WO1992001824A1

WO1992001824A1 - The protection of cementitious material

Info

Publication number: WO1992001824A1
Application number: PCT/GB1991/001179
Authority: WO
Inventors: Kenneth Brian Pithouse
Original assignee: Kenneth Brian Pithouse
Priority date: 1990-07-17
Filing date: 1991-07-16
Publication date: 1992-02-06
Also published as: AU8220591A; GB9015743D0

Abstract

A protective anode for incorporation in a concrete or like cementitious structure so as to provide anodic protection against corrosion of metal reinforcement is in the form of an assembly (10) which comprises an electrode (11) embedded within and sheathed by a body (12) of cementitious material.

Description

THE PROTECTION OF CEMENTITIOUS MATERIAL

This invention relates to the protection of cementitious material, and especially to the inhibition of corrosion of reinforcement in structures of masonry or cementitious material, for example concrete, by cathodic protection.

Masonry or cementitious materials usually have compressive strength, but little tensile strength. It is therefore necessary when using concrete, for example, as a structural material to incorporate reinforcing members (usually metal, preferably steel) to impart the required strength. The reinforcing members may be placed under tension to form "pre-stressed" or "post-tensioned" concrete structures. if the reinforcement corrodes, it may expand and cause internal stresses within the concrete. This ultimately- leads to cracking of the concrete which is then liable to start to break up. The cracking may also result in the reinforcement being further exposed to water and atmospheric oxygen, accelerating the corrosion process.

In order to protect such structures surface mounted anodes are often applied to the structure in order to maintain the reinforcement members at a cathodic potential. However, in many cases, for example where the surface is relatively inaccessible, it is necessary to employ a point-source anode to provide the necessary potential for cathodic protection of the reinforcement members. Point-source anodes are usually in the form of an elongate anode that is inserted into a hole that is bored in the structure, an anode being inserted every 200 to 500 mm in each direction of the surface of the structure. One point-source anode, for example, employs graphite rods embedded in a carbon paste, which fill holes drilled into the structure.

The known point-source anodes, however suffer from the doisadvantage that they lead to acidification of the cementitious material and to gas formation due to the oxidation of carbon to carbon dixoide. In addition, such carbon-based anodes need to operate at a relatively high voltage which tends to drive water away from the anode and thereby reduce the conductivity of the cementitious material. Also, the relatively high anodic potential tends to attract chloride ions to the anode which are oxidized to C10~ ions leading to further acidification of the cementitious material .

If a point-source anode is to be provided every 200 to 500 mm in the concrete structure, it will need to supply a current of at least 150, preferably 170, especially 190, and in the preferred case at least 220

2 mA per m of electrode. However, in order to operate continuously without failure for a number of years, a point-source anode should not exceed a value of about

110 mA per square metre of anode active surface. If the anode cannot provide a minimum of about 150 mA per of anode active surface, then those parts of the reinforcement furthest away from the anodes may remain unprotected. However, the current is limited to about 110 A per square metre of anode active surface by NACE (National Association of Corrosion Engineers (Committee T-3K-2) ) above which acid degradation of the cementitious material may occur.

The invention provides an anode assembly for protecting metal reinforcement within a cementitious structure, in the form of an electrode embedded in and sheathed by a body of material having a resistivity compatible with the resistivity of cementitious material.

The present invention further provides a cementitious structure having a metal reinforcement, the structure being protected from corrosion of the reinforcement by means of a point source anode array that comprises one or more point source anode assemblies, the or each of which contains an electrode that is embedded in cementitious material, the electrode having a surface area of electrode material in contact with the cementitious material that is at least 1.3 times as great as the overall surface area of the electrode. The structure according to the present invention has the advantage that it can be cathodically protected against corrosion for long periods of time without any undue degradation of the cementitious material or without an unduly large number of point-source anodes needing to be embedded in the structure. In certain forms of structure up to 1 to

2.5m 2 of steel reinforcement may be used per m2 of surface area of the cementitious material, with the result that it may be difficult to bore a hole for a point-source anode without hitting a reinforcement member. If a carbon based anode according to the prior art is employed for cathodic protection, the carbon composite that is used will form a direct electrical connection with the reinforcement member if the bore hole does hit a reinforcement member, with the result that, not only are the reinforcement members not protected against corrosion, they will actually be subject to accelerated corrosion due to the impressed anodic potential thereon by the point-source anode.

Thus, according to another aspect of the invention, there is provided a point source anode assembly for protecting a reinforced structure from corrosion, which comprises an electrode that is embedded in a low resistivity material having substantially the same resistivity as cementitious material so that the outer surface of the anode comprises substantially entirely the low resistivity material.

Although in theory any material having the same or similar resistivity as that of cementitious material may be used to embed the electrode, it is preferable to employ cementitious material itself as the low resistivity material, for example to employ a cementitious paste as a grout that encloses the electrode or at least provides the surface of the anode assembly. Such materials generally have a volume resistivity in the range of from 20 kohm cm to 60 kohm cm and usually from 30 kohm cm to 50 kohm cm, the preferred materials having a resistivity of about 40 kohm cm.

This aspect of the invention has the advantage that if the hole that is bored in the structure for locating the anode assembly does touch a reinforcement member, as is quite likely, the reinforcement member is isolated from direct electrical contact with the electrode by cementitious material or its electrical equivalent, with the result that the cathodic protection system will function as required.

The anode assembly according to the invention has the further advantage that, since its surface that is in contact with the concrete or other cementitious material is also formed from cementitious material, the propensity of the anode assembly to form a surface of weakness where it bonds to the concrete is reduced. Such a surface may weaken the structure or may disbond, thereby imparing the effectiveness of the cathodic protection of the reinforcing members. In addition, because the surface of the anode assembly is formed from cementitious material, any disbonding of the anode assembly from the structure that may occur will not cause the electrode/cementitious material surface area to be decreased, and so the current density of the electrode/cementitious material boundary will not be increased. Preferably the electrode has a metal surface area in direct contact with the cementitious material^' or its equivalent (herein referred to simply as cementitious material) that is at least 1.3 times the overall surface area of the mesh. The electrode surface area that is in direct contact with the cementitious material as employed herein is the total surface area of the electrode material used in the anode assembly that is in contact with the cementitious material, while the overall area of the electrode is simply the product of the major dimensions of the electrode. Preferably the electrode has a surface area that is at least 1.4 times, and especially at least 1.5 times the overall area of the electrode. The electrode may be formed in a number of ways in order to increase its surface area. For example, it may have a corrugated or otherwise undulating surface, it may be in the form of a perforated sheet, an expanded metal, or it may be in the form of a sponge. Preferably the electrode comprises fibres, for example as a mat, weave or mesh. As a typical example, a mesh may comprise metal strands, for example 0.5 mm diameter, and at a density of 500

2 strands per m in each direction, which will result in a total electrode area of about 1.5 m 2 per m2 overall area of the mesh. In preferred instances, the total electrode surface area can be even higher, for

2 example at least 1.6 or at least 1.8 m overall area of the electrode.

The electrode can be formed from a number of materials, for example from ceramics or metals. The metal used for the mesh may comprise platinum or valve metal base. e.g. titanium. Certain metals e.g. platinum may be uncoated but preferably the metal is coated with an appropriate material for example an oxide or an electrocatalytic coating for example a spinel ceramic or other conductive material containing a noble metal catalyse. Semiconductive ceramics e.g. titanates or conductive polymers may be employed especially in conjunction with catalytic coatings.

The mesh may be formed in any convenient shape for use in the anode. Usually the anode will be elongate in form, and have a circular cross-section in order to be able to be inserted in a bore hole in the cementitious structure, in which case the anode mesh will normally be formed into a circular cylinder, cone or conical frustum.

The grouting material is preferably ionically conductive and especially contains water. It preferably includes at least one hydroscopic (or deliquescent) material to assist in maintaining a desirable level of electrical conductivity.

Preferred grouting materials and electrode materials are described in International patent application No. PCT/GB 89/00404 to M.J. Webb, the disclosure of which is incorporated herein by reference.

The point-source anode assembly and array according to the invention may be employed as the sole anode in a cathodic protection system of a structure, or it may be employed together with another anode assembly, for example a surface mounted anode assembly or an overlay system, for protection of the structure. Examples of surface mounted systems include the system described in the above international application, preformed anode cementitious tiles, conductive paint and flame-sprayed zinc. One example of an overlay system is an expanded catalysed titanium mesh anode within a cementitious render.

One form of point-source anode and anode assembly will now be described by way of example with reference to the accompanying drawings in which:-

Fig. 1 is a schematic view of part of a structure that is protected by means of an anode array according to the present invention;

Fig. 2 is a schematic side view of an anode assembly according to the invention employed in the structure of Figure 1; Fig. 3 is a view similar to Fig. 2 , but showing a modified anode assembly; and

Fig. 4 shows the modified assembly in use. Referring to the accompanying drawings, a concrete structure 1 comprises a reinforced concrete pile 2 having a reinforced concrete pile cap 3 at its head. On top of the pile cap 3, a floor 4 is layed.

In order to protect the pile cap 3 cathodically against corrosion of the reinforcement bars therein, it is necessary to provide the pile cap with a number of point-source anode assemblies 10 at intervals of between 200 and 500 cm, and to impress a current in

2 the range of from 3 to 20 mA per m steel surface area of the structure.

Each point-source anode assembly 10 is of the general structure shown in Figure 2. The assembly 10 comprises an electrode mesh 11 formed from titanium coated with a cobalt spinel catalyst. The mesh is formed from a lay of titanium wires of diameter approximately 0.5mm and separated by approximately 2mm in each direction, which corresponds to an electrode surface of about 1.57 m 2 perm2 of overall mesh.

The electrode mesh is formed into a generally cylindrical shape and is embedded in a cementitious material 12 having a frusto-conical shape, so that the narrow end 13 of the cementitious material has a slightly larger diameter than that of the electrode mesh while the wider end of the cementitious material is about 30% larger than that of the electrode mesh. The end of the electrode mesh located in the wider diameter end of the cementitious material is connected to the current supply by means of a titanium bus which may be insulated by a moulded or heat-recoverable

¹⁰ sleeve 14.

In order to install the anode assembly array in the concrete structure, a number of holes are bored in the structure to a depth of about 250 mm to 1000 mm depending on the structure, and the holes are then

¹^ filled with an ionically conductive grouting material . Before the grouting material has had time to dry or set, the anode assemblies 10 are pushed into the holes until their ends are substantially flush with the surface of the structure, so that the on electrode mesh is in good electrical contact with the concrete via the cementitious material of the assembly and the grouting material.

In operation a current of about 2.5 to 20mA may be applied to the electrode by means of a constant 5 current source, depending on the size of the electrode mesh. In the preferred embodiment a current of

_2 approximately 200 mA m of overall mesh is applied, which corresponds to a current of approximately 127 mA

_2 m of electrode material in contact with the cementitious material.

The cementitious material making up the sheet around the electrodes can incorporate calcium hydroxide to increase is pH so as to enhance the electrical performance of the assembly. Further, the grouting material used to secure the assembly into a structure to be protected can also have an addition of calcium hydroxide for the same reason. Referring now to Figures 3 and 4, a modified assembly 15 is shown which is the same shape as the assembly 10. The assembly 15 includes a generally cylindrical mesh electrode 16 with its associated lead 17. The assembly 15 is formed with a longitudinal grouting passage 18.

Fig. 4 shows the assembly 15 in use within a cementitious structure 19 which includes reinforcement bars 20,21. In the structure 19 has been formed a cylindrical cavity 22. In use, grout 23 from a source nozzle 24 is pumped down the passage 18 to exit at the inner end of the assembly 15. As it flows from the outlet 25 of the passage 18 it forms an effective annular piston between the assembly and the walls of the cavity 22 and as it travels outwardly expels air from the space between the walls of the cavity and the assembly. This greatly tends to reduce the amount of air inclusions which can reduce the electrical contact between the assembly and the structure 19.

Fig. 4 also illustrates one of the advantages of the invention. It will be seen that in protecting an existing cementitious structure the cavity 22 has been bored so as to intersect a reinforcing bar 20.

In the case of prior art point-source anodes such reinforcing bar 20 would immediately be placed into direct electrical contact with the introduced anode assembly (either directly or via , e.g. high conductivity carbon-based grout) and would itself

(and any other reinforcing members in electrical contact with it) become an anode leading to increased corrosion and weakening of the structure.

With the present invention, however, it will be seen that the electrode 16 is spaced from the reinforcement 20 at least by the material of the body sheathing the electrode and possibly also by the grout material 23 thus ensuring that it cannot become an anode. A further preferred anode assembly consists of a cylinder of platinum coated titanium, this being in the form of an expanded metal mesh of specific surface area 1.61 sqM/sq M of expanded mesh. The finished cylinder had external dimensions of 30 mm diameter and 500 mm length. The mesh was formed from titanium sheet having a thickness of 0.7 mm.

To this expanded metal mesh cylinder, a 99.95% pure titanium rod of 4 mm diameter, was welded such that the rod extended from the cylinder to a distance of 500 mm, and the rod was welded along the principle longitudinal axis. As means of insulation a heat recoverable polyethylene based sleeve was recovered over the 500 mm length of the titanium rod.

The above finished assembly was then centralised in a paper/card based tube, this having an internal diameter of 50 mm diameter. At one end of the tube a plastic plug was affixed this to prevent any of the potting medium from escaping from the tube at the time of casting. The titanium cylinder was so arranged that the tube form extended 500 past either end of the cylinder.

With the cylinder arranged and held vertically the following potting medium was poured into the card tube so fully encapsulating the titanium anode cylinder:-

Anhydrous Calcium sulphate 60%

Calcium hydroxide powder 25% Calcium oxide 5%

Ground pumice 10%

Note the ground pumice had an average partical size of lmm.

Water was then added to the mixture to produce a smooth blend capable of being poured and such that it would fully encapsulate the anode cylinder.

After a period of one day the potting medium had fully set and the finished anode assembly was ready for use. Given the anode specific surface area, the current output from this assembly was 17 mA per linear meter of the finished assembly.

In a variation an assembly as described above was modified in that along the central axis of the anode cylinder a nylon tube was positioned, this having an outside diameter of 10mm and wall thickness of 1mm. This nylon tube runs the whole length of the card form and extends to 300 mm outside it. Following encapsulation as described above, the nylon tube can be used to pressure grout the anode into its final location within the concrete structure to be protected.

The grouting material is ionically conductive and preferably contains water. It preferably includes at least one hygroscopic (or deliquescent) material to assist in maintaining a desirable moisture content and hence a desirable level of electrical conductivity. Such materials which can be used both for the encapsulating sheathing body and for the grout can be non-cementitious and can include for example calcium chloride, or premixed commercially available materials such as the grout Thora CP (Trade Mark) supplied by the Thora Company. The ionic electrical conductivity as determined by the four pin probe method (described in a paper presented by R.P. Brown at "Corrosion 83" Symposium, Anaheim, California, U.S.A. during 18-22 April 1983 incorporated herein by

-5 reference) is preferably within the range from 10 to

_i _5 _ι

0.02 S cm , more preferably 4 x 10 to 0.02 Scm , and especially 6.6 x 10 -5 to 0.02 Scm-1. Examples of potentially suitable grouting materials include a mixture of portland cement and fine aggregate mixed with water to produce a pouring consistency without segregation of the constituents.

The type of cement is chosen from those shown in

Table 1, to suit the particular working environment of the anode assembly.

TABLE 1

Portland Cement Types and Basic Composition

Portland Cement Type

2

58

16.2

7.1

11.9

For example type 5 cement would be selected in those instances in which the anode assembly would be asked to work in a high sulphate containing environment.

Further to the above, inclusion of pozzolanic materials in the cement can be considered, for example the use of type IP cement which contains pozzolan to between 15 and 40% by weight of the total cement.

Pozzolan is defined as a siliceous or siliceous-and-aluminous material which in itself possesses little or no cementitious value, but which in finely divided form will react with calcium hydroxide in the presence of moisture.

Use of this type of grout material will have the added value that a degree of acid resistance is imparted to the grout, this being useful around the anode due to the electrochemically generated acidic by-products of normal operation.

Other examples of grouting materials include polymer-modified cement mortars, for example the commercially available materials such as: Sika Grout, Icement 503, ES08 and 508 as supplied by Sika InterCol; Excem as supplied by Celtite Selfix Ltd; and calcium sulphate based plasters, e.g. plaster of paris. Preferred are those which are initially flowable, for convenient introduction into the apparatus and then tend to solidify.

The invention is not limited to the precise details of the following and variations may be made within the scope of the following claims.

Claims

1. An anode assembly, for protecting metal reinforcement within a cementitious structure, in the form of an electrode embedded in and sheathed by a body of material having a resistivity compatible with the resistivity of cementitious material.

2. An assembly as claimed in claim 1, wherein the body has a volume resistivity in the range from 20 kohm cm to 60 kohm cm.

3. An assembly as claimed in claim 2 wherein the resistivity is from 30 kohm cm to 50 kohm cm.

4. An assembly as claimed in claim 3 wherein the resistivity is 40 kohm cm.

5. An assembly as claimed in any preceding claim wherein the body is of cementitious material.

6. An assembly as claimed in any preceding claim wherein the electrode has a metal surface area in direct contact with the body of material at least 1.3 times the overall surface area of the electrode.

7. An assembly as claimed in claim 6 wherein the electrode has a surface area that is at least 1.4 times and desirably at least 1.5 times the overall area of the electrode.

8. An assembly as claimed in any preceding claim wherein the electrode is corrugated.

9. An assembly as claimed in any of claims 1 to 7 wherein the electrode is in the form of a perforated sheet.

10. An assembly as claimed in any of claims 1 to 7 wherein the electrode is in the form of an expanded metal web.

11. An assembly as claimed in any of claim 1 to -7 5 wherein the electrode is in the form of a sponge.

12. An assembly as claimed in any of claims 1 to 7 wherein the electrode comprises fibres.

13. An assembly as claimed in claim 12wherein the fibres are arranged in a mat, weave or mesh.

^^υ 14. An assembly as claimed in any preceding claim wherein the electrode is made from metal.

15. An assembly as claimed in claim 14 when appendent to claim 13 wherein the fibres are metal strands.

16. An assembly as claimed in Claim 15 and in the -^ form of a mesh having a density of 500 strands per square metre in each direction resulting in a total electrode area of at least 1.5 ^• m2_.per m2 overall area of the mesh.

17. An assembly as claimed in any of claims 1 to 13 on wherein the electrode is formed from a conducting ceramic.

18. An assembly as claimed in claim 14 wherein the metal is platignum or titanium.

19. An assembly as claimed in claim 14 or 18, 15 or 16 wherein the metal is coated with an oxide.

20. An assembly as claimed in claim 14 or 18, or 15 or 16 wherein the metal is coated with an electrocatalytic coating.

21. An assembly as claimed in claim 20, wherein the electrocatalytic coating contains a noble metal catalyst.

22. An assembly as claimed in claim 21 wherein the electrocatalytic coating is spinel ceramic.

23. An assembly as claimed in any of claims 1 to 13 wherein the electrode is made from semi-conductive ceramic such as titanate or conductive polymer.

24. An assembly as claimed in any of claims 13 to 23 wherein the mesh is cylindrical.

25. An assembly as claimed in any preceding claim and having a through passage for the introduction of grout.

26. An electrode assembly as claimed in any preceding claim and shaped to be generally frusto-conical having a smaller diameter inner end, and a lead extending from an outer end.

27. An electrode assembly substantially as hereinbefore described with reference to and as illustrated within the accompanying drawings.

28. A cementitious structure having metal reinforcement and protected against corrosion of the reinforcement by a point-source anode, comprising an assembly as claimed in any preceding claim.

29. A cementitious structure having metal reinforcement and protected against corrosion of the reinforcement by an array of points-source anodes. each anode comprising an assembly as claimed in any of claims 1 to 27.

30. A cementitious structure as claimed in claim 28 and comprising further cathodic protection.

31. A cementitious structure as claimed in claim 30 wherein the further cathodic protection is provided by a surface mounted, or an overlay, anode assembly.

32. A cementitious structure substantially as described with reference to the accompanying drawings.

33. A method of providing cathodic protection for a ° cementitious structure having metal reinforcement including the steps of forming a cavity in the structure, introducing into the structure an assembly as claimed in any of claims 1 to 27 and grouting between the assembly and the cementitious material ⁵ with compatible grout.

34. A method as claimed in claim 33 wherein the grout is ionically conducting.

35. A method as claimed in claim 33 wherein the grout is cementitious.

36. A method as claimed in claim 33, 34 or 35 wherein the grouting material includes hygroscopic or deliquescent material.

37. A method as claimed in any of claims 33 to 37 wherein the grout includes calcium hydroxide.

38. A method as claimed in any of claim 33 to 37 wherein the assembly has a through passage and grout is introduced into the space between the assembly and the cavity through said passage, an inlet to the passage being at an outer side of the assembly and an outlet of the passage being at an inner side of the assembly, so that grout travels, in the void between the assembly and the cavity in an outward direction, expelling air as it travels.

39. A method of protecting a cementitious structure against corrosion substantially as hereinbefore described with reference to the accompanying drawings.