IL104837A - Method for electrochemical treatment of reinforcing steel in concrete having embedded steel reinforcement - Google Patents

Description

A METHOD FOR ELECTROCHEMICAL TREATMENT OF REINFORCING STEEL IN CONCRETE HAVING EMBEDDED STEEL REINFORCEMENT Embedded steel in reinforced concrete is normally protected against corrosion by virtue of a dense oxide film which forms on the steel surface in alkaline environments. This film acts as a barrier to aggressive agents. However, when concrete becomes contaminated with chloride ions, or when its alkalinity is reduced by absorption of carbon dioxide from the air, the passivating oxide film may break down, thus rendering the embedded steel subject to corrosion.

Much research has been done to examine the causes and mechanisms involved in the corrosion of steel reinforcement in concrete. The general consensus today is briefly that the corrosion process is electro-chemical in nature, in that sites where the passive oxide film is broken form anodes, and the surrounding areas where the film is still intact form cathodes. The anodic and cathodic areas together form corrosion cells leading to the dissolution of iron at the anodic areas.

Various electro-chemical methods have been developed in an effort to control this corrosion, or to neutralize its causes. One well known such method is that of cathodic protection whereby the embedded steel is brought to and maintained at an electrical potential at which it cannot corrode. Cathodic protection installations have been shown to be workable, but suffer from a number of adverse factors, not the least of which is their necessarily being permanent installations requiring ongoing monitoring and maintenance. Other disadvantages are high cost, the extra structural loading introduced by heavy concrete overlays, and the difficulty of ensuring correct current distribution.

Another such method is that of chloride extraction, in which chloride ions are caused to migrate under the influence of an electric field to an external electrolyte where they accumulate in, and eventually are removed with, the electrolyte. The Vennesland et al. U.S. Patent no. 4,032,803 is an example of such processes. The chloride extraction process, though effective and less costly than cathodic protection, and thus a substantial improvement thereover, nevertheless suffers from the difficulty of predicting the time necessary for treatment to be completed. Because of this, frequent sampling and analysis of the concrete is required to determine remaining chloride levels. This difficulty is compounded by there so far being no residual chloride level which is generally accepted by the industry as being safe with regard to future chloride attack. These factors can make it difficult to calculate the cost and time necessary to reach a particular treatment target. In some cases, this time can also be unacceptably long from a practical aspect, especially since it is difficult to plan for in advance.

A third such method, which is applied to carbonated concretes, is the impregnation of the carbonated zones by the electro-migration of alkaline substances from an external source. The Miller et al. U.S. Patent No. 4,865,702 is illustrative of this process. This latter method, though successful in carbonated concretes which are low in chloride, can become inefficient, or even fail, when the concrete contains significant amounts of ionic substances such as chlorides. Also, when the concrete contains blast furnace cement, or where pozzolans have been added to the mix, the treatment time can become unreasonably long.

The present invention overcomes the difficulties of the above mentioned methods by being highly predictable with regard to treatment time, by eliminating the necessity for sampling and chloride analysis, by being quicker and hence more economical to apply, and by being equally applicable to almost any kind of concrete, carbonated or not, chloride contaminated or not, pozzolanic or not, and whether or not blast furnace cement has been used.

Summary of the Invention The present invention is based upon the discovery and recognition that the electro-chemical treatment of concrete 104,837/2 does not have to be controlled as a function of the chloride content, for example, or as a function of the degree of carbonation. Rather, the invention is based upon the recognition that the electro-chemical processing of concrete is optimally controlled as a function of the surface area of the embedded steel reinforcement. In a given' structure, the surface area of the embedded reinforcement is either known from the construction records, or is the subject of close approximation. Pursuant to the invention, electro-chemical treatment can be set up more or less in a known manner disclosed by the Vennesland et al. U.S. Patent No. 4,032,803, or the Miller U.S. Patent No. 5,228,959, Significantly, however, instead of periodically taking core samples of the concrete structure to evaluate residual chloride levels, for example, the process is controlled by reference to the accumulated current flow in relation to the total surface area of the embedded reinforcing steel. The process is continued until a minimum of 500 ampere-hours of current flow per square meter of surface area of the embedded steel has been realized. The process can be discontinued at that stage (and preferably is discontinued before the current . flow significantly exceeds 2000 ampere-hours per square meter of surface area) , regardless of the residual chloride level or carbonation level at various points in the concrete.

The process may be discontinued at this stage with a high level of confidence that the embedded reinforcing steel will be protected for a significant period of time. As compared with previously known procedures, processing according to the present invention can be accomplished with less than half the energy input and processing time.

For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment of the invention and to the accompanying drawing.

Description of the Drawing; Fig. l is a schematic illustration of an installation of steel reinforced concrete set up for treatment in accordance with the process of the invention.

Fig. 2 is a graphical representation illustrating the increasing passivity (and therefore protection) of embedded steel reinforcement over a period of time after treatment in accordance with the invention.

Description of Preferred Embodiment: Referring now to the drawing, 10 represents a concrete structure, comprised of set and hardened concrete 11 in which is embedded steel reinforcement 12, which can be of a known and conventional type. Depending on the engineering requirements of the structure, the amount of reinforcing steel per unit of concrete may vary rather widely. For the purposes of this invention, it is assumed that the concrete structure is a mature installation, in which the body of the concrete 11 has become contaminated by chloride ion penetration, carbonation or other circumstance tending to create conditions favoring corrosion of the reinforcing steel 11.

To carry out the process of the invention, electrical connections are made to the reinforcement steel to be protected, and to a temporary distributed anode placed externally in an electrolytic mass or liquid in contact with the surface of the concrete to the treated. In the illustrated arrangement, a DC power source, designated by the letter "G" , is connected at its positive side to a distributed electrode structure 13 , arranged in electrical communication with an exposed surface of the concrete structure 10, and at its negative side to the embedded reinforcing steel. As many connecting points as desired may be established, with the objective of realizing a relatively uniformly distributed current flow between the reinforcing steel and the distributed electrode.

To advantage, the electrode structure 13 may comprise a mesh like material of suitably conductive material, such as steel wire mesh or titanium mesh, for example. In the illustrated form of the invention, the electrode structure is embedded in an electrolytic medium 14 arranged in intimate contact with the exposed surface 15 of the concrete structure 10.

In appropriate cases, when the surface 15 is upwardly facing and horizontal (or nearly so) , the electrolytic medium can be a liquid, appropriately pooled to cover the concrete surface. More preferably, the electrolytic medium is a self-adherent conductive mass, such as a sprayed-on mixture of cellulosic pulp fiber and water or other electrolyte. The fiber mass is applied in a first layer, prior to mounting the electrode structure 13, and in a second layer thereafter, to completely embed the electrode structure within the conductive mass. A self-adherent electrolytic mass is desireable in many cases, as where the exposed concrete surface is vertical or downwardly facing, for example.

Other arrangements of distributed electrode are possible, such as conductive surface coatings, foil layers placed in direct contact with the concrete surface, spongy blankets in certain cases, etc. The particular form of distributed surface electrode is not critical to the invention, as long as it functions effectively to distribute the current flow effectively over the surface area of the embedded steel reinforcement. Generally this objective is realized by distributing the current from the external distributed electrode 13 relatively uniformly over the exposed surface of the concrete structure.

In carrying out the process of the invention, a direct electric current of at least 0.1 amperes per square meter of surface area of the embedded steel reinforcement 12 is caused to flow between the reinforcement steel, which is negatively connected, and the external electrode, which is positively connected to function as an anode. The output voltage of the DC power source "G" may vary between wide limits, but it should be designed to deliver sufficient charge at the minimum current density mentioned above. In practice, it has been found convenient to use a power source "G" capable of being adjusted to between 5 and 40 Volts DC output, and with sufficient current capacity to deliver between 0.5 and 10 amperes per square meter of surface area of the embedded steel 12. The output of the power source can be monitored by suitable voltage and current meters "V" and "A" as shown.

Pursuant to the invention, the current is passed for the time necessary to give a total charge of at least about 500 ampere-hours per square meter of surface area of the embedded steel reinforcement 12. Preferably, the total charge should not exceed about 2000 ampere-hours per square meter of steel surface area, because the energy consumed is largely wasted and does not achieve a significant benefit. A total charge of as high as 10,000 ampere-hours per square meter of steel surface area can actually be detrimental, causing degradation of the concrete .

The actual time taken to achieve the desired total charge per unit of steel area will of course depend on the available DC power source and, within extremely wide limits, is not significant.

After a sufficient total charge has been passed to the embedded reinforcing steel 12, the current is switched off, the entire installation is removed, and the external conductive material, if removable, is removed. The steel will then have been given long term protection by being conditioned to become strongly passivated.

An explanation of the treatment given to the steel by the process of the present invention is as follows: The application of a current charge at a density of not less than 0.1 ampere per square meter or surface are of the reinforcing steel results in a phenomenon known as cathodic stripping. That is to say, any existing oxide or other films present on the steel surface are completely removed leaving a perfectly clean, active steel surface. At the same time, since the steel in question is very strongly charged negatively, chloride ions, if any are present in the concrete, are strongly repelled from the steel surface. This repulsion leaves the steel surface chloride free. In addition, the surrounding concrete is also rendered essentially chloride free to a distance of usually at least 10 mm from the steel. Simultaneously, the electro-chemical cathodic reactions caused by the action of the current at the steel surface lead to the production of sodium hydroxide which is produced in sufficient quantities to impregnate the pores of the concrete surrounding the steel and thus render the environment highly alkaline. These cathodic reactions are believed to be generally as follows: 02 + 2H20 + 4e" —> 4OH' 2H20 + 2e' —> H2 + 20H' Na+ + e' —> Na 2Na + 2H20 —> 2NaOH + H2 When the current is then switched off, after a suitable treatment charge has been delivered, the steel will begin to repassivate by virtue of it now being in a clean, active condition in a chloride-free, highly alkaline environment. Under these relatively ideal conditions, the steel will oxidize to produce the dense oxide film necessary to protect the steel from corrosion. This oxidation process is actually a special form of corrosion which results in the formation of the very dense protective oxide film known as the passivating film.

If desired, the formation of this film is easily followed by monitoring the electrical potential of the steel in relation to a standard reference half-cell 16, such as silver/silver oxide, lead/lead oxide, copper/copper sulphate, etc. The reference cell 16 should preferably, though not necessarily, be installed in a fixed position near the steel to be monitored, for example by grouting into a drilled hole 17 in the concrete.

A diagram can then be drawn up showing the change in potential with time, an example of which is shown in Fig. 2 of the drawings. Such a diagram will show that the passivation process, which commences as soon as the processing current is discontinued, extends over a long period of time. If the reference cell monitoring is sufficiently prolonged, it will show when the steel gains the potential commonly considered as being safe from a corrosion point of view. Indeed, if sufficiently prolonged, it can also show if the steel ever again becomes subject to corrosion, which would be indicated by the potential again passing the value associated with corrosion, but from the opposite direction.

As shown in Fig. 2, the reference potential, measured with a suitable volt meter 18, between the lead/lead oxide half cell 16 and the steel reinforcement 12, increases slowly, over a period of several months. Starting from an initial potential of about -400 millivolts, the reference potential gradually increases to about +500 millivolts (considered relatively safe, from a corrosion standpoint) , in a period of around seven weeks. After a year, the reference potential has continued to increase to a level of around +700 millivolts.

It has been found in practice that the corrosion protection imparted in this way is long lived, is robust against new penetration by chloride ions, and even, surprisingly, that the corrosion protection provided eventually spreads to areas of embedded steel in concrete outside of the treated area, but in metallic contact with the steel within the treated zone, and that this occurs even after the current has been switched off and the installations removed.

The process of the present invention, while related to the procedures described in the beforementioned Vennesland et al and Miller patents, has surprising and unexpected advantages in that, by controlling the processing in accordance with current flow in relation to surface area of the embedded steel reinforcement, extraordinary processing economies are realized.

At the same time, there is greater assurance that the protection/rehabilitation sought is effectively achieved within a targeted processing period. Thus, processing in accordance with the present invention may, in a typical case, achieve reliable results in about half the time required to achieve a chloride level which could be regarded as reasonably safe.

The process of the present invention, in processing according to the surface area of the embedded reinforcement, enables the processing time to be accurately predicted in advance, whereas controlling in accordance with remaining chloride levels of the concrete requires the periodic taking and testing of core samples from the material under treatment and cannot be predicted in advance. Moreover, by the time the testing of the core samples indicates that the chloride levels have been reduced to targeted levels, it can be expected that processing will have been carried on for a time far beyond that required to achieve ampere-hour per square meter of surface area levels known to be effective under the present invention.

As will be appreciated, concrete structures may vary widely in the amount of internal embedded reinforcement per unit of concrete. Depending upon engineering requirements, steel-to-concrete area ratios can vary between 0.2 and 2 square meters of steel surface area per square meter of concrete surface. A more typical range is between 0.3 and 1 square meter of steel surface area per square meter of concrete surface. Accordingly, it will be appreciated that controlling treating time in accordance with surface area of the reinforcing steel can lead to significantly different end results than controlling time in accordance with concrete core samples.

It should be understood, of course, that the specific form of the invention herein illustrated and described is intended to be representative only, as certain changes may be made therein without departing from the clear teachings of the disclosure. Accordingly, reference should be made to the following appended claims in determining the full scope of the invention.

Claims (5)

- 15 - 104,837/2 WHAT IS CLAIMED IS:

1. A process for electrochemical treatment of a predetermined area of reinforced concrete having steel reinforcement embedded therein, to protect the embedded steel reinforcement against corrosion, wherein an electroconductive material is applied to an exposed surface area of the concrete to form a distributed electrode, a source of DC voltage is applied to said electroconductive material, as a positive terminal, and to said embedded steel reinforcement, as a negative terminal, and wherein said reinforced concrete incorporates approximately 0.2 to 2.0 square meters of surface area of reinforcing steel per square meter of concrete surface area, characterized by: a) initially ascertaining the approximate surface area of said steel reinforcement embedded in said predetermined area of concrete; b) applying said DC voltage to impart a distributed current flow to said electroconductive material, as an anode, and to said embedded steel reinforcement, as a cathode ; c) continuing said DC voltage and said distributed current flow until at least about 500 ampere-hours of current per square meter of surface area of said embedded steel reinforcement has flowed between said terminals, and d) controlling said process as a function of ampere-hours of current flow per unit of surface area of reinforcing steel and, pursuant thereto, discontinuing said DC voltage and ending said treatment before said current flow substantially exceeds 2000 ampere-hours per square meter of surface area of said embedded steel reinforcement, without regard to residual chloride levels or residual carbonation levels at various points in said predetermined area of concrete. - 16 - 104,837/2

2. A process according to claim 1, further characterized by: said DC voltage being applied at a level to impart a distributed current flow of from 0.5 to 10 amperes per square meter of surface area of the embedded reinforcement.

3. A process according to claim 1, further characterized by: said electroconductive material including a liquid electrolyte.

4. A process according to claim 1, further characterized by: a) said electroconductive material comprising a removable self-adherent material, and b) said material is removed from the surface of said concrete after discontinuation of said treatment.

5. A process for electrochemical treatment of reinforcing steel in concrete, substantially as hereinbefore described and with reference to the accompanying drawings. for the Applicant: WOLFF, BREG AN AND GOLLER ^Υ: si n