WO1999001406A1

WO1999001406A1 - A process for the electrochemical treatment of concrete

Info

Publication number: WO1999001406A1
Application number: PCT/GB1998/001966
Authority: WO
Inventors: Moira Helen Decter
Original assignee: Fosroc International Limited
Priority date: 1997-07-03
Filing date: 1998-07-03
Publication date: 1999-01-14
Also published as: AU8230198A; GB9714048D0; ZA985880B

Abstract

A process for the electrochemical realkalization of reinforced concrete comprises (i) mixing a composition of potassium carbonate and an organic water retaining material with water to form an adhesive mixture containing potassium carbonate solution and applying the mixture as a coating to the surface of the concrete which is to be realkalized, the amounts of potassium carbonate and water being such that the concentration of potassium carbonate is at least 0.3 Molar and (ii) passing a direct electric current between an anode in contact with the coating and a cathode which is located internally in the concrete, to cause the internal pH of the concrete to increase and a surface layer of concrete to be be impregnated with the electrolyte solution. The advantage of using potassium carbonate is that efflorescence on the concrete surface is avoided.

Description

A PROCESS FOR THE ELECTROCHEMICAL TREATMENT OF

CONCRETE .

Field of the Invention. This invention relates to a process for the electrochemical treatment of reinforced concrete, to a composition for use in the process and to a method for the preparation of the composition.

Background of the Invention In reinforced concrete the steel reinforcement is normally protected against corrosion by the alkaline environment in the concrete mass. Gradually however the alkalinity is reduced by the action of carbon dioxide and other gases in the atmosphere such as oxides of sulphur. The term given to this process is carbonation and concrete which has been subjected to the action of atmospheric gases is referred to as carbonated. The effect is a gradual decrease in the pH and, if allowed to continue the pH will continue to fall and when it reaches a value of about 9.5 the steel is no longer protected against corrosion. Corrosion of the reinforcement may then take place leading to weakening of the reinforcement and spalling of the concrete . A process for increasing the alkalinity of concrete which has been carbonated has been previously described, for example in European Patent No 264,421 and United States Patent No 4,865,702. In this process an electric current is passed between an external anode which is in contact with an alkaline electrolyte applied to the external surface of the concrete and the internal reinforcement of the concrete as cathode. During the process there are two effects: the alkaline electrolyte moves into the concrete and alkalinity is increased in the region of the cathode. The benefit of this is that the steel is repassivated and a layer of electrolyte which has permeated into the concrete maintains a cover zone over and around the steel of sufficiently high pH to keep the steel passive. This process has become known as realkalization.

As the alkaline electrolyte for this process there is proposed in European Patent No 264 421 an aqueous solution of calcium, sodium and/or potassium salts either in a liquid or absorbed in a porous medium such as rock wool, cellulose, sawdust, sand, clay and the like or the electrolyte can be strongly retarded concrete, mortar, cement grout or lime paste. Further in GB Patent Application No 2,279, 664A there is proposed an electrochemical realkalization process in which a gel compound incorporating alkaline metal salts, (constituents of which might be any one or more of the elements, calcium, lithium, or potassium) in solution with a permeable material upon which an electrode is superimposed.

In the operation of the process on a commercial scale the alkaline electrolyte that is usually used is an aqueous solution of sodium carbonate. In some embodiments of this process the sodium carbonate is applied to the surface of the concrete with an adherent coating of cellulosic fibre, conveniently by spraying .

Problem to be solved by the Invention Although the results of the process using sodium carbonate solution as the alkaline electrolyte have been generally very successful, the surface of the concrete after the treatment, frequently shows efflorescence. This is a heavy crystalline salt which is unsightly and is unsuitable for the application of a decorative coating. Consequently it is necessary to thoroughly clean the concrete after treatment to remove the efflorescence. A solution to the above described problem has now been invented in which a solution of potassium carbonate is employed as the electrolyte with the adherent coating and in which the potassium carbonate is premixed with the material that is to form the adherent coating before the addition of water.

Summary of the Invention

According to the present invention a process for the electro-chemical realkalization of reinforced concrete comprises : mixing a composition containing potassium carbonate and an organic water retaining material with water to form an adhesive mixture containing potassium carbonate solution and applying the mixture as a coating to the surface of the concrete which is to be realkalized, the amounts of potassium carbonate and water being such that the concentration of potassium carbonate is at least 0.3 Molar and passing a direct electric current between an anode in contact with the coating and a cathode which is located internally in the concrete, to cause the internal pH of the concrete to increase and a surface layer of concrete to be impregnated with the electrolyte solution.

Advantageous Effect of the Invention The advantage of the present invention obtained by the use of an aqueous solution of potassium carbonate as the alkaline electrolyte is that efflorescence on the external surface of the concrete after treatment is reduced or eliminated. This advantage could not have been predicted from the prior art and is therefore unexpected. In addition the formation of a composition of potassium carbonate with the water retaining material means that at the work site only water needs to be added.

Detailed Description of the Invention

By external surface is meant a surface that is exposed to the atmosphere. The term electrolyte is intended to refer to the aqueous solution of potassium carbonate .

The concentration of the potassium carbonate solution is preferably at least 0.5 Molar and concentrations from 0.5 Molar up to saturation are particularly suitable.

Preferably the electrolyte will be substantially free of ions other than those derived from potassium carbonate and water although small quantities of other ions e.g. sodium, calcium, or lithium for example in amounts that occur as impurities in commercially available forms of potassium carbonate are acceptable. It is therefore convenient to employ potassium carbonate (as the compound) to make up the electrolyte. The potassium carbonate may be a general industrial grade material for example one containing at least 97% by weight of potassium carbonate on a dry basis .

It is preferred that the amount of sodium ions, if present, is less than 5% by weight of the potassium carbonate based on the dry weight of the potassium carbonate, because the inventors have discovered that sodium carbonate is the material which causes the efflorescence on the concrete surface after treatment. Preferably the pH of the electrolyte at the commencement of the process is in the range 10.5 to 12.5 preferably 10.9 to 12.0.

Conveniently the adhesive mixture is applied by spraying and the mixing of the water with the composition is effected in the spraying process.

The organic water retaining material may be a cellulosic fibre and the composition of cellulosic fibre and potassium carbonate suitably contains at least 10% of potassium carbonate based on the dry weight of the cellulosic fibre preferably from 30 to 150% by weight of potassium carbonate based on the dry weight of the cellulosic fibre.

The adhesive mixture conveniently contains more potassium carbonate than is required to saturate the water present in the mixture. In this case the coating will contain undissolved potassium carbonate which facilitates replenishment mentioned below.

During the process the electrolyte may be replenished. This may be done by spraying e.g. at intervals potassium carbonate solution onto the adherent coating or, when the coating contains undissolved potassium carbonate, spraying water onto the coating. The water retaining material forms an adhesive mixture when mixed with water. By adhesive is meant that it is capable of adhering to a concrete surface which may be an inclined surface.

The adherent coating can be as described in European Patent No 2000428 and the water retaining material can be as described in European Patent No 398117 or US Patent Nos 5,198,082; 5,228,959; and 5,407,543.

Conveniently the water retaining material can hold at least 100% its own weight of water and preferably at least 200%, more preferably at least 300% for example 300 to 500%.

Preferably the water retaining material is a cellulosic fibre, hereafter referred to as fibre, and it is a feature of the present invention that the fibre is premixed with the potassium carbonate, e.g. at a factory so that on the site only water needs to be added to the composition of fibre and potassium carbonate . The dry fibre is conveniently mixed with the potassium carbonate (which may be in solid form) in the process for the preparation of the fibre e.g. the milling of the cellulose and supplied to the job site as a composition where it is mixed with water, for example, by supplying the cellulose fibre/potassium carbonate composition and water as two components to a suitable spray nozzle in which they are mixed and from which is emitted a spray containing a mixture of the two. The cellulosic fibres may be recycled or reconstituted cellulose pulp.

Conveniently the cellulose pulp is derived from newsprint or other waste paper.

Processes for the production of cellulose fibres are known in the art and are in commercial operation. Cellulose fibre is known as a replacement for asbestos fibres in a number of applications such as panels, tile adhesive, refractory linings and especially fibre cement panels. In a typical process for the preparation of cellulose fibres the feed in the form of waste newsprint in sheet form is passed to a shredder from where the shredded paper is passed through the first of two hammer mills such as a Jacobson mill. The mill has rotating hammers or blades which together with air suction force the material through a perforated metal screen. The material, which at this point is partly fibrised, is passed to a second hammer mill. At a point between the two hammer mills chemicals such as fire retardants are added. In the preparation of fibre for use in the present invention the potassium carbonate is added at this point . The material is then passed through the second hammer mill in which it is further fibrised. The product is then compressed and extruded into bags for storage . The fibres after leaving the second hammer mill usually have a length of between 0.5 and 2.0mm. The freeness of the fibres may be in the range 45 to 75o SR (Shopper-Riegler) .

At the job site (where the realkalization is to be carried out) water is mixed with the composition of fibre and potassium carbonate. The quantity of water mixed is desirably such as to produce the required adhesiveness and consistency. The amount of water added and the amount of potassium carbonate in the mixture are such as to provide a concentration of potassium carbonate solution of at least 0.3 Molar, preferably at least 0.5 Molar. More preferably the relative amounts of water and potassium carbonate are such that there will be an excess of potassium carbonate over that required to saturate the water.

The fibre will therefore contain undissolved potassium carbonate and replenishment of the electrolyte solution can be effected by adding water e.g. by spraying to the coating. The cathode is conveniently connected to, or provided by, the reinforcement of the reinforced concrete.

The process is particularly suitable for use with concrete that has been carbonated to a pH of 10.5 or less, or 10.0 or less, especially 9.5 or less, since at this last mentioned pH the steel of the reinforcement is no longer protected against corrosion.

The process can be carried out as described in European Patent No 264,241 and U.S. Patent No. 4,865,702.

For example the applied voltage between anode and cathode can conveniently be from 3 to 40 volts, preferably from 5 to 20 volts to provide a current density of from 0.15 to 6, preferably from 0.5 to 2.5 amps per square metre of concrete surf ce. The alkalinity can be monitored by measuring the pH e.g. by means of an indicator sprayed onto the freshly broken concrete and when the desired pH has been reached e.g. a pH of greater than 10.5, usually greater than about 11, the process can be stopped.

Since loss of alkalinity is caused by atmospheric gases such as carbon dioxide, for a concrete that has weathered, the pH at or near the surface will often be lower than that further into the concrete body and the pH in the immediate vicinity of the reinforcement may still be sufficiently high to provide passivation of the steel. It is within the scope of the present invention to realkalize such concrete. Although the pH of the concrete may vary through its thickness any zone may be chosen as the zone whose pH is measured to determine whether to realkalize the concrete . The zone whose pH is measured to determine the end of the process will usually be the same distance from the surface as the first chosen zone.

The process may comprise measuring the pH of a chosen zone of the concrete and when the pH is 10.0 or less, carrying out the process as described above and, after a period of time, measuring the pH again, and when the pH is 10.5 or more stopping the passage of the electric current .

The anode can comprise wires, cords, plates, foil or sheet metal . The anode and its associated electrolyte can be preformed and applied to the concrete surface as an assembly.

Alternatively however the anode is applied to the concrete surface and then coated with an electrolyte-containing medium, for example as described in United States Patent No 5,320,722.

For example the anode may be first mounted on the concrete and the surface sprayed with an electrolytic composition, comprised of a fibrous cellulosic pulp mixed with the potassium carbonate electrolyte to form a self adherent cohesive electrolytic medium. The pulp may be formed of natural cellulose fibres with addition of mineral fire and smoulder retardants such as borax, magnesium and aluminium oxides and certain silicates. Desirably the dry fibre is mixed with the fire retardants.

As described in European Patent No 398117B the cellulosic fibre may be in the form similar to papier-mache and when mixed with the aqueous solution of potassium salt forms a pulp. The concrete surface may be moistened with water before the application of the cellulosic fibre. Because of the porous character of the concrete, the cellulosic pulp-water or pulp-solution mixture adheres to the concrete surface. The pulp-solution is desirably comprised of chopped pulp mixed uniformly in a ratio of from 2 to 1 to about 8 to 1 of water or other solution per kg of dry fibre . Advantageously a first layer of the papier-mache like material is sprayed onto the concrete surface to a depth of approximately equal to the thickness of wooden battens i.e. about 2 cms (where these are employed) . After this first layer a distributed electrode grid may be attached to the exposed surfaces of the battens and then additional pulp-solution mixture sprayed over the grid to provide a total layer thickness of 3 to 5 cms. Altogether, the pulp fibre-solution of each layer is applied to the concrete surface in amounts of about 1 kg of dry fibre together with 2 to 8 litres of water or solution.

A minimum level of moisture should be maintained and this can be accomplished by spraying the surface of the coating with water as often as necessary, or by running water through the fibre from the outside edged of the treatment zone, usually twice per day is adequate . At the job site the cellulose fibre and potassium carbonate mixture and water are supplied to a suitable spray nozzle as two components in which they are mixed and from which is emitted a spray containing a mixture of the two i.e. wet fibrous material with the desired electrolyte content .

Particularly suitable cellulosic materials are available commercially from Excel Industries, Ltd of Ebbw Vale Gwent Wales under the trade designations NCT FG 2000 and NCT FW 2000. The material typically has a loose bulk density of about 25 to 35kg/cubic metre. The material is formulated to provide for a water absorption of approximately 1500 per cent of the dry fibre, to wet out rapidly and have slow drainage characteristics.

The process of the present invention can use as the anode a consumable metal such as steel or an inert metal such as titanium.

According to another aspect of the present invention a composition suitable for application to the surface of reinforced concrete to provide an adherent layer for an anode comprises: cellulosic fibres and at least 10% preferably from 20 to 150% by weight of potassium carbonate based on the dry weight of the cellulosic fibres .

The composition may be prepared by grinding the fibres and potassium carbonate together. Referring to the drawing a body of concrete 2 has an external surface 4, steel reinforcement 6 and a zone 8 adjacent to the surface 4 which has been carbonated. Maintained in contact with the external surface 4 by an adherent coating 11 is an electrolyte 10 which is a saturated aqueous solution of potassium carbonate. The coating 11 contains undissolved potassium carbonate to provide a reservoir to facilitate replenishment of the electrolyte. Embedded in the coating 11 is a steel anode 12. In operation of the process of the present invention a voltage is applied between the reinforcement 6 as cathode and the steel anode 12 to provide a direct current at a current density of 1 amp per square metre of concrete surface. Electrolyte 10 penetrates the surface 4 of the concrete and into the concrete as indicated by the downward pointing arrows. A front of advancing electrolyte is indicated at 16. This is shown as a mixture of potassium carbonate and bicarbonate because potassium carbonate is converted by carbon dioxide in the atmosphere to a mixture (not necessarily in equimolar proportions) . Under the influence of the electric current the ions move in the directions indicated ie potassium and sodium ions move towards the cathode and hydroxyl ions towards the anode .

The invention is illustrated by the following Example.

Example 1 A reinforced concrete structure that has been carbonated by exposure to the atmosphere over a period of years and whose pH in the vicinity of the steel reinforcement has fallen to about 9.5 is subjected to the following treatment . Wooden battens are secured to the surface of the concrete using mushroom fixings. The battens have a thickness of 25mm. A steel mesh is fixed to the battens using U shaped nails. A mixture of cellulosic fibre and potassium carbonate is combined with water in the spraying equipment and sprayed onto the concrete, through the steel anode mesh to a depth to cover the mesh by approximately 10mm.

The mixture sprayed onto the concrete contains (i) 8 % by weight of potassium carbonate, (ii) 15 % by weight of cellulosic fibre and (iii) 77 % by weight of water all % being based on the combined weight of (i) , (ii) and (iii) . The steel anode is connected to the positive terminal of a source of direct current whilst the steel reinforcement of the concrete structure is connected to the negative terminal.

A voltage of about 12 volts was applied and adjusted to provide a current density of one amp per square metre of concrete surface and maintained over 4 days . The pH of the concrete in the vicinity of the reinforcement was measured at regular intervals using a phenolphthalein indicator made up in water and ethanol . The phenolphthalein indicator solution is sprayed onto a core taken from the treated concrete and broken down its length prior to application of the indicator. When the pH rises to a value high enough to turn the indicator pink (approximately pH 11) the current is switched off. The steel anode, wooden battens and cellulosic fibre layer are removed and, after drying, the external surface of the concrete inspected. No sign of efflorescence is observed.

The example is repeated exactly as described above except sodium carbonate is used in place of the potassium carbonate. After removal of the steel anode, wooden battens and cellulosic fibre layer and drying, inspection of the surface revealed some efflorescence on the concrete surface . The advantage of premixing the potassium carbonate and fibre, eg at the factory, is that on the site it is necessary to add only water. This can be done by supplying the fibre and potassium carbonate as one component to a spray nozzle (which will be the spray nozzle used for applying the adherent coating to the concrete) , the other component being water which is also supplied to the nozzle. Mixing is thereby achieved by the nozzle. It is therefore not necessary to dissolve the potassium carbonate separately. Experimental work has found that potassium carbonate has f rther advantages over sodium carbonate :

(i) potassium carbonate solution penetrates into the concrete faster than sodium carbonate under identical conditions and molar concentrations. This means that the pH of the concrete layer adjacent to the surface is increased more quickly.

(ii) potassium carbonate has far better solubilility properties at low temperatures. For example at 4 degrees Centigrade sodium carbonate at a saturated concentration is below 1 Molar. Potassium carbonate however has a saturated solution of over 5 Molar at this temperature. This is significant since one of the main applications of the invention is the for the treatment of the external surfaces of buildings and other concrete structures and means that potassium carbonate can be more reliably used during the winter.

Claims

CLAIMS :

1. A process for the electro-chemical realkalization of reinforced concrete comprising: mixing a composition containing potassium carbonate and an organic water retaining material with water to form an adhesive mixture containing potassium carbonate solution and applying the mixture as a coating to the surface of the concrete which is to be realkalized, the amounts of potassium carbonate and water being such that the concentration of the potassium carbonate solution is at least 0.3 Molar and passing a direct electric current between an anode in contact with the coating and a cathode which is located internally in the concrete, to cause the internal pH of the concrete to increase and a surface layer of concrete to be impregnated with the electrolyte solution.

2. A process as claimed in claim 1 wherein the adhesive mixture is applied by spraying and the mixing of the water with the composition is effected in the spraying process .

3. A process as claimed in claim 1 or claim 2 wherein the organic water retaining material is a cellulosic fibre and the composition of cellulosic fibre and potassium carbonate contains at least 10% of potassium carbonate based on the dry weight of the cellulosic fibre .

4. A process as claimed in claim 3 wherein the composition contains from 30 to 150% by weight of potassium carbonate based on the dry weight of the cellulosic fibre.

5. A process as claimed in any one of the preceding claims wherein the adhesive mixture contains more potassium carbonate than is required to saturate the water present in the mixture.

6. A process as claimed in claimed in any one of the preceding claims wherein the applied voltage between anode an cathode is from 3 to 40 and the current density from 0.15 to 6 amps per square metre of concrete surface .

7. A process as claimed in claim 6 wherein the voltage is from 5 to 20 and the current density from 0.5 to 2.5 amps per square metre of concrete surface.

8. A composition suitable when mixed with water for application to the surface of reinforced concrete to provide an adherent layer said composition comprising: (i) an organic material capable of retaining water, said organic material forming an adhesive mixture when mixed with water and

(ii) at least 10% by weight of potassium carbonate based on the dry weight of the organic water retaining material .

9. A composition as claimed in claim 8 wherein the organic material is a cellulosic fibre and the amount of potassium carbonate is from 30 to 150% by weight based on the dry weight of the cellulosic fibres.

10. A composition as claimed in claim 8 or 9 which further contains water in an amount to provide an adhesive composition suitable for application to a concrete surface by spraying.

11. A method for the production of a composition as claimed in claim 8 or 9 which method comprises adding the potassium carbonate to the cellulosic fibres during the milling of the cellulosic fibre.

12. A method as claimed in claim 11 wherein the cellulosic fibres are obtained from waste paper and the method comprises shredding the paper and adding the potassium carbonate when the paper is at least partly shredded.