GB2430938A

GB2430938A - Backfill

Info

Publication number: GB2430938A
Application number: GB0600661A
Authority: GB
Inventors: Adrian Charles Roberts; Nigel Davison; Gareth Kevin Glass
Original assignee: CONCRETE PRESERVATION TECHNOLO
Current assignee: GLASS, GARETH K; ROBERTS, ADRIAN C
Priority date: 2005-10-04
Filing date: 2006-01-13
Publication date: 2007-04-11
Anticipated expiration: 2026-01-13
Also published as: GB2430938B; GB0600661D0

Abstract

A method of accommodating the pressure arising from the delivery of a large current density off a sacrificial anode metal [1] located in a cavity [2] in concrete [3] is disclosed. It comprises substantially surrounding the anode with a pliable viscous backfill [4] such as a lime putty or clay and providing a space into which the backfill may move. The space may be provided by venting the backfill to space outside the cavity through an opening [5] or by including a void space within the backfill [6] or a void space within the cavity [7]. The void space in the backfill or cavity may be generated using a weak foamed polymer.

Description

BACKFILL

FIELD

This invention is concerned with the protection of steel in concrete using sacrificial metal anodes and, in particular, the accommodation of voluminous products resulting from the action of a sacrificial metal embedded in a cavity in the concrete.

BACKGROUND

Discrete sacrificial anodes have been embedded in cavities in concrete to protect the reinforcing steel. In the process the anode metal dissolves to form products that often have a greater volume than the metal from which they were derived. As a result, pressure is exerted on the surrounding concrete which can lead to damage of the concrete. A backfill is a material surrounding a sacrificial anode that maintains an electrolytic contact between the electrolyte in the environment and the surface of a sacrificial anode. An anode is an electrode that supports a net oxidation reaction on its surface. The backfill should preferably be capable of accommodating the products of the anodic reaction.

One commercially available sacrificial anode assembly based on WO 94/29496 comprises a zinc anode activated by hydroxyl ions in a weak porous material that surrounds the zinc. The zinc corrodes to form soluble products that precipitate out in the pores of the surrounding material.

Accommodating the voluminous products of sacrificial metal dissolution is addressed directly in WO 03/027356 and WO 2005/035831. In addition to features of the commercially available product based on WO 94/29496, WO 03/027356 describes a void behind the sacrificial metal into which the metal will be displaced as voluminous products of metal dissolution are formed, and the use of fibres in the anode encapsulating mortar or backfill.

WO 2005/035831 describes the use of a layer of reinforcement to restrain the expansive pressure from the voluminous products and compress a porous anode assembly.

PROBLEM TO BE SOLVED

When sacrificial metal dissolution is accelerated using a DC power supply, high volume products will be produced at a rate much greater than that encountered in the more conventional use of sacrificial anodes. As a result an improved method of accommodating this relatively rapid expansion is needed.

SUMMARY OF THE INVENTION

A method of protecting steel in concrete comprises forming a cavity in the concrete, placing a puttylike ionically conductive backfill in the cavity, inserting an anode consisting of a metal less noble than steel into the backfill, providing a space into which the backfill may move when subjected to pressure and passing a current from the anode to the steel in the concrete. The space may be provided by venting the backfill to space outside the cavity or by including void space within the cavity or within the backfill. The backfill is a pliable, viscous material that retains its pliable, viscous properties while a high current density is delivered off the anode. The backfill preferably hardens slowly to form a weak porous material that can accommodate the longer term, lower expansion rates resulting from the reduced rate of forming products at the anode after an initial high current treatment. The conductivity of the backfill primarily arises from one or more dissociated salts within the backfill. Possible backfills comprise a mixture of fine solid particles in water. A preferred backfill will consist at least in part of lime putty and may also contain a weak foamed polymer filler to trap air voids within the backfill. A weak foamed polymer may also be used to trap air within the cavity.

ADVANTAGEOUS EFFECT OF THIS INVENTION

The combination of a puttylike backfill and a space can accommodate the relatively fast generation of high volume product arising from the delivery of a high current density off a sacrificial anode embedded in a cavity in concrete. The delivery of a high current off the anode may only be required for a relatively brief initial period when a space into which the backfill may move may be provided outside the cavity. After this time the putty can harden slowly to form a weak porous material that can accommodate additional expansive product generated at a lower rate within the pore system of the backfill without placing the surrounding concrete at risk. The use of a putty as opposed to a liquid allows the anode to be installed in cavities in sides and soffits of concrete structures as the putty may hold the anode in position in the cavity. The putty also retains electrolyte in the longer term to ensure anode function.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a discrete sacrificial metal anode embedded in a puttylike backfill in a cavity in concrete together with various spaces to accommodate movement of the backfill.

Figure 2 shows the current density driven off an aluminium anode embedded in a lime putty in concrete using a 12V DC power supply connected between the anode and embedded steel.

Figure 3 shows the galvanic current density off an aluminium anode connected to the steel after completing the impressed current treatment described in Figure 2.

MODE OF THE INVENTION

Brief high current electrochemical treatments have been developed to arrest and prevent steel corrosion in concrete. A brief treatment may involve the delivery of a charge to the steel of the order of 50 to 500 kC/m2 (charge per unit area of steel) in a short period. It is possible to do this in as little as 48 hours using a power supply but it will typically take a longer time.

A sacrificial anode metal may be used in an impressed current mode in the delivery of a high current for a brief initial period. Referring to Figure 1, an anode [1] is located in a cavity [2] in concrete [3]. The anode consists of a metal less noble than steel. The principal anodic reaction is the dissolution of the anode metal. It is preferably selected from aluminium, zinc or magnesium or an alloy of aluminium, zinc or magnesium.

The anode is substantially surrounded by a pliable viscous backfill [4]. The backfill is not rigid and it is also not a runny fluid. The properties of the backfill mean that it can move into adjacent available space when subjected to pressure. The backfill retains is pliable viscous properties while a high rate of production of voluminous products at the anode persists. The rate of production of products at the anode is related to the current delivered off the anode.

High initial impressed current densities are likely to persist for at least 2 to 3 days and will more typically be delivered for one week. High initial current densities may extend to 3 months. Thus the backfill should retain its pliable viscous properties for at least 48 hours and will preferably retain these properties for up to 3 months.

The backfill is ionically conductive to support the metal dissolution reaction on the sacrificial metal anode. The conductivity of the backfill substantially arises from one or more dissociated salts within an electrolyte contained in the backfill. The resulting ions in the electrolyte preferably assist with the dissolution of the sacrificial metal. Examples of such ions include hydroxyl ions, sulphate ions and halide ions.

The backfill may slowly harden in time, but after hardening it will preferably form a weak porous material capable of continuing to accommodate the voluminous products of the anodic reaction that are generated at a slower rate. It is preferable that the compressive strength of the backfill does not exceed 10 N/mm2 and more preferably does not exceed 2 N/mm2.

Examples of the backfill include gels, clays, putty, and heavily retarded cement or fine mortar paste. Cement products harden by reaction with water (hydration). They can therefore harden under water. In this respect they differ from other suggested backfill materials which may only harden when exposed to the air and, in some cases, the pliable viscous properties may partially be restored when the backfill is re-hydrated.

Gels typically contain more than 60% water. As noted in US 6254752, a very high water content is an advantage in temporary electrochemical treatments designed to deliver high currents for a brief period after which the anode system is removed. However dehydration of the gel results in shrinkage that can isolate the anode from the surrounding concrete if the anode is also installed for longer term use. This will be the case when the sacrificial metal remaining after an initial impressed current treatment is connected to the steel to provide sacrificial protection.

Improved dimensional stability may be achieved by reducing the water content. This may be achieved using a dispersion of fine solid particles in water. Clay particles are less than 5 microns in diameter and some clays contain less than 50% water when fully saturated. Silt particles will have diameters of up to 50 microns and sand particles will be larger. The inclusion of larger particles (silt and sand) improves dimensional stability and results in a courser backfill.

Concretes and mortars include substantial quantities of sand and larger aggregate particles as well as relatively little water. When they are based on the use of hydraulic cements like Portland cement, the reaction between the cement and water will typically produce a rigid material in less than 12 hours. This reaction may be retarded by adding a retarding agent that retards the setting reaction of the cement to retain the pliable viscous properties of the concrete or mortar or cement paste mix for a longer period. Cement based mixes harden to produce relatively strong materials but the strength may be reduced and the porosity increased by increasing the water content of the mix A preferred backfill contains lime putty produced by slaking quicklime (CaO) to form a colloidal dispersion of fine calcium hydroxide crystals in water. Matured lime putty has a relatively consistent volume and reacts with carbon dioxide in the air to form a weak porous material consisting mainly of calcium carbonate that has a compressive strength of less than 0.5NImm2. Lime putty may be blended with other materials to improve the properties of the backfill. Lime putty like clay does not set while it is waterlogged and the puttylike characteristics can be partially restored after a short period of dehydration if it is mixed with water.

A space is provided into which the backfill will move when subjected to pressure. The space may be provided outside the cavity through an opening [5] connecting the cavity to the external environment. A wide opening from the external environment to the cavity may be partially filled with a sealing material [8] such as a cement or mortar paste, to inhibit rapid moisture loss from the cavity. At the end of a brief high current treatment when the formation of voluminous products slows down, it is preferable to seal the opening to the external environment and to use other space within the cavity to accommodate the voluminous products.

A space may be provided by including voids within the backfill [6] or voids within the cavity [7]. The void space may be created using a filler material that traps a compressible fluid like air within the putty or within the cavity. An example of a filler material is a weak foamed polymer such as polystyrene foam.

The anode and backfill may be assembled as a separate unit prior to installation in a concrete structure. This may be achieved by forming a porous container or mould with an opening to facilitate placing the backfill and anode in the container. The porous mould or container may be made using a layer of hydraulic cement or mortar formed into an appropriate shape. Excess water in the cement results in the formation of capillary porosity as the cement hydrates. The mould or container may also be formed from a material like cardboard or a porous cloth or even one or more layers of thin absorbent paper impregnated with a hydraulic cement with a high water to cement ratio. The use of a cement that sets to form a porous material such as hydraulic cement results in a rigid container or mould.

The pliable viscous backfill is installed within the container and the anode is inserted into the backfill. The container then forms an outer porous layer of the anode assembly. A conductor connected to the anode protrudes from the container to facilitate making a connection to the anode. The opening to the container may be sealed after the backfill and anode are installed in the container. If an opening venting the backfill to the external environment is left open, it is preferable that the container is a rigid container. If no opening is left, it is preferable that one or more features is present from the list comprising, a portion of the container or seal is easily broken to create an opening when the anode is used, a portion of the container or seal is elastomeric, a compressible void space is present in the container, a compressible void space is present in the backfill These features are needed to accommodate expansion within the anode assembly.

Example

An anode 15mm in diameter and 100mm long comprising a bar of the aluminium alloy known as US Navy specification MIL-A-24779(SH) that was cast around a titanium wire to facilitate the electrical connection to the aluminium was embedded in a lime putty in a 25mm diameter by 130mm deep hole in a concrete block. The basic arrangement is shown in Figure 1. The concrete block measuring 380 by 270 by 220mm was made using graded all-in-one 20mm aggregate and ordinary Portland cement in the ratio 8:1. The water to cement ratio was 0.6 and 4% chloride ion by weight of cement was added to the mix by dissolving sodium chloride in the mix water. A sheet of steel with a surface area of 0.125m2 was included in the concrete block. The lime putty was produced by slaking and maturing quicklime and was sourced from a manufacturer of lime putty and lime mortars. The hole in the concrete block containing the lime putty and the anode was left open to the air. The concrete block was stored in a dry indoor environment and the temperature varied between 10 and 20C.

The anode and the steel were connected to a I2Volt DC power supply for a period of 13 days during which a charge of 65kC was delivered from the anode to the steel. The current density delivered off the anode for the first 11 days is given in Figure 2. For most of this time, the current delivered off the anode was greater than 5000mA/m2. During this period water and corrosion products accumulated at the location of the anode and moved out of the hole containing the anode and the putty onto the surface of the concrete.

At the end of the period of impressed current treatment, the DC supply was removed and the anode was connected to the steel. The galvanic current out of the anode was measured using a 1 ohm resistor as a current sensor in the connection between the anode and the steel. The current density delivered off the anode acting purely in a galvanic mode for the next 40 days is given in Figure 3. For most of this period, the current density delivered off the anode was between 500 and 600 mNm2.

Claims

1. A method of protecting steel in concrete that comprises forming a cavity in the concrete and placing an ionically conductive backfill in the cavity and inserting a compact discrete anode consisting of a metal less noble than steel into the backfill and passing a current from the anode to the steel in the concrete characterised in that the backfill is a pliable viscous material and the pliable viscous properties of the backfill are retained for at least 48 hours after exposure to air and the compressive strength of the backfill does not exceed iON/mm2 within 7 days of exposure to air and the conductivity of the backfill primarily arises from one or more dissociated salts in an electrolyte contained within the backfill and a space is present to accommodate backfill movement when under pressure.

2. A method as claimed in claim 1 where the cavity contains a space to accommodate backfill movement.

3. A method as claimed in any of claims 1 or 2 where an opening connects the backfill to a space outside the cavity.

4. A method as claimed in any of claims 1 to 3 where the compressive strength of the backfill is less than 2 N/mm2.

5. An ionically conductive anode backfill for use in cavities in concrete with a discrete anode less noble than steel that substantially comprises fine solid particles in water characterised in that the fine solid particles are less than 30 micron in diameter and the water content is less than 60% of the weight of the backfill and the conductivity of the backfill primarily arises from one or more dissociated salts in the water contained within the backfill and the backfill is a pliable viscous material that retains its pliable viscous properties for at least 48 hours after exposure to air and the compressive strength of the backfill does not exceed 2 N/mm2 within 7 days of exposure to air and the backfill retains its pliable viscous properties while it remains waterlogged for a minimum of 3 months.

6. An ionically conductive anode backfill for use in cavities in concrete with a discrete anode less noble than steel that consists at least in part of lime putty characterised in that the lime putty is a colloidal suspension of fine calcium hydroxide crystals in water.

7. An ionically conductive anode backfill for use in cavities in concrete with a discrete anode less noble than steel that substantially comprises a pliable, viscous material and a weak filler material characterised in that the conductivity of the backfill primarily arises from one or more dissociated salts in the water contained within the backfill and the backfill retains its pliable viscous properties for at least 48 hours after exposure to air and the compressive strength of the backfill does not exceed 2N/mm2 within 7 days of exposure to air and the weak filler material traps a compressible fluid within the backfill.

8. The production of a separate unit that consists of an anode less noble than steel and a pliable viscous backfill and a porous outer layer comprising forming a porous container and at least in part filling the container with a pliable viscous backfill that retains its pliable viscous properties for at least 48 hours after exposure to air and inserting an anode less noble than steel into the backfill.

9. The production of a unit as claimed in claim 8 where the opening to the porous container is sealed and the unit includes one or more features present from the list comprising a portion of the container or seal is easily broken to create an opening, a portion of the container or seal is elastomeric, a compressible void space is present in the container, a compressible void space is present in the backfill.

10. An anode assembly for protecting steel in concrete substantially as herein described above and illustrated in the accompanying drawings.