Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 59
59 Criterion 1: Crack width less than 0.012 in. Port spacing can depend on the crack width and the amount of Action recommended: No action is recommended. pressure applied. Professional judgment from an experienced injector should be used. The ports should be at least 8 in. apart. With a crack width this narrow, no repair is required. However, However, if the crack passes through the entire web, the spac- if the owner requires repair, steps for Criterion 2 can be followed. ing should not exceed the thickness of the web. After the ports are installed, the exterior of the cracks are to be sealed with an epoxy paste and allowed to harden. This is to prevent the injected Criterion 2: Crack width 0.012 in. to 0.025 in. epoxy from leaking out of the crack. With cracks that extend on Action recommended: Apply cementitious packing materi- both sides of the girder, the opposite side of the injection should als to cracks between 0.012 in. and 0.025 in. Apply surface be sealed as well. If the cracks on each side do not connect, epoxy sealant to the end 4 ft as recommended in this manual. injection should be performed on each side individually. After confining the cracked area is completed, the epoxy can be It is recommended that the crack be filled with a cementitious mixed and the injection can begin from the bottom up. Injection packing material and covered with a water-resistant surface sealant should be performed with an epoxy injecting machine. The low- to keep water contaminated with corrosion-inducing chemicals est injection port should be filled with epoxy first until it begins from reaching the steel inside the girder. to come out of the next port, which is slightly higher than the first The area in question should be cleaned and cleared of any debris port. The used port is to be plugged so the epoxy does not leak out. such as dirt, dust, grease, oil, or any other foreign material. This will Then, the process can be repeated until epoxy begins to come aid in the bonding of the material to the concrete. Cleaning prod- out of the next port in line. This process continues until the top ucts that are corrosive should not be used. port is reached, and the crack is completely filled. The final port It is best that the packing material used to fill the cracks be should be placed a few inches away from the termination point cementitious, slightly viscous, and easily worked by hand. The of the crack, but this remaining portion of the crack should still material should be rubbed into the cracks either by hand or by be filled with the last injection. brush until the entire outer opening is filled and a surface is created that is even with the original girder web surface. Excess Criterion 4: Crack width greater than 0.050 in. material should be wiped off so the surface remains even. The surface sealant should be water resistant and highly flow- Action recommended: Reject girder, unless shown by de- able. Its application should result in a smooth surface. The sealant tailed analysis that structural capacity and long-term dura- should be applied with either a brush or a roller so the side faces bility are satisfactory. of the girder are fully covered. The top face of the girder where it normally is connected with a cast-in-place concrete slab should Cracks exceeding a width of 0.050 in. may be symptomatic of not be covered with sealant. It is recommended that a minimum causes beyond the normal effects of bursting forces due to pre- length of 4 ft at each end of the girder be covered. stress release. All aspects of material quality, reinforcement qual- Examples of acceptable patching and sealant materials to be ity and quantity, and production practices must be examined. If used are provided in Section 3.4 of this report. a loss of structural capacity were to occur, typical methods of epoxy injection may not be sufficient to measurably return the Criterion 3: Crack width 0.025 in. to 0.050 in. girder back to its intended strength, especially if cracking causes Action recommended: Epoxy injection of all cracks larger excessive loss of prestress. than 0.025 in. Apply surface sealant to the end 4 ft as recom- mended in this report. 3.7 Improved Crack Control For cracks wider than 0.025 in., epoxy injection is recom- Reinforcement Details mended. It is important that this be performed such that the for Use in New Girders crack is completely filled and that the epoxy is effectively bonded to both surfaces of the crack. Cracks of this size in the web gen- Most designers follow the provisions of Article 188.8.131.52 erally exist in the full width of the web and appear on both side of the AASHTO LRFD Bridge Design Specifications (18). faces of the member. Injection must be done in accordance with However, states with recently introduced I-girder shapes that proven practices and epoxy manufacturer's specifications. Epoxy pressure should be high enough to fully penetrate the crack depth, can accommodate a relatively large amount of prestressing yet the pressure should not cause a blow out of the epoxy paste (as many as sixty-eight 0.6-in. diameter strands) have devel- material used to confine the epoxy. oped supplementary requirements for end zone reinforcement. Before injection, the surface and interior of the crack should Factored Bursting Resistance, as given in Article 184.108.40.206 of be cleared of all debris such as dirt, dust, grease, oil, moisture, or any other foreign material without using corrosive chemicals. the AASHTO LRFD (18), indicates that the end reinforcement If loose particles have entered the crack, they can be blown out resistance shall not be less than 4% of the prestressing force at with filtered high-pressure air equipment, as long as they do not transfer. The end zone reinforcement is designed for 20 ksi introduce oil into the fissure. Water, solvents, or detergents should allowable stress to control the crack size and is located within not be used because they may compromise the ability of the epoxy to bond to the concrete. h/4 from the end of the girder, where h is total girder depth. When applying the epoxy, the crack should first be exam- The following recommendations offer improvements ined to determine the ideal placement for the injection ports. to the AASHTO provisions, especially for cases with high
OCR for page 59
60 prestressing levels. Effective and simplified reinforcement significantly, the proposed details lend themselves to optimal detailing is proposed. bar detailing with minimized end zone reinforcement conges- The following recommendations are based on experience tion. The team has found it to be most effective to have a large in Nebraska and Washington State where very large amounts area of vertical steel as close as possible to the end of the girder, of prestressing have been provided on some projects. A re- with the steel area gradually diminishing as the distance search project conducted for NDOR resulted in recommen- from the end is increased. The reinforcement must be dations published in a 2004 PCI Journal article (16). Results anchored well enough into the bottom and top flanges to of this project have shown that the end zone reinforcement assure no slippage at the design stress level of 30 ksi. The bot- closest to the member end is the most stressed and would tom flange must also be confined with a minimum amount correspond to the widest crack, as shown in Figure 3.73. of confinement steel to help resist strand slippage and bound- Also shown is that the stress in the vertical reinforcement ary zone cracking. drops sharply at a distance h/8 away from the girder end, The five proposed requirements are as follow: with steel beyond the h/2 distance having little influence on cracking. (1) Provide reinforcement in the end (h/8) to resist at least Since the end zone reinforcement is provided to mini- 2% of the prestressing force, using an allowable stress mize the crack width, and not for strength, there is no need limit of 20 ksi. to develop the full yield strength beyond the locations of (2) Provide reinforcement in the end (h/2) to resist at least 4% the top and bottom cracks, which are assumed for design to of the prestressing force, using an allowable stress limit of be at the junction between the web and the flanges. 20 ksi. The reinforcement in the zone between the h/8 and The results of this research, along with additional recom- h/2 sections must not be less than shear reinforcement mendations from the Nebraska and Washington producers requirement as stipulated in (3) below. involved in NCHRP Project 18-14, have been used in the full- (3) Beyond the (h/2) zone, provide reinforcement to meet scale testing in this project (see Section 3.2 of this report), shear requirements at the nearest critical section. where these recommendations have been compared with the (4) Determine the bar anchorage into the flanges for a max- AASHTO LRFD provisions as well as other local practices in the imum stress of 30 ksi. four states supplying eight full-scale girder specimens. (5) Confine the strands in the bottom flange with at least the The full-scale testing confirmed that, although the AASHTO equivalent of #3 bars at 3-in. spacing for a distance equal LRFD requirements provided acceptable performance in all to at least 60 strand diameters. The #3 bars must totally cases, the proposed details provided better performance. More enclose the bottom flange strands. Welded wire reinforce- 25.0 h/8 h/8 h/4 h/4 h/4 20.0 Design Steel Stress (ksi) 14.8 ksi 15.0 10.7 ksi fs = 2.4/(z/h + 0.1) 10.0 8.3 ksi 6.9 ksi 5.0 5.1 ksi 4.0 2.8 0.0 0.000 0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000 1.125 1.250 Distance from the Beam End/Girder Height (z/h) Source: Reprinted with permission from Tuan, C.Y., Yehia, S.A., Jongpitaksseel, N., and Tadros, M.K., "End Zone Reinforcement for Pretensioned Concrete Girders," PCIJournal, Vol. 49, No. 3, May-June (2004), Figure 15. Figure 3.73. Average measured stress in end zone reinforcement versus distance from the member end.