Click for next page ( 15

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 14
14 CHAPTER THREE APPLICATION OF CATHODIC PROTECTION BRIEF HISTORY OF USE IN NORTH AMERICA and the wirings in cut outs on the reinforced concrete deck, thereby reducing their susceptibility to damage during replace- The very first application of cathodic protection to reinforced ment of the asphalt riding layer. By 1978, the asphaltcoke concrete elements was reported in 1959 by Caltrans (5). Dur- breeze overlay system had become one of the three standard ing the late 1940s and the 1950s, Caltrans had spent more than procedures that MTO used for rehabilitation of reinforced con- $1 million in repairing damage caused by corrosion on the crete bridge decks and as many as 30 systems were installed 7-mile San MateoHayward Bridge located in San Francisco by 1984 (17,18). and were forced to look for ways to control or stop corrosion. An experimental system was installed on the reinforced con- Although the basic concept of the asphaltcoke breeze crete beams of this structure. It used carbons rods and a con- system was sound and the systems (especially the designs ductive backfill, which were placed in wooden troughs and employed by Ontario) were working well, the focus of the attached on to the concrete beams. Later, in 1972, based on this cathodic protection industry shifted to other materials and proof of concept, Caltrans installed a full-scale cathodic pro- methods of installation, and these systems fell out of use tection system on the deck of the Sly Park Bridge near Placer- in the United States. Recently, MTO also discontinued the ville, California (6). This system used silicon iron anodes use of this type of system. As slots cut on the deck surface embedded in a layer of coke breeze (see Figure 14). The coke provided a convenient mechanism to install anodes with- breeze was used as a conductor to distribute the current uni- out adding any additional dead load, the focus of research formly over the bridge deck. As the coke breeze mix did not and development shifted to anode and backfill materials that have the requisite material properties to serve as the riding would be used in slots. surface, an overlay of asphalt was placed on top of the coke breeze. Based on the initial success at the Sly Park Bridge, In the late 1970s, two separate efforts evaluated the appli- Caltrans installed seven more asphaltcoke breeze overlay cation of zinc anodes in the form of ribbons and sheets in cathodic protection systems during 1974 and 1975. An eval- Illinois. The first effort evaluated zinc ribbons placed in lon- uation of these seven bridge deck cathodic protection sys- gitudinal slots on the deck and backfilled with cementitious tems was performed and its report in 1981 established the mortar and was conducted by the Illinois DOT. The anode feasibility of using such a system on bridge decks for miti- spacings evaluated in this effort were 24 and 48 in., and this gating corrosion (14). study concluded that the zinc ribbons were only capable of throwing the protective current to a distance of 3 in. on either With the application of the first system on Sly Park, the side of the slot (19). The second effort was performed under cathodic protection industry for reinforced concrete struc- NCHRP and it evaluated the zinc ribbons in the slot and zinc tures was born. Several concurrent efforts for developing sheets on the surface of the deck. This effort was unable to new technologies and implementing the one demonstrated come to any conclusions on the feasibility of using zinc on at Sly Park were initiated in the 1970s. A majority of these bridge decks as a cathodic protection anode material (20). efforts focused on the implementation of cathodic protection on bridge decks. FHWA led the way with Demonstration The platinum niobium wire that was commercially avail- Project 34 (DP-34), Cathodic Protection for Reinforced Con- able became the next material of choice as the primary anode. crete Bridge Decks, which began in 1975 and funded the It provided the necessary electrical properties, was durable, installation of 14 asphaltcoke breeze systems by 1982 (15). and could provide a reasonable service life. Research by Other reports indicated that by 1984 a total of 22 systems FHWA and others indicated that spacing between slots not were operational in 11 states (16). exceed 12 in. for effective current distribution by this anode material (2123). The first evaluation of this system in 1977 In 1976, MTO reported installation of asphaltcoke breeze used portland cement mortar as the backfill material and systems on two structures; one was a ramp at a major inter- topped it off with a polymer modified mortar (21). The back- change in Ontario and the other was the Duffins Creek Bridge. fill material failed owing to the generation of gasses and acid Ontario improved on the original design developed by Cal- at the platinum wire, which resulted in acid attack. Several trans by changing the mix design of the coke breeze layer and other materials, such as a proprietary conductive cementi- making it more stable. In addition, they installed the anodes tious non-shrink grout (24) and a conductive grout mixture,

OCR for page 14
15 FIGURE 14 Conductive coke breeze system with pancake anodes. were tested and they too could not withstand the acid attack. led with the maximum number of installations. In Missouri, An industry supplier proposed a calcined petroleum coke back- the earlier slotted systems were overlaid with asphalt. Later, fill topped with a flexible sealant. Although this combination the asphalt overlays were replaced by cementitious overlays. was able to withstand the acid attack it was not durable enough In 1985, one of the largest slotted cathodic protection systems for use on bridge decks (15,21). Continued FHWA research was installed on the elevated sections of I-64 in Charleston, led to the development of the conductive polymer grout, which West Virginia. The system is still operational and the last time was able to meet all the requirements needed of the backfill it was evaluated in 2005 it was found to provide adequate pro- material (21,25). tection against corrosion (26). Figure 15 documents the instal- lation of one such system. From 1979 to 1984, slotted systems were installed on a total of 15 bridges distributed over 11 states. With the devel- The successes of the conductive polymer grout in the slots opment of the FHWA conductive polymer grout, the number led to the development of the mounded system. The conduc- of bridges increased to more than 100 by 1989 (16). Missouri tive polymer grout was laid out in a grid on the surface of the FIGURE 15 Various stages of construction of a slotted FHWA conductive polymer cathodic protection system.

OCR for page 14
16 judged as the best paint material tested and was used in a field application. In the early 1980s, MTO also conducted feasi- bility tests using different conductive coatings (33). At first, the conductive paints developed for other applications such as television tubes were used. Later, paints were specifically developed for use on reinforced concrete structures. The ser- vice life of the paint system is limited by the durability of the paint and its ability to weather. The durability of the paint in wet, freezethaw, and splash zones was a significant concern and was evidenced by the Ontario study in which the paint started to weather in 3 to 6 years. Conductive paint systems installed on two bridges in Virginia started to exhibit signif- icant deterioration of the paint within 10 years of operation (2734); however, properly designed and well-installed sys- tems have provided adequate protection in humid environ- FIGURE 16 Construction of mounded conductive polymer cathodic protection system. ments (28,30,33). Conductive paints have performed much better in parking garage structures (35). deck after deteriorated concrete was removed and patched Arc sprayed zinc was developed as a conductive coating and the deck was scarified (see Figure 16). The platinum wire anode by Caltrans in 1983 and was used as an impressed was encapsulated in the grout in one direction of the grid and current anode in the first field trial on Pier 4 of the Richmond carbon fibers were used in the other direction of the grid. Car- San Rafael Bridge located in San Francisco Bay, Califor- bon fiber was used to reduce the cost of the system. Latex- nia (36). Using the arc spray technique, the application rate modified concrete or a conventional concrete overlay was was significantly increased over the older flame sprayed then placed over the grid to provide a riding surface. The first method. In the arc sprayed technique, an electrical arc between such system was installed in 1983 on the 42nd Street Bridge two zinc wires is used to melt the metal, which is applied in South Minneapolis, Minnesota. This system was moni- to the concrete surface by a stream of air. As this coating is tored for five years under an FHWA research program and a metal, it is very conductive and a limited number of elec- was found to be operational until July 1996. The evaluation trical contact points to the power supply are required if used in May 1998 suggested that the system had fallen into dis- as an impressed current anode. Unlike the conductive paints, repair and was powered down (27). A few more of these sys- the zinc coating is comparable in color to concrete and requires tems were installed on bridge decks and one was installed in no decorative overcoat. The second largest application of a parking garage. arc sprayed zinc is the Yaquina Bay Bridge in Oregon (37), where it has been used as an impressed current anode. Oregon For applications on concrete elements that are not subjected was instrumental in the development of good specifications to traffic, conductive paint and mastics were developed in the for the application of arc sprayed zinc systems and quality late 1970s (2831). The conductive coating type anode sys- control. The section of the structure that was receiving the arc tem completely covers the concrete surface and provides effi- sprayed zinc was enclosed to control the environment for the cient current distribution. It is easy to install using a variety application of the zinc coating and to contain the dust gener- of common installation methods (spray, roll, or brush) and its ated during concrete repair (see Figure 17). In the response to low initial cost makes it a desirable system. The conductive a survey question, the state of Oregon stated that "In Oregon, coating is black, and a decorative overcoat latex paint is often impressed current cathodic protection with arc sprayed zinc required for safety and cosmetic purposes. NCHRP Project anodes appears to fill a niche market for the preservation of 12-19 identified several commercially available conductive our historic bridges along the Pacific coastline. The effective paints that showed promise. The trial was conducted at a U.S. life of the zinc anodes and the requirements for renewal or Army Corp of Engineers building at Ft. Lee, Virginia (31). replacement of anodes is the subject of on-going research." The paint was used as a secondary anode and the platinum With the completion of ongoing construction, Oregon will wire was used as the primary anode with the FHWA conduc- have 1.17 million square feet of concrete under arc sprayed tive polymer providing the electrical contact between the plat- zinc cathodic protection making it the largest user of this type inum wire and the paint. Since 1975, the Florida DOT (FDOT) of system in North America. has been involved in conductive paint and mastic cathodic protection work on pilings, piers, caps, beams, and deck FDOT started to use it on sub- and superstructure elements undersides and its work also indicated that conductive paints as a galvanic anode. By 2002, it had been applied to 13 bridges and mastics can be a viable anode material (32). A follow-up with a combined protected concrete surface of approximately NCHRP effort, Project 12-19B, was charged with further eval- 350,000 square feet (38). In a typical application, the system uation of conductive paint cathodic protection systems (28). is installed without any concrete restoration and the connec- This effort identified another conductive coating that was tion to steel is achieved by applying the zinc directly on to

OCR for page 14
17 FIGURE 17 Yaquina Bay Bridge, Newport, Oregon, has the largest arc-sprayed zinc-impressed current cathodic protection system. the exposed reinforcement. The most common uses include This anode is composed of a titanium base upon which propri- structures where the deterioration is several feet above the etary mixed metal oxides are sintered. The mixed metaloxide tidal zone and on structures where only isolated areas need catalyst is specific to the evolution of oxygen rather than chlo- to be provided with corrosion control. The service life of this rine, and the operating voltage is 0.5V below the theoretical galvanic anode has been observed by FDOT to range from voltage required to drive the oxidation of chloride ions to chlo- 5 to 10 years depending on the environmental conditions at rine gas (40). This reduces acid attack on the concrete as a the site, location above water level, and the type of reinforce- result of the generation of chlorine gas at the anode/concrete ment being protected. interface. The anode is supplied primarily in two forms: mesh and ribbon. In the mesh form, a titanium expanded mesh with The ferex anode became available in 1984. This anode diamond-shaped openings is used as a base and the mixed used a copper conductor covered by a flexible polymer anode metal oxides are sintered on all exposed surfaces of the tita- material. Woven into a mesh, the anode was placed on the deck nium. The mesh is supplied in 4-ft wide rolls, and can be or the substructure element and encapsulated by a cementi- installed on a horizontal or a vertical surface and is usually tious overlay material. Up to 50 systems were installed on encapsulated in a cementitious overlay material. Power is bridge decks. However, by 1990 many of these systems were supplied to the anode by means of titanium conductor bars exhibiting anode deterioration and the anode is no longer welded to the anode at appropriate locations. The ribbon is used (16). There were several reasons for the failure of this available as either a solid ribbon or an expanded mesh ribbon anode material, ranging from deficient system design to and is usually installed in slots, which are then backfilled inability to withstand the high alkaline environment in the with a cementitious material. The first experimental bridge concrete. decks were constructed with this system between 1986 and 1987. Its first use on a substructure was in Ontario, Canada, FDOT developed the concept of conductive rubber and where it was encapsulated with an acrylic polymer-modified the rubber industry was able to manufacture it for use as an anode material. This rubber was produced as sheets with cor- rugation on one face and could be positioned on the pile sur- face and held in place by a compression jacket manufactured from fiber-reinforced polyester. The compression jacket used stainless steel bands to produce the requisite pressure to hold the system in place (see Figure 18). The first such system was installed in 1987 on the piles of a bridge carrying US-90 over the Intracoastal Waterway in Jacksonville, Florida. Results of 2 years of monitoring indicated uniform distribution of cur- rent on marine pilings (39). An update in 2002 reported that this system had been installed on three bridges and the sys- tems were providing adequate protection (38). This system is no longer used owing to the availability of better alternatives. The mixed metal oxide anode was developed in 1985 and has been used on both bridge decks and substructure elements. FIGURE 18 Conductive rubber anodes.