Activating Matrix For Cathodic Protection

Abstract

The galvanic cathodic protection of reinforced concrete structures such as bridges, buildings, parking structures, piers, and wharves, is enhanced by the use of an inert water absorbent solid. The absorbent solid and chemicals are mixed with a cementitious binder to form an activating matrix. This matrix surrounds a sacrificial metal anode such as zinc, or aluminum or their alloys. The metal anode is electrically connected to the ferrous reinforcing member by a metallic conductor. The water absorbent solid may be a clay such as bentonite or a hydrated mineral such as vermiculite. It is preferably in the form of discrete particles dispersed throughout the binder. The inclusion of the absorbent solid in the activating matrix serves to increase the protective current, thereby reducing corrosion of the reinforcing components of the concrete structure.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] This invention generally relates to the field of galvanic cathodic protection of steel embedded in concrete structures, and is particularly concerned with the performance of embedded sacrificial anodes, such as zinc, aluminum, and alloys thereof.

[0003] 2. Background Art

[0004] The problems associated with corrosion-induced deterioration of reinforced concrete structures are now well understood. Steel reinforcement has generally performed well over the years in concrete structures such as bridges, buildings, parking structures, piers, and wharves, since the alkaline environment of concrete causes the surface of the steel to "passivate" such that it does not corrode. Unfortunately, since concrete is inherently somewhat porous, exposure to salt over a number of years results in the concrete becoming contaminated with chloride ions. Salt is commonly introduced in the form of seawater, set accelerators, or deicing salt.

[0005] When the chloride reaches the level of the reinforcing steel, and exceeds a certain threshold level for contamination, it destroys the ability of the concrete to keep the steel in a passive, non-corrosive state. It has been determined that a chloride concentration of 0.6 Kg per cubic meter of concrete is a critical value above which corrosion of the steel can occur. The products of corrosion of the steel occupy 2.5 to 4 times the volume of the original steel, and this expansion exerts a tremendous tensile force on the surrounding concrete. When this tensile force exceeds the tensile strength of the concrete, cracking and delaminations develop. With continued corrosion, freezing and thawing, and traffic pounding, the utility or integrity of the structure is finally compromised and repair or replacement becomes necessary. Reinforced concrete structures continue to deteriorate at an alarming rate. In a recent report to Congress, the Federal Highway Administration reported that of the nation's 577,000 bridges, 266,000 (39% of the total) were classified as deficient, and that 134,000 (23% of the total) were classified as structurally deficient. Structurally deficient bridges are those that are closed, restricted to light vehicles only, or that require immediate rehabilitation to remain open. The damage on most of these bridges is caused by corrosion. The United States Department of Transportation has estimated that $90.9 billion will be needed to replace or repair the damage on these existing bridges.

[0006] Many solutions to this problem have been proposed, including higher quality concrete, improved construction practices, increased concrete cover over the reinforcing steel, specialty concretes, corrosion inhibiting admixtures, surface sealers, and electrochemical techniques, such as cathodic protection and chloride removal. Of these techniques, only cathodic protection is capable of controlling corrosion of reinforcing steel over an extended period of time without complete removal of the salt-contaminated concrete.

[0007] Cathodic protection reduces or eliminates corrosion of the steel by making it the cathode of an electrochemical cell. This results in cathodic polarization of the steel, which tends to suppress oxidation reactions (such as corrosion) in favor of reduction reactions (such as oxygen reduction). Cathodic protection was first applied to a reinforced concrete bridge deck in 1973. Since then, understanding and techniques have improved, and today cathodic protection has been applied to over one million square meters of concrete structure worldwide. Anodes, in particular, have been the subject of much attention, and several different types of anodes have evolved for specific circumstances and different types of structures.

[0008] The most commonly used type of cathodic protection system is impressed current cathodic protection (ICCP), which is characterized by the use of inert anodes, such as carbon, titanium suboxide, and most commonly, catalyzed titanium. ICCP also requires the use of an auxiliary power supply to cause protective current to flow through the circuit, along with attendant wiring and electrical conduit. This type of cathodic protection has been generally successful, but problems have been reported with reliability and maintenance of the power supply. Problems have also been reported related to the durability of the anode itself, as well as the concrete immediately adjacent to the anode, since one of the products of reaction at an inert anode is acid (H.sup.+). Acid attacks the integrity of the cement paste phase within concrete. Finally, the complexity of ICCP systems requires additional monitoring and maintenance, which results in additional operating costs.

[0009] A second type of cathodic protection, known as galvanic cathodic protection (GCP), offers certain important advantages over ICCP. GCP uses sacrificial anodes, such as zinc and aluminum, and alloys thereof, which have inherently negative electrochemical potentials. When such anodes are used, protective current flows in the circuit without need for an external power supply since the reactions that occur are thermodynamically favored. GCP therefore requires no rectifier, external wiring or conduit. This simplicity increases reliability and reduces initial cost, as well as costs associated with long term monitoring and maintenance. Also, the use of GCP to protect high-strength prestressed steel from corrosion is considered inherently safe from the standpoint of hydrogen embrittlement. Recognizing these advantages, the Federal Highway Administration issued a Broad Agency Announcement (BAA) in 1992 for the study and development of sacrificial anode technology applied to reinforced and prestressed bridge components. As a result of this announcement and the technology that was developed because of this BAA, interest in GCP has greatly increased over the past few years.

[0010] In PCT Published Application WO94/29496 and in U.S. Pat. No. 6,022,469 by Page, a method of galvanic cathodic protection is disclosed wherein a zinc or zinc alloy anode is surrounded by a mortar containing an agent to maintain a high pH in the mortar surrounding the anode. This agent, specifically lithium hydroxide (LiOH), serves to prevent passivation of the zinc anode and maintain the anode in an electrochemically active state. In this method, the zinc anode is electrically attached to the reinforcing steel causing protective current to flow and mitigating subsequent corrosion of the steel.

[0011] In U.S. Pat. No. 5,292,411 Bartholomew et al discloses a method of patching an eroded area of concrete comprising the use of a metal anode having an ionically conductive hydrogel attached to at least a portion of the anode. In this patent it is taught that the anode and the hydrogel are flexible and are conformed within the eroded area, the anode being in elongated foil form.

[0012] In U.S. Pat. No. 6,471,851 based on application Ser. No. 08/839,292 filed on Apr. 17, 1997 by Bennett, the use of deliquescent or hygroscopic chemicals, collectively called "humectants" is disclosed to maintain a galvanic sprayed zinc anode in an active state and delivering protective current. In U.S. Pat. No. 6,033,553, two of the most effective such chemicals, namely lithium nitrate and lithium bromide (LiNO.sub.3 and LiBr), are disclosed to enhance the performance of sprayed zinc anodes. And in U.S. Pat. No. 6,217,742 B1, issued Apr. 17, 2001, Bennett discloses the use of LiNO.sub.3 and LiBr to enhance the performance of embedded discrete anodes. And finally, in U.S. Pat. No. 6,165,346, issued Dec. 26, 2000, Whitmore broadly claims the use of deliquescent chemicals to enhance the performance of the apparatus disclosed by Page in U.S. Pat. No. 6,022,469.

[0013] In U.S. Pat. No. 7,160,433B2 issued Jan. 9, 2007, a method of cathodic protection of reinforcing steel is disclosed comprising a sacrificial anode embedded in an ionically conductive compressible matrix designed to absorb the expansive products of corrosion of the sacrificial anode metal.

[0014] In U.S. Pat. No. 6,572,760 B2, issued Jun. 3, 2003, Whitmore discloses the use of a deliquescent material bound into a porous anode body, which acts to maintain the anode electrochemically active, while providing room for the expansive products of corrosion. The same patent discloses several mechanical means of making electrical connection to the reinforcing steel within a hole drilled into the concrete covering material. Many of these means involve driven pins, impact tools, and other specialized techniques. These techniques are all relatively complex and difficult to perform.

[0015] Finally, in U.S. Pat. No. 6,193,857, issued Feb. 27, 2001, Davison, et al describes an anode assembly comprising a block of anode material cast around an elongated electrical connector (wire). Other claims disclose making contact between the elongated connector and the reinforcing steel by winding the connector around the reinforcing steel and twisting the ends of the connector together using a twisting tool.

[0016] The anodes described above and the means of connection disclosed have become the basis for commercial products designed to extend the life of patch repair and to cathodically protect reinforced concrete structures from corrosion. But some embodiments, such as the use of high pH to maintain the anode in an electrochemically active state as described by Page, result in protective current that is small and often inadequate to mitigate corrosion. Use of the chemicals disclosed by Bennett, such as lithium nitrate and lithium bromide, result in a higher current, but even this current is sometimes inadequate in cases of high chloride contamination and the presence of strong corrosion of the reinforcing steel.

[0017] It would be of great benefit to increase the protective current higher than was previously possible using the prior art as described in the patent literature above.

DISCLOSURE OF THE INVENTION

[0018] The present invention relates to an apparatus for cathodic protection of reinforced concrete, and more particularly, to an apparatus for improving the performance and service life of embedded anodes prepared from sacrificial metals such as zinc, aluminum, and alloys thereof. The present invention more specifically relates to an apparatus for cathodic protection wherein the performance of the sacrificial anode is enhanced by the use of a mixture of chemicals and an inert water absorbent solid in a cementitious grout, thereby forming an activating matrix surrounding the sacrificial anode.

[0019] The chemical component of the matrix may be any one, or a combination of, the chemicals previously disclosed in the prior art. Particularly advantageous are lithium nitrate, lithium bromide and their mixtures or any one of several chemicals intended to raise the pH of the matrix to a value greater than about 13.5.

[0020] The inert water absorbent solid may be any solid capable of readily absorbing and retaining moisture. Particularly advantageous are clays, such as bentonite, and hydrated minerals such as vermiculite. The inert water absorbent solid is preferably in the form of small discrete particles dispersed, together with the chemicals, in a cementitious binder. The binder, chemicals, and water absorbent solid together form a continuous matrix that substantially surrounds the sacrificial anode.

[0021] The apparatus for cathodic protection also incorporates an elongated metallic conductor that serves to electrically connect the sacrificial anode to the reinforcing steel, or other metal to be protected, thereby providing an electrical path for the flow of protective current. The elongated metallic conductor may be attached to the reinforcing steel by one of several methods, such as wrapping, twisting, resistance welding, tig welding, mechanical compression and the like.

[0022] The present invention also relates to a method of cathodic protection of reinforced concrete, and more particularly, to a method of improving the performance and service life of embedded anodes intended to apply cathodic protection to reinforcing steel and other metals embedded in concrete.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Further features of the present invention will become apparent to those skilled in the art to which the present invention relates from reading the following specification with references to the accompanying drawings, in which:

[0024] FIG. 1 illustrates the results of the tests described in Example 1. This Figure shows protective current delivered by sacrificial anodes operated in galvanic mode as a function of time. The data labeled "Control" was obtained by a standard control test block, as described in Example 1, and is generally considered good performance. The data labeled "50% Bentonite" was obtained from a test block in which 50% by weight of the cement in the matrix surrounding the anode was substituted with bentonite, one of a group of highly absorptive clays.

[0025] FIG. 2 illustrates the results of the tests described in Example 2. This figure shows cell voltage of test blocks operated in accelerated mode using an impressed current of 5 mA as a function of time. The data labeled "Control" was obtained by a standard control test block, as described in Example 1, and is again considered good performance. The data labeled "8.6% Vermiculite" was obtained from a test block in which 8.6% by weight of the matrix surrounding the anode consisted of vermiculite, a phyllosilicate mineral resembling mica.

MODES FOR CARRYING OUT THE INVENTION

[0026] The present invention relates broadly to all reinforced concrete structures with which cathodic protection systems are useful. Generally, the reinforcing metal in a reinforced concrete structure is carbon steel. However, other ferrous-based metals can also be used.

[0027] The activating matrix of the present invention relates to galvanic cathodic protection (GCP), that is, cathodic protection utilizing anodes consisting of sacrificial metals such as zinc, aluminum, magnesium, or alloys thereof. Of these materials, zinc or zinc alloys are preferred for reasons of efficiency, longevity, driving potential and cost. Sacrificial metals are capable of providing protective current without the use of ancillary power supplies, since the reactions that take place during their use are thermodynamically favored. The sacrificial metal anodes may be of various geometric configurations, such as flat plate, expanded or perforated sheet, or cast shapes of various designs. It is generally beneficial for the anodes to have a high anode surface area, that is, a high area of anode-concrete interface. Preferably, the anode surface area should be from 3-6 times the superficial surface area, whereas the anode surface area for plain flat sheet is 2 times the superficial surface area (counting both sides of the sheet).

[0028] Since sacrificial metal anodes tend to passivate in the alkaline environment of concrete, it is necessary to provide an activating agent to maintain the anode in an electrochemically active and conductive state. In U.S. patent application Ser. No. 08/839,292 filed on Apr. 17, 1997 by Bennett, the use of deliquescent or hygroscopic chemicals, collectively called "humectants" is disclosed to maintain a galvanic sprayed zinc anode in an active state and delivering protective current. In U.S. Pat. No. 6,033,553, two of the most effective such chemicals, namely lithium nitrate and lithium bromide (LiNO.sub.3 and LiBr), are disclosed to enhance the performance of sprayed zinc anodes. And in U.S. Pat. No. 6,217,742 B1, issued Apr. 17, 2001, Bennett discloses the use of LiNO.sub.3 and LiBr to enhance the performance of embedded discrete anodes. It has been found that a mixture of lithium nitrate and lithium bromide is particularly effective to enhance the performance of zinc anodes.

[0029] In the present invention, the performance of the sacrificial anode is enhanced by incorporating an inert water absorbent solid into a cementitious grout surrounding the sacrificial anode. The inert water absorbent solid may be one of several materials that are not chemically reactive in this environment, and readily absorb water when exposed to liquid water. They are not deliquescent materials, as claimed in U.S. Pat. No. 6,165,346 by Whitmore, deliquescent materials being those materials that liquefy due to the absorption of moisture from the air at normal conditions of relative humidity. Nor are they typically hygroscopic materials, hygroscopic materials being those materials that readily absorb, become coated with, and retain moisture from the air. But the water absorbent solid materials of the present invention retain moisture when directly wetted by liquid water. The use of such materials in this context has not been heretofore reported or contemplated.

[0030] Two such inert water absorbent solid materials known to be effective in this regard are bentonite and vermiculite. Bentonite is a rock comprised essentially of mixtures of montmorillonite and beidellite with the former predominating. Bentonite is also a trade name given to highly absorptive clays or drilling muds. Vermiculite is a phyllosilicate mineral resembling mica. Vermiculite mined in the US is a hydrated phlogopite or biotite mica that expands many times its volume when heated, a process called exfoliation. Vermiculite is commonly used as packing material, especially for hazardous liquids.

[0031] Bentonite may be used in the present invention by substituting about 10% to 50% of the cement in the mortar surrounding the sacrificial anode with bentonite. Substitution in this manner has been found to result in higher performance in the form of a greater level of protective current delivered to the anode. Performance has been found to increase with the increase of percentage of bentonite. Substitutions higher than 50% of the cement with bentonite would still be effective, but the cementitious matrix becomes increasingly weak with increasing substitution of the cement.

[0032] Vermiculite may be used in the present invention by incorporating into the matrix in the amount of about 3% to 15% of total weight. Such substitution has been found to be particularly effective for increasing performance of sacrificial anode. The exact reason for the effectiveness of this material is not known.

Example 1

[0033] A steel reinforced 12.times.12.times.4-inch (30.5.times.30.5.times.10.2 cm) concrete test block was constructed using concrete with the following mix proportions:

TABLE-US-00001 Type 1A Portland cement - 715 lb/yd.sup.3 Lake sand fine aggregate - 1010 lb/yd.sup.3 No. 8 Marblehead limestone - 1830 lb/yd.sup.3 Water - 285 lb/yd.sup.3 Chloride (added as NaCl) - 5 lb/yd.sup.3 Airmix air entrainer (0.95% oz/CWT) - about 6.5% air

[0034] The test block contained about 24 inches (60 cm) of #4 (12 mm dia.) reinforcing bar, or about 0.25 square feet (240 square centimeters) of steel surface area Each test block was cast with two blockouts for two test cells, each blockout forming a circular test cavity about 4 inches (10 cm) in diameter .times.2.75 inches (7 cm) deep.

[0035] An anode was first constructed by soldering 40 grams of pure zinc to galvanized tie wires. The zinc was then cast into a mixture containing 65% sand, 15.2% Type III cement, and 19.8% lithium liquid mixture, prepared by combining 40% by volume saturated lithium bromide solution and 60% by volume saturated lithium nitrate solution. The mixture surrounding the anode was allowed to cure, and the anode was then placed into a cavity in the test block and mortared in place with Eucopatch, a one-part cementitious repair material produced by The Euclid Chemical Company. The anode was connected to the reinforcing bars in the test block with a 10 ohm resistor, which facilitated measurement of the flow of protective current.

[0036] The flow of protective current to the reinforcing bars is shown by the line labeled "Control" on FIG. 1. Current began a little over 1 milliamp (mA), and slowly decreased to about 0.10 mA after one year.

[0037] A second anode was prepared in the same manner, except that 50% of the cement was substituted with bentonite. After curing of the mixture surrounding the anode, the anode was placed into a test cavity and mortared in place with Eucopatch. This anode was connected to the reinforcing bars in the same manner as the Control. The flow of protective current to the reinforcing bars is shown by the line labeled "50% Bentonite" on FIG. 1. In this case, current began a little over 1.5 mA, and slowly decreased to about 0.26 mA after one year. The improvement in the flow of protective current as a result of the use of bentonite was significant and consistent. This improvement is expected to result in a higher polarization of the steel surrounding the anode, a greater level of cathodic protection, and a longer effective service life of the anode.

Example 2

[0038] A steel reinforced test block was constructed as in Example 1 above, and a control anode was also prepared as described in Example 1.

[0039] This anode was subjected to 5 mA of impressed current in constant current mode of operation. In this way, a total charge equivalent to several years of service life can be impressed on the anode in a period of about 60 days. The effectiveness of the anode can be determined by observation of the cell operating voltage. Lower operating voltage indicates that an anode will deliver a higher level of protective current when operated in galvanic mode.

[0040] The operating voltage of the control anode is shown by the line labeled "Control" on FIG. 2. Operating voltage began at about 1.0 volt, and increased to about 5.0 volts after 60 days.

[0041] A second anode was prepared in a similar manner, except that the matrix surrounding the anode contained 8.6% vermiculite by weight. After curing of the mortar surrounding the anode, the anode was placed into a test cavity and mortared in place with Eucopatch. This anode was connected to the reinforcing bars in the same manner as the Control. The operating voltage of the anode surrounded with the vermiculite mixture is shown by the line labeled "8.6% Vermiculite" on FIG. 2. In this case, operating voltage began at about 0.5 volts, and increased to only about 1.5 volts after 60 days. This improvement is again expected to result in a higher polarization of the steel surrounding the anode, a greater level of cathodic protection, and a longer effective service life of the anode.

INDUSTRIAL APPLICABILITY

[0042] The invention is useful for prolonging the life of concrete structures such as bridges, buildings, parking structures, piers, and wharves, employing galvanic cathodic protection of the steel embedded in the concrete for reinforcement.

Claims

1. An apparatus for cathodic protection of a reinforced concrete structure, comprising: at least one sacrificial anode member; an ionically-conductive material into which is bound an electrochemical activating agent at least partly covering the sacrificial anode member(s); at least one elongated metallic conductor bonded to the sacrificial anode member(s); characterized in that at least one inert water absorbent solid is dispersed into the ionically-conductive material surrounding the anode.

2. The apparatus of claim 1 wherein the sacrificial anode member is zinc or a zinc alloy.

3. The apparatus of claim 1 wherein the sacrificial anode member is a high surface area structure having an actual surface area from 3 to 6 times that of its superficial surface area.

4. The apparatus of claim 1 wherein the ionically-conductive covering material is a cementitious-based material.

5. The apparatus of claim 1 wherein the electrochemical activating agent is an alkaline hydroxide present in sufficient amount to raise the pH of the covering material above about pH 13.3.

6. The apparatus of claim 1 wherein the electrochemical activating agent is a deliquescent or hygroscopic material.

7. The apparatus of claim 6 wherein the electrochemical activating agent is lithium nitrate, lithium bromide, or combinations thereof.

8. The apparatus of claim 1 wherein the inert water absorbent solid is bentonite, vermiculite, or combinations thereof.

9. The apparatus of claim 1 wherein the inert water absorbent solid is bentonite.

10. The apparatus of claim 1 wherein the inert water absorbent solid is vermiculite.

11. An method for the cathodic protection of a reinforced concrete structure, comprising: at least one sacrificial anode member; an ionically-conductive covering material surrounding said sacrificial anode member(s), into which is bound an electrochemical activating agent; at least one elongated metallic conductor bonded to the sacrificial anode member(s) with a carbon loaded organic-based mastic; and, connecting the elongated metallic conductor to the reinforcing steel of the reinforced concrete structure, thus causing protective current to flow; characterized in that an inert water absorbent solid material is dispersed into the ionically-conductive covering material.

12. The method of claim 11 wherein the sacrificial anode member is zinc or a zinc alloy.

13. The method of claim 11 wherein the sacrificial anode member is a high surface area structure having an actual surface area from 3 to 6 times that of its superficial surface area.

14. The method of claim 11 wherein the ionically-conductive covering material is a cementitious-based material.

15. The method of claim 11 wherein the electrochemical activating agent is an alkaline hydroxide present in sufficient amount to raise the pH of the covering material above about pH 13.5.

16. The method of claim 11 wherein the electrochemical activating agent is a deliquescent of hygroscopic material.

17. The method of claim 16 wherein the electrochemical activating agent is lithium nitrate, lithium bromide, or combinations thereof.

18. The method of claim 11 wherein the inert water absorbent solid material is bentonite, vermiculite, or combinations thereof.

19. The method of claim 11 wherein the inert water absorbent solid material is bentonite.

20. The method of claim 11 wherein the inert water absorbent solid material is vermiculite.