GB2400121A - Cathodic protection of wall ties or the like - Google Patents

Abstract

Protection is provided for discontinuous metal objects positioned in a cavity between two surfaces, e.g. wall ties 5, by introducing an electro- conductive material 10 between the surfaces and causing the metal to become cathodic and prevent corrosion thereof. Material 10 may be a conductive foam, introduced via bore 11. The metal may be made cathodic by applied potential or by use of a sacrificial anode, which may be unitary or may be metal particles dispersed in the foam, which may be heat and sound -insulating, and which may provide a Faraday cage to protect a building from electromagnetic radiation.

Description

Improvements Relating to Protection of Discontinuous Metal Ties The

present invention relates to a method of protecting discontinuous metal objects or spans or ties or bridges positioned between two surfaces and projecting into a space between the two surfaces so as to improve the corrosion resistance of said metal objects, the method being of particular use in the protection of, especially but not exclusively, metal cavity wall ties and is thus of great utility to the building construction industry. The method of the present invention utilises electro- conductive material so as to provide an electrical connection between the ties or bridges. It is envisaged that the method of the present invention may also be applicable to other industries where it is desired to provide corrosion protection to discontinuous metal objects, by introduction of a suitable electro-conductive medium.

Background to the Invention

In the UK, and other countries, buildings were traditionally constructed with external walls consisting of a single leaf of masonry, typically 225mm of solid brickwork. In the early l900s external cavity walls consisting of two leaves of masonry became common. These were usually constructed using two leaves of 1 02mm brickwork with a cavity therebetween. From about 1945 cavity walls became the dominant form of external wall construction. More recently, block-work (and in some cases timber), have been used in the inner leaf. A cavity wall has the advantage of reducing water penetration and improving the thermal efficiency of the wall.

Wall ties, usually mild steel, have been built into cavity walls to tie the two leaves together and to ensure the structural integrity the wall. Many different forms of ties, including vertical twist fish-tail ties, butterfly ties and double triangle ties have been used. Until recently these have normally been galvanised to limit corrosion, however within the last 10 to 20 years, stainless steel ties have been generally adopted, and these are significantly more resistant to corrosion than the earlier mild steel ties.

A problem associated with wall ties especially the older type is corrosion. Corrosion of earlier examples of cavity wall ties has been known for over 25 years and has been attributed to factors such as inadequate protection (or no protection) of the steelwork, the use of aggressive mortars or severe environmental exposure. The ultimate consequence of corrosion of the cavity wall ties is the loss of structural integrity and potential for structural failure. However, as a precursor to failure, damage is caused to the wall structure and hence building due to the expansive forces created by the formation of rust (rust is approximately x 5 greater in volume than the metal from which it was formed).

It is suggested that the maximum life expectancy of early galvanised cavity wall ties is of the order of some 50 years. Therefore, any cavity wall ties in buildings constructed before or during the 1950s are likely to have reached the end of there life expectancy and will be corroding.

UK Government statistics presented in the 'Office of Deputy Prime Minister, English House Condition Survey, 2001' suggests that in England approximately 100,000 cavity wall dwellings were built each year in the 1950s. It is therefore likely that year on year 100,000 dwellings might present with the problem of wall tie corrosion. A typical small detached might have 200 to 300 wall ties, all of which may be corroding and it will be appreciated that the problem of wall tie corrosion is not limited to the UK. Accordingly, wall tie corrosion represents a substantial problem to both the householder and the construction /building industry.

Remedial measures for dealing with corroded wall ties and the associated expansion caused by the formation of rust can be dealt with in a number of ways depending on the type of the original wall ties and the extent of the corrosion and the nature of the existing installation. Methods currently employed to rectify the problem of corroding wall ties, can be summarised as follows: À Completely remove corroded ties, by first removing bricks at each tie location; À Structurally isolate corroded ties, by first cutting mortar away from around each tie; À Crop of ties, by first removing bricks at each tie location; À Saw away joint and tie or remove joint and bend back ties; À Leave ties in place (this is only applicable if wall tie corrosion is not causing damage); and À Reconstruction of inner andlor outer leaf.

All of the current remedial methods also include some facility for reinstating the tying action between the two leaves, such as drilling and grouting new ties or drilling and installing expansion ties or injecting polyurethane foam to 'stick' the two leaves of the cavity wall together. However, it is advised that if a foam is used, the ties still need to be removed as foam is unlikely to stop corrosion of ties (1).

A problem with current methods is that not only are they are highly intrusive, requiring the removal of existing masonry, and can leave the property with inherent structural weaknesses but as noted hereinbefore, a typical small detached house might have 200 to 300 wall ties, all of which may be corroding and may need brickwork to be removed to allow treatment hence there is a great deal of disturbance to the dweller.

The corrosion of metals consists of chemical reactions between the metals and oxygen to form metal oxides, in the presence of an electrolyte. In the case of iron and steel, the iron reacts with oxygen, usually in the presence of water, to form iron oxide, i.e. rust. The reactions between the metals and oxygen involve the loss of electrons from areas of the metal, which are referred to as the anodic areas. These electrons are transferred to other areas, referred to as cathodic areas, which do not corrode.

Cathodic Protection (CP) is an established method of providing corrosion protection to metals and has been used successfully for various land and marine applications (2).

CP involves introducing an electrical charge to the metal, thus forcing the metal to become cathodic, preventing the loss of electrons and hence preventing corrosion.

There are two general techniques for providing CP, the "impressed current method" and the "sacrificial anode" method. In the former impressed current method of CP the electrical charge is provided by an external power source. In the latter sacrificial anode method the electrical charge is created by the potential difference between the metal to be protected and the sacrificial anode, typically comprising a more reactive metal than the one to be protected.

CP is carried out on metal objects that are electrically continuous (such as metal pipelines) or are surrounded in continuous electrolytes (such as seawater). A disadvantage and limitation of CP is that CP cannot be carried out on a group of discontinuous metal objects i.e. metal objects that are not surrounded by an electrolyte or are electrically unconnected. Thus, although CP could be carried out on individual independent wall ties it would be an impractical and highly time consuming not to say a prohibitively expensive method of protection.

There is therefore a need, especially but not exclusively in the construction industry, to provide an alternative economical and less intrusive way of providing corrosion resistance and preventing further corrosion to metal objects such as wall ties that project into the space between two surfaces or connect two surfaces or leaves together.

Statement of the Invention

According to a first aspect of the invention there is provided a method of providing protection to at least one discontinuous metal object, the metal object being positioned in a space or cavity defined by two surfaces, the method comprising introducing an electro-conductive material between said two surfaces so as to create a continuous connecting medium between said surfaces and metal object and creating an electrical charge in the metal object thereby forcing the metal object to become cathodic so as to prevent loss of electrons therefrom and so prevent corrosion of same.

Reference herein to metal objects includes metal spans or ties or bridges.

Reference herein to discontinuous is intended to refer to the metal objects being isolated from one another and that the metal objects that are not surrounded by an electrolyte or are electrically unconnected prior to the application of the method of the present invention.

Preferably, protection is provided simultaneously to a plurality of discontinuous metal objects.

In one embodiment of the invention the metal object is associated with or embedded in both surfaces and acts to connect them together. Alternatively, the metal object is associated with or embedded in only a first surface and projects into the cavity between the two surfaces without touching the second surface, for example in the instance where the metal object is an angled lintel supporting one surface only.

Alternatively, the metal object is not in contact with either surface, for example in the instance of the metal object being a conduit. It will be appreciated that the method of the present invention is applicable to each situation.

Reference herein to a surface includes a rough or smooth surface constructed of brick, plaster, mortar, plastic, wood, glass, ceramic and so on. The surfaces do not necessarily have to be constructed of the same material merely separated from one another with a discontinuous metal object positioned in the cavity between the two surfaces.

Preferably, the metal object is rendered cathodic by either an impressed current or by the creation of a sacrificial anode by CP. It will be appreciated that either, application of an impressed current or creation of a sacrificial anode by CP, is appropriate to execute the method of the present invention and that neither "route" of CP is intended to limit the scope of the application.

It should be understood that the impressed current method of CP is where the electrical charge is provided by an external power source. The sacrificial anode method of CP is where the charge is created by the potential difference between the metal to be protected and the sacrificial anode, the sacrificial anode typically being composed of a more reactive metal than the metal to be protected.

An advantage of the method of the present invention over the prior art is that existing brickwork can remain in position. Accordingly, the method of the present invention is less intrusive, and creates less disturbance for the building occupants, moreover the structural integrity of the existing building is retained, thus there is less reinstatement, less remaining visual disturbance, limited redecoration being required, reduced energy consumption during installation and less potential danger to operatives.

This present invention provides a method of CP of metal objects such as and without limitation, cavity wall ties, utilising injected an electroconductive material such as foam to provide electrical connection between the metal objects/ties the leaves and adjacent metal objects.

This invention provides a new method of achieving electrical conductivity between, for example, wall ties to permit the use of CP which hitherto has been limited to use in protection of discontinuous metal object protection. The method of imparting electrical conductivity of the present invention to metal objects is provided by, preferably injecting an electro-conductive material/foam into a cavity wall or space.

The composition and manufacture of the electro-conductive material has been developed by a specialist manufacturer specifically for the purposes of this invention.

Preferably, the electro-conductive material of the present invention further includes an acid etch or other components to ensure a complete electrical connection between the two wall leaves or surfaces and around embedded or connected metal objects.

S Foams in the construction industry have typically been employed to provide thermal insulation or stability for cavity walls. It is known from the prior art that urea formaldehyde (UF) foam systems are suitable for thermal insulation of cavity walls with masonry or concrete inner and outer leaves.

lO Preferably, the electro-conductive material of the present invention also provides thermal insulation properties and/or structural support and/or acoustic insulation between said two leaves/surfaces, thus promoting the use of a single foam or agent for multi-purpose use.

Preferably, the electro-conductive material is non-toxic and has fire resistance properties.

The manufacture of electro-conductive foams is a known technology, but has not previously been applied to foams intended for introduction/injection into cavity walls. For example, JP2002319315, discloses an electro-conductive foam with specific physical properties however the foam is not intended for use in cavity walls.

It is believed that such a type of foam would be applicable in the method of the present invention. However, it will be appreciated that other foams or injectable agents that may foam on introduction or on subsequent initiation or do not foam at all but expand to fill the cavity into which they have been introduced will be appropriate to perform the present invention so long as they retain electro-conductive properties In some instances it may be useful to use electro-conductive agents with adhesive properties so as to enhance connection between the two sheets/walls/leaves. In one embodiment of the invention in the instance where the sacrificial anode method of CP is employed the more reactive metal may be dispersed within the electro- conductive material itself.

Depending on the nature of the existing installation, including the wall condition, the cavity thickness and the condition of the existing ties, a number of variations in the repair/replacement process/method of the present invention may be required and it not intended to limit the scope of the present application.

A general description of the detailed method of the present invention is as follows: Firstly the external walls are surveyed to determine the nature of the existing installation; then depending on the nature of the existing installation, a limited number of remedial wall ties may be required to ensure structural stability during the subsequent installation process; holes may be drilled in the external walls from the outside of the property at centres and locations, as determined by the nature of the existing installation and structural requirements; electro- conductive material, preferably in the form of a foam, can then be injected into the wall cavity so as to provide electrical conductivity between the metal objects/bridges /spans/ties or any other embedded or connected metal objects, such as lintels. The electro-conductive material or foam may optionally require an 'acid etch' or other component to ensure a complete electrical connection to the embedded metal; depending on the nature of the existing installation, either the impressed current method of CP or the sacrificial anode method of CP may be adopted. Preferably, in the case of the sacrificial anode method, the anode may be provided by one or more isolated pieces of sacrificial material or by the inclusion of sacrificial anode 'filings' incorporated within the body of the electro-conductive material/foam; the adopted electro-conductive material/foam when set or in a semi-solid or solid form provides a tying force to stick' the two leaves of the cavity wall together thereby negating the requirement for the introduction of any further remedial wall ties finally; drill holes are made good and redecorated.

An additional advantage of this method of dealing with corrosion of wall ties arises due to compliance with the British Standards referred to above and includes, greatly improved thermal insulation, and increased structural support and stabilization.

Although an objective of the method of the present invention is to permit CP of, for example, cavity wall ties in existing buildings, where the original wall ties are corroding, it will be appreciated that the method of the present invention can also be applied as a corrosion preventative treatment in existing or new construction. Other potential advantages of the method of the present invention include, creation of a so called "Faraday Cage" which allows for the protection of the inside of the building from external electro-magnetic influences. In this respect the method of the present invention would be of particular utility in dwelling in proximity of high external EM- influences such as pylons and electric sub-stations and the like.

According to a further aspect of the invention there is provided a method of protecting the inside of a building from external electro-magnetic influences comprising: (i) injecting or introducing an electro-conductive material into a cavity or space between an outer and inner wall so as to create a continuous connecting medium between said walls and any metal objects placed therebetween and: (ii) introducing an electrical charge to the metal object so as to cause the metal object to become cathodic thereby creating a "Faraday Cage" effect within the cavity or space between said two walls.

The principles of the method of the present invention may also be applicable to other situations in the construction industry and other industries to provide CP of discontinuous metal objects, by introducing a suitable electro-conductive medium.

These industries include for example and without limitation the aircraft, shipping, transport and transport industries or any other industry in which two surfaces are connected together by metal bridges or connection means.

According to a yet further aspect of the invention there is provided a method of protecting a plurality of wall ties positioned between two surfaces or leaves, at least one of an end of said tie being embedded in or connected to at least one of said surfaces, the method comprising: (i) injecting or introducing an electro-conductive material between said two leaves so as to create a continuous connecting medium between said leaves and said metal tie(s) and: (ii) introducing an electrical charge to the metal tie so as to cause the metal tie(s) to become cathodic thereby preventing loss of electrons and hence preventing corrosion of said ties by cathodic protection.

Preferably, the method of the second and third aspects of the invention includes any one or more of the features as hereinbefore disclosed.

According to a yet further aspect of the invention there is provided a method comprising CP (as hereinbefore described) for the protection and prevention of corrosion of discontinuous metal objects.

The present invention will now be described by way of example only with reference to the following figures wherein: Figure l shows a schematic diagram of a typical existing wall tie construction; Figure 2 shows a schematic diagram of a typical prior art remedial methods of repair; and Figure 3 shows a schematic representation of the method of the present invention.

Detailed Description of the Invention

With reference to Figure 1 there is shown a schematic representation of a typical traditional wall (1) consisting of a rendered outer leaf (2) of typically 102mm thick brickwork, a 50mm wide cavity (3) and a plastered inner leaf of 102mm brickwork (4). These dimensions are not intended to limit the scope of the application but merely to exemplify typical wall thickness and composition. The two leaves of the wall are tied together using a galvanised vertical twist fish-tail tie (5) (an example of one of the many types of wall ties that are currently available). The wall tie as seen is l O partially corroded causing expansion and cracking in both the outer and inner leaves of masonry (6A and 6B).

Figure 2 shows a schematic representation of a typical prior art method of wall tie replacement. A new remedial wall tie (7) is first installed at a site remote from the corroded tie (originally at position 5). A brick (8) is removed from the outer leaf at the tie location, the corroded tie is completely removed (S), the brickwork is reinstated, the render is reinstated, the existing cracks in the internal plaster are made good and the walls are redecorated (9A and 9B).

With reference to Figure 3 there is shown a schematic representation of the method of the present invention, whereby electro-conductive material such as in the form of a foam is injected into the cavity (10) at an injection point (11). Cathodic Protection is applied using either the impressed current or the sacrificial anode method, the existing cracks in the internal plaster and external render are made good and walls are redecorated (12A and 12B). It will be appreciated from the foregoing that the method of the present invention obviates the need for removing the existing tie and therefore no requirement to remove any existing bricks. Depending of the nature of the existing installation there is unlikely to be any requirement for the introduction of any remedial wall ties. In this way the ties are protected from further corrosion and there is less disruption for the dweller.

The method of the present invention provides an easy and rapid means of protecting discontinuous metal objects such as wall ties with the added advantages that the injected electro-conductive material may also be used to "stick" walls together, to thermally insulate the building or to provide acoustic protection or protection from electro-magnetic influences.

References 1. Building Research Establishment Digest 329 (2000) 2. British Standard BS 7361-1:1991, Cathodic protection - Part 1: Code of practice for land and marine applications.

Claims (7)

CLAIMS -

1. A method of providing protection to at least one discontinuous metal object, the metal object being positioned in a space or cavity defined by two surfaces, the method comprising introducing an electro-conductive material between said two surfaces so as to create a continuous connecting medium between said surfaces and metal object and creating an electrical charge in the metal object thereby forcing the metal object to become cathodic so as to prevent loss of electrons therefrom and so prevent corrosion of same.

2. A method according to claim 1 wherein protection is provided simultaneously to a plurality of discontinuous metal objects.

3. A method according to claim I wherein the metal object is associated with or embedded in both surfaces and acts to connect them together and/or is associated with or embedded in only a first surface and projects into the cavity between the two surfaces without touching the second surface and/or the metal object is not in contact with either surface.

4. The method of any preceding claim wherein the metal object is rendered cathodic by an impressed external current thereon.

5. The method of any of claims 1 to 3 wherein the metal object is rendered

cathodic by introduction of a sacrificial anode.

6. The method according to claim 4 wherein charge is created by a potential difference between the metal object to be protected and a sacrificial anode comprising a more reactive metal.

7. The method according to either claims 5 or 6 wherein the sacrificial anode is in the form of piece of metal composed of a more reactive metal than the metal object 1..

7. The method according to either claims 5 or 6 wherein the sacrificial anode is in the form of piece of metal composed of a more reactive metal than the metal object which it is desired to protect or is in the form of metal particles dispersed within the electro-conductive material.

8. The method according to any preceding claim wherein the electroconductive material between said two surfaces and surrounding said metal object is a foam.

9. The method according to any preceding claim wherein the electroconductive material further includes an acid etch or other component to ensure a complete electrical connection to an embedded metal object and/or an adhesive agent.

l O. The method according to any preceding claim wherein the electroconductive material also provides thermal insulation properties and/or structural support and/or acoustic insulation between said two surfaces.

11. A method according to any preceding claim wherein the electroconductive agent foams on introduction to the cavity or space or on a subsequent initiation signal or expands without foaming to fill the cavity or space into which it has been introduced.

12. A method according to any preceding claim wherein the electroconductive agent is non-toxic and has fire resistance properties.

13. A method of protecting the inside of a building from external electro magnetic influences comprising: (i) injecting or introducing an electro-conductive material into a cavity or space between an outer and inner wall so as to create a continuous connecting medium between said walls and any metal objects placed therebetween and: (ii) introducing an electrical charge to the metal object so as to cause the metal object to become cathodic thereby creating a "Faraday Cage" effect within the cavity or space between said two walls.

14. A method of protecting a plurality of wall ties positioned between two surfaces or leaves, at least one end of said tie being embedded in or connected to at least one of said surfaces, the method comprising: (i) injecting or introducing an electro-conductive material between said two leaves so as to create a continuous connecting medium between said leaves and said metal tie(s) and: (ii) introducing an electrical charge to the metal tie so as to cause the metal tie(s) to become cathodic thereby preventing loss of electrons and hence preventing corrosion of said ties by cathodic protection.

15. The method of either claims 13 or 14 further including any one or more of the features recited in claims 2 to 12.

16. Use of the method of claim 1 and further including any one or more of the features of claims 2 to 12 in the aircraft, shipping, and/or transport industries or any other industry in which two surfaces are connected together by metal bridges or connection means by providing CP protection of discontinuous metal objects.

Amendments to the claims have been fled as follows 1. A method of providing protection to at least one discontinuous metal object, the metal object being positioned in a space or cavity defined by two surfaces, the S method comprising introducing an electro-conductive material between said two surfaces so as to create a continuous connecting medium between said surfaces and metal object and creating an electrical charge in the metal object thereby forcing the metal object to become cathodic so as to prevent loss of electrons therefrom and so prevent corrosion of same.

2. A method according to claim 1 wherein protection is provided simultaneously to a plurality of discontinuous metal objects.

3. A method according to claim 1 wherein the metal object is associated with or embedded in both surfaces and acts to connect them togetherand/orisassociated with or embedded in only a first surface and projects into the cavity between the two surfaces without touching the second surface and/or the metal object is not in contact with either surface.

4. The method of any preceding claim wherein the metal object is rendered cathodic by an impressed external current thereon.

5. The method of any of claims 1 to 3 wherein the metal object is rendered

cathodic by introduction of a sacrificial anode.

6. The method according to claim 5 wherein charge is created by a potential difference between the metal object to be protected and a sacrificial anode comprising a more reactive metal.