GB2279664A - Anode for impressed current re-alkalization and dechlorination of reinforced concrete subjected to carbonation attack or aggressive ion penetration - Google Patents

Abstract

In a method for realkalising and dechlorinating corroded areas of reinforcement steel in concrete and masonry electrochemically, the anode is composed of a preformed permeable synthetic material e.g. a sponge impregnated with alkaline metal salts in solution to which has been added a setting agent to form a gel and upon which is superimposed an electrically conductive material eg. conductive paint or metal mesh. The anode can be preformed and placed in situ on the surface of the affected concrete.

Description

REALKALIZATION AND DECHLORINATION OF CONCRETE BY SURFACE MOUNTED ELECTROCHEMICAL MEANS DESCRIPTION Field of the Invention The present invention is concerned with the treatment of affected concrete in the vicinity of steel reinforcement by electrochemical means. For the better understanding of the invention herein described, the expression "affected concrete" means, where the context so requires, concrete or masonry which has been subjected to carbonation attack or aggressive ion penetration.

Backaround Steel corrosion problems occur as the result of its tendency to revert to its lower energy ore state when it is exposed to moisture and oxygen. During this process it develops an oxide surface scale or rust. A small amount of surface rusting will not generally impair the tensile strength, whereas localised pitting corrosion, if severe, will have that effect.

In alkaline solutions, steel corrosion may be prevented by the formation of a thin passive oxide film. As the result of the presence of cement, the environment within concrete is usually highly alkaline. Cement when hydrated produces an alkaline pore solution with a high pH value which may exceed 13. High pH means very small quantities of hydrogen ions, which, in turn, means large quantities of hydroxyl ions. Such high alkalinity promotes the formation of a passive oxide film which prevents further metal dissolution.

Steel remains passive at these high pHs; the limited oxidation required to retain a passive film having no practical affect on the steel or its surrounding concrete. However, this passivity will be impaired by a reduction of the pH to below 11.5, or by the presence of aggressive ions such as chloride ions. If the passive oxide breaks down, allowing metal ions to migrate through the defective lattice of iron oxide, fresh oxide deposits at the metal interface will be manifest as rust. When steel starts to corrode, the process is accelerated by the hydrolysis of the corrosion products, which results in a further reduction in the pH at the sites of active metal dissolution.

Thus the corrosion attack on reinforcing steel can continue within affected concrete.

Rust has a greater volume than the steel from which it is derived. This increase in volume will cause tensile stresses, which if allowed to go unchecked, will lead to disruption and spalling of the concrete with attendant risks to the public, and possible serious deterioration in the strength of the structure.

Passive film breakdown mainly occurs in two ways, either from carbonation, or from the ingress of soluble chloride ions.

Carbonation occurs by the ingress through permeable and cracked concrete of carbon dioxide, which, when combined with water, attacks the hydration products of cement thereby depressing the pH of the cement matrix at the reinforcement level to a value below pH 9.

Chloride ions are highly mobile, and the mechanics of chloride induced corrosion are highly complex. In sufficient quantities, chloride ions are particularly prone to cause corrosion because of their ability to damage the passivity of the oxide film.

Indeed, it has been found that even very high pH concrete cannot provide corrosion protection to steel exposed for long periods to chloride-bearing environments.

At ambient temperatures, the process of corrosion is predominantly electrochemical. An oxidation reaction at the anode is balanced by the reduction reaction at the cathode, and is accompanied by the flow of electrons between the anode and the cathode. Consequently, an efficacious counter-measure is some form of electro-chemical intervention.

One method preventing iron dissolution is the treatment known as cathodic protection. The principle is that a continuing unimpeded current flowing at a required strength from the new anode to the reinforcement will overcome the natural corrosion current, and previously anodic areas of the steel become cathodic.

One beneficial side-effect of cathodic protection is that chloride ions migrate away from the steel towards the anode.

Another beneficial effect is that oxygen is converted into hydroxyl ions, raising the pH at the concrete to steel interface.

One of the first attempts to utilise the beneficial affect of chloride removal, took place in November 1976, when Mr. J.E.

Slater using a calcium hydroxide solution electrolyte, successfully removed chloride ions from steel reinforced concrete. However, a relatively high voltage and current was necessary, and the apparatus could only be used on a horizontal flat surface.

Another example of a method for the removal of chlorides from steel reinforced concrete is disclosed and claimed in European Patent No. 0 200 428, where lower voltage and current levels are involved, and the temporary external anode net is contained within electrolytic material which is applied by means of a spray. This treatment may be preceded and followed by sandblasting; the second occasion being in preparation for the application of repair materials to the treated concrete.

A method for the realkalisation of a zone of carbonated steel in concrete is disclosed and claimed in European Patent 0 264 421. Low voltage electric currents are passed between a temporary anode within a suitable electrolytic medium, such as a papier-mache poultice, containing alkaline salts, and, the steel exposed within the carbonated concrete to be treated, with each having opposite polarity, whereby alkaline hydroxyl ions migrate to the reinforcement in the carbonated zone.

Both methods require lengthy and extensive preparation, and, are not wholly sympathetic to the environment, because of the high concentration and accumulation of waste materials resulting from the process, in particular, the wasteful degeneration of the electrolytic medium.

SUMMARY OF THE INVENTION One object of the present invention is to provide a method whereby a gel compound incorporating alkaline metal salts, (constituents of which might be any one or more of the elements sodium calcium, lithium, or potassium), in solution with a permeable material upon which an electrode is superimposed as an integral part for the purpose of generating alkalinity at the cathodic areas of reinforcement steel.

Another object of the invention is to provide a method whereby chloride ions are removed from the concrete.

The instant process is a unique and flexible system. It minimises expenditures by enabling the electrode to be readyformed at any appropriate location, convenient for the site. In this context, the electrode may be recycled as many times as are necessary and effective, without the need for disassembly, or, the necessity for increments of alkaline salts to be applied for absorption, but simply by ensuring its placement in an environment capable of preserving a moist condition prior to refixing in a new location. It observes health and safety, and environmental considerations by avoiding the use of chemicals in concentrations that would otherwise be hazardous. In use, the system is much cleaner, requiring no heavy dosing of the electrode either to keep it wet, or, for the removal of waste.

The system is less noisy, raises less dust, and leaves less waste, than other known systems. At the conclusion of the treatment there are no visible corrosion stains of the structure or the electrode. A further advantage of the instant process is that it is compatible with state of the art concrete technology and techniques.

These and many other attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention will be described with reference to the accompanying drawings wherein: FIG 1. is a perspective view showing the basic structure of the electrode in section.

FIG 2. is a schematic diagram showing the electrode in situ and in connection with reinforcement in affected concrete.

FIG 3. is a graphical representation indicating current density and drive voltage plotted as a function of time in the application of another aspect of the invention.

FIG 4. is a graphical representation indicating current density plotted as a function of time in the application of one aspect of the invention.

FIG 5. is a graphical representation indicating pH as treatment progresses and at its conclusion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig 1, the invention in its most general aspects comprises an electrode (anode) of either a pre-formed permeable synthetic sponge-like material, impregnated with an admixture of alkaline salts in solution, to which has been added a setting agent to form a gel, and upon which is superimposed an electrically conductive material such as a conductive paint or metal mesh.

Preparation of the electrode (anode) commences with the application of the conductive material td one surface of preformed permeable synthetic sponge-like or rigid material.

A primary anode, such as a carbon fibre tape of sufficient dimension to facilitate the required density of electric current to pass over the surface of the (primary) electrode is then superimposed to complete the electrode.

The prepared permeable material is then immersed, upside down, in the prepared gel solution, and compressed to expel all air before the solution is allowed to cool and set As indicated above, the electrode (anode) in its preferred form will be ready-formed, prior to its placement in situ for the commencement of the treatment to affected concrete as set out in this description.

In some instances to facilitate to secure fixing of the electrode to the affected concrete to be treated, it may be expedient to fix the conductive material to a rigid board material, such as plyrod, which is used to hold the electrode against the affected concrete.

Before fixing the electrode to the area of the affected concrete, it may be preferable to prepare its surface by applying an alkaline gel, such as calcium carbonate, to reduce acid generation and improve electrical contact.

As regards Fig. 2, the negative terminal of the power supply is connected to the steel in the affected concrete. The positive terminal is connected to the electrode located on the surface of the structure. When the current is switched on, electrochemical reactions take place on the surface of the electrode, (anode) and at the affected concrete to steel interface (cathode). At the steel bar (cathode - negative terminal), the passage of current converts water to hydrogen gas and hydroxyl ions, or oxygen plus water to hydroxyl ions.

Simultaneously, the oxidation process converts water into oxygen gas and hydrogen at the surface of the electrode (anode positive terminal), and ions migrate between the electrode and the steel through the electrode material and the affected concrete. The generation of hydroxyl ions results in an increase in alkalinity at the cathode.

Positive ions which move toward the steel under the influence of the current, have the secondary affect, of securing the hydroxyl ions which produce the alkalinity within the affected concrete surrounding the steel.

The increase in alkalinity can be calculated from the magnitude of the current passing, the length of time for which it is applied, and the rate at which hydroxyl ions are pulled away from the steel bar under the influence of the current flow.

Alternatively, the rate at which positive ions are transported into the affected concrete may be used to calculate the minimum increase in overall hydroxyl ion concentration.

Besides the beneficial effect of an increase in alkalinity, chloride ions which are also charge carriers, migrate towards the anode under the influence of the imposed electric field.

Chlorides will migrate towards the anode if they lie in the path of the current, and are not bound as insoluble complexes.

Conversely, chlorides not lying in the current path, will not be removed by this technique. This emphasises the need for an adequate feasibility survey to be efficiently carried out.

Thus the object of the invention is to control and utilise electro-chemical principles to achieve residual alkalinity in the affected concrete and to remove chloride from the affected concrete.

In one example, an electrode is prepared containing 2% by weight agar powder, or, carrageenin, a generic term for a gel agent prepared from sea weed, and sodium bicarbonate solution at a concentration of 1 molar or saturation, whichever is the lesser.

A current density of lA/m2 is applied.

In another example a proprietary gel agent such as "Gelling System SRM" may be used in the preparation of the electrode, to which a current density of lAmp/m2 is applied.

Fig 3 gives the current and drive voltage as a function of time.

It will be noted that the drive voltage may rise to 30 volts to maintain lAmp/m2 current.

In another example, an electrode again containing 2% by weight agar powder, or, carrageenin, but with 0.3 Molar potassium chloride solution is prepared, to which a current density of lAmp/m2 is applied.

Fig. 4 shows the current density as a function of time, with a drive voltage limited to a maximum of 18 Volts.

It should be noted that in both examples, no water is added to the electrode (anode), after commencing the application of the current. Before and after current application the pH of the electrode is determined using a universal indicator. Initially, a pH of 6 was indicated prior to the application of the current.

After completion of the current application, the affected concrete adjacent to the steel may be tested for pH changes, using either a universal indicator or phenolphthalein, when a rise in excess of 12pH will be indicated.

Fig 5. shows in contour the pH gradients at the electrode, and interface of steel to affected concrete.

While preferred embodiments of the invention have been shown and described in detail, it would be readily understood and appreciated that numerous omissions changes and additions may be made without departing from the spirit and scope of the present invention as defined in the following claims.

Claims (8)

1. A method for treating affected reinforced concrete or masonry, characterised as an anode comprising permeable synthetic material impregnated with a gel containing alkaline metal salts, (constituents of which might be any one or more of the elements sodium calcium, lithium, or potassium) in solution, with electrically conductive material superimposed, and whereby an electric current is passed between the said anode, and the steel reinforcement, and whereby alkalinity is generated at the cathodic areas of steel reinforcement, and whereby chloride ions migrate away from cathodic areas of steel reinforcement toward the anode.

2. An anode according to Claim 1 characterised as being readyformed prior to its placement in situ for the commencement of treatment to affected concrete.

3. A method according to any of the preceding Claims characterised in that chloride ions are extracted electrochemically under the influence of electric current flow from the cathodic areas of steel reinforcement.

4. An anode according to Claim 1 to which electrically conductive material is applied to the upper surface of permeable synthetic sponge-like material to form into an electrically durable layer.

5. An anode according to Claim 1 to which electrically conductive coating material is applied to the upper surface of permeable material according to Claim 4 adapted to incorporate a layer of material capable of securely fixing the said anode to the affected concrete to be treated.

6. An anode according to Claim 1 of sufficient dimension to facilitate an electric current of required density to pass over the surface of it.

7. An anode according to any of the preceding claims, characterised as capable of being recycled and reconstituted.

8. A method where at least one anode according to any one of the preceding Claims is applied.