CA2160575A1 - Method for treating reinforced concrete and/or the reinforcement thereof - Google Patents

Abstract

A process for the electrochemical treatment of reinforcing steel in concrete having embedded steel reinforcement (12) including applying and electroconductive material (14) to an exposed surface of the concrete to form a distributed electrode, and applying a DC
voltage to said electroconductive material as a positive terminal, and to said embedded steel reinforcement, as a negative terminal, and characterised by effecting passivation of the embedded steel and/or by imparting a predetermined modification to the bond strength between said embedded steel and said concrete by applying said DC voltage to the terminals so as to impart a distributed current flow between said electroconductive material as an anode and said embedded steel reinforcement as a cathode; and by continuing application of said DC
voltage and said distributed current flow in accordance with a predetermined current flow/time treatment regime which is such that the flow of current per square metre of surface area of said embedded steel reinforcement that passes between said terminals within a predetermined time period lies within predetermined limits.

Description

W094/~68 ~ 6 ~ 5 7 S PCT/GB94/00802 METHOD FOR TREATING REINFORCED CONCRETE
AND/OR THE REINFORCEMENT THEREOF

BACKGROUND AND FIELD OF THE INVENTION

Embedded steel in reinforced concrete is normally5 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 electrochemical in nature, in that sites where the passive oxide film is broken form anodes, and the surrounding areas where the film is 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 neutralise 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 W094/~68 PCT/GB94/00802 = -2-2~a~
maintenance. Other disadvantages are high cost, the extra structural loading introduced by heavy concrete overlays and the difficulty of ensuring permanent correct current distribution.

S 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 10 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 great and economically expensive practical difficulty of predicting 15 the time necessary for the 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 20 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 25 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 30 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 WOg4/~68 216 ~ 5 ~ PCT/GB94/00802 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. Further problems arise where chloride setting accelerators have been used in the originating concrete mix whereby the chloride is consequently distributed throughout the concrete mass.

In practice, it has been noted that it is difficult and economically not cost effective for many treatment 10 situations to reduce the chloride content to below some 50% of the original content. Also, in practlce, the documenting, monitoring and controlling of the chloride removal entails the taking of numerous core samples by, for example, diamond core drilling and then analysing the 15 cores for chloride content. As is well known concrete is a notoriously inhomgeneous material so that statistically significant numbers of core samples need to be taken and analysed and documented to ensure effective monitoring of the chloride removal. Furthermore, the 20 taking of a core sample leaves a hole which needs to be filled.

Furthermore, since the removal of the chloride content of concrete is never completely predictable there is always the uncertainty of the time required to achieve a desired 25 chloride level. Similar considerations apply to the realkalisaton of concrete in that core drilled samples are required for phenolphthalin testing and sodium and potassium determination.
.

It has additionally been found that in steel reinforced structures it is desirable to be able to modify the bond strength between the reinforeing steel and the set and hardened concrete since in reinforced structures it has been noted that over a period it is possible for such bond W094/~68 ~ PCTIGB94/00802 strength to depart from a satisfactory bond.

The present invention overcomes the difficulties of the above mentioned methods by be-ing 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.

SUMMARIES OF THE INVENTION

According to a first aspect of the invention there is provided a process for the electrochemical treatment of reinforcing steel in concrete having embedded steel reinforcement, including applying an electroconductive material to an exposed surface of the concrete to form a distributed electrode, and applying a DC voltage to said electroconductive material as a positive terminal, and to said embedded steel reinforcement, as a negative terminal, and characterised by effecting passivation of the embedded steel and/or by imparting a predetermined modification to the bond strength between said embedded steel and said concrete including the steps of:-(a) applying said DC voltage to the terminals so as toinpart a distributed current flow between said electroconductive material, as an anode and said embedded steel reinforcement as a cathode; and by (b) continuing application of said DC voltage and said distributed current flow in accordance with a predetermined current flow/time treatment regime which i~
such that the flow of current per square metre of surface area of said embedded steel reinforcement that passes W094/~68 5?1 6 0 S 7~ PCT/GB94/00802 between said terminals within a predetermined time period lies within predetermined limits.

In accordance with a particular aspect of the invention, the application of said DC voltage and said distributed current flow is continued until at least about lOO
ampere-hours of current, per square metre of surface area of said embedded steel reinforcement, has passed between said terminals; and said treatment is discontinued before said current flow substantially exceeds 3000 ampere hours, per square metre of surface area of the embedded steel reinforcement, regardless of residual chloride levels and residual alkali levels in the concrete.

Thus, the present invention is based upon the discovery and recognition that the electrochemical treatment of concrete does not have to be controlled, for example, as a function of the chloride content or as a function of the degree of carbonation. Rather, the invention is based upon the recognition that the electrochemical processing of concrete is optimally controlled as a function of the surface area of the 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 a close approximation. Pursuant to the invention, eletrochemical treatment can be set up more or less in a known manner as is disclosed by the Vennesland et al,. U.S. Patent No 4,032 ,803 or by 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 r~lation to the total surface area of the embedded reinforcing steel. The process is continued until a W094/~68 PCT/GB94/00802 2~&~ 6- ~

minimum of lO0 ampere-hours of current flow per square metre of surface area of the embedded steel has been realised. The process can be discontinued at that stage (and preferably is discontinued before the current flow significantly exceeds 300~ ampere hours per square metre of surface area of the embedded steel), 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 hitherto required.

For a more complete understanding of the invention, reference should be made to the following detailed description of a preferred embodiment of the invention and to the accompanying drawings:

Figure l is a schematic illustration of reinforced concrete set up for treatment in accordance with a first aspect the present invention;

Figure 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 first aspect of the invention;

Figure 3 is a simplified cross sectional illustration of a concrete structure illustrating the application of a ~0 second aspect of the invention; and W094/~68 7 21 G ~ 5 7~ PCT/GB94/00802 Figure 4 is a representative graph illustrating the relationships between treatment time according to the invention and its effect upon the bond strength between concrete and steel embedded therein.
.

DESCRIPTION OF PREFERRED EMBODIMENTS:

Referring now to the drawing lO represents a concrete, comprised of set and hardened concrete ll in which is embedded steel reinforcement 12, which can be of a known and conventional type. Depending on the engineering requirements for 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 ll has become contaminated by chloride ions, carbonation or other circumstances tending to create conditions favouring corrosion of the steel reinforcement l2.

To carry out the process of the invention, electrical connections are made to the reinforcement steel to be protected, and to a temporary anode placed externally in an electrolytic mass or liquid in contact with the surface of the concrete to be treated. In the illustated arrangement a, D.C. 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 lO, and at its negative side to the embedded reinforcing steel. As many connecting points, as desired, may be established with the objective of realising a relatively uniformly distributed current flow between the reinforcing steel and the distributed electrode.

W094/~68 PCT/GB94/00802 p~G~5 -8- ~

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. As is shown in Figure 1 the electrode structure is embedded in an electrolytic medium 14 arranged in intimate contact with the exposed surface 15 of the concrete structure lO.

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 10 concrete surface. More preferably, the electrolytic medium is a self adherent conductive mass, such as a sprayed on mixture of cellulosic pulp fibre and water or other electrolyte. The fibre mass is applied in a first layer, prior to mounting the electrode structure 13 and in 15 a second layer thereafter to embed completely the electrode structure within the conductive mass. It will be understood that other suitable materials could be used to form the requisite electrolytic mass.

A self-adherent electrolytic mass is desirable in many 20 cases, as where the exposed concrete is, for example, vertical or downwardly facing or where the surface of the concrete is convoluted or very rough.

Other arrangements of the distributed electrode are possible, such as conductive surface coatings, foil layers 25 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 30 area of the embedded steel reinforcement. Generally this objective is realised by distributing the current from the externsl distributed electrode 13 relatively uniformly over the exposed surface of the concrete structure.

WO 94124068 ~1 6 o~ 7~ PcT~GB94/00802 In carrying out the process of the invention, a direct electric current of at least 0.1 amperes per square metre 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 metre of surface area of the embedded steel. The output of the power source can be monitored by suitable voltage and current meters "V" and "A" as shown.

Pursuant to this aspect of the invention, the current is passed for the time necessary to give a total charge of at least about 100 amperes per square metre of surface area 20 of the embedded steel reinforcement 12. Preferably the total charge should not exceed about 3000 ampere-hours per square metre of steel surface area, because the energy consumed is largely wasted and does not achieve significant benefit. If the treatment of the invention is 25 required only to deal with chloride removal the total charge can be in the order of 1000 amperehours per square metre of steel surface area. Whilst higher charge levels can be used it is useful to note that a total charge of as high as 10,000 ampere-hours per square metre of steel 30 surface area could actually be detrimental, causing degredation of the concrete.

The actual time taken to achieve the desired total charge per unit of steel will of course depend on the available W094/~68 PCT/GB94/00802 2~605~ -10-DC power source and, within extremely wide limits, is not significant.

After a sufficient total ch~rge 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 this aspect of the invention is as follows:

The application of a current charge at a density of not less than O.l amperes per square metre of surface area 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 lOmm. from the steel. Simultaneously, the electrochemical cathodic reactions caused by the 25 action of the current at the steel surface lead to the production of for instance 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.

30 These cathodic reactions are believed to be generally as follows:

W094/~68 ~ 75 PCT/GB94/00802 (la) 2 + 2H2O + 4e --~40H

(lb) 2H2O + 2e~ --~H2 + 20H

(2a) Na~ ~ e~ --~Na (2b) 2Na + 2H2O --~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 oxidise 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 to 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 Figure 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 W094/~68 PCT/GB94/00802 ~ 12-corrosion point of view. Indeed, if sufficiently prolonged, it can also show whether or not the steel ever again becomes subject to corrosion, by noting whether the potential again passes the value associated with corrosion, but from the opposite direction.

As shown in Figure 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 0 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 should be noted that depending upon the nature of the concrete involved other potential values could be applicable.

It has been found in practice that the corrosion protection imparted 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 the embedded steel in concrete outside the treated area, but in metallic contact with the steel within the treated zone, and tha~ this occurs even after the current has been switched off and the installation removed.

The process of the invention, while related to the procedures described in the before mentioned Vennesland et al and Miller patents, has surprisingly 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 realised. At the same time, W094/24068 -13- 21 6 ~ ~ 7 ~ PcTlGB94loo8o2 there is greater assurance that the desired protection/rehabilitation 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, involving processing - of a concrete structure according to the surface area of the embedded reinforcement, enables the processing time to be accurately predicted in advance, whereas controlling in accordance with the remaining chloride levels 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 indicates that 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 the ampere-hour per square metre of surface 2~ levels known to be effective under the proposals of 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 ratios vary between 0.2 and 2 square metres of steel surface area per square metre of concrete surface. A more typical range is between 0.3 and 1 square metre of steel surface area per square metre of concrete surface. Accordingly, it will be appreciated that controlling treatment time in accordance with the surface area of the reinforcing steel can lead to significantly different end results than those achieved when controlling time in accordance with concrete core samples.

W094/~68 ~ PCT/GB94/00802 ~6~ -14- ~

This situation is further enhanced by the fact that, as has been found in practice, it is not necessary to treat the whole of a concrete structure subject to embedded reinforcement corrosion in order to achieve an effective treatment i.e., passivation of the reinforcing steel. It has been found that the above mentioned lasting protection can be achieved by subjecting only a part of the structure to a predetermined current/time treatment regime.

In other words, satisfactory lasting protection can be gained merely by subjecting a selected part of the concrete structure to a said current flow/time treatmemt regime by passing the direct current through the selected part only.

In a practical instance, should the corrosion protection given by a previous corrosion protection treatment by chloride extraction or realkalisation breakdown, for example, by the egress of new chlorides or by the washing out of alkali, the required corrosion protection can be achieved by treating the defective part of the structure only.

A further feature of the process of the invention arises in practical installations in which no part of the structure is readily available for sufficient time for adequate treatment to be effected (for example, as would 25 be the case of a bridge structure having to be closed to traffic for the duration of a treatment lasting several weeks). In such situations, in practicing the process of the invention it is possible to construct a prolongation to a part of a structure to be treated by, 30 for example, adding to a reinforced structure an additional reinforced concrete part sufficiently in physical contact which ensures that an adequate ion path W094/~68 15 ~ 575 PCT/GB94/00802 is provided in the concrete across the join between the concrete structure and the additional part and such that effective electical connection is made with the embedded steel to enable the required electrochemical treatment of the actual concrete structure to take place.
..
As will be understood this aspect of the invention enables a main structure to be given a lasting corrosion protection by treatment of a part of the actual structure it is required to treat or by treatment of an extension to the actual structure it is required to treat.

As has been indicated above the amount of current flow necessary to give a lasting protection is a function of the area of a structure available for treatment, and that it is desirable to restrict the maximum current to a level that can safely be used without introducing any undesirable side effects. Thus in general it has been found advantageous, for the purposes of chloride removal or realkalisation, to maintain current densities below 5 ampere/hours per square metre of concrete surface. It has been noted that the use of e~cessively higher current densities can cause detrimental effects. However, in the case of any concrete part added to a structure for the purposes above mentioned higher densities may be used.

This makes it possible to treat large areas of a structure by the passage of higher current densities in the reinforced prolongation part, i.e., outside the actual main structure.

It has been noted that, in practice, in order to provide a lasting protection i.e., five to ten years in a particular application of the concepts of the invention, the steel reinforcement needs to be charged at a level corresponding to lOO0 ampere/hours per square metre thereof. This -W094/~68 6~$ 16- PCT/GB94/00802 to lO00 ampere/hours per square metre thereof. This protection can for example, in practice, be achieved by treating the whole area at one ampere/hour per square metre for lO00 hours, by treating one half of the area at 2 ampere/hours per square metre for 500 hours, by treating one tenth of the area at lO ampere/hours per square metre for lO0 hours, or by any other proportional conbination.
It is to be noted that the above numerical data figures are for guidance only since precise figures will depend upon the geometry of the structure and other parameters such as the chemical/physical nature of the concrete.

An additional aspect of the present invention relates to a realisation that the application of the DC voltsge between the embedded steel and the concrete can be used to modify and establish targeted bond strengths between the concrete and embedded steel. That is to say the process for the electrochemical treatment of hardened concrete can be used in order to modify (i.e., by increasing or decreasing) the bond strength between hardened concrete and internally embedded steel, particularly reinforcing bars, pretensioning or post tensioning rods or cables.
Heretofore, this has been impossible, since there has been no known procedure for controllably changing the steel-to-concrete bond, in situ, in hardened concrete.

An additional aspect of the invention involves the modification of the steel-to-concrete interface in a hardened concrete structure to enhance the seal at such interface. Frequently, the interface seal between embedded steel reinforcing or tensioning elements is less than perfect, due to accumulation of bleeding water at the steel surface during the initial hardening of the concrete, or possibly due to insufficient compaction of W094/~68 -17_ 2 1 6 0 5 7 5 PCT/GB94/00802 the concrete when initially poured. Imperfections in the seal at the steel-to-concrete interfaces can result in seepages, in structures exposed to water pressure, or a possible carbonation of the concrete surfaces adjacent to the steel, with consequent corrosion of the steel.

The present aspect of the invention is based partly upon the discovery that, during the electrochemical treatment of concrete for the reasons so far discussed, by utilising the internally embedded steel as a cathode, and a 10 distributed electrode structure structure spaced therefrom, typically at an exposed surface of the concrete as an anode, a marked change occurs in the bond between the embedded steel and the surrounding concrete, as a function of the electrical charge applied. During an 15 initial phase of the treatment, there is a progressive and significant reduction in the bond strength to a level far below the initial bond strength. This is followed, with continued treatment, by a progressive and significant increase in bond strength. It has been observed that this 2a variation in bond strength is both predictable and repeatable for given types of concrete. Accordingly, by establishing a simple database of relationships between a given treatment time and its effect upon the steel-to-concrete bond strength, it becomes possible 25 predictably to modify such bond strength in an existing structure.

In the case of pre-tensioned or post-tensioned concrete structures, for example, it may be desirable to decrease the bond strength at the steel-to-concrete interface.
30 This would tend better to accommodate flexing of the compressed concrete structural element. With static steel reinforcing bars, on the other hand, it may be desirable to effect an increased bond at the steel-to-concrete interface.

W094/~68 PCT/GB94/00802 This aspect of the invention is not essentially a rehabilitive process as has been previously discussed above, but is to be considered as being directed to controlling and modifying - the bond at the steel-to-concrete interface.

In this aspect of the process of the present invention, treatment conditions and controlling parameters are different than for those involved in other aspects in the treatment of a concrete structure.

10 For a better understanding of the invention of this aspect of the invention reference should be made to the following to Figures 3 and 4 of the accompanying drawings Referring now to Figure 3, the bonding at the steel-to-concrete interface is modified by passing an 15 electrical current between the embedded steel and a distributed electrode associated with the concrete, at a location spaced from the embedded steel. Figure 3 shows a typical and advantageous arrangement for the accomplishment of that objective. In Figure 3, the 20 reference numeral lO designates a reinforced (or pre-tensioned of post-tensioned) concrete structure. In the illustration, a concrete body is provided with a plurality of reinforcing bars 12, which are embedded in and surrounded by the concrete.

25 A source "G" of DC voltage is connected at its negative side to the embedded steel elements 12 and at its negative side to a distributed electrode element 13, which may be in the form of a conductive wire mesh, for example, of steel or titanium. In the illustrated system, the 30 electrode element 13 is embedded in an electrolytic mass 14, which advantangously may be a cellulosic pulp fibre, WOg4/~68 21 6 0 5 7 5 PCT/GB94/00802 for example, maintained moist with water or electolytic solution. Where the cellulosic pulp fibre is employed, it typically is sprayed onto the outer surface 15 of the concrete 11 in two layers. The fibrous material is self-adherent to the surface of the concrete, and thus may be applied to the vertical or even downwardly facing surfaces. After applying the first layer, the mesh electrode 13 is installed, and a second layer of the fibre is applied over the top of the electrode substantially as shown in Figure 1, The particular form of the distributed electrode is not significant to the invention. Where the character and orientation of the concrete admits, the electrode 13 may be submerged in a pool of liquid, or embedded in a wet spongy mass or blanket, for example. Likewise, where appropriate, the surface of the concrete may be coated with a conductive layer (or placed in contact with a conductive foil). The principal requirement, for the purposes of the present invention, is to provide an area-distributed electrode arrangement, to accommodate a distributed flow of electricity between the internally embedded steel elements 12 and the opposite electrode.

The operating capacity of the voltage source "~" is not critical. Practical considerations, however, suggest 25 that DC voltage may be made available at from 5 to about volts DC, preferably adjustable. 50 volts is a ~ convenient upper limit for safety purposes. The system desirably has a sufficient current capacity to deliver between 0.5 and 10 amps of current, per square metre of 30 surface area of embedded steel in the area being processed.

With reference to Figure 4 of the drawings, there is shown a typical curve of values of steel-to-concrete bond W094/~68 ~6~ 20- PCT/GB94/0080 strength, in MPa (Megapascals) in relation to the total electrical charge applied to the embedded steel, in terms of ampere-hours per square metre of surface area of the embedded steel. In the',illustration of Figure 4, the solid line represents an ,average of values for a concrete of typical composition. The upper and lower dotted lines represent typical deviations from the average values represented by the solid line.

As will be evident in Figure 4, during the first stages of 10 processing in accordance with the present invention, and up to a point where between 4000 and 5000 ampere hours per square metre of steel surface area have been caused to flow, the bond strength between the embedded steel and the ~urrounding concrete progressively diminishes. In the 15 illustration, the starting bond strength is approximately 1.8 MPa, and this progressively reduces to a value of around 0.6-0.7 MPa, after a current flow of around 3000 ampere hours per square metre of steel surface area.

Upon continued flow of current between the embedded steel 20 and the distributed anode, the bond strength between the embedded steel and the surrounding concrete begins to increase. With continued current flow, the bond strength increases dramatically above initial levels finally reaching a maximum limit. In the data illustrated in 25 Figure 4, maximum bond strength is reached at a level of about 5.7 MPa, after current flow of approximately 12,000 ampere hours per square metre of surface area of embedded steel.

After reaching its maximum values, bond s~rength again 30 begins to decrease with continued current flow, although it ultimately levels off and becomes relatively stable at current flow in the range 14000-lS000 ampere hours per square metre of surface area. Normally, there would be no -~094/~68 ~ 6 ~ ~ ~ 5 PCT/GB94/00802 reason to carry the process beyond the point of maximum bond strength. Indeed, it may be detrimental to do so.
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The data reflected in Figure 4 of the drawings, represents _ a smoothed-out curve based upon actual data readings from a concrete of average quality. Similar databases can be developed for any sp`ecific concrete mixture, although the curve of Figure 4 is suitable for most practical cases.

In the course of treatment in accordance with this aspect of the invention, it is observed that, when treatment has 10 continued to the point where bond strength has increased above initial values, the interstices of the concrete, at the steel-to-concrete interface and immediately adjacent thereto, have been impregnated with substances produced by the electrochemical reactions at the steel surface. These 15 are thought to be mixtures of various compounds, including calcium hydroxide and calcium carbonate. This impregnation with reaction compounds, renders the interface zone impervious and sealed, for all practical purposes.

20 The process of the invention achieves remarkable and unexpected results in enabling for the first time, the in situ modification of steel-to-concrete bond strength in a hardened concrete structure. Depending upon the requirements of a particular installation, the bond strength may be controllably decreased, as may be desired in installations utilising pre-tensioned or post tensioned tendons, or increased, as in the case of standard static reinforcing bars embedded in a typical concrete structure.

A database of values for a typical concrete composition is 30 easily produced and can serve acceptably for most types of concrete. For particularly critical structures and/or for unique concrete formulations, a relatively simple set wo 94,24068 ~51~ -22- PCT/GB94/00802 of tests can be performed to establish a specific database of values for a specific composition of concrete. These values can then be followed in controlling the process as applied to a particular structure utilising the special composition.

In addition to providing for precise modification and control of steel-to-concrete bond strength, the process of the invention can also be utilised to seal effectively the steel-to-concrete interface against the ingress of water 10 and atmosphere. This is the result of the precipitation of reaction products in the interstices of the concrete at and lmmediately surrounding the steel-to-concrete interface, which makes the concrete in this area relatively impenatrable to external liquids and gases.

15 The process of the invention is simple and economical to apply, and utilises known technology and known equipment.
In a typical case, the external electrode means can be installed on an exterior surface of the structure and then washed away or otherwise removed upon completion of the 20 procedure.

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 25 of the disclosure. Accordingly reference should be made to the following appended claims in determinng the full scope of the invention.

Claims (18)

1. A process for the electrochemical treatment of reinforcing steel in concrete having embedded steel reinforcement including applying an electroconductive material to an exposed surface of the concrete to form a distributed electrode, and applying a DC voltage to said electroconductive material as a positive terminal, and to said embedded steel reinforcement, as a negative terminal, and characterised by effecting passivation of the embedded steel and/or by imparting a predetermined modification to the bond strength between said embedded steel and said concrete including the steps of:-(a) applying said DC voltage to the terminals so as to impart a distributed current flow between said electroconductive material, as an anode and said embedded steel reinforcement as a cathode; and by (b) continuing application of said DC voltage and said distributed current flow in accordance with a predetermined current flow/time treatment regime which is such that the flow of current per square metre of surface area of said embedded steel reinforcement that passes between said terminals within a predetermined time period lies within predetermined limits.

2. A process as claimed in claim 1, and characterised in that the predetermined current flow/time regime is such as to effect passivation of the embedded steel.

3. A process as claimed in claim 1, and characterised in that predetermined current flow/time regime is such as to impart a predetermined modification in the bond strength between said embedded steel and said concrete.

4. A process as claimed in claim 1, and characterised in that predetermined current flow/time regime is such as to effect a predetermined relationship between passivation of the embedded steel and a predetermined modification to the in the bond strength between said embedded steel and the concrete to said concrete

5. A process according to claim 1, and further characterised by said current/time treatment regime requiring discontinuing said treatment before said current flow substantially exceeds 3000 ampere hours per square metre of surface area of the embedded steel reinforcement, regardless of residual chloride levels and residual carbonation levels in the concrete.

6. A process according to claim any preceding claim and further characterised by said current/time regime requiring said DC voltage to be applied at a level to impart a distributed current flow of from 0.5 amperes per square metre of surface area of the embedded reinforcement.

7. A process according to any preceding claim and characterised by forming said electrolytic material as a liquid electrolyte.

8. A process according to any of claims 1 to 6, and characterised by forming said electrolyte as a removable self-adherent material, and removing the electrolytic material from the surface of the concrete after discontinuation of said treatment.

9. A process according to any one of the precedding claims, and characterised by applying the DC voltage between selected part or parts of a concrete structure under such conditions as to effectively treat the the part or parts and other parts or the whole of the structure.

10. A process as claimed in claim 6, and characterised by; physically connecting an additional reinforced concrete structure to the structure to be treated, the I additional structure having associated therewith a distributed external electrode; and applying the DC
voltage to the additional structure.

11. A process for the electrochemical treatment of a steel reinforced concrete structure by modifying the bond i between a body of set and cured concrete and internally embedded steel which comprises providing a source of DC
voltage, connecting said internally embedded steel to a negative termainal of said voltage source, forming as distributed electrode means in association with said concrete body, and connecting said distributed electrode means to a positive terminal of said voltage source, characterised by (a) establishing, if necessary, and providing a data base, applicable to said concrete, indicating progressive relationships between steel-to-concrete bond strengths and total electrical flow between said steel and said distributed electrode means, per unit of embedded steel, (b) causing said voltage source to effect a current flow between said embedded steel and said distributed electrode means, and (c) terminating said process when the total charge per unit of steel is such, as calculated from said data base, to impart a predetermined modification in the bond strength of said embedded steel to said concrete.

12. A process as claimed in claim11, and further characterised by said data base providing progressive relationships between the total current charge per unit of surface area of the embedded steel and steel-to-concrete bond strength.

13. A process as claimed in claim 11 or 12, and characterised by terminating said process at a point at which the total electrical charge provided to said embedded steel is such as to effect a reduction in bond strength between said steel and said concrete.

14 A process as claimed in claim 11, 12 or 13, and further characterised by terminating said process at a point at which the total electrical charge provided to said embedded steel is such as to effect an increase of bond strength between said steel and said concrete.

15. A process as claimed in claim 11, 12, 13 or 14 and further characterised by said voltage source providing from about 5 to about 50 volts DC with sufficient capacity to deliver from about 0.5 to about 10 amperes per square metre of surface area of the embedded steel under treatment.

16. A process as claimed in any one of the preceding claims 9 to 12, and characterised in that when said embedded steel comprises tension elements for maintaining surrounding concrete under compression the process is controlled to reduce bond strength between said tension elements and said concrete.

17. A process as claimed in any one of claims 9 to 12, and characterised in that when said embedded steel comprises untensioned reinforcing elements the process is controlled to increase bond strength between the untensioned eleents and said concrete.

18. A process as claimed in any preceding claim and further characterised by said process being continued sufficiently to effect significant sealing of the steel-to-concrete interface by filling of interstices of the surrounding concrete with reaction products of the electrochemical treatment.