AU682690B2 - Realkalization and dechlorination of concrete by surface mounted electrochemical means - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION For a Standard Patent

ORIGINAL

r o Name of Applicant: Actual Inventor(s): Address for Service: CONCRETE REPAIRS LIMITED JOHN FRANCIS DREWETT WRAY ASSOCIATES, Primary Industry House, 239 Terrace, Perth, Western Australia, 6000.

Adelaide Attorney code: WR Invention Title: "Realkalization Concrete by Electrochemical and Dechlorination of Surface Mounted Means" The following statement is a full description of this invention, including the best method of performing it known to us:- 1 s= 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 5 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.

:Background 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 Snot generally impair the tensile strength, whereas localised 15 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 fbrmation of a passive oxide film which prevents further metal dissolution.

Steel remains passive at these high pH's; 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 15 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 deierioration in the strength of the structure.

Passive film breakdown mainly occurs in two ways, either from 20 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 25 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.

uc~l"~ 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 and porewater converts into hydroxyl ions, raising the pH at the concrete to steel interface.

EARLY WORK AND PRIOR ART In the early part of this century, work was done in the United States to investigate serious damage done to certain concrete buildings and bridges under the influence of stray electric current fr electric railways and other power sources. Applying electrolytic principles, in March 1913, E.B.Rosa, Burton McCollum, and O.S.Peters of the US Bureau of Standards reported at length in

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a paper entitled "No 18 Electrolysis in Concrete" In the course of this work, Rosa et al. discovered inter alia (a) that the quantity of electricity that passes through concrete, does not alone determine the damage it may do, but that the rate at which the current flows is the important factor, (but damage 3 i i' ~4 P°~B~HDBlls~Rlllr~R~li~X~Lll~nu~~rr*~i~- decreases with decreasing voltage); that solutions produced at the cathode are strongly alkaline; that positive alkali metal ions move toward the cathode, and negative hydroxyl ions created at the cathode move toward the anode.

One of the first known attempts to apply these principles and 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, (Vennesland and Opsahl), 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.

The movement of hydroxyl ions from the interior of concrete towards the carbonated surface zone exposed to the atmosphere by diffusion is described by Page and Treadway at page 112 in Nature Vol. 297 No. 5862 dated 13th May 1982. The philosophy of increasing the alkalinity of concrete which had undergone a degree of carbonation was first proposed in European patent 0 221 744 Miller) albeit by saturation and diffusion a phenomenon recognised by Rosa et al. The movement of alkali by electro-migration was known due to the works of Rosa et al, as described above.

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 obje-t of the present invention is to provide a method whereby a gel ci .,und incorporating alkaline metal salts, (constituents of which might be any one or more of the elements sodium calcium, lithium, or potassium), in solution is impregnated into 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 anode assembly to be ready-, formed at any appropriate location, convenient for the site. In this context, the electrode anode assembly may be removed for reuse as many times as are necessary and effective, without the need 2o 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 re-fixing in a new location. It observes health and safety, and *I environmental considerations by avoiding the use of chemicals in concentrations that would otherwise be hazardous. In use, the electrode anode assembly is much cleaner, requiring no heavy dosing of electrolyte either to keep it wet, or, for the removal of waste.

The electrode anode assembly is less noisy, raises less dust, and I-i ;I~ 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 anode assembly. A further adv,' -ge of the instant process is that it is compatible with state o. 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 anode assembly in section.

FIG 2. is a schematic diagram showing the electrode anode assembly 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 'o 25 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 the use of an electrode anode assembly of a pre-formed permeable synthetic sponge-like material, having a porosity capable I_ of absorbing and retaining liquids by capillary action, impregnated or saturated with an aqueous electrolyte having 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 assembly commences with the application of the conductive material to one surface of said permeable synthetic 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 upon the coating while still wet, and allowed to dry into an electrically durable layer.

The prepared permeable material is then immersed, uncoated side *down, in the prepared electrolyte solution and compressed to expel all air before the electrolyte solution is allowed to cool and set.

As indicated above, the anode assembly in its preferred form will be ready-formed, prior to it's placement in situ for the commencement of the treatment to affected concrete as set out in this description.

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

Before fixing the anode assembly to the area of the affected concrete, it may be preferable to prepare either the concrete 7 i ~L surface, or the surface of the electrode anode assembly, by applying to them, alkaline metal salts in a gel solution, to reduce acid generation and improve electrical contact.

As regards Fig. 2, the negative terminal of the power supply is connected to the steel which must be electrically continuous 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 S 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.

8 \C'Vr^ I 1 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 electrochemical 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 IA/m 2 (concrete area) 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 1A\m 2 (concrete area) 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 lA/m 2 (concrete area) 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 lA/m2 (concrete area) is applied.

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^Y OLl~tlL1PIILPZs~P~~-L*- 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 c 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 (4)

1. A method for treating and/or preventing corrosion in concrete or masonry reinforced with steel comprising the steps of: forming an anode assembly by superimposing a layer of electrically conductive material over a permeable synthetic material impregnated with gel having a porosity capable of absorbing and retaining liquids by capillary action, attaching a strip of conductive material over a selected area of said layer of electrically conductive material; immersing said permeable porous synthetic material assembled with the said layer of electrically conductive material into an aqueous electrolyte solution incorporating alkali metal salts or the alkaline earth metal calcium to cause said porous material to becoMe impregnated with electrolyte; pre-treating the surface of concrete or masonry upon which the anode assembly is placed with an alkaline metal salt solution to reduce acid generation and enable an effective reclamation procedure to be carried out; placing said formed anode assembly containing impregnated electrolyte over an area of concrete or masonry to be treated so that impregnated permeable material lies Scontiguous to said concrete or masonry; connecting said conductive material to the positive terminal of a power supply; and connecting the negative terminal of such power supply to the reinforcing steel in said concrete or masonry, whereby by passing ain electric current between the said formed anode assembly and reinforcing steel, simultaneous "chloride removal and alkalisation occur at the reinforcing steel cathode.

2. A method according to the preceding claim characterised in that the said conductive material is composed of carbon fibre.

3. A method according to claim 1 whereby said electrolyte S solution further comprises a gelling agent.

4. A method as defined in any one of the preceding Claims wherein at least one anode assembly is applied. Dated this twenty third day of July 1997 Concrete Repairs Limited Applicant Wray Associates Perth, Western Australia _Patent Attorneys fori the Applicant pBf"Y ~IT~au~nmn~u~l~Y-Y-~I~YYPYlls~s~,~ ABSTRACT OF THE DISCLOSURE A method for realkalising and dechlorinating corroded areas of reinforcement steel in concrete and masonry electrochemical means, characterised by an electrode (anode) composed of alkaline salts upon which is superimposed electrically conductive material. When the current is switched on, alkalinity is generated at the cathode (steel bar), by the production of hydroxyl ions, while chloride ions at the cathode (steel bar), being electrical charge carriers, also migrate toward the anode under the influence of the current flow. *0 *,eo i- i i~ ~I