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5 Post-Tensioning and Stay Cable Systems Introduction This chapter is comprised of four major sections. The first section presents an overview of the bridge post-tensioning system, which consists of the internal and the external post- tensioning systems. The second section discusses the vari- ous components of the post-tensioning system. The third section includes an overview of the stay cable systems, and the fourth section discusses the various components of the stay cable system. Overview of Bridge Post-Tensioning Systems In the late 1920s, Eugene Freyssinet, a French civil engi- neer, pioneered prestressed concrete technology. He patented prestressed concrete technology in 1928 and is considered the father of prestressed concrete (Emmanuel 1994). Although Freyssinet pioneered prestressed concrete, C. W. Doehring pat- ented prestressing methods as early as 1888. Freyssinet recog- nized that only high-strength prestressing wire could counteract the effects of creep, develop anchorage, and improve other load-carrying attributes, which helped in the widespread use of prestressed concrete technology in many structural systems, including long-span segmental bridges. Two types of stress- ing technologies are commonly used: pre-tensioning, where the stress is applied before the placement of concrete; and post- tensioning, where the stress is applied after the concrete cures. Post-tensioned (PT) structural elements are used quite often in bridges due to their ability to achieve long spans economi- cally while providing an aesthetically pleasing structure. In addi- tion, the use of post-tensioning systems in long-span bridges has become influential since they reduce the overall cost of the sub- structure. This is especially significant when substructure costs increase due to complexities in span conditions such as bridges spanning seas, valleys, and developed urban areas. Post- tensioning systems also are preferred in bridge construction because they greatly increase structural capacities and are fairly easy to implement effectively. The following discussion pro- vides more information on bridge post-tensioning systems. Post-Tensioning Systems PT construction consists of prestressing steel tendons that are placed in longitudinal ducts and tensioned after the con- crete cures. Post-tensioning can be an economical alternative for long-span bridge structures, as well as reduce construction time. Post-tensioning is typically applied in both segmental bridge construction, and the rehabilitation and strengthen- ing of bridges. Figure 2-1 shows a PT segmental bridge under construction. While the button-head tendon system typically was used for bridges until the late 1970s, most modern con- struction uses a tendon system where seven-wire strands are seated with anchor wedges within the anchor head. Based on the location of the tendons, post-tensioning sys- tems are classified into two groups, internal and external post- tensioning systems. A tendon placed inside the concrete is defined as an internal tendon, while a tendon placed outside the concrete is defined as an external tendon. In general, segmental PT bridges may have either or both of these tendon systems. Figure 2-2 shows a schematic of a cross-section at mid-span of a typical PT bridge girder. In this figure, tendons T1âT3 are external tendons, whereas tendons T4âT7 are internal tendons. Tendons are also classified as bonded or unbonded tendons. A tendon that is in direct contact to the adjacent concrete/grout is defined as a bonded tendon, whereas a tendon that is not in direct contact with concrete or cannot transfer the stress through surface bonding is defined as an unbonded tendon. In general, when ducts are filled completely with strands and grout without any voids, external tendons are considered unbonded tendons and internal tendons are considered bonded tendons. However, internal tendons are also unbonded if injected with flexible filler. Voids, if present, can cause discontinuity in the transfer of stress to the adjacent concrete along the tendon length. C h a p t e r 2
6Internal Post-Tensioning Internal post-tensioning is characterized by placement of the steel tendons in longitudinal ducts that are within con- crete elements. Figure 2-3 shows examples of internal tendons in precast concrete elements. The tendons are stressed and the ducts grouted. While the anchorage fittings transfer the main prestressing force to the concrete, the grout acts to improve the transfer of stress in the event that a member is overloaded to its ultimate flexural strength. This is accomplished by the grout bonding the tendon to the inner wall of the duct, while the duct itself is bonded to the surrounding concrete. External Post-Tensioning External post-tensioning refers to cases where the ducts containing the steel prestressing strands are not bonded to the concrete member. Rather, the PT tendons run through voided regions within a girder, such as through the interior space of a segmental box section as shown in Figure 2-4. The desired tendon profile is typically maintained by passing the tendons through sleeves cast within diaphragms and devia- tors. The tendons are grouted within the duct to protect the steel strands from corrosive elements. As the tendons are not embedded inside the cured concrete section, the monitoring, repair, and maintenance of external post-tensioning systems are not as complex as those for inter- nal post-tensioning systems. However, because of the absence of concrete cover protection and the possible presence of unwanted air voids, external tendons can be more vulnerable to corrosion than internal tendons within the same bridge segment. Although post-tensioning systems provide many advantages for designers and contractors, these systems have raised con- cerns regarding corrosion of the PT tendons. Unlike conven- tionally reinforced concrete systems where corrosion distress can be identified by visual staining, cracking, or spalling of the concrete cover, corroding post-tensioning systems sel- dom show these surface distress indicators. These distress indicators are typically not visible as the tendons are embed- ded in ducts far from the external surface of the structure, or the ducts confine the corrosion processes, or the corrosion processes form in voids within the ducts. Even so, the degree Figure 2-1. PT segmental bridge under construction (DSI 2012). Figure 2-2. Cross-section of a PT box girder with internal and external tendons (Trejo et al. 2009). Figure 2-3. Ducts for internal tendons (Tinkey and Olson 2007). Figure 2-4. External post-tensioning system (VSL 2013).
7 of corrosion of PT tendons is more critical to the structural performance of post-tensioning systems than conventionally reinforced systems. In addition, the cost to replace tendons can exceed several hundred thousand dollars per tendon. Table 2-1 compares the advantages and disadvantages of internal and external post-tensioning systems. Components of Post-Tensioning System A post-tensioning system comprises of several components such as the tendons, ducts, grout, and the anchorage system. Tendons Tendons are the main components of the post-tensioning system. The tendons are comprised of multiple high-strength steel wire strands. The strands in turn are formed from seven individual wires, where six wires are helically wound around a center wire. All strands are grade 270 low-relaxation seven- wire strands. Typical strands have a nominal diameter of 0.5 in. or 0.6 in. The tendons in a post-tensioning system are installed after the concrete had sufficiently cured to a prescribed initial compressive strength. During the jacking operation, the ten- dons are subject to tensile forces. Upon release of the jacking forces, the tendons induce compressive stresses in the concrete. Ducts Originally ducts were used to form a continuous void through the concrete structure for the later placement of inter- nal post-tensioning strands. However, with time the ducts have doubled up their role as a barrier to corrosive environment to protect the strands from corrosion. There are several types of ducts that are used in practice. ⢠Corrugated steel ducts: These ducts are spirally wound from galvanized strip steel. These ducts are manufactured with welded or interlocking seams with sufficient rigidity to maintain their profile between supports during fresh concrete pour. ⢠Smooth rigid steel pipes: These ducts are typically used in external tendons when the tendons deviate through seg- mental bridge pier segment diaphragms and deviators. ⢠Corrugated plastic: These ducts are manufactured from polyethylene or polypropylene, and are typically used for internal tendons. ⢠Corrugated plastic tight-radius duct: These ducts are especially formulated for tight-radii applications. They Advantages Disadvantages Internal Post- Tensioning Allow for possible reanchoring of the strands, losing only locally the prestress force in the event of failure of a section Tendons can follow well the moment diagram Low production costs System reserves available Concentration of vent hoses at the road surface (can be avoided) Design experience necessary High quality requirements, especially during grouting Grout characteristics are critical Expensive maintenance Very difficult to investigate and impossible to replace External Post- Tensioning Webs free of tendons Low weight High quality No vent hoses Tendons with high level corrosion protection Restressable Strengthenable Theoretically replaceable More easy to investigate Can be damaged by external conditions Exposed to atmospheric influences Reserve is missing due to no bond Full loading action on the anchorage Critical assembly operations More expensive construction and demolition More sensitive to fire More vulnerable to vandalism With any failure the prestressing force disappears over the overall length of the tendon The buckling and whipping generated by a sudden rupture create risks for the inspection staff and for the other tendons Design experience necessary Table 2-1. Advantages and disadvantages of internal and external post-tensioning systems (Pereira 2003).
8are typically used in anchorage blisters, external tendon deviation points, and as looped vertical tendons in precast PT piers. ⢠Smooth high-density polyethylene (HDPE) pipe: These ducts are typically used for external tendon applications. These pipes are protected against ultraviolet light by the small amount of carbon present in them. Grout Cement grout is chemically basic and provides a protective environment around the post-tensioning strands. The grout also bonds the internal tendons to the structure. There are four classes of grouts that can be used for most post-tensioning applications as specified by the specification for grouting of PT structures (PTI M55.1-12, 2012). The four classes of grout and their recommended exposure conditions are: ⢠Class A â nonaggressive: indoor or nonaggressive outdoor. ⢠Class B â aggressive: subject to wet/dry cycles, marine envi- ronment, deicing salts. ⢠Class C â nonaggressive or aggressive (pre-packages). ⢠Class D â determined by engineer. Anchorage Systems The anchorage system mainly constitutes of the trumpet, bearing plate, wedge plate, wedges, and the permanent grout caps. Figure 2-5 shows a basic bearing plate anchorage system. Basic bearing plate anchorages consist of a flat plate bearing directly against the concrete face. The plates may be square, rectangular, or round, and are sheared or torch cut from any readily available ASTM A36 steel plate. This is used in conjunc- tion with trumpets that are made of galvanized steel metal or plastic, to transition from the strand spacing in the wedge to the duct. Wedges are used to hold the post-tensioning strands in place and prevent them from slipping through the anchor head holes. Wedge performance is crucial for the proper anchoring of strands. Case-hardened low-carbon or alloy steel with ductile core are used for the wedge to bite into the strands and conform to the irregularity between the strand and wedge hole. Grout caps are used to protect the tendon ends in the anchor- age zones. The grout caps are typically made of noncorrosive materials such as fiber-reinforced plastic, stainless steel, or galvanized ferrous metals. Overview of Bridge Stay Cable Systems The cable-stayed bridge consists of a bridge deck supported by a system of inclined cables, or stays, passing over or attached to towers located at the main abutments or piers. Cable-stayed bridges are not a recent development, but have not seen wide- spread application until recently. Due to economy in weight, material, and cost, as well as its versatile concept that lends itself to a wide variety of geometric configurations, modern cable- stayed bridges have been successfully built in Europe since the mid-1950s and in the United States since the late 1970s. In par- ticular, cable-stayed bridge technology has become an increas- ingly popular choice for bridge engineers in the United States and worldwide since the 1990s. Figure 2-6 illustrates the num- ber of cable-stayed bridges built in the United States from 1955 to 2005. It can be seen that the number of cable-stayed bridges has significantly grown since the 90s. Stay Cable Systems A typical elevation of a multi-span cable-stayed bridge is shown in Figure 2-7. The major components of cable-stayed bridges are cables, a deck, anchors, and towers. In a typical arrangement, the towers and deck are subjected to compressive stresses, whereas the stay cables are under tensile stresses. The Figure 2-5. Basic bearing plate anchorage system (VSL 2013). Figure 2-6. Number of cable- stayed bridges built in the United States (Tabatabai 2005).
9 cables of a cable-stayed bridge must have large tensile strength and high fatigue resistance. There are several systems for cable-stayed bridge cables that have been used in the past and continue to be used today. The main components of a stay cable system are discussed in the following sections. Components of Stay Cable System Stay cable systems consist of several basic components such as MTEs, sheathing, corrosion protection systems, anchorage systems, and saddles. Main Tension Elements MTEs are the fundamental tension resisting elements in the stay cable. There are several types of MTEs, including: structural ropes, helical locked coil strands, structural strands, parallel prestressing wire cables, parallel prestressing strand cables, and parallel prestressing bar cables. Some of the MTE systems are shown in Figure 2-8. In the United States, the parallel seven-wire strand system is the most commonly used stay cable system (Hamilton III 1995). Sheathing Sheathing is the outer enclosure for the MTE and serves as a barrier between the environment and the MTEs. Sheathing also holds the grout around the MTEs. The most common sheath- ing is HDPE, while steel, stainless steel, aluminum, and copper have also been used. Sheathings were first used in 1961 in the Schillerstrasse pedestrian bridge in Stuttgart, Germany. They encase the steel from anchorage region to anchorage/saddle region and serve two main purposes. First, they provide an ini- tial barrier to protect the strand from the outside environment. Secondly, sheathings allow grout to be injected into the cable which serves as a corrosion resistant layer. Grouted cables will always be encased within a sheathing but cables with a sheath- ing, are not all necessarily grouted, as other corrosion protec- tion systems may be used with sheathings (Weseman 1994). Two commonly used types of sheathing materials are steel pipe and HDPE. Steel pipe sheathing is only used in three bridges in the United States: Dame Point Bridge in Florida, Sunshine Skyway Bridge in Florida, and the C and D Canal Bridge in Delaware (Tabatabai 2005). Black steel pipe (ASTM A53) is generally lifted into place and fixed by butt-welding in the field (Weseman 1994). The strands are passed through the pipe and stressed to design loads. The tensile strength of the steel pipe is not accounted for in the strength of the MTEs. Typically, grouting is used as the corrosion protection sys- tem when steel pipe sheathing is used (Weseman 1994). The grout bonds the strand to the steel sheathing, which causes the sheathing to oscillate as the MTEs oscillate and in turn, experience the same fatigue loadings as the strands. This is the primary reason that steel pipe is the least commonly used sheathing (Hamilton III 1995). HDPE is more commonly used when compared to steel pipe as a sheathing material. HDPE is advantageous in that it is both nonreactive and almost impermeable, assuming that it is undamaged. The HDPE sheathing is lifted into place segmentally and either fusion welded or coupled together in order to form a continuous stay. Corrosion resistance systems used with HDPE ducts include but are not limited to grouting, greasing and sheathing the individual strands, or epoxy coat- ing the strand. The latter two corrosion protection systems are applied during the manufacturing process and not in the field. When HDPE sheathing is grouted, the hardened grout is sus- ceptible to thermal expansion within the sheathing, causing expanding hoop stresses on the sheathing. These expanding hoop stresses, acting simultaneously with bending stresses due to cable oscillations, can lead to cracking of the HDPE sheath- ing, allowing possible intrusion of water, chlorides, or other contaminants (Hamilton III 1995). Figure 2-7. Typical multi-span cable-stayed bridge (Vannemreddi 2010). (b) Helical strands (c) Parallel seven-wire strands (d) Parallel wires (a) Locked coil strands Figure 2-8. Various cross-sections of the MTEs of stay cable systems (Tabatabai 2005).
10 Corrosion Protection Systems The corrosion protection system is just as important as the structural properties of the strand. Corrosion pro- tection systems can include various combinations of gal- vanizing, epoxy coatings, corrosion inhibiting greases and waxes, sheathing, and other materials. In Europe and Japan, galvanizing is frequently used for coating strands while epoxy coating and, most recently, greased-and-sheathed or waxed-and-sheathed strands, are commonly used in the United States. Corrosion can both reduce the strength and decrease the fatigue life of the strands and is currently a major issue in many cable-stayed bridges across the world. The impor- tance of corrosion protection is more known today than in the past and new methods of protecting the MTEs from the environment have been developed over the past decades. A worldwide survey of bridge owners concluded that the average expected life of bridge stay cables is 75 years, which requires a very effective corrosion protection sys- tem (Hamilton III 1995). The three corrosion protection systems that are most commonly used include grouting of the tendons, greasing and sheathing the strands, and epoxy coating the strands. Grouting of the tendons is the most widely used corrosion protection system, particularly in older bridges. The basis for this system is twofold. First, a duct filled with grout cov- ers the MTEs and thus does not allow the bare strand to be exposed to corrosive environment. Second, Portland cement used in grout is alkaline in nature and is able to prevent/ mitigate corrosive activity. Prior to 1999, grouts were com- posed of cement, water, and an expanding admixture. But, a study by Schokker et al. (1999) recommended the addition of fly ash and the removal of the expanding admixture. In 2003, the first generation of pre-bagged thixotropic grouts became available, which is a Type C and most recommended type of grout today per the Post-Tensioning Institute (PTI). There are several different specifications for grout provided by different entities, including PTI, ASTM, AASHTO, DOTs, and possibly bridge owners (Merrill 2014). Typically, the strands are placed in a duct and the duct is then injected with a cement grout to prevent corrosion. Grouting is an ideal corrosion protec- tion system in a laboratory but proper grouting procedures may not be followed in the field, and quality of the grouting depends on many factors like grout quality, weather, grouting team experience, etc. (Merrill 2014). Improper grouting pro- cedures may lead to conditions such as grouting voids, water infiltration, or other detrimental grouting conditions such as soft grout. These unfavorable conditions allow water and oxy- gen to enter into the duct over time, nullifying the alkaline environment of the grout and creating a potentially corrosive environment. Greasing and sheathing of prestressing strands is a corro- sion protection system applied to the steel in the manufactur- ing plant. The process is done by lubricating the strand with a corrosion-inhibiting grease, wax, or epoxy, and extruding a high-density polyethylene over the strand. The sheathing is placed as tightly around the strand as possible in order to ensure that no differential movement between the strand and the sheathing occurs during stressing and that no air voids exist in the space between the strand and sheathing (Weseman 1994). This sheathing also protects the strand from fatigue problems due to rubbing against other strands or the duct within the sad- dle regions. There are several advantages to this system. First, as the corrosion protection is applied at the manufacturing plant, the strand is not only protected during its service life, but also during storage, shipping, and installation. Second, as no grouting is used within the sheathing, it is possible to individually de-tension, remove, and replace each strand if maintenance is required (Weseman 1994). Greasing and sheathing of strands does require special corrosion protec- tion in the anchorage regions because the teeth of the wedges have to penetrate through the sheathing and grease, expos- ing the strand to a potentially corrosive environment (Shinichi 2006). Epoxy coating of prestressing steel is one of the most com- monly used factory applied corrosion protection systems and must conform to ASTM A882. It entails applying a thick layer of corrosion-resistant epoxy to the exterior of the strand (0.02â0.04 mm according to PTI recommendations) as well as within the interstices (Weseman 1994). This epoxy must adhere to the strand in order to prevent the epoxy from peeling off as it rubs with mechanical equipment or adjacent strands. As with the greasing and sheathing system, epoxy coating also protects the strand during storage, shipping, and installation. Overall, epoxy coating is extremely efficient at protecting the strand from corrosion. In a corrosion test performed by Sumiden Wire Products Corporation, corrosion resistance of epoxy-coated strand was compared to that of bare strands and galvanized strands when subjected to water, several chemical solutions, and a salt spray. The bare strand showed rust in three of the five test conditions, the galvanized strand showed rust in all five of the test conditions, and the epoxy-coated strand showed no visible corrosion in any of the tests (Shinichi 2006). Epoxy coating has several advantages to grouting: there is typi- cally higher quality control of corrosion protection in a manu- facturing plant than in the field, it saves labor cost of grouting, and may be inspected more easily. A primary disadvantage to coating the strand is that nicking of the coating can lead to peeling of the coating, exposing the bare strand. Similar to the greased-and-sheathed strands, the teeth of the wedges can pen- etrate the coating when gripping the strand leading to potential corrosion problems in the anchorage regions, if not properly accounted for (Shinichi 2006).
11 Saddles Saddles are used to avoid tower anchorages and enable a continuous stay cable passing over the tower. This requires that the MTEs be jacked simultaneously at the two deck anchor- ages on each side of the pylon, both during construction and future maintenance. In addition, in the event that a MTE needs to be replaced, twice the length of the MTEs needs to be replaced when compared to individually anchored MTEs. The use of saddles has not been encouraged by an FHWA Technical Advisory (Weseman 1994). Anchorage Systems Anchorage systems consist of all components required to anchor prestressing steel and transfer the prestressing force to the towers and to the superstructure. In the United States, point anchorages, HiAm-type anchorages, and bond socket-type anchorages are commonly used for stay cables. Point anchor- age systems consist of a conical wedge with a toothed center hole to grab and anchor the strand. In HiAm-type anchor- ages, a steel socket filled with zinc dust, steel balls, and epoxy is used to anchor the MTEs. In bond socket anchorages, a wedge is used to terminate MTEs at an anchorage plate and a conical socket is filled with either cement grout or epoxy (Tabatabai 2005). Closing Remarks An overview of the bridge post-tensioning system, which includes internal and external post-tensioning, and the stay cable systems are presented in this chapter. The various com- ponents that constitute the two systems are also presented.
