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One of the major advancements in bridge construction in the United States in the second half of the twentieth century was the development and use of prestressed concrete. Prestressed concrete bridges, offer a broad range of engineering solutions and a variety of aesthetic opportunities. The objective of this Manual is to provide guidance to individuals involved in the installation or inspection of post-tensioning work for post tensioned concrete bridges including post-tensioning systems, materials, installation and grouting of tendons.
The tensile strength of concrete is only about 10% of its compressive strength. As a result, plain concrete members are likely to crack when loaded. In order to resist tensile stresses which plain concrete cannot resist, it can be reinforced with steel reinforcing bars. Reinforcing is selected assuming that the tensile zone of the concrete carries no load and that tensile stresses are resisted only by tensile forces in the reinforcing bars. The resulting reinforced concrete member may crack, but it can effectively carry the design loads (Figure 1.1).
Figure 1.1 - Reinforced concrete beam under load
Although cracks occur in reinforced concrete, the cracks are normally very small and uniformly distributed. However, cracks in reinforced concrete can reduce long-term durability. Introducing a means of precompressing the tensile zones of concrete members to offset anticipated tensile stresses reduces or eliminates cracking to produce more durable concrete bridges.
The function of prestressing is to place the concrete structure under compression in those regions where load causes tensile stress. Tension caused by the load will first have to cancel the compression induced by the prestressing before it can crack the concrete. Figure 1.2 (a) shows a plainly reinforced concrete simple-span beam and fixed cantilever beam cracked under applied load. Figure 1.2(b) shows the same unloaded beams with prestressing forces applied by stressing high strength tendons. By placing the prestressing low in the simple-span beam and high in the cantilever beam, compression is induced in the tension zones; creating upward camber.
Figure 1.2(c) shows the two prestressed beams after loads have been applied. The loads cause both the simple-span beam and cantilever beam to deflect down, creating tensile stresses in the bottom of the simple-span beam and top of the cantilever beam. The Bridge Designer balances the effects of load and prestressing in such a way that tension from the loading is compensated by compression induced by the prestressing. Tension is eliminated under the combination of the two and tension cracks are prevented. Also, construction materials (concrete and steel) are used more efficiently; optimizing materials, construction effort and cost.
Figure 1.2 - Comparison of Reinforced and Prestressed Concrete Beams
Prestressing can be applied to concrete members in two ways, by pretensioning or post-tensioning. In pretensioned members the prestressing strands are tensioned against restraining bulkheads before the concrete is cast. After the concrete has been placed, allowed to harden and attain sufficient strength, the strands are released and their force is transferred to the concrete member. Prestressing by post-tensioning involves installing and stressing prestressing strand or bar tendons only after the concrete has been placed, hardened and attained a minimum compressive strength for that transfer.
Compressive forces are induced in a concrete structure by tensioning steel tendons of strands or bars placed in ducts embedded in the concrete. The tendons are installed after the concrete has been placed and sufficiently cured to a prescribed initial compressive strength. A hydraulic jack is attached to one or both ends of the tendon and pressurized to a predetermined value while bearing against the end of the concrete beam. This induces a predetermined force in the tendon and the tendon elongates elastically under this force. After jacking to the full, required force, the force in the tendon is transferred from the jack to the end anchorage.
Tendons made up of strands are secured by steel wedges that grip each strand and seat firmly in a wedge plate. The wedge plate itself carries all the strands and bears on a steel anchorage. The anchorage may be a simple steel bearing plate or may be a special casting with two or three concentric bearing surfaces that transfer the tendon force to the concrete. Bar tendons are usually threaded and anchor by means of spherical nuts that bear against a square or rectangular bearing plate cast into the concrete. For an explanation of post-tensioning terminology and acronyms, see Appendix A.
After stressing, protruding strands or bars of permanent tendons are cut off using an abrasive disc saw. Flame cutting should not be used as it negatively affects the characteristics of the prestressing steel. Approximately 20mm (¾ in) of strand is left to protrude from wedges or a certain minimum bar length is left beyond the nut of a bar anchor. Tendons are then grouted using a cementitious based grout. This grout is pumped through a grout inlet into the duct by means of a grout pump. Grouting is done carefully under controlled conditions using grout outlets to ensure that the duct anchorage and grout caps are completely filled. For final protection, after grouting, an anchorage may be covered by a cap of high quality grout contained in a permanent non-metallic and/or concrete pour-back with a durable seal-coat.
Post-tensioning and grouting operations require certain levels of experience, as outlined in Appendix B.
Many proprietary post-tensioning systems are available. Several suppliers produce systems for tendons made of wires, strands or bars. The most common systems found in bridge construction are multiple strand systems for permanent post-tensioning tendons and bar systems for both temporary and permanent situations. Refer to manufacturers' and suppliers' literature for details of available systems. Key features of three common systems (multiple-strand and bar tendons) are illustrated in Figures 1.3, 1.4 and 1.5.
Figure 1.3 - Typical Post-Tensioning Anchorage Hardware for Strand Tendons
Figure 1.4 - Typical Post-Tensioning Bar System Hardware. (Courtesy of Dywidag Systems International)
Figure 1.5 - Typical Post-Tensioning Bar System Hardware. (Courtesy of Williams Form Engineering Corporation)
Bridges of this type have a superstructure cross-section of solid or cellular construction.
They are built on-site using formwork supported by temporary falsework (Figure 1.6). Formwork creates the shape of the concrete section and any internal voids or diaphragms. Reinforcement and post-tensioning ducts are installed in the forms and then the concrete is placed, consolidated and cured. When the concrete attains sufficient strength, post-tensioning is installed and stressed to predetermined forces.
Figure 1.6 - Cast -In-Place Post-Tensioned Construction in California
Longitudinal post-tensioning typically comprises multi-strand tendons smoothly draped to a designed profile. In continuous spans, the tendon profile lies in the bottom of the section in the mid-span region and rises to the top of the section over interior supports. In simple spans and at the expansion ends of continuous spans, post-tensioning anchors are arranged vertically so that the resultant of the tendon anchor force passes close to the centroid of the section. A draped profile of this type provides the most effective distribution of internal prestress for this type of construction.
Precast, post-tensioned AASHTO and bulb-T girders are usually pre-tensioned sufficiently at the precast plant to carry their own self weight for transportation to the site and erection. On site, girders are first erected as simple spans. However, over the interior piers of a three or four-span unit, they are made continuous by cast-in-place joints that connect the girder ends and form transverse, reinforced diaphragms.
Post tensioning ducts cast into the webs are spliced through the cast-in-place joints. The ducts follow a smoothly curved, draped profile along each girder line, rising to the top of the girders over the interior piers and draping to the bottom flange in mid-span regions. Before the deck slab is cast, some or all of the tendons running the full length of the multi-span unit are installed and stressed, making each simple span I-girder into a series of continuous spans. When the deck slab has been cast and cured, additional tendons may be installed and stressed on the fully composite section. Tendons may be anchored in a variety of configurations at the ends of each continuous unit.
Longer spans can be built using similar techniques. A variable depth girder section cantilevering over a pier can be spliced to a typical precast girder in the main and side-spans. An example is shown in Figure 1.7
Figure 1.7-Spliced Haunched I-Girder of Main
Temporary supports are needed at the splice location in the side spans. The ends of girders have protruding mild reinforcing to help secure the girder to the closure concrete and ducts that splice with those of other girder components to accommodate tendons over the full length of the main unit. The variable depth girder sections are placed over the piers, aligned with the girders of the side spans, and closures cast. Usually, temporary strong-back beams support the drop-in girder of the main span while closures are cast.
The sequence for erecting and temporarily supporting this type of I-girder construction is illustrated in Figure 1.8. After all closures have been cast and have attained the necessary strength, longitudinal post-tensioning tendons are installed and stressed. To maximize the efficiency of the post-tensioning, phased stressing is necessary. Some of the longitudinal tendons are stressed on the I-girder section alone (i.e. while it is non-composite). The remaining tendons are stressed after the deck slab has been cast and act upon the full composite section.
Figure 1.8 - Erection Sequence and Temporary Supports for Spliced I- Girder
An example of cast-in-place balanced cantilever construction using form travelers is shown in Figure 1.9. Form travelers support the concrete until it has reached a satisfactory strength for post-tensioning. Longitudinal post-tensioning comprises cantilever tendons in the top slab at supports and continuity tendons in both top and bottom slabs through the mid-span regions.
Figure 1.9 - Cast-In-Place Segmental construction using Form Travelers
Cast-in-place balanced cantilever construction was adopted for four bridges on the Foothills Parkway in Tennessee designed by the Eastern Federal Lands Division of the Federal Highway Administration (Figure 1.10).
Precast segmental balanced cantilever construction involves the symmetrical erection of segments about a supporting pier. When a segment is lifted into position, adjoining match-cast faces are coated with epoxy and temporary post-tensioning bars are installed and stressed to attach the segment to the cantilever. Typically, after a new, balancing segment, is in place on each end of the cantilever, post-tensioning tendons are installed and stressed from one segment on one end of the cantilever to its counter-part on the other. Consequently, as segments are added, more top cantilever tendons are added.
Figure 1.11 - Precast Segmental Balanced Cantilever Construction.
Figure 1.11 shows two typical methods of placing precast segments in balanced cantilever; using cranes with stability towers at each pier and using an overhead launching gantry. When all segments of a new cantilever have been erected and tendons stressed, a closure joint is made at mid-span. Continuity post-tensioning tendons are installed and stressed through the closure to make the cantilevers continuous.
Figure 1.12 - Typical Balanced Cantilever Segment
Figure 1.12 offers a perspective showing various features of a typical precast cantilever segment, tendon locations and anchors. These are briefly as follows.
Longitudinal post-tensioning tendons for cantilever construction are contained within the top slab, usually spaced in a single layer over each web. For long spans, a second layer of tendons in the thickened haunch of the top slab may be required. The layout pattern of the ducts is always the same at each match-cast joint and ducts shift sideways or up and down within a segment to make up the full tendon profile from an anchor at one end of the cantilever to that at the other. Tendons terminate at anchors by a shift of the duct from its row in the slab to an anchorage. Relative to each segment, cantilever tendons always anchor in the same location. This may be in the end face of the segment or within an anchor block (or "blister") on the interior of the segment.
To complete a span, the ends of two adjacent cantilevers are connected by a cast-in-place closure at or near mid-span of interior spans. In end spans, the closure joint is usually nearer to the end expansion joint. When the closure concrete attains sufficient strength, longitudinal post-tensioning (continuity) tendons are installed, tensioned and grouted. Figure 1.13 depicts typical locations and layouts for bottom continuity tendons at mid-span.
Figure 1.13 - Bottom Continuity Tendons for Balanced Cantilever Construction
Span-by-span construction involves the erection of all segments of a span on a temporary support system with small closure joints cast at one or both ends next to the segments over the pier. Figure 1.14 shows typical phases for span-by-span construction.
Figure 1.14 - Span-By-Span Construction
Tendons, usually external, are installed and stressed from the pier segment at one end of the span to that at the other (Figure 1.15). The tendons drape between the piers, being anchored near the top of the section over the piers but deviated to the bottom of the section within the mid-span region.
Figure 1.15 - Interior Span Post -Tensioning for Span-By-Span Construction.
In order to achieve continuity with the next span, the tendons from one span overlap with the tendons of the next in the top of the pier segment. At the very ends of each continuous unit, the ends of the tendons anchor in the diaphragm of the expansion joint segment with anchors dispersed vertically and approximately parallel to the web of the box.
For bridge decks, transverse post-tensioning is used in cast-in-place solid slabs and to transversely connect spans made of precast-prestressed slabs placed side-by-side by means of narrow cast-in-place longitudinal joints. Transverse post-tensioning is frequently used in deck slabs of cast-in-place or precast boxes, diaphragms, transverse ribs and similar applications. For further information and examples, see Appendix C.
Substructures for standard AASHTO I-girders, Bulb-T's, spliced girders, cast-in-place post-tensioned and many segmental structures are typically built using reinforced concrete construction. However, for large bridges or to accommodate other special construction needs, post-tensioned substructures may be appropriate. Post-tensioned substructures may be used for bridges of all types of superstructures. Some of the more typical applications are shown in the following sections.
Transverse post-tensioned tendons using strand or bar tensile elements provide an effective reinforcing scheme for Hammerhead Piers (Figure 1.16). This is especially true for large hammerheads with significant cantilevers or where vertical clearances restrict the available depth. The tendons are internal to the concrete and are stressed and grouted after the pier concrete has reached sufficient strength.
Figure 1.16 - Post-Tensioning in Hammerhead Piers
Straddle bents are often required to support upper level roadways in complex multi-level interchanges (Figure 1.17). Limited vertical clearances often restrict the depths of the straddle bent caps, resulting in a post-tensioned rather than conventionally reinforced concrete member.
In a typical straddle bent, tendons drape to a prescribed profile that may be similar to the drape in a beam on simple supports, or it may rise over the columns where a monolithic connection is made to transfer moments into the columns and provide frame action. The columns may be reinforced or post-tensioned, depending upon the magnitude of the forces and moments induced in the frame.
Tendons in straddle bents are internal and grouted during construction. However, it is possible to apply external tendons of a similar type to repair, or rehabilitate a damaged structure.
Figure 1.17 - Post-Tensioning in Straddle Bents
Cantilever piers (C-piers) are often used in multi-level interchanges or in flyover bridges where a concentric column would intrude into a horizontal clearance associated with an underlying roadway. For structural efficiency and economy, a typical cantilever pier usually contains transverse and vertical post-tensioning (Figure 1.18) rather than solely being reinforced.
Detailing of cantilever piers should provide for proper development of prestressing forces in the cantilever, column and footing. Anchors at corners must cross in an effective manner to oppose tension and develop pre-compression all around the exterior of the pier. An alternative would be to use a continuous tendon rather than two separate tendons.
Tendons are internal, stressed and grouted during construction. Similar external tendons may be used for repair or rehabilitation. Special attention would be needed, however, to anchor them and develop forces around the top corner and into the footing.
Figure 1.18 - Post-Tensioning in Cantilever Piers.
Hollow section, precast concrete segmental piers have been used on several projects. Vertical post-tensioning usually consists of PT bars for short to moderate heights, up to about 12M (40 feet). Strand tendons are usually needed for taller piers. Bars are typically anchored in footings and extend to the pier caps. Strand tendons are usually continuous and extend from an anchor in the cap on one side of the pier, down the pier, loop through the footing and up the opposite side to another anchor in the cap. Post-tensioning bars are also used to temporarily secure precast segments and compress epoxy in the joints as they are erected prior to installing permanent strand tendons. Hollow precast, oval section segments with an aesthetically shaped octagonal exterior with concave faces, were used for the Linn Cove Viaduct on the Blue Ridge Parkway in North Carolina (Figure 1.19).
Figure 1.19 - Precast Hollow Segmental Piers, Linn Cove Viaduct, North Carolina
Precast segmental piers with an I-section were used for the Mid-Bay Bridge in Florida. The taller piers were post-tensioned with strand tendons, looping through the foundations, (Figure 1.20).
Figure 1.20 - Precast I-Piers.
Precast concrete hollow box section segments were used for the main arch ribs of the Natchez Trace Parkway Bridge in Tennessee (Figure 1.21). These were erected using temporary cable stays to the central pier column, which in turn were balanced by tie-backs anchoring in the adjacent hillsides. Temporary post-tensioning bars were used to secure each new segment to that previously erected to compress the epoxy joint.
Figure 1.21 - Natchez Trace Parkway Arches, Tennessee.
Large concentrated bearing loads on the top of piers induce local transverse tensile stresses. These stresses may be resisted by mild steel reinforcement or by transverse post-tensioning. Because tendon lengths are typically short, bar tendons are typically used in this application. Special conditions may call for the use of strand tendons. An example of this is the transverse post-tensioning tendons in the tops of the large elliptical piers of the main span unit of Sunshine Skyway Bridge in Florida. Internal multi-strand transverse tendons were used in a hoop layout to provide the required transverse prestressing.
Temporary post-tensioning bars are a key feature of precast cantilever erection. In cantilever erection, each new precast segment added to the cantilever is first secured to the previous segment using temporary post-tensioning bars to squeeze the epoxy joint and hold the segment until the main cantilever tendons can be installed. Construction operations are arranged to make it possible to lift a segment, apply epoxy, install temporary bars and squeeze the joint before the epoxy begins to set.
Depending on the size of the segment, there may be four to eight temporary bars distributed around the cross section. In most precast cantilever bridges, there is at least one temporary PT bar in a duct in the concrete wing of the segment. In some bridges, temporary PT bars anchor in blocks on the underside of the top slab and on the top of the bottom slab. Alternatively, bars may be installed in ducts within the top and bottom slabs and anchored in blockouts at the segment joints (Fig. 1.22)
Figure 1.22 - Temporary PT Bars for Segment Erection
Temporary PT bars are usually needed for span-by-span erection in order to squeeze the epoxy. In such cases, the bars may be anchored at temporary blocks (blisters) on the interior of the section or at diaphragms and deviators, passing through them in ducts. Using slow-set epoxy, it is possible to erect and epoxy several segments of a span at one time.