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9 face of the member. This type of cracking generally would strands and the debonding lengths, residual stress from cur- be controlled with bottom flange confinement of the con- ing, restraint of forms during curing, and using forceful crete around the strands. means to remove the side forms and bulkheads. From the · Use of large strands: With the increasing use of concrete survey responses, the most commonly cited cause was strand with high strength, a number of state highway agencies distribution (72%), and the second most commonly cited have begun using 0.6-in. diameter strands at the standard cause was detensioning (50%). More discussion on sources of 2-in. spacing in place of the conventional 0.5-in. diameter end zone reinforcement is given in Appendix A, Literature strands. Also, a demonstration project by the Nebraska Review, of this report. Department of Roads is under way to implement the use of 0.7-in. strands on the Pacific Street overpass over Inter- 1.2.3 Design of End Zone Reinforcement state I-680 in Omaha. A full-scale specimen was fabricated for the University of Nebraska research team. The spec- Design of end zone reinforcement details is typically done imen, an NU 900 (36-in. deep) I-girder was prestressed by (1) estimating the bursting force (vertical tensile force with twenty-four 0.7-in. strands at 2.2 in. horizontally and developed at ends of pretensioned precast concrete girders 2.25 in. vertically. This prestress was the same amount during prestress release) as a percentage of the total prestress- required for a two-span bridge, 100-ft span, 10-ft, 10-in. ing force just before release, (2) setting a limit on the stress of spacing. Previous research at the university established the required end zone reinforcement that allows the designer that cracks are more extensive with the larger 0.6-in. and to control the size of the cracks and keeps them within accept- 0.7-in. strands than with the 0.5-in. strands. able limits, and (3) providing a scheme on how to distribute · Inadequate design of end zone reinforcement: Increased the end zone reinforcement. vertical reinforcement concentrated at the ends of the girder A literature search has shown that most of the design meth- has been shown to reduce the lengths and widths of end ods require that the end zone reinforcement be designed to zone cracks. Therefore, insufficient amounts of end rein- resist about 4% of the total prestressing force at transfer, and forcement or misplacement of the bars too far away from that the reinforcement must be designed for a service stress the edges may increase the amount of cracking experienced. not exceeding 20 ksi. However, there is no agreement on how Also, the lack of confinement stirrups around the prestress- to distribute this reinforcement in the end zone areas. For ing strands may increase cracking. It should be noted that example, Article 5.10.10.1 of the AASHTO LRFD specifications the end zone reinforcement is not presented to eliminate (18) states that it should be located within h/4 (one-fourth end zone cracking but to control it. of the depth of the girder) from the end of the girder, while · Concrete type: Lightweight concrete has a reduced tensile recent research conducted by the University of Nebraska (16), strength capacity and modulus of elasticity, and is there- and adopted by Alberta DOT, Canada, has recommended that fore less able to withstand the extreme prestressing forces. 50% of this reinforcement should be placed h/8 (one-eighth This leads to longer, wider, and a larger quantity of crack- of the depth of the girder) from the end of the beam and the ing along the ends. remainder should be placed between h/8 and h/2 from the · Low concrete release strength: The concrete must be end. In addition to the disagreement on the distribution of allowed to set and cure long enough to reach certain strength the end zone reinforcement, some highway authorities require before release. This strength value, known as the minimum a specific way of anchoring the end zone reinforcement. For release strength, assures that the concrete is strong enough example, the Illinois Department of Transportation (IDOT) to handle the prestressing forces. If the concrete does not requires that the end zone reinforcement should be made of reach this strength, it may be too weak to resist the prestress- 3 /4-in. diameter threaded rods that are welded to a 1-in.-thick ing forces, leading to cracking. plate embedded in the bottom flange, as shown in Figure 1.3. · Strand distribution: Girders with a large number of draped Also, the threaded rods are anchored at the top surface of the strands appear to have more extensive cracking than girders girder through a 3/4-in.-thick plate with nuts. Another exam- with fewer or no draped strands. The concentration of the ple is the bursting reinforcement detail recommended and prestressing force at the top of the web and the bottom used by Central Pre-Mix Prestress Co. of Spokane, Washing- flange increases the bending of the section and the vertical ton, where a single #8 bar that travels vertically through the tensile stresses. center of the girder is bent back into the interior of the beam at both the top and bottom, as shown in Figure 1.4. Other proposed variables related to end zone cracking More discussion on end zone reinforcement details used include form geometry, beam length, the number of strands, by various highway authorities is given in Appendix A, Liter- thermal and shrinkage stresses, the number of debonded ature Review, of this report.