WO1992013116A1

WO1992013116A1 - Rust preventive material and method of application

Info

Publication number: WO1992013116A1
Application number: PCT/US1992/000624
Authority: WO
Inventors: Dodd S. Carr; Rodney G. Powers; Robert M. Langley; Ivan R. Lasa
Original assignee: International Lead Zinc Research Organization, Inc.
Priority date: 1991-01-25
Filing date: 1992-01-24
Publication date: 1992-08-06
Also published as: AU1342392A; CA2101251A1

Abstract

Perforated sheets (13) of rolled zinc, held against steel reinforced concrete highway piers (11) in seawater intertidal zones (30), provide sacrificial cathodic protection to the embedded steel (12). Unique grooved, recycled plastic/wood composite extrusions (14) press the perforated zinc sheets firmly against the four faces of the concrete piers by means of durable stainless steel binding strips (16).

Description

Rust Preventive Material and Method of Application i Background of the Invention

Steel reinforced concrete structures, subjected to the corrosive effects of saltwater immersion, as in highway piers, begin to show signs of catastrophic degradation after about 12-15 years of service. This 5 is manifested by cracking and spalling of the concrete away from the corroding steel reinforcing bars (rebars) embedded in the concrete. When the saltwater migrates through the concrete and reaches the steel rebars at concentration of about 3-5 lb/yd 3 , the steel develops

10 successive layers of rust which impart a large expansive stress in the concrete (rust occupies a volume about ten times larger than the original steel) . Although concrete is extremely strong in compression, it is very weak in tension. Thus, the expanding rust

15 layers on the steel rebars cause the concrete to crack in tension and fall away from the rebars, especially in the intertidal zones between mean low tide and high tide.

Many methods have been employed to repair such

20 damaged reinforced concrete structures and to protect them against further degradation. For example, the cracked concrete may be chipped away from the corroding steel rebars; the exposed areas may be thoroughly sandblasted to remove any rust and loose concrete; and

25 then a fresh concrete cover is cast over the f. sandblasted area. However, this expensive procedure must be repeated whenever the ingress of saltwater t causes the steel rebar to corrode again.

Another approach to the problem is to provide the

30 steel rebar with a protective polymeric barrier film prior to embedding it in concrete. For example, a fusion bonded epoxy film is used to encapsulate the deformed (ridged) steel rebar with a corrosion resistant coating. However, in practice, the epoxy film is damaged during shipment; during bending; during cutting to length; and during installation with bare or coated steel tie wires. As a result, saltwater corrosion proceeds rapidly through the many defects in the epoxy film on the steel rebar. This causes the concrete to crack and spall in the same way as with bare steel rebars.

In recent years, the process of cathodic protection, by means of an impressed current, has been partially successful in protecting steel rebars against corrosion in saltwater environments. Thus, for example, a titanium mesh anode is firmly attached around the concrete pier in the intertidal zone and a direct current is then impressed upon the cathodic steel rebar. This causes the steel rebar to become polarized and thereby protected against saltwater corrosion.

Other anode materials that have been used for impressed current cathodic protection of steel rebars include platinum meshes; conductive rubber; conductive polymers; and flame-sprayed zinc coatings on the dry concrete surface above the tidal zone. See in this regard U.S. Patent No. 4,699,703 and published European Patent Application EP 0 174 471. However, such impressed current systems are time consuming, costly, difficult to install and hard to control. Thus, unless the impressed current is uniformly distributed over the steel rebar network in the concrete, corrosion may still occur. Also, if the current density is too high, gassing may occur, which disbonds conductive polymeric coatings, and acidic conditions develop, which attack the concrete and the anode. Summary of the Invention

Now, unexpectedly, it has been found that steel rebars in concrete structures, located in saltwater environments (immersion areas; intertidal zon.es; and splash zones) , can be economically and effectively protected against corrosion by cathodic protection afforded by means of perforated zinc sheets held firmly against the concrete piers. The chemical composition of the zinc is environmentally acceptable; the perforations provide a means of maintaining a moist condition in the concrete, thereby offering a necessary conductive path between the sacrificial zinc anode and the embedded steel rebars in the concrete; no expensive rectifier and electrical wiring system is needed; the protective current to the steel rebar is self controlled by the natural corrosion potential of zinc in saltwater; problems of gassing and acid formation are avoided, and the protective system of perforated zinc is easily, rapidly, and economically installed. Although the use of perforated zinc sheets illustrates the present concept of cathodic protection of steel rebars in concrete, other forms of zinc sheets may be employed with equal efficacy. For example, expanded zinc mesh, prepared by slitting zinc sheets at longitudinal intervals and then stretching the sheets transversely, may be used effectively. Moreover, any other metallic elements above mild steel in the electromotive series in seawater could be employed; for example, cadmium, aluminum, and magnesium. However, these alternative materials would not be preferred because of their toxicity (cadmium) or rapid rate of consumption (aluminum and magnesium) . Brief Description of the Drawings

Figure 1 is a side view of concrete pier structure with a cathodic protection system according to the present invention. Figure 2 is a cross-sectional view of Figure 1 taken at plane A-A.

Figure 3 is a graph depicting E-log-I test criteria.

Figure 4 is a graph depicting depolarizing test results.

Figure 4a is a graph also depicting depolarizing test results.

Figure 4b is a graph also depicting depolarizing test results.

Description of the Preferred Embodiments

As shown in Fig. 1, a concrete pier 11 has imbedded therein steel rebars 12. A perforated zinc sheet 13 is wrapped around the pier in the intertidal zone 30 (i.e. the zone encompassing mean low tide and mean high tide water levels) and is held firmly in place by mats 14. The mats, as shown in Fig. 2, have vertical grooves 15 which allow seawater to irrigate the zinc sheet. The mats are held in place by straps 16, preferably of stainless steel. As can be seen in Fig. 2, the mats serve to space the straps away from the zinc sheet at the corners of the piers thus relieving a potential stress and excessive corrosion point. An electrical connection is maintained between the zinc sheet and the rebar, e.g., by a wire 17, and a bore hole 18 in the concrete may be made for this purpose.

Example 1 Strips of perforated zinc sheets, nine inches wide by 0.049 inch thick by four feet long, were fabricated into two open cage structures large enough to wrap around the four sides of two steel reinforced concrete highway bridge piers, in saltwater (B.B. McCormick Bridge, U.S. Highway 90, Jacksonville, Florida) . The composition of the perforated zinc sheet was as follows:

Metal Percent by Weight

Cadmium 0.010 Maximum

Lead 0.010 Maximum Iron 0.008 Maximum

Copper 0.6 - 1.0

Titanium 0.005 Maximum

Aluminum 0.001 Maximum

Zinc Balance The perforations were approximately 0.75 inch in diameter and the solid area between holes was about 0.05 inch. A solid zinc strip, 0.83 inch X 0.05 inch x about six feet long, was used to support eight parallel perforated zinc sheets vertically and to provide electrical contact with each sheet. One end of each perforated zinc sheet was bent tightly around the solid zinc strip and crimped. Then, soft solder (rosin core) was used to join the splice areas of the perforated sheets to the solid zinc supporting strip. At the bridge site, each cage structure was placed around a concrete pier (piers A and E) ; tapped firmly around each corner of the concrete piers; and electrically connected to the steel rebar through a bore hole in the concrete above the high tide mark (a hole was drilled and tapped into the exposed rebar through the bore hole) . The midline of the perforated zinc sheet cage was positioned at the mean high tide level of the concrete pier.

To hold the perforated zinc sheet cage firmly against the concrete pier, four plastic-wood attachment devices were employed, one on each face of the pier. The composite plastic-wood retainers were fabricated from planks of recycled plastic (50%) ; each retainer was 15.0 inches wide by 1.75 inches thick with a tapered profile thinner toward the corners of the pier (to assure uniform pressure distribution across each perforated zinc sheet) ; a one-half inch radius of curvature was used longitudinally along each side of each four-foot long plastic retainer (to avoid sharp bends in the stainless steel binder straps) ; and the flat, contact surface of the plastic retainer was ribbed with parallel longitudinal grooves 3/16 inch wide by 3/16 inch deep. The grooves provide tidal irrigation of saltwater to the perforations in the zinc sheets to flush away any acidic films that may have formed, and increase the compressive forces exerted on the zinc-concrete interface by reducing the compression-transmitting contact areas.

Mitigation of corrosion utilizing the herein described system was achieved by allowing direct ionic current flow from the zinc sheets onto the surface of the reinforcing steel and electronic current flow through the direct connection between the zinc and the steel. The direct current was supplied by the zinc sheets, which maintain a higher electromotive potential in relation to the rebar, without requiring an external power source.

The flow of electrons to the steel rebars, by the direct current supplied by the dissolution of perforated zinc sheets, prevents any corrosion current from flowing between anodic and cathodic areas within the steel, resulting in polarization of the rebars.

To polarize the rebars to a cathodically protected state, it is necessary to shift their electromotive potential level by a minimum of approximately 100 millivolts in the negative direction. The precise minimum amount of current required to cathodically protect the steel was determined using the E-log-I test criteria (Figure 3) .

The E-log-I criteria for cathodic protection states that by graphical analysis of the curve produced by the plotting of the polarized voltage potentials against the log of the current applied, the minimum required current for cathodic protection can be determined. The current supplied by the anode (perforated zinc sheets) was applied to the structure in small increments, producing a polarized potential shift at the cathode (steel) . Typically, at small current values the potentials do not shift significantly. However, as current values are increased, the potentials shift to a point where a direct relationship exists between the potential and the logarithm of the current, producing a linear segment in the polarization curve. From this linear relationship (Tafel Slope) , the minimum amount of current required for cathodic protection (Icp) and its corresponding cathodic protection potential (Ecp) as well as the corrosion current (Icorr) can be graphically determined.

The system performance during and after E-log-I test demonstrated the capability of the perforated zinc anode to shift and maintain the potentials of the steel at levels substantially above the minimum cathodic protection requirement (Ecp) with no indication of overprotection.

Example 2 To further corroborate the cathodic polarization level achieved, four identical test probes were embedded in the concrete at the same depth of the reinforcing steel on each pile. Polarization was verified by interrupting the direct current flow while monitoring the open circuit potential decay for a period of four hours. The results of this test are shown in the following Tables 1(a) and 1(b):

TABLE Ifa)

Pier E

Static Energized Total Current Probe No. Potential Potential Current Density

(mV) (mV) (mA) (mA/ft*)

1 -230 -430 .120 8.64 2 -294 -682 .120 8.64 3 -560 -960 .260 18.72 4 -540 -900 .140 10.80

AFTER 24 HOURS ENERGIZED

1

2

3

4 STRUCTURE DRY -294 -688 WET

As shown in Figures 4(a) and 4(b), depolarization exceeding 100 mV was observed. This procedure serves as a means of corroborating cathodic protection at any time after the corrosion prevention device has been installed on the structure. In addition, Tables 1(a) and 1(b) show a decrease of current density in relation to time typical of an effective cathodic protection system with efficient polarization characteristics.

Example 3 Complementary to the herein described corrosion control system, additional sacrificial zinc can be applied to the structure surface at elevations above the perforated zinc panels if corrosion levels at those elevations so require. This can be achieved by arc or flame sprayed zinc metalizing on the concrete surface. This method applies the zinc by means of equipment designed to feed two zinc wires through a handheld gun provided with the capability of maintaining an electric arc or a continuous flame which melts the zinc wires. A trigger type switch on the gun activates the preset air nozzle which sprays the molten zinc onto the concrete surface. By connecting the metalized areas to the reinforcing steel, the sprayed zinc provides the protection current for the steel at this elevation while the perforated zinc protects the tidal areas elevation. Moisture is present at intertidal areas precluding the use of sprayed molten zinc. Therefore, the sprayed zinc application by itself is not an effective corrosive prevention method at the intertidal zone without the incorporation of the zinc sacrificial device (zinc perforated sheets) .

This sprayed zinc coating is depicted in Fig. 1 at the area 19. The sprayed zinc surface may be electrically connected to the rebars by a wire or the like (not shown) .

Example 4 When corrosion protection requirements of steel rebars in concrete bridge piers extend to elevations below the perforated zinc sheet system, additional sacrificial zinc bulk anodes 21 (Fig. 1) can be incorporated below the intertidal zone and electrically connected to the zinc sheets by connection means 22. By placing the bulk anodes in permanent direct contact with the tidal waters, additional protection current can flow through the water onto the concrete and then onto the embedded reinforcing steel surface. This procedure will extend the service life of the perforated zinc sheets by supplying the necessary current to protect the steel below the tidal area which otherwise would use current from the perforated zinc during high tides. Example 5 According to a preferred embodiment, installation of the perforated zinc sheets around steel reinforced concrete piers in saltwater environments is greatly facilitated by means of a novel plastic-wood attachment device. Such a device is manufactured by Riverhead Milling Inc., Philadelphia, PA, and employs a composite of recycled plastics and wood. It is designed to apply uniform pressure of the perforated zinc sheet against the faces of the concrete pier; it floats in saltwater and can be easily retrieved if dropped; it provides a unique means of irrigating the zinc-concrete interface; and it can be easily and rapidly installed and held in place with corrosion resistant stainless steel straps. Summarizing, it will be understood from the foregoing disclosure that the corrosion prevention device will mitigate corrosion of reinforcing steel embedded in concrete at the intertidal zone. Moreover, the perforated zinc sheets may be fabricated to conform to different shapes and dimensions of various structures. Thus, the fabrication being relatively simple, the material derived from recycled products, and the elimination of extensive electrical work and maintenance costs, it is believed that the hereby presented corrosion prevention device exhibits a superior cost effective system in relation to other conventional devices for steel protection in marine environments.

Concluding, it should be evident from the foregoing description of the present invention that the perforated zinc sheet corrosion prevention device exhibits enhanced performance and the capability to adapt to many variables without deviation from the scope of the following claims.

Claims

1. A corrosion resistant steel reinforced structure in a marine environment comprising a. a steel reinforced concrete structure located at least in the intertidal zone between low and high tide in a marine environment, b. a perforated zinc anode sheet firmly held in place against the surface of the steel reinforced concrete structure at least in the intertidal zone of the structure, c. means for maintaining an electronic electrical connection between the steel reinforcement and the zinc anode thereby causing a direct electrical current to be passed from the zinc anode sheet to the steel reinforcement substantially solely by the higher electromotive potential of zinc in relation to the steel reinforcement in said marine environment, said zinc anode thereby being selectively sacrificed and protecting the steel reinforcement from corrosion.

2. A structure according to claim 1 wherein said zinc sheet is adapted to permit the irrigation of the zinc-concrete interface with water.

5. A structure according to claim 1 wherein the zinc sheet is firmly held in place by mats, said mats have means to allow free passage of water to the interface between the zinc sheets and the concrete surface. 6. A structure according to claim 5 wherein the mats are comprised substantially of recycled wood and plastic.

7. A structure according to claim 5 wherein said means to allow the free passage of water comprise grooves in the surface of the mats facing the zinc sheet.

8. A structure according to claim 5 wherein the mats are held in place by straps.

9. A structure according to claim 1 wherein the steel reinforcement comprises rebars embedded in the concrete, and wherein said means for maintaining an electrical connection comprise means passing from said zinc sheet through a bore hole in said concrete to the rebars.

10. A structure according to claim 9 wherein said means for maintaining an electrical connection is a wire.

11. A structure according to claim 5 wherein said device comprises a plurality of said zinc sheets electrically connected to each other.

12. A structure according to claim 5, further comprising means for additional cathodic protection positioned above the intertidal zone of said structure, said means comprising zinc deposited directly on said concrete structure in the form of a molten zinc spray, said means being maintained in electrical contact with said steel reinforcement. 13. A structure according to claim 9, further comprising means for additional cathodic protection positioned below the intertidal zone of said structure, said means comprising at least one bulk zone anode maintained in direct contact with the tidal water, said bulk zinc anode being maintained in electrical contact with said zinc sheet and said steel reinforcement.