WO2005080637A1

WO2005080637A1 - Anode assembly and means of attachment

Info

Publication number: WO2005080637A1
Application number: PCT/US2005/004403
Authority: WO
Inventors: John E. Bennett; Andrew J. Niksa
Original assignee: Bennett John E
Priority date: 2004-02-17
Filing date: 2005-02-14
Publication date: 2005-09-01

Abstract

The performance of sacrificial anodes such as zinc, aluminum and their alloys in providing galvanic cathodic protection of steel embedded in concrete is improved by clamping the anode to the steel. Spring action clamps, 'C' clamps and band clamps are described. The electrical contact between the anode and the steel can be further improved by tack welding, or the use of one or more knife edges, points or protrusions into a portion of the clamp that contacts the steel. An electrochemical activating agent, such as lithium nitrate and/or lithium bromide, acts as a humectant and, as such, enhances the performance of the sacrificial properties of the anodes.

Description

ANODE ASSEMBLY AND MEANS OF ATTACHMENT

Technical Field 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. Background Art 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 a kaline 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. When the chloride reaches the level of the reinforcing steel, and exceeds a certain threshold level for contarnination, 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 two and one-half to four 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 the United States 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. 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. 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. 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 (IT¹). 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. 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. In PCT Published Application WO94/29496 and in US Patent 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 above about 14 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. In US Patent 5,292,411, Bartholomew et al disclose 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. In US Patent Application No. 08/839,292, filed on April 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 US Patent 6,033,553, two of the most effective such chemicals, namely lithium nitrate and lithium bromide (LiNO₃ and LiBr), are disclosed to enhance the performance of sprayed zinc anodes. In US Patent 6,217,742 Bl, issued April 17, 2001, Bennett discloses the use of LiNO₃ and LiBr to enhance the performance of embedded discrete anodes. And in US

Patent 6,165,346, issued December 26, 2000, Whitmore broadly claims the use of deliquescent chemicals to enhance the performance of the apparatus disclosed by Page in US Patent 6,022,469. In PCT application Serial No. PCT/US02/30030, filed September 20, 2002, 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. In US Patent No. 6,572,760 B2, issued June 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. Finally, in US Patent 6,193,857, issued February 27, 2001, Davison, et al describe 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. This form of connection is simpler, and easier to execute than those of Whitmore, but is still laborious and time-consuming on site. 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. Also, the means of electrical attachment of the anode to the reinforcing steel has been a problem. Means used to date have either been unreliable, or complex and difficult to use, or time consuming and labor-intensive. DISCLOSURE OF INVENTION The present invention relates to an apparatus and a method of cathodic protection of reinforced concrete and, more particularly, to a method of 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 cathodic protection wherein the performance of the sacrificial anode is enhanced by the use of deliquescent or hygroscopic chemicals, known collectively as humectants. The preferred humectant materials are lithium bromide, lithium nitrate, or combinations thereof as previously described in issued

US Patent 6,217,741. The present invention discloses electrical attachment of the sacrificial anode to the reinforcing steel using a clamp designed to fasten onto the reinforcing bar with sufficient force to provide a reliable low-resistance electrical contact. The clamp in one embodiment is a circular metallic member incorporating a spring action, and designed to closely conform to the round reinforcing bar. In another embodiment of the present invention, the clamp is a band clamp, which may be metallic or polymeric, and which fastens around the reinforcing bar and secures tightly to the body containing the sacrificial anode. A screw device may be incorporated into the band clamp to increase contact pressure and promote electrical contact. In another embodiment, the clamp is an open clamp, such as a "C" clamp, incorporating a screw device to enhance pressure and electrical contact. Electrical contact may be improved using any of these embodiments by incorporating knife-edges, points, or other protrusions into the portion of the clamp that comes into contact with the reinforcing bar. These edges or protrusions, when clamped onto the reinforcing bar, provide a small contact area with high force, promoting electrical contact with low ohmic drop (low IR contact). The clamp may be attached to the sacrificial anode member by metallurgical connection means, such as soldering or welding. The present invention also relates to an anode assembly comprising at least one sacrificial anode member, a material covering the sacrificial anode member containing at least one deliquescent or hydroscopic chemical, and a clamp for attachment of the anode assembly to the reinforcing steel. The present invention also resides in a reinforced concrete structure utilizing at least one sacrificial anode assembly prepared according to the method of the present invention. BRIEF DESCRIPTION OF DRAWINGS 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: Figure 1 is a drawing illustrating one embodiment of the present invention in which a sacrificial metal anode or anodes 1 are embedded within a covering of cementitious mortar 2 containing a deliquescent or hydroscopic chemical (not shown) dissolved or dispersed throughout. The anodes 1 are attached to a metal member 3 by a suitable means, such as soldering or welding. The metal member 3 has knife-edges 4 on its surface, which are placed in contact with a reinforcing bar 5. The metal member 3 and knife-edges 4 are secured tightly to the reinforcing bar 5 by a high tensile strength cable tie 6, which may be of either metal or polymeric construction. The cable tie 6 is routed through the cementitious mortar 2 and behind the metal member 3. Figure 2 is a drawing illustrating another embodiment wherein sacrificial anode or anodes 1 are again embedded within a covering of cementitious mortar 2 containing a deliquescent or hygroscopic chemical. A metal spring clamp 7 is attached to the sacrificial anode 1 by a threaded connection 8. The metal spring clamp 7 is secured by pressing around a reinforcing bar 5, and is held firmly in place by tension provided by the spring clamp 7. A spot or resistance weld 9 between the clamp 7 and the reinforcing steel 5 may also be used to provide an exceptionally low resistance contact. Figure 3 is a drawing illustrating another embodiment wherein sacrificial anode or anodes 1 are again embedded within a covering of cementitious mortar 2 containing a deliquescent or hygroscopic chemical. The anode 1 is attached to a metal member 3 by a suitable means, such as soldering or welding. The metal member 3 has knife-edges 4 on its surface, which are placed in contact with a reinforcing bar 5. The metal member 3 and knife-edges 4 are secured tightly to the reinforcing bar 5 by hose or band clamp 10. The band clamp 10 has a series of holes for quick connection around the reinforcing bar 5, and the clamp is then secured tightly to the reinforcing bar 5 using a thumbscrew 11. Figure 4 is a drawing showing yet another embodiment of this invention in which a sacrificial metal anode 1 is again embedded within a covering of cementitious mortar 2 containing a deliquescent or hydroscopic chemical (not shown). The anodes 1 are attached to a metal member 3 by a suitable means, such as soldering or welding. The metal member 3 may also have knife-edges on its surface, such as shown in Figure 1 and Figure 3. The metal member 3 is secured tightly to a reinforcing bar 5 by use of a thumbscrew 11, which is inserted into an open "C" clamp 12. These figures are intended only to illustrate possible configurations of the present invention. Other configurations utilizing various types of clamps will become apparent to those skilled in the art by reading the following description of preferred embodiments. MODE(S) FOR CARRYING OUT THE INVENTION 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. The anode assembly and method of connection 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. Sacrificial metal anodes are consumed anodically, fomiing in the process oxides that typically take up more volume than the parent metal. 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 three to six times the superficial surface area, whereas the anode surface area for plain flat sheet is two times the superficial surface area (counting both sides of the sheet). In the present invention, as was also previously disclosed, an electrochemical activating agent is desirable to maintain the sacrificial anode in an electrochemically active state. One activating agent is an alkali hydroxide, such as lithium hydroxide, to maintain the pH of the mortar surrounding the anode above about pH 13.3. The preferred electrochemical activating agents used in the present invention are those that are known collectively as humectants, that is, chemicals that are either hygroscopic or deliquescent. Such chemicals have been shown to effectively enhance the performance of sacrificial metal anodes by imparting a very high ionic conductivity to the mortar surrounding the anode, and, in some cases, by mamtaining the anode in an electrochemically active state. Examples of such chemicals are lithium acetate, zinc bromide, zinc chloride, calcium chloride, potassium chloride, potassium nitrite, potassium carbonate, potassium phosphate, ammonium nitrate, ammonium thiocyanate, lithium thiocyanate, lithium nitrate, lithium bromide, and the like. Other effective chemicals for this purpose will become obvious to those skilled in the art. Lithium nitrate, lithium bromide, and combinations thereof have been found to be particularly effective electrochemical activating agents for zinc anodes. Lithium nitrate, lithium bromide, and combinations thereof have been particularly effective in the range of 0.05 to 0.4 grams dry basis per cubic centimeter. Also, it is necessary to provide a secure low-resistance electrical connection between the sacrificial metal anode and the reinforcing steel or other metal to be protected. In the past, this connection has usually been provided in the form of a wire, typically steel wire known as "tie wire," or in the form of steel pins driven or welded onto the reinforcing steel. Such connections sometimes failed to provide a reliable long-lasting contact, and are laborious and time- consuming in the field application. In the present invention, new and advantageous methods are proposed for making a reliable, long-lasting, and low resistance connection between the sacrificial anode and the reinforcing steel. The present method uses a clamp that fastens onto the reinforcing bar and conforms to the size and shape of the reinforcing bar being used. Such clamp may be entirely metallic, or may be, for example, partly polymeric and partly metallic, the metallic portion being that which rests in contact with the reinforcing bar. Low resistance contact is enhanced if the clamp is provided with ridges or sharp edges on the contacting surface to penetrate surface oxides, dirt, or oils. Said metallic clamp must have sufficient resilience or spring quality to maintain sufficient pressure to assure long-term reliability of the contact. The present invention also contemplates soldering or welding, such as tack welding or resistance welding, to further establish a reliable metallurgical bond. One type of clamp disclosed for this application is similar to that known as a "cable tie" clamp, as illustrated in Figure 1. Cable ties are readily adjustable to any size of reinforcing bar, and can provide high tension and contact pressure. Cable tie installation tools make installation and tensioning of the cable ties quick and repeatable. Another type of clamp disclosed for this application is a "hose clamp" or "band clamp." The band clamp illustrated in Figure 3 has a series of square holes in the band. The band is then wrapped around any size of reinforcing bar, and is hooked through one of the square holes. The band is tightened and contact pressure is provided by tightening a thumbscrew that is threaded through the band and into contact with the reinforcing bar. Other types of band clamps utilize worm-drives, quick release buckles, or snap-grips. A third type of clamp disclosed for this application is an "open clamp." Open clamp designs rely on the bending strength of bars to provide high contact pressure. Some open clamps, such as that shown in Figure 4, utilize thumbscrews or sliding jaws secured by threaded nuts. Other types of open clamps that may be utilized include those referred to as hammer clamps, bench clamps, and dog clamps. A final type of clamp disclosed for this application are those referred to as "spring clamps," which snap onto the reinforcing steel and are held in place by their strength and elasticity. One type of spring clamp is illustrated by the drawing in Figure 2. The metal spring may not be flexible enough to accommodate several diameters of reinforcing steel, so the spring may be interchangeable for the specific size of bar in place. The material of the spring clamp should be partially tempered high yield steel, similar to spring steel. In cases where extra security is needed, or to provide a very low resistance electrical contact, the clamp material or wire may be resistance or spot-welded onto the reinforcing bar. Other types of clamps beyond those described in the preferred embodiments above will be apparent to those skilled in the art. INDUSTRIAL APPLICABILITY The industrial use of the invention relates to steel reinforced concrete structures, such as bridges, buildings, parking structures, piers and wharves. From the foregoing description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.

Claims

Having described the invention, the following is claimed: 1. An apparatus for cathodic protection of a reinforced concrete structure, comprising: at least one sacrificial anode member; said anode member embedded in an ionically-conductive covering material into which is bound an electrochemical activating agent; the sacrificial anode member(s) attached to a metallic clamp; and said metallic clamp designed to be secured firmly onto the reinforcing bar, providing a stable low-resistance contact.

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 of 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 further including an ionically-conductive compressible material sufficient to absorb the expansive products of corrosion of the sacrificial anode and positioned on at least one side of the sacrificial anode member(s).

9. The apparatus of claim 1 wherein the metallic clamp is provided with ridges or sharp edges in the area of the contact with the reinforcing bar.

10. A method for cathodic protection of a reinforced concrete structure, comprising: providing at least one sacrificial anode member; embedding the sacrificial anode member in an ionically conductive covering material into which is bound an electrochemical activating agent; creating a metallurgical bond between the sacrificial anode member and a steel member; and clamping the steel member firmly to a reinforcing bar, thereby establishing a low-resistance contact therebetween.

11. The method of claim 10 wherein the sacrificial anode member is zinc or a zinc alloy.

12. The method of claim 10 wherein the sacrificial anode member is a high surface area structure having an actual surface area from three to six times that of its superficial surface area.

13. The method of claim 10 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.

14. The method of claim 10 wherein the electrochemical activating agent is a deliquescent or hygroscopic material.

15. The method of claim 14 wherein the electrochemical activating agent is lithium nitrate, lithium bromide, or combinations thereof.

16. The method of claim 10 wherein the metallurgical bond is provided by soldering or welding;

17. The method of claim 10 wherein the clamping is achieved using a clamp provided with ridges or sharp edges in the area of the contact with the reinforcing bar.

18. The method of claim 10 wherein clamping is achieved using a cable clamp.

19. The method of claim 10 wherein clamping is achieved using a band clamp.

20. The method of claim 10 wherein clamping is achieved using an open 'C clamp.

21. The method of claim 10 wherein clamping is achieved using a spring clamp.

22. The method of claim 10 wherein clamping is achieved by resistance welding a clamp to the reinforcing bar.

23. The method of claim 10 wherein the ionically-conductive covering material is a cementitious-based material.