At this rate it would take eighteen weeks to react all Ca(OH), in 12cm³ zone of concrete per week, and, of course, correspondingly longer time periods the lower the current passed or the volume of concrete affected.
The calculations involved are. approximate but while advanced at no detriment to the claims of the patent, are advanced as an indication of the magnitude of the problem of acid attack around anodes in concrete.
Some indication of the effect of porous anodes in removing acidity has been obtained from laboratory experiments in which porous anodes were cemented to a reinforced concrete block and were kept saturated with water artificially by means of a surrounding shallow rim. With application of current to rebars within the concrete, acidity develops on the outer surface of the porous anode and after five hours of operation at 80mA at 8 volts the pH of the distilled water fell to 2.7.
It should be noted that H⁺ ions generated at the anodelconcrete interface diffused away from the cathode into water to the outer side of the anode, presumably because of the favourable concentration gradient
In a further experiment, hydraulically porous Ti.O, material was used to divide the volume of a glass beaker into two approximately equal volumes. In one side was placed sodium chloride solution and a titanium metal strip cathode. In the other (representing the atmosphere side in the bridge deck) was put distilled water. With the hydraulically porous Ti₄O₇ separator connected as an anode and anodically polarised with respect to the titanium cathode, acidity developed with time in the initially distilled water compartment Such acidity develops in spite of the good diffusivity of H⁺ ions in the sodium chloride side, ie H⁺ ions diffused in the "wrong" direction away from the cathode.
Referring now to the drawings, as illustrated schematically in Figure 1, many road bridges are based on a series of upstanding pillars 1, 2 supporting a cross member 3. Members 1, 2 and 3 are formed of reinforced concrete. The cross members 3 carry plurality of substantially rectangular section steel girders ₄, 5 which carry the actual road bed 6.
As shown in Figures 2 and 3 there is provided a mechanism for cathodically protecting the rebars in the structure 3. The rebars 7 (more clearly shown in Figure 3) are connected to a source of eletrical current 8 as cathodes. A series of hydraulically porous TiO_x tiles 9, 10 are grouted to the exterior of the concrete structure 3 and an electrical connection is made to the ties in a suitable manner. The preferred value for x is 1.75, but tiles where x is predominantly in the range 1.75 to 1.8 are acceptable. Electrodes of this material are described in US Patent 4₄22917, the description of which patent is incorporated herein by way of reference.
As the TiO_x material will conduct electricity as an anode it may be used to pass an electrical current through the moisture in the concrete into the rebars 7. During operation anions such as Cr are attracted to the anodes and by using porous TiO_xmaterial the anions can diffuse through the TiO_x to be oxidised by the air in the atmosphere or to be washed away by the irrigation of the anodes which will occur from rain water washing over the surface of the support structures, or by applied water irrigation.
It will be appreciated that tubular anodes can be used. If necessary water could be directed through the anodes from gulleys on the road deck.
By irrigating the anodes, excessive anion build-up in the concrete can be reduced.
One method of connecting the anodes to the electrical conductor line is shown with reference to Figure 4. In Figure 4 the anode plate 11 is provided with an upstanding ear ₁2 integral with the plate. A hole 13 exposed through the ear 12. Electrical contact is made by bolting or clamping a suitable wire through the aperture 13.
An alternative method of making a connection is illustrated in Figure 5. In the design of Figure 5 a tile 14 of circular or eliptical shape is provided in the centre with three up-standing ears 15, 16 and 17. The ears are provided with slots 18, 19 and 20. It will be seen with the slots 18 and 20 face in the opposite direction the the slot 19.
A lead coated wire having a copper core is threaded around the ears 15, 16 and 17 and the slots are so positioned that tightening of the lead covered wire causes the wire to bite into the slots 18, 19 and 20 to make an electrical contact
An alternative mode of operation may be used containing TiO_x as the electrical conducting constituent formed by packing powder into grooves in the concrete back-filled into grooves in the concrete.
The current density which needs to be applied to the anodes is very low. Typically a current density of the order of 20 milliamps/m² will protect the steel rebars. Thus for concrete structure 1 1 2 m²by 14m long, a total current number across being 1-2t amps would be sufficient Operating 15cm x 15εm tiles at 5. amps/m² would permit each tile to pass 0.1 amp so that 15-20 tiles per cross-beam would be sufficient
It will be appreciated that the number of tiles used would be determined by the required cathodic protection throwing power. Each anode would be electrically connected to the power source used for the impressed current cathodic protection system.
The hydraulically porous TiO_x material, particularly material having a density in the range 2.3 to 3.5g/cc is readily bonded by cement to concrete and has the distinctly advantageous property of not being effected by water freezing within the pores of the material. As material is hydraulically porous water as a liquid (rather than merely as a vapour) can pass through it; for example material of 3mm thickness with a head of water of 30cm will pass one litre of water per 5cm² of exposed surface area per day.
Instead of using lead coated wires as connectors titanium strips may be used. The titanium strips may themselves have a coating to restrict corrosion of the strips in which case an oxide film formed by anodising is preferred. This arrangement is shown more clearly in Figures 6 to 13.
Referring to Figure 6 this shows the concrete cross beam 3 supported on the pillars 1, 2. The beam is a conventional steel rebar reinforced concrete structure. The pillars 1 and 2 are also conventional concrete with steel rebars. Extending along the length of the beam 3 is a titanium strip 21 and extending vertically from the strip is a plurality of strips, two of which are shown at 22, 23. The entire length of the concrete beam 1 will be covered by strips 22, 23. The strips 5, 6 are spot welded, riveted, bolted or otherwise connected to the strip 21 and a vertical strip 24 is connected to the strip 21. Strip 24 is connected to a suitable source of electrical current as an anode and a suitable connection is made to the steel reinforcement within the beam 3 as a cathode. Thus electricity can be conducted along strips 24 and 21 to the plurality of vertically extending titanium strips 22, 23. A plurality of TiO ₁.₇₅ anodes are bolted to the strips 22, 23 as shown in Figure 7. The anodes 25 are of a ceramic-like material and are bolted to tabs formed integrally in the strip 26. The tabs are shown in more detail as 27 and 28 in the strips 29 and 30 shown in Figures 8 and 9.
As can be seen in Figure 10 the tabs can be formed as connectors for adjacent strips 31, 32 and also to connect in an anode 33. The anodes are secured by means of titanium nuts and bolts 34.
The anodes may be embedded into holes drilled into the concrete and are grouted into position as shown in Figure ₁₁. The anode 3₄ is surrounded by grout 36 located in the concrete structure 37. The anode is bolted to titanium strip 38.
By this system the permanently connected anodes can be distributed over the surface of the beam 3 and of course, if required, over the surface of the upright pillars 1, 2. Any other structure can simultaneously be protected.
Because the concrete grout securing the anodes may well become weak during operation, even if it is one chosen for its acid resistance such as a high alumina cement, it is probably desirable to use mechanical means to hold the anodes in situ in addition to the grout The titanium strips can carry out both functions of supplying current to the anodes and holding the anodes in situ. When such a system is used, slotted strips 39 (Figures 12 and 13) can be used. To install such a system, initially the position of the shear (longitudinal) rebars in the material is located, marked out with chalk, and thus enabling the location of the hydraulically porous tile anodes to be marked in approximate position. The hydraulically porous tile used measured 50mm x 50mm and have a hole drilled in one location to take a titanium metal nut 40 and bolt 41. The connector strip 39 of slotted titanium had a width of 20mm and was 0.5mm thickness. By means of a self tapping screw, the strip is located at its upper end, leaving slot locations for the anode and defining positions for the self tapping holding screws 42. Because of large aggregate in the concrete cover, it is difficult to drill holes exactly in the concrete, thus the use of the slots facilitated installation. Then the anode tiles are grouted at locations down the strip and the titanium nuts 40 screwed loosely in position and the self tapping screws 42 screwed into position while the acid resisting cement is still soft. This sequence is progressed along the side of the beam.
When the electrically conducting and chemically resistant grout has hardened, the nuts to the anodes are tightened, and a horizontal linking titanium strip connector applied.
The horizontal connector will also be attached to the concrete structure, but only after all other positioning had been completed. The system can then be used to protect the rebars.
Obviously the cathodic protection system could be used to protect rebars in concrete in any situation, for example car parks, foundations, marine structures etc. In the case of bridges the system can be installed on the underside of the bridge deck itself to protect the bridge deck. This installation can be done without interfering with the traffic flow.

Claims

1. A cathodic protection system for the protection of iron or steel reinforcement bars in concrete which includes a source of direct electrical current connected to the reinforcement bars and to an anode, so that, in use, the reinforcement bars are connected as a cathode, wherein the anode is a hydraulically porous material bonded to the concrete so as to make electrical contact therewith and being exposed to the environment over part of its surface area.

2. A cathodic protection system for the protection of iron or steel reinforcement bars in concrete which includes a source of direct current connected to the reinforcement bars and to an anode, wherein the anode is a hydraulically porous ceramic material, preferably a ceramic, bonded to the concrete so as to make electrical contact therewith.

3. A cathodic protection system as claimed in Claim 1 or Claim 2 in which the hydraulically porous material is directly bonded to the concrete by cement.

4. A cathodic protection system as claimed in Claim 1 or Claim 2 in which the porous material is embedded in a conducting backfill.

5. A cathodic protection system as claimed in any one of Claims 1 to 4 in which a preferred material for the anode is porous TiO _x where "x" is in the range 1.67 to 1.95.

6. A cathodic protection system as claimed in Claim 5 where "x" is in the range of 1.75 to 1.8.

7. A cathodic protection system as claimed in Claim 5 or Claim 6 in which the porous TiO_x material has a thickness in the range 2-3 mm.

8. A cathodic protection system as claimed in Claim 5, 6 or 7 in which the density of the material is in the range 2.3 to 3.5.

9. A cathodic protection system as claimed inOaim 5 w Claim 6 in which the porous TiO_x material is in the form of a tube passing into a hole in the concrete structure.

10. A cathodic protection system as claimed in Claim 1 or Claim 2 in which the porous material is graphite or porous magnetite, porous high silicon iron or porous sintered zinc sheet.

11. A cathodic protection system for the cathodic protection of steel reinforcement bars embedded in concrete, the system comprising a plurality of anodes embedded in spaced location in the concrete, the anodes being formed of hydraulically porous TiO_x where x is in the range 1.67 to 1.95, the anodes being electrically interconnected by strips of titanium conductors, the anodes being anodically polarised relative to the steel reinforcement bars by the titanium strip conductors.

12. A cathodic protection system as claimed in Claim 11 in which the titanium is in the form of strips having sections cut and bent out of the plane of the strips to form tabs to which the anodes are connected.

13. A cathodic protection system as claimed in Claim 11 or Claim 12 in which the anodes are connected to the titanium strips by nuts and bolts of titanium or by a titanium rivet

14. A cathodic protection system as claimed in any one of Claims 11 to 13 in which the strips may mechanically locate the anodes on the concrete.

15. A cathodic protection system as claimed in Claim 14 in which the strips are provided with slots.

16. A cathodic protection system as claimed in any one of Claims 1₁to 14 in which there is a plurality of strips with the strips being bolted, riveted, welded or otherwise joined together in electrical contact