GB2461360A

GB2461360A - Sacrificial anode system

Info

Publication number: GB2461360A
Application number: GB0904034A
Authority: GB
Inventors: Nigel Davison; Gareth Kevin Glass; Adrian Charles Roberts
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-03-10
Filing date: 2009-03-09
Publication date: 2010-01-06
Anticipated expiration: 2029-03-09
Also published as: GB0804389D0; GB2461360B; EP2262926A1; GB2458268A; GB0904034D0; GB2461360B8; WO2009112857A1; GB2461360A8

Abstract

The use of a discrete sacrificial anode 4, backfill 3, tape 5 and an adhesive, to protect steel 8 in concrete 2 is described. The backfill 3 is preferably placed in a shallow cavity in the concrete surface 2 and the sacrificial anode 4 is inserted into the backfill 3. The cavity is covered with a tape 5 that extends over the adjacent concrete surfaces on opposite sides of the sacrificial anode 4 and backfill 3 and the tape 5 is attached to the concrete surface 2 with the adhesive. The tape 5 and the adhesive holds the anode 4 in place and prevents a weathering environment from damaging the backfill 3. A temporary impressed current may also be passed through the steel 8 via a titanium wire 9. A water repellent primer may also be used in conjunction with the adhesive.

Description

DISCRETE SACRIFICIAL ANODE ASSEMBLY

Technical Field

1100011 This invention is related to the protection of steel in concrete construction using sac- rificial anodes and in particular to the use of elongated discrete sacrificial anodes dis-tributed across a concrete structure.

Background Art

1100021 Conosion of steel in reinforced concrete is a major problem. Electrochemical treatments have been used to treat this problem. These treatments have been grouped into temporary and permanent treatments. These treatments may be further grouped into impressed current and galvanic (sacrificial) treatments.

1100031 Permanent treatments are expected to provide a protective effect while the treatment is applied. Permanent treatments are often associated with a protection criterion that is assessed during the life of the protective effect. By contrast, temporary treatments are expected to provide a protective effect that persists after the treatment has ended. This results in a significant difference in their practical application. Temporary treatments are applied in a brief application period and do not require the sustained intervention of an expert in the art to sustain the protective effect. A temporary treatment is applied as part of an installation process that has a foreseen ending and the period of application is typically less than 3 months. Examples of permanent and temporary treatments are given in GB2426008.

1100041 As noted above electrochemical treatments may also be grouped into impressed current and galvanic (sacrificial) treatments. In impressed current electrochemical treatments, the anode is connected to the positive terminal and the steel is connected to the negative terminal of a source of DC power. In galvanic electrochemical treatments, the protection cunent is provided by one or more sacrificial anodes that are directly connected to the steel.

1100051 Anodes for electrochemical treatments may be grouped into inert anodes and sac-rificial anodes. The anodic reaction on inert anodes substantially comprises the conversion of water into oxygen. An example of an inert anode system is given in EP0479337. The anodic reaction on sacrificial anodes substantially comprises the dis- solution of a sacrificial metal element which is preferably less noble than steel. Sac-rificial anodes have been used in both impressed cunent and galvanic treatments but inert anodes cannot be used in galvanic treatments.

1100061 Sacrificial anodes for concrete structures may be divided into discrete anodes that are normally embedded within cavities in the concrete (ACT Repair Application Procedure 8 -Installation of Embedded Galvanic Anodes (www.concrete.org/general/RAP-8.pdf)) or continuous anodes that are attached to the concrete surfaces (US529241 1, BS EN 12696:2000). In a continuous anode, the an-odically active surfaces are connected together by conductors with anodically active surfaces. A single anode protects a large element of a concrete structure with a surface area of typically up to 200m2. Discrete sacrificial anodes are individually distinct elements and anodically active surfaces are usually connected to each other through a conductor that is not anodically active. They also may be connected directly to the steel. More than one discrete anode may be installed in a concrete surface of im x im.

1100071 Sacrificial anodes attached directly to the concrete surface are accessible to facilitate anode replacement. However such anodes often lose adhesion to the concrete surface.

Discrete sacrificial anodes may be buried in cavities formed for this purpose. Anodes buried in cavities in the concrete are strongly attached to the concrete. However they have a small surface area and usually require some form of activation to maintain a high cunent output. Sacrificial anode systems include an anode and a supporting electrolyte. An activating agent may be located in a porous sunounding material or backfill to promote current output.

1100081 A backfill provides space to accommodate the products of anodic dissolution. The backfill is weak and is normally separated from a weathering environment by a layer of repair mortar or concrete that will typically be 10mm thick. This results in the need for a relatively deep cavity compared to the anode size in which to install the anode.

However the size of the cavity is often limited by the location of the steel rein-forcement within the concrete and it is sometimes impractical to install embedded anodes that are distributed within a concrete structure to distribute the protection current to the steel.

1100091 This invention is related to the use of discrete sacrificial anodes that are less noble than steel. It is also related to the use of said discrete sacrificial anodes in temporary impressed current treatments and in galvanic treatments and combinations of temporary and galvanic treatments. The problem to be solved by this invention is to improve the method of attaching discrete sacrificial anode systems to the concrete to reduce the amount of concrete removed when installing discrete sacrificial anode systems.

Summary of Invention

1100101 This invention discloses the use of a sacrificial anode and a backfill and a tape and an adhesive to protect steel in concrete. The backfill is preferably placed in a shallow cavity in the concrete surface and the sacrificial anode is inserted into the backfill. In this preferred arrangement, the cavity is covered with a tape that extends over the adjacent concrete surfaces on opposite sides of the cavity and the tape is attached to the concrete surface with the adhesive. The tape and the adhesive holds the anode and backfill in place and prevents a weathering environment from damaging the backfill.

The sacrificial anode and backfill may alternatively be located on the concrete surface to form a profile that extends away from the concrete surface.

Detailed description of the preferred embodiments

1100111 This invention provides in one aspect the use of a discrete sacrificild anode and a backfill and a tape and an adhesive to protect steel in concrete which use comprises placing the backfill in contact with the concrete and placing the sacrificial anode in contact with the backfill and covering the backfill and sacrificial anode and the adjacent concrete on opposite sides of the sacrificial anode and backfill with the tape and attaching the tape to the adjacent concrete with the adhesive wherein the sacrificial anode comprises a metal less noble than steel and the backfill is adapted to accommodate the products of sacrificial metal dissolution.

1100121 Sacrificial anode assemblies have the advantage that they can provide galvanic protection to the steel and do not require the maintenance of a power supply. However sacrificial anode assemblies attached to the concrete surface tend to lose adhesion to the concrete. Possible reasons for this include the generation of corrosion product at the anode interface and the difference in thermal expansion and contraction properties between the anode metal and the concrete substrate. In the product based on U5529241 1 adhesion to the concrete surface was preferably provided through an adhesive gel that also acted as the activating backfill. Current passes from the sac-rificial anode through the adhesive gel to the concrete surface. However, even in this case adhesion problems were encountered. This was attributed to water affecting the ionically conductive adhesive. These anodes have been pinned to the concrete surface to maintain adhesion.

1100131 An alternative solution is to use a stronger adhesive. An example of a very strong adhesive that is also used to bond fibre reinforcement to concrete to increase the concrete tensile strength is an epoxy adhesive. Other polymers may also provide adequate adhesion. The surface of the concrete is preferably cleaned and primed for application of the adhesive. An example of a primer is a water repellent coating such as a silane that inhibits the collection of water in the concrete behind the adhesive. Strong adhesives will restrict ionic current flow and will effectively be insulators. Indeed it is preferable to have a non-conducting adhesive as this prevents electrochemical reactions from damaging the adhesive properties. Such non-conducting adhesives cannot be used to electrically connect the sacrificial metal element to the concrete.

However the anode and backfill may be held in place through the strategic use of the adhesive. In one example a covering covers and extends past a discrete sacrificial anode and backfill and the extensions are glued to the concrete surface with the adhesive.

I00 141 The discrete sacrificial anode is preferably in an elongated form, examples of which include a ribbon, a wire, a tube and a bar, and is connected to the concrete through a backfill. Several sacrificial anodes are preferably connected to each other using a conductor such as a copper core electric cable. A preferred conductor is a titanium wire. The conductor may then be connected to either the steel in the concrete to provide galvanic protection, or to the positive terminal of a power supply to provide a temporary impressed cunent treatment.

I00 151 The covering is preferably a tape that is preferably a fibre tape with an open weave to allow the adhesive to penetrate the tape. An example of a covering is a 50mm wide builder's crack bridging (scrim) tape. The tape and adhesive attaches the anode and backfill to the concrete surface on opposite sides of the anode/backfill assembly.

I00 161 Part of the concrete surface is covered with the adhesive and this reduces the area of active sacrificial anode on the concrete surface. This may reduce the cunent off the sacrificial anode. The sacrificial anode system therefore preferably includes an ac-tivating agent and/or the sacrificial anode is preferably fitted with an impressed cunent connection to facilitate the delivery of a temporary impressed cunent treatment from the anode. An impressed current connection is a connection that does not corrode when it is driven to positive potentials using a power supply (GB2426008).

I00 171 A temporary impressed cunent treatment is preferred over a permanent impressed current treatment as a power supply is only maintained for a brief period by skilled personnel. A temporary treatment may be used to restore steel passivity in the event that a risk of steel corrosion is detected. An example of this is given in GB2426008. A temporary treatment will generate hydroxide at the steel which promotes steel passive film formation. A temporary treatment will also draw chloride to the sacrificial anode to promote sacrificial anode activity. A temporary treatment has a foreseeable ter-mination point and will preferably last less than 3 months and more preferably last less than 3 weeks. By comparison to the temporary impressed current treatment, the galvanic cunent treatment from the sacrificial anode is described as long term or permanent with a period measured in years and an applicator would normally leave the treatment running at the end of an application contract. Other known examples of treatments described as temporary and permanent are given in European Federation of Corrosion, Publication 24, (Electrochemical Rehabilitation Methods for Reinforced Concrete Structures -A state of the art report, ISBN 1-86125-0827, 1998).

I00 181 The sacrificial anode and backfill may be located in a cavity formed in the concrete cover over the reinforcing steel. This increases the surface area of the anode that delivers cunent into the concrete and reduces the profile of the assembly that extends away from the concrete surface. An example of such a cavity may be a slot or chase cut into the concrete surface. The cavity is preferably covered with the tape and the adhesive to isolate the backfill from the external environment to prevent weathering of the weak backfill material. Thus the anode and backfill can be located close to the concrete surface as there is no need to cover the anode and backfill with a repair mortar.

IOO 191 This invention provides in another aspect a use of a sacrificial anode and a backfill and an insulating adhesive to protect steel in concrete which use comprises placing the backfill on the concrete and covering the backfill with the sacrificial anode wherein the sacrificial anode extends past the backfill on opposite sides of the backfill and attaching the sacrificial anode to the concrete with the insulating adhesive.

Brief Description of Drawings

IOO2O1 This invention will now be further described with reference by way of example to the drawings in which Figure 1 shows an arrangement using a slot in a concrete surface and Figure 2 shows an anode assembly located on a concrete surface and Figure 3 shows another arrangement located on a concrete surface and Figure 4 shows an array of discrete anodes connected to a power supply and Figure 5 shows the arrangement used to test the assembly and Figure 6 shows the drive voltage and recorded current density of an anode in a specimen prepared using an epoxy adhesive and Figure 7 shows the drive voltage and recorded current density of an anode in a specimen prepared using a PVA adhesive.

Mode(s) for Carrying Out the Invention IOO2 11 Referring to Figure 1, slots I 11 are cut into a concrete surface I21. Only one slot is shown but slots will preferably be distributed over the anode surface to distribute the protection current to the steel. In most cases it is preferable to fill the slots with a backfill I31, an example of which is given in W02007/039768, and then to insert the sacrificial anode I41 into the backfill. This order may be reversed in some cases. In this example the sacrificial anode is a bar with a rectangular section. The concrete surface adjacent to the slots is preferably cleaned and a primer is applied. While the primer is still tacky, a tape I51 comprising a fibre with an open weave is located over the slot.

The adhesive is then applied to the tape to glue the tape to the concrete surface and to isolate the backfill and anode from the external environment.

IOO221 The anode may be connected to a titanium wire to form an impressed cunent connection 161. This is preferable if the anode is also to be used as an impressed current anode to deliver a temporary electrochemical treatment. The location of the titanium wire -anode connection is preferably on the edge of the anode closest to the concrete surface. The connection is preferably isolated from contact with the electrolyte in the environment. This may be achieved with the tape and the adhesive.

100231 The titanium wire may be connected to the positive terminal of a DC power supply 171 and the steel I81 may be connected to the negative terminal of the power supply through electrical connections I9l. Alternatively the anode may be directly connected 101 to the steel to deliver galvanic protection. A schematic switching arrangement 1 1 to switch between the galvanic and impressed current options is included in Figure 1. Any sequential combination of impressed current and galvanic treatments is possible. However, if an impressed current treatment is used, it is preferably a temporary impressed cunent treatment as the power supply does not need to be maintained in the long term in this case and can be removed at the end of the treatment.

I00241 Figure 2 shows a backfill 131 located on a concrete surface I21 and a sacrificial anode 141 located on the backfill. In this example a sacrificial anode ribbon is shown. The ribbon anode is attached to a tape 151 at least at opposite edges of the ribbon anode and the tape extends away from the ribbon anode. The tape may be attached to the ribbon anode in advance and the backfill may be located on the ribbon anode prior to placing the assembly on the concrete surface. After the assembly is located on the concrete surface, the tape is glued to the concrete surface with an adhesive. The steel and the connections between the anode and the steel are not shown.

I00251 Figure 3 shows a backfill 131 located on a concrete surface I21 and a ribbon anode 141 located on the backfill such that the edge of the anode extends away from the backfill on opposite sides of the backfill. The edge of the anode is then glued to the concrete surface using an adhesive 1121.

I00261 Figure 4 shows several sacrificial anode assemblies I2 11 wherein the sacrificial anode is inserted into a backfill located in slots in a concrete deck I221. The sacrificial anodes and the backfill are covered with a tape attached to the concrete surface with an adhesive. The anodes are connected together through conductors I231 that may also be located within the concrete cover. Both serial and parallel connections to the anode as-semblies are possible and a single slot may contain several anode assemblies. In the example in Figure 4 the conductor and connections are suitable for impressed current use and the conductor is connected to the positive terminal of a power supply I241 to facilitate the delivery of a temporary impressed cunent electrochemical treatment. The connections to the negative terminal of the power supply are not shown.

I00271 Figure 5 shows the layout used to test the assembly in the laboratory. A single 5mm wide slot, 15mm deep and 320mm long was cut into a 66 x 100 x 430mm concrete prism 311. The concrete prism was a pre cast concrete lintel used in house building and contained a single steel bar I321 that protruded from the bottom edge of the specimen. An electrical cable was connected to the steel bar. The slot was filled with a lime putty backfill described in W02007/039768. A zinc ribbon measuring 300 x 12 x 2 mm was connected to a copper core cable and the connection was insulated to protect it during an impressed cunent treatment. The zinc ribbon 331 was then inserted into the lime putty. The concrete surface around the slot was cleaned and treated with a water repellent liquid known as Thompson's� WaterSeal�. While the surface was still tacky, a 50mm wide open weave scrim tape I341 was placed over the anode and backfill. An adhesive was applied to fix the tape to the concrete. Two specimens were made and a different adhesive was used for each specimen. The adhesives used were an epoxy resin sold as a coating for concrete structures and an exterior grade PVA. In both cases the steel was connected to the negative terminal and the anode was connected to the positive terminal of a power supply I35I using the conductors I36I connected to the steel and the anode. A voltage was applied between the anode and the steel and the voltage was varied over a period of several weeks.

I0028I The applied voltage and the cunent density delivered from the zinc ribbon for the specimen that used the epoxy adhesive are given in Figure 6. The cunent density off the anode was greater than 2000 mA/rn2 (typically 4000 mA/m2) for at least 25 days.

This is a relatively high current density for a specimen that contained no chloride and represents the current density applied in a temporary electrochemical treatment.

I0029I The applied voltage and the cunent density delivered from the zinc ribbon for the specimen that used the PVA adhesive are given in Figure 7. The cunent density off the anode was much lower than that delivered from the specimen prepared with the epoxy adhesive. This probably reflects the less effective barrier of the PVA adhesive that allows the backfill to dry in a laboratory environment.

I0030I In both specimens, no sign of loss of adhesion of the tape was observed despite several weeks of impressed current. The tape and the adhesive held the anodes securely in place in both examples.