1. In Figure 1.2 (b), the designer balances the effects of load and prestressing in such a way that tension from the loading is compensated by __________ induced by the prestressing. Compression Tension 2. In figure 1.2 ( c ) , tension is eliminated under the combination of the two and tension cracks are prevented. Compression Tension 3. Precast, post-tensioned AASHTO and bulb-T girders are usually _________ 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. The girder segments are made continuous over multiple spans (typically three or four spans) by cast-in-place joints that connect the girder ends and are incorporated as a part of transverse diaphragms at the piers. Pre-tensioned Post-tensioned 4. In Example shown in fig 1.7; 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. True False 5. Fig 1.8 shows: Erection sequence and temporary supports for spliced I-Grider Pre-tensioning the cable in the slab. 6. Figure 1.9 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 into continuous spans. Other methods of erecting the precast segments are available, including deck mounted beam and winch systems and tire mounted segment lifters. True False 7. Figure 1.10 is : Typical balanced cantilever segment Typical vertical grider 8. In continuity tendons in end spans (188.8.131.52) , Although continuity tendons may not always be needed in the top, it is good practice to provide at least two, one over each web. Continuity tendons may be stressed from the expansion joint if access is available or they may be stressed at anchor blisters within the superstructure. True False 9. Span-by-span construction involves the erection of all segments of a span on a ________ support system with small closure joints cast at one or both ends next to the segments over the pier. Figure 1.19 shows typical phases for span-by-span construction. Temporary Permanent 10. The erection schematic shown in Figure 1.19 shows __________ erection trusses used to support the precast segments during construction. Overhead erection gantries are also commonly used to support the segments during erection. Temporary Permanent 11. Top slabs of precast and cast-in-place segmental, and similar boxes cast-in-place on falsework are often transversely post-tensioned. Transverse post-tensioning typically comprises internal, multi-strand tendons grouted after stressing. Tendons are spaced at regular, frequent intervals, approximately 2 to 3 feet along the length of the structure. Tendons anchor in the block-outs in the edges of the top slab cantilever wings. Blockouts are subsequently filled with concrete and are usually covered with a traffic barrier. Figure 1.24 shows a perspective view of typical transverse post-tensioning tendons in a box girder. In precast segments, top slab transverse tendons are usually tensioned and grouted while the segment is in storage in the casting yard. True Fase 12. Vertical post-tensioning bars are occasionally added to webs (Figure 1.28), usually in the high ____ zone near the piers, to control principal tension stresses and mitigate or avoid associated cracking. Shear Tensile 13. Transverse post-tensioned tendons using strand or bar tensile elements provide an effective reinforcing scheme for Hammerhead Piers (Figure 1.29). 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. True False 14. Precast concrete hollow box section segments were used for the main arch ribs of the Natchez Trace Parkway Bridge in Tennessee (Figure 1.34). These were erected using ________ cable stays to the central pier column, which in turn were balanced by tie-backs anchoring in the adjacent hillsides. Temporary Permanent 15. Satisfactory performance of post-tensioned bridges depends upon the appropriate selection, design, specification and fabrication of various materials and components that make up the post-tensioning system. True False 16. Samples for testing should be furnished at the job site for each manufacturer of prestressing steel and bar. Each sample furnished for testing should be accompanied by certification showing: Coil/reel number (strand only); Heat number; Chemical composition; Yield strength; Breaking strength; and Elongation properties True False 17. An example of typical frequencies of sampling and LOT designations are, as follows: For strand: three randomly selected samples, 1.5m (5ft) long, per manufacturer, per size of strand, per shipment, with a minimum of one sample per ten delivered reels and minimum of one per LOT/HEAT. For bar: three randomly selected samples, 1.5m (5ft) long, per size of bar, per heat of steel with a minimum of one sample per shipment.
One of each of the samples furnished to represent a LOT should be tested in accordance with appropriate ASTM standard. The remaining samples should be properly identified, tagged, and stored. In the event of a loss or failure, the stored samples should be used for evaluation. For acceptance of the LOT represented, test results must demonstrate 100% of f
pu. True False 18. Wedge performance is critical to the proper anchoring of strands. Different wedges have been developed for particular systems and applications such that there is no single standard wedge. However, wedges for post-tensioning systems should have the following characteristics: Wedge length at least 2.5 times the strand diameter. Wedge angle of 5 to 7 degrees. Internal serrated teeth for gripping the strand. Case-hardened low carbon or alloy steel. Two or three parts with a spring wire retainer clip or o-ring in a groove around the thick end. True False 19. Post-Tensioning Institute document “Specification for Grouting of Post-Tensioned Structures,” PTI M55.1-12, is an excellent resource for the grouting of tendons. This specification delineates four classes of grout that can be used for most post-tensioning applications. These four classes of grout and their recommend exposure conditions are: Class A – Nonaggressive: indoor or nonaggressive outdoor. Class B – Aggressive: subject to wet/dry cycles, marine environment, deicing salts. Class C – Nonaggressive or aggressive (pre-packages) Class D – Determined by Engineer True False 20. Grouts should be tested according to ASTM C939 “Standard Test Method for Flow of Grout, modified by filling the flow cone above the standard level, and measuring the time to fill a one-liter container. The efflux time should be between 5 and 30 seconds immediately after mixing. After allowing the grout to stand for 30 minutes without agitation and then remixing for 30 seconds, the efflux time should be less than 30 seconds. It is recommended that some of the laboratory qualification tests be run at the ends of this spectrum. True False 21. Vibrators can displace ducts when they are not properly secured. Also, over aggressive or improper use of internal vibrators may lead to local duct deformation or damage. Care must be exercised. Place concrete in relatively small lifts of only two to three feet and allow internal vibrators to only penetrate sufficient to consolidate the lifts. Use care not to get a vibrator permanently lodged in the rebar cage (Figure 3.25). True False 22. In aggressive environments, extreme care should be taken to ensure that water does not get into the ducts prior to installation of the prestressing steel. In the unlikely event that a duct may be contaminated with chlorides, flushing of the duct with water may be necessary to remove the chlorides before installing tendons. Only clean water should be used for flushing. The ducts should be well drained. All water should be removed before grouting. Flushing water should be blown out of the ducts before installing tendons, followed by swabbing the duct dry. All low point drains should be open during drying. Flushing should not be considered a standard practice and should only be done in emergency situations with the approval of the Engineer. True False 23. Figure 3.27, Mono-strand tendons are often used in buildings, where each tendon is a single strand in an extruded sheath. In bridges, mono strand jacks are normally used to stress transverse tendons, usually comprising 3 or 4 strands, in deck slabs or similar applications. Also, mono-strands are often used for repair or rehabilitation. True False 24. When a strand tendon has been jacked to the required force and the jack is released, the wedges are ________ the wedge plate until they bite and secure the strand. Typically the amount of anchor set is between 1/4 inch and 3/8 inch (Figure 3.34). Drawn into Pushed out 25. In figure 4.2 the upper left picture is: Void volume measurement Digital volumeter Grout pump Vacuum grout injection