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13 Pylon Pylon Deck Deck Cable terminated at pylon Cable goes thru saddle Pylon Saddle Cable (a) FIGURE 13 Saddles in stay cables. STAY CABLE MATERIALS In this section, the materials used in cable systems are dis- cussed, and the importance of detailing and issues of material suitability and compatibility are presented. MTE Materials Steel (b) Today, steel is the predominant MTE material used for stay cables (100% of cable-stayed bridges in the United States FIGURE 12 Field assembly of cables for the Cape Girardeau Bridge (courtesy: Missouri DOT). and Canada). According to the latest edition of the PTI Rec- ommendations for Stay Cable Design, Testing and Installa- tion (2001), steel wires used as MTEs must conform to the · A more difficult cable removal and replacement requirements of ASTM A421/A421M, Standard Specifica- process would be required should that become neces- tion for Uncoated Stress-Relieved Steel Wire for Prestressed sary; Concrete, Type BA. Strands must conform to ASTM A416/ · Precluding slip at the saddles would require special A416M, Standard Specification for Steel Strand, Uncoated considerations; Seven-Wire for Prestressed Concrete, and must be weldless, · In large single saddles, the application of protective low-relaxation grade. Bars must conform to ASTM A722/ tape may become difficult in the vicinity of large sin- A722M, Standard Specification for Uncoated High-Strength gle saddles as spaces between cables are reduced; Steel Bar for Prestressing Concrete. · Steel pipe at large single saddles should not participate in load transfer to the pylon (i.e., tension in the steel pipe Fiber-Reinforced Polymers controlled); and · Geometric control through cable length would be more In recent years, a number of exploratory efforts have focused difficult. on the use of fiber-reinforced polymers (FRPs) in prestress- ing applications and stay cables. These investigations have In 1993, a worldwide survey of stay cable practitioners by generally focused on glass, aramid, or carbon fiber-reinforced Hamilton and Breen (1995) indicated that the majority of polymers (GFRP, AFRP, and CFRP). Epoxy-based resins respondents did not favor the use of saddles, with European are typically used as the matrix for the composite, and the respondents having the highest rate of objections at 76%. FRP is made using a pultrusion process. Fisher and Bassett The results of the questionnaire in this study showed a total (1997), Christoffersen et al. (1999), Roos and Noisternig of seven bridges with saddles (21%), six of which were in the (1999), and Noro et al. (2001) provided information on the United States and one in Canada (Figure 14). properties of FRP composites and their comparison to steel.
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14 100.0 Percent of Bridges 80.0 U.S. Canada 60.0 40.0 20.0 0.0 yes no no answer Saddle Use FIGURE 14 Use of saddles. Tables 3 and 4 show reported comparisons of different oped. There is a cushioning layer used between the jaws and the material properties. rods (Bridge Design and Engineering 2005). The main advantages of FRP composite cables are corro- A number of demonstration projects have been built with sion resistance and lighter weight. For CFRP, the coefficient FRPs. However, there are currently no known cable-stayed of thermal expansion is much lower than steel (approximately bridges in the United States and Canada with FRP cables. one-sixtieth), and the strain at rupture is reported to be 1.6% According to Seible and Burgueno (1997), the first all-com- as compared with 6% for steel (Roos and Noisternig 1999). posite cable-stayed pedestrian bridge was built in Aberfeldy, The main disadvantages of FRP composites are their high Scotland, in 1993, with aramid fiber stay cables. These cost and very low shear strength (both transverse and inter- authors also reported on the design of a vehicular cable- laminar shear strengths). The low shear strength seriously stayed composite bridge on the campus of the University of affects the gripping ability at anchorages (Christoffersen et al. California, San Diego (I5/Gilman). However, this bridge has 1999). Fisher and Bassett (1997) reported that although com- not been constructed. posite materials do not rust, "they can corrode when inte- grated into structures with incompatible materials." They Christoffersen et al. (1999) reported on the construction of report that carbon fiber can be subjected to galvanic corro- a CFRP cable-stayed bridge in Denmark. To protect against sion with metals and thus should be insulated from metallic possible damage to cables from fire, impact, or vandalism anchorage components. Similarly, glass fiber prestressing (saw cutting), the designers used stainless steel sheathing over tendons "can be susceptible to corrosion under sustained an extruded HDPE sheath. The design was also based on the loads when exposed to water or salt water." ability to sustain static failures of two adjacent cables or a sud- den failure of one cable. Provisions were made for periodic Various manufacturers have devised anchorage solutions. replacement of an original cable at 5-year intervals. These solutions, an example of which is shown in Figure 15, are typically based on a conical steel socket filled with a pot- The Storchenbrücke (Stork) Bridge in Winterthur, Switzer- ting material such as epoxy. However, a wedge-type anchorage land, incorporates two CFRP stay cables, each consisting of system for the carbon fiber composite cables of a pedestrian 241 parallel pultruded CFRP rods of 5 to 6 mm in diameter bridge (the Laroin Bridge in southern France) has been devel- (Hooks et al. 1997). The other 22 stay cables on this bridge TABLE 3 TYPICAL PROPERTIES OF THE MOST COMMON FRP MATERIALS AND STEEL Tensile Strength Young's Modulus Density Material ksi (MPa) ksi (GPA) lb/ft3 (kg/m3) CFRP (carbon) 245435 (17003000) 2030043500 (140300) 100 (1600) AFRP (aramid) 175305 (12002100) 725017400 (50120) 81 (1300) GFRP (glass) 218 (1500) 7250 (50) 150 (2400) Steel 270 (1860) 29000 (200) 490 (7850) Source: Christoffersen et al. (1999).
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15 TABLE 4 Figure 16 shows the results of the survey with respect to QUALITATIVE COMPARISON OF FRP PROPERTIES the type of MTE coatings used, if any, on bridges in the Properties GFRP AFRP CFRP United States and Canada. Strands and wires that are coated Environmental resistance -- + + with temporary protection oils, as described previously, are considered bare in the survey. Cables with bare MTEs repre- Tensile strength + + ++ sent 43% of the survey bridges in the United States, whereas Fatigue strength 0 -- ++ no Canadian bridges use bare MTEs. Young's modulus -- -- ++ Creep/relaxation -- 0 ++ Stress fatigue -- -- ++ Galvanizing Density + ++ ++ Material price ++ -- -- A very common coating for strands that is used extensively in Europe and Japan is zinc coating (hot dip galvanizing). Notes: -- = not good, 0 = neutral, + = good, ++ = very good. Galvanizing is a sacrificial form of cathodic protection against Source: Christoffersen et al. (1999). corrosion and can be consumed with time, especially in an aggressive environment. In the United States, however, gal- have steel MTEs. The stiffness of the anchorage filler material vanized MTEs have not been used very often for stay cables, was varied along the length of the anchorage by adding alu- except for the Sacramento River (Meridian) Bridge in Cali- minum oxide pellets with varying thicknesses of epoxy coat- fornia and the two early bridges in Alaska, including the old- ing. The cables passed qualification fatigue and static testing est cable-stayed bridge in the United States, the Sitka Harbor (Hooks et al. 1997). Bridge. The main concern has been that the galvanizing process, especially with strands in contact with grout, could Roos and Noisternig (1999) reported on fatigue and static lead to hydrogen embrittlement. Corrosion and other electro- testing of CFRP stay cables with up to 91 wires using PTI rec- chemical processes can lead to evolution of hydrogen. Ab- ommendations. The cable sustained two million cycles of sorbed hydrogen can reduce the ductility of steel, through a fatigue loading without wire failure, but reached only a maxi- phenomenon known as hydrogen embrittlement (Barton et al. mum of 78% of nominal capacity and thus did not meet the 2000). On the other hand, 61% of Canadian bridges in the requirements. survey used galvanized MTE members. However, none of the Canadian bridges included galvanized MTE in contact MTE Coatings with cement grout. Various MTE coatings are available worldwide. These coat- It was also believed that the process of galvanizing would ings are mainly provided for the corrosion protection of MTE. degrade the tensile strength of strand and its fatigue life ("Cable In earlier stay cable designs when uncoated strands and cement Stays . . ." 1994). The concern for contact between galvanized grouts were used, it was assumed that grout would provide the strand and cement is widely held (Ito 1999). However, PTI necessary protection. However, given that the time between recommendations state that "galvanized prestressing strand stressing of strands and grouting could be several months or may be used in contact with cement grout provided the steel years, it soon became clear that the strands would be left has been manufactured in accordance with the latest ASTM unprotected and could corrode within that time period. One A416, BS 5896, or EN 10138 standard. Experience has shown of the early steps taken to address this issue was to use water- that strand manufactured to these standards is not susceptible soluble oil sprays on the strands (Funahashi 1995). Later, a to hydrogen embrittlement" (Recommendations for Stay Cable protective/lubricant coating (a petroleum microcrystalline Design . . . 2001). The PTI document does not refer to other wax based product) was applied to the strands. references that form the basis for that statement. The PTI recommendations also include the following: Galvanized strand is made from either as-galvanized wires (in Japan) or drawn-galvanized wires (in Europe). The advantage of as-galvanized wire is heavier coating weight (300 g/m2) or more for better corrosion protection. The advantage of drawn-galvanized wire, on the other hand, is improved fatigue performance and tighter control on tolerance. In Europe, galvanized wires and strands are routinely used for ungrouted stay cables, and special manufacturing processes are adopted that reportedly ensure compliance with the strength FIGURE 15 One type of CFRP anchorage and fatigue requirements including those of the standards (Roos and Noisternig 1999). listed by the PTI document. In the United States, the market
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16 70.0 Percent of Bridges 60.0 U.S. Canada 50.0 40.0 30.0 20.0 10.0 0.0 bare greased- epoxy- epoxy- galvanized stainless other no answer and- coated coated in steel sheathed outside only and out MTE Coating FIGURE 16 Types of MTE coatings used. conditions have reportedly not yet justified local production of 284 ksi (1960 MPa) for 5 mm/0.197 in. wires have been galvanized strands of sufficient quality (fatigue and strength) developed in Japan. They reported good fatigue and low- for use in stay cables. The Buy America Act enacted by the temperature response and elongations of 6% to 7%. Tauri U.S. Congress in 1933 has so far effectively prevented the et al. (2001) attributed the loss of strength in galvanized importation of stay cable-quality galvanized strands. Accord- wires to the "spheroidizing of cementite, resulting in the col- ing to the Buy America Act, all federal construction contracts lapse of the lamellar structure of ferrite and cementite." They that are undertaken within the United States must use domes- reported that the silicon and chromium elements can suppress tic construction materials, subject to a few exceptions. There- this loss of strength. fore, galvanized wires and strands are currently not being used in U.S. stay cables. It should be noted that galvanized strands individually sheathed with HDPE are also available and have been used Suzumura and Nakamura (2004) studied environmental overseas. According to a worldwide survey of the stay cable factors affecting corrosion of galvanized steel wires for sus- industry performed by Hamilton and Breen (1995), the pension bridges. They concluded that galvanized steel wires galvanized-and-sheathed strand is the most highly rated by did not corrode when kept in an environment with a relative the respondents. humidity of less than 60%. The corrosion rate increased sig- nificantly with temperature. They reported that for a wire kept in a wet environment the zinc layer (350 g/m2) would be con- Individually Sheathed Strands with Corrosion sumed within 10 years. In 100% and 60% relative humidity Inhibiting Coating environments, the consumption of zinc would be complete in 34 years and 211 years, respectively. Figure 17 shows the PTI provides detailed recommendations for such strands, effects of relative humidity and sodium chloride on the cor- which are typically referred to as greased-and-sheathed or rosion rate. Figure 18 shows the effect of temperature on the waxed-and-sheathed strands (Recommendation for Stay Cable corrosion rate. Design . . . 2001). The grease or wax is believed to reduce potential for fretting fatigue resulting from interwire contact Tarui et al. (2001) reported that galvanized wires with (Frank and Breen 2004). These strands are individually coated strengths of 256 ksi (1770 MPa) for 7 mm/0.276 in. wires and and then covered with HDPE or high-density polypropylene FIGURE 18 Relative corrosion rate--wet condition FIGURE 17 Corrosion rate for galvanized wire without chloride exposure (Suzumura and Nakamura (Suzumura and Nakamura 2004). 2004).
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17 (HDPP) that is extruded over the strand. These systems have strand." According to the questionnaire response received, been common in all recently constructed cable-stayed bridges moisture has been found in the cable anchorages on this bridge. in the United States. Examples of bridges using these types of strands are the Cape Girardeau Bridge in Missouri and the Sixth Street Viaduct Bridges in Wisconsin. Cable Sheathings and Wraps Options for cable sheathings include HDPE, steel, stainless Epoxy Coating steel, or aluminum (Ito 1999). The most common cable sheath- ing is HDPE (Figure 20). Seventeen U.S. cable-stayed bridges The use of epoxy-coated strands became popular in the United included in the survey responses (61%) have HDPE pipes States in the early to mid-1990s, and was used on at least four around the cables. However, other bridges such as the Dame bridges in the United States in that decade. Three types of such Point Bridge (Florida), Maumee River Bridge (Ohio), and the strands were originally available for stay cables. In one, an Sunshine Skyway Bridge (Florida) have steel pipes. The new epoxy coating with a smooth surface was applied on the out- Maumee River Bridge is designed with stainless steel pipes. side perimeter of the seven-wire strands, thus leaving air voids The cable sheathing, when used, serves as the first line of in the interstitial spaces between the six outside wires and defense, a barrier against damage or intrusion of harmful sub- the center wire (Figure 19). The second type of epoxy-coated stances from the outside. In cases where grout or other fillers strand produced was similar to the first, except for a grit- are used, the sheathing also serves as a container for the filler. impregnated surface to improve bond with grout. The third The survey indicates that three bridges in the United States and type of strand had epoxy in all interstitial spaces in addition to nine bridges in Canada do not have any external sheathing. the outside surface. The FHWA advisory ("Cable Stays . . ." 1994) and the current PTI provisions (Recommendation for The HDPE pipes include approximately 2% to 3% carbon Stay Cable Design . . . 2001) recommend that only epoxy- to protect against ultraviolet radiation (Saul and Svensson coated strands with filled interstices should be used for stay 1991; Ito 1999). However, considering that the coefficient of cables. thermal expansion of HDPE is much higher than the grout or steel (Funahashi 1995), and that the basic color of HDPE with Qualification tests in the early 1990s on unfilled strands carbon is black, the issue of increased surface temperatures indicated that pressurized grout water could infiltrate the void had to be addressed. Saul and Svensson (1991) reported that spaces inside the strands and remain there as free water, the surface temperature of black pipes can reach more than resulting in extensive corrosion and fatigue fractures in the 149°F (65°C) owing to direction solar radiation, whereas the time period of the test (Tabatabai et al. 1995). Although a surface of a white pipe under the same condition would reach complete determination of the path of water was not made, it only 104°F (40°C). Paint does not adhere well to HDPE. was clear that one likely source was the penetration (and Until recently, new HDPE-covered cables were commonly breach) of the epoxy coating at the wedges. Corrosion tests by wrapped with a light color self-adhesive polyvinyl fluoride Hamilton et al. (1998b) also showed corrosion inside unfilled (PVF) tape (mostly referred to by the commercial name epoxy-coated strands, but no corrosion was found in the filled Tedlar®), which was spirally wrapped around the HDPE strands. pipe. Typically, a 50% overlap is provided. Saul and Svensson (1991) reported that during the installa- Some damage has been reported on wrapped tapes in some tion of stay cables for the Quincy Bridge in Illinois, "it became bridges (based on the survey results). In one case, the Pasco apparent that the ends of the strands must be sealed with an Kennewick Bridge in Washington State, the damage was epoxy coating in order to prevent moisture rising due to capil- reported to be extensive. In that case, polyvinyl chloride lary pressure through the full height of the cables in the (PVC) tapes were first used over the pipes, and these tapes interstices between the individual seven wires forming each became brittle after several years and began to flake off (Saul and Svensson 1991). In other cases, the damage was reported to be minor. A laminated tape consisting of a trans- lucent Tedlar tape with a color PVC tape backing was also developed (Saul and Svensson 1991). In recent years, co- extruded HDPE pipes with bright surface colors have entered the market, and recently constructed bridges use this approach in lieu of the PVF tape. Figure 21 shows the results of the sur- vey with respect to damage to the protective tape. In tests performed by Hamilton et al. (1998), clear HDPE sheathing was used to allow assessment of grout condition FIGURE 19 Epoxy-coated strand inside the sheathing. There is no information available that (Funahashi 1995). would indicate if clear HDPE sheathing has ever been used
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18 80 70 Percent of Bridges 60 U.S. Canada 50 40 30 20 10 0 HDPE steel Pipe no sheathing other no answer Cable Sheathing FIGURE 20 Types of cable sheathing used. on stay cables in the field to facilitate inspections. An impor- thickness of the pipe. On some of the early bridge projects, such tant challenge would be the required resistance to ultraviolet as the ZarateBrazo Largo Bridges in Argentina and the Luling radiation. Bridge in Louisiana, there have been problems with grouting operation that reportedly contributed to the cracking of the Figure 22 shows the results of the survey with respect to the HDPE pipe (Saul and Svensson 1991). The authors discussed cracking of cable sheathing or sheathing connections. Four HDPE stresses as a result of coiling and uncoiling, effects of bridges or 14% of the respondents in the United States (and grouting pressures, and effects of high temperatures at the time none in Canada) reported problems with the sheathing or con- of grouting. nections. In the C&D Canal Bridge in St. Georges, Delaware, cracking of the steel sheathing was noted on one of the stay Steel pipe segments are typically welded together in the cables. This cracking was attributed to the position of a grout field. The external pipe is generally bolted to the anchorage vent hole at a high-stress location near a pylon. The respon- pipe. The axial and flexural stiffness of the steel pipe is far dents to the questionnaire also identified two bridges that had greater than that of the HDPE. In a grouted system, sufficient splitting of HDPE [Quincy (Illinois) and Luling (Louisiana)]. bond between the grout and the steel pipe can be developed, thus transmitting some of the fluctuating cable stresses into In field-fabricated cables, the HDPE pipe segments are typ- the sheathing (owing to strain compatibility). In some quali- ically assembled and welded together (HDPE welding) by spe- fication tests, steel sheathing connections developed fatigue cial machines on the bridge deck before being lifted into place. fractures as a result of this unintended effect (Tabatabai et al. In some shop-fabricated cable systems, the cable assemblies 1995). Saul and Svensson (1991) also reported that "some (including HDPE) are assembled, coiled, and then shipped on welded connections have failed in the past" without elaborat- large reels. The coiling and uncoiling of HDPE pipes at low ing. Steel sheathings must also be periodically painted to pro- temperatures can lead to cracking (Funahashi 1995). In newer shop-fabricated Japanese cable systems, the HDPE is extruded tect against corrosion. over the MTE bundle, thus creating a tight fit between the sheathing and MTE. Fillers and Blocking Compounds When a cable with HDPE sheathing is grouted, the pipe Fillers refer to materials placed inside the sheathing and must resist grouting pressures. This would increase the required around the MTE. In United States practice, the most common 80 70 U. S. Canada Percent of Bridges 60 50 40 30 20 10 0 yes no not known not applicable no answer Wrapping Tape FIGURE 21 Damage to wrapping tape.
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19 70.0 Percent of Bridges 60.0 50.0 U. S. Canada 40.0 30.0 20.0 10.0 0.0 ns le no n h ng er w ot ab tio sw hi no -b lic at c an tk ne he pp s ye no no on -s ta -c no s ye s ye Cracking FIGURE 22 Cracking of cable sheathing and connections. type of filler within the HDPE pipe and the MTE has been the · The increased mass owing to the grout helps with damp- cement grout. Table 5 shows the results of the survey with ing and vibration control. respect to the type of fillers used, if any, in the stay cables. Fifty-four percent of U.S. bridges in the survey include The disadvantages are: some type of cement grout in the free length of the cable. None of the Canadian bridges use cement grout. Table 6 shows · Stress fluctuations in the cable and grout shrinkage can responses to the survey with respect to the filler materials used result in the cracking of the grout. This cracking can in the anchorage zones. lead to intrusion of moisture if the external sheathing is breached. · Grouting adds to the cost of cables. · Grouting could complicate many types of NDT and Portland Cement Grout inspections. · Grout water and bleed water could present internally There have been a variety of opinions on the merits of cement driven corrosion danger when not properly controlled. grouts for stay cables. As stated earlier, the practice of grout- Voids could be introduced inside grouted ducts. ing stay cables comes from the post-tensioning technology, and not from the suspension cable technology. Grouting has Tabatabai et al. (1995) performed qualification tests on not, in general, been very popular in Europe (Hamilton and some grouted stay cable specimens with uncoated (bare) Breen 1995). The main advantages typically given for cement strands. The dissection of cable specimens after fatigue and grout in stay cables are: static tests indicated transverse cracking in the grout. Corro- sion was noted on the strand at the intersection of the grout · Cement grout provides a physical barrier for the MTE cracks and the strand, some with surface pitting. Fatigue frac- that is not easily breached. tures were also noted at those locations, thus establishing that · Grout provides an alkali environment for the bare steel the cracking occurred early in the fatigue test and not in the and protects it against corrosion. subsequent static test. Figure 23 shows corrosion at the loca- tion of grout cracking. Ito (1999) refers to the presence of grout cracks and how they may be associated with potential for "fretting corrosion" of steel wires. TABLE 5 SURVEY RESULTS--TYPE OF GROUT USED? (Question 4.7) U.S. % U.S. Canada % Canada % total TABLE 6 Grout not used 6 21.4 12 92.3 43.9 SURVEY RESULTS--ARE FILLER MATERIALS USED IN THE ANCHORAGE ZONE? (Question 4.8) Cementwater 5 17.9 0 0.0 12.2 Cementwater 9 32.1 0 0.0 22.0 U.S. % U.S. Canada % Canada % total admixtures Yes--grout 6 21.4 0 0.0 14.6 Commercial pre- 1 3.6 0 0.0 2.4 Yes--grease 10 35.7 0 0.0 24.4 packaged grouts Yes--other 7 25.0 2 15.4 22.0 Not known 6 21.4 0 0.0 14.6 No filler 0 0.0 11 84.6 26.8 Not applicable 0 0.0 1 7.7 2.4 Not known 4 14.3 0 0.0 9.8 No answer 1 3.6 0 0.0 2.4 No answer 1 3.6 0 0.0 2.4
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20 these bridges. Ito (1999) reported that grout cracking has been observed on some cable-stayed bridges, but does not provide additional information. Saul and Svensson (1991) also reported grout cracking on cable test specimens. Hamilton et al. (1998a,b) reported on accelerated corro- sion tests done on eight grouted stay cable specimens, five of which were uncoated (bare) strands, one with epoxy-coated strands (some filled and some unfilled), one with a galva- nized strand, and one with a greased-and-sheathed strand. These specimens were loaded and grouted inside clear pipes to allow visual examination of grout surface. Windows were cut into the pipes to simulate damage to the HDPE. Wet and dry saltwater ponding cycles were initiated to represent long- FIGURE 23 Corrosion of strand at transverse crack in grout. term ingress of chloride-laden air and moisture through the openings. The main objective was to determine if the cement grout can provide positive secondary protection if the first protective layer (HDPE) is breached. Hamilton et al. (1998a,b) There are conflicting results from examination of grout concluded that a relatively low level of loading above the conditions on four bridges. In these bridges, the Cochrane grout injection load would result in grout cracking. The salt Bridge (Alabama), PascoKennewick Bridge (Washington solution was able to reach almost any location in the speci- State), Talmadge Memorial (Georgia), and the Fred Hartman mens, and the primary mechanism for corrosion was crack- Bridge (Texas), the HDPE sheathing was partially removed ing of grout at sheathing breaks. According to the authors, (windows cut) during inspections to allow examination of the galvanized, greased-and-sheathed, and filled epoxy-coated condition of grout and wires (Grant 1991; Tabatabai et al. strands provided vast improvement over the bare strands. 1998; Dowd et al. 2001; and survey results). In the Cochrane Corrosion was observed inside the unfilled epoxy-coated Bridge, no cracking of grout or corrosion of MTE was reported strands. Therefore, the authors concluded that the traditional (Tabatabai et al. 1998a). Grant (1991) also reported no grout grout-bare-strand HDPE system could no longer be consid- cracks for the PascoKennewick Bridge. However, Frank and ered a redundant system. Frank and Breen (2004) concluded Breen (1994) reported that in the PascoKennewick Bridge that the use of portland cement grout has not proven to be an there were small, closely spaced grout cracks perpendicular to effective corrosion protection barrier. the stay (Figure 24). The inspection of grout for the Fred Hart- man Bridge indicated "fine, intersecting transverse and longi- When wires of bare strands are encased in grout fracture, the tudinal cracks, with spacing between 12 and 19 mm." These force in the broken wire redevelops a relatively short distance cracks were not readily evident. away from the fracture. Some qualification tests have shown multiple fractures on the same wire over a length of a few It is not known whether longitudinal cracking of HDPE inches (Tabatabai et al. 1995). Therefore, the overall cable axial pipes in some bridges (such as ZarateBrazo Largo in stiffness would not necessarily change when limited numbers Argentina) affected the integrity of the grout, because no of individual wire breaks occur, especially when those breaks examination of the grout was reported in the literature for are spread over some distance. This can be viewed as both pos- itive and negative; positive because cross-section strength at locations away from fracture would remain unchanged and negative because monitoring of cable force changes (or deck profile deflection changes) would not indicate loss of section because stiffness has not been affected substantially. The global stiffness of the cable would remain essentially unchanged even when moderate wire section losses occur. Other Fillers Ito (1999) reported that cement grout plasticized with poly- urethane has been used in some bridges. A synthetic resin material based on polybutadiene was used on two Japanese bridges (Ito 1999). Grease and wax have also been used. In FIGURE 24 Exposed cable on the PascoKennewick Bridge the Alex Fraser (Annacis) Bridge in British Columbia, petro- (Frank and Breen 1994). leum wax blocking compound was used inside the sheathing.
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21 Wax is injected at high temperatures and solidifies when cooled, resulting in shrinkage and cracking. Ito (1999) reported that a type of petroleum wax that could be applied at ambient temperature has been developed. Hemmert and Sczyslo (1999) reported that red lead is com- monly used in locked coil cables. A coating of paint is some- times used over the locked coil cables. One option is not to have any fillers inside the HDPE pipe. That is the approach used on the Charles River Cross- ing Bridge in Boston, Maumee River Bridge in Ohio, Sixth Street Viaduct Bridges in Wisconsin, and Cooper River Bridge in South Carolina, where individually coated and sheathed strands (or epoxy-coated strands) are used inside HDPE pipe without cement grout. The Charles River Bridge is believed to be the first ungrouted parallel strand stay cable system built in the United States and marks a major shift in the stay cable technology in this country. In response to the ques- tionnaire, the cable manufacturer for the Charles River Bridge, Freyssinet LLC, stated that the ungrouted system FIGURE 25 Anchorage detail including neoprene washers for would improve inspectability and allows for future replace- the Cochrane Bridge (Alabama) (Telang et al. 2000). ments. All of the major cable suppliers in the United States currently offer cable systems with the no-grout option. neoprene ring) and its reliability is subject to debate, it is clear Little et al. (2001) discussed fungal-influenced corrosion that problems with neoprene rings can exacerbate cable vibra- of post-tensioned tendons. They reported that bacteria have tion problems. Telang et al. (2000) reported on problems with been implicated in corrosion of tendons in structures. Their the washers on the Cochrane Bridge that likely contributed to experiments showed the fungal degradation of lubricating excessive rainwind vibrations. The cable was not centrally grease, which produced formic and acetic acids resulting in located in the middle of the steel anchor pipe (or the guide corrosion of steel cables. Fusarium sp., Penicillium sp., and pipe); thus, the thickness of neoprene around the HDPE was Hormoconis sp. were isolated from corroding tendons in a variable. Also, there were gaps between the neoprene ring and post-tensioned structure and used in testing. The test speci- the cable along the perimeter (see Figure 26). This would mens were coated with "metal soap hydrocarbon grease" be- reduce the effectiveness of the washer both in reducing bend- fore insertion into PVC sheathing. There were no indications ing stresses and in damping vibrations. of chlorides in the energy-dispersive X-ray analysis system spectra of the grease. This article did not refer specifically to The steel rings that typically hold the washers in place stay cables. ("keeper rings") can fail and result in the dislocation and mis- alignment of the neoprene ring (Figure 27). Neoprene Rings A stay cable is subjected to lateral movements as a result of vibrations. These vibrations create bending stresses at the two ends of the cable, thus increasing the potential for fatigue. To address this issue, most cable-stayed bridges in the United States have what are termed neoprene rubber "washers," "rings," or "donuts" placed around the HDPE pipe within the guide pipe (anchor pipe) near the ends of the cables. Figure 25, a diagram of the anchorage for the Cochrane Bridge in Alabama, shows the typical position of the neo- prene washer with respect to the other components of cable anchorage. In addition to reducing bending stresses at the anchorages, the neoprene rings also contribute to the vibration damping of FIGURE 26 Cap between cable and neoprene washer (Telang the cables. Although the level of damping (attributed to the et al. 2000).