Durability of Precast Segmental Bridges Final Report (1998) / Chapter Skim
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Chapter 2 Findings
Chapter 2 Findings
Pages 6-24
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From page 6... ...
The structure was 32 years old with no evidence of distress prior to failures. The Bickton Meadows Footbridge collapsed as a result of severe corrosion ofthe top tendons.5 However, both the precast units and the thin mortar joints were of extremely poor quality.
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From page 7... ...
Literature Review. A durability survey of segmental concrete bridges was conducted under the sponsorship of the American Segmental Bridge Instituted The survey identified 109 precast segmental bridges, most built in the U.S.
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From page 8... ...
Improper grouting and detailing was blamed for corrosion problems with the post-tensioned Walnut Lane Bridge. The Sixth South Street Viaduct experienced corrosion distress ofthe post-tensioning, but the tendons were in a galvanized steel duct without the presence of grout; thus the tendons were unhanded and unprotected.~3 Overall, there have been no reported problems with corrosion of precast segmental structures in North America.
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From page 9... ...
Segment Joint Conditions Internal Tendons Cast-in-Place 8°/0 Match-Cast Epoxy 92% Figure 2-] Joint Conditions for Segmental Bridges - Internal Tendons 9
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Segment Joint Conditions External Tendons 1~1 Match-Cast Dry 31°/0 Match-Cast Epoxy 69% Figure 2-2 Joint Conditions for Segmental Bridges - External Tendons 10
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All those who responded to the question indicated that the AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges was used. Table 2-1 Segment Joint Types (from survey)
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From page 12... ...
joints properly sealed with epoxy resin, are satisfactory in durability terms".3 Thin mortar joints are specifically not allowed. However, although the Working Party states that match-cast epoxy joints are acceptable from a durability standpoint, they add that "special consideration has also to be given to the continuity of the ducts across the joints".3 Until the Working Party is satisfied with a detail to guarantee duct continuity, or equivalent corrosion protection can be assured by other means, the ban on precast segmental construction with internal tendons will remain in effect in the U.K.
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From page 13... ...
Currently, all bridge design and construction is governed by the AASHTO Standard Specifications for Highway Bridges.~5 Precast segmental bridges also fall under the jurisdiction of the AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges. Although this document is only a guide specification, results from the survey indicate that, as a rule, designers and owners are adopting and using the document as part of the design and project specifications.
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From page 14... ...
Examination of draft copies of this specification indicate that it will provide more definitive guidance than is currently available in the PTI PostTension~ng Manual that is currently referenced by the AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges. Although there is not a comprehensive specification available, there are a number of papers that document research on the testing and development of high performance grouts for the corrosion protection of post-tension~ng.~~27 The studies attempt to optimize both grout material properties and placement techniques.
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From page 15... ...
The first large-scale application of this NOT method to evaluate grouted tendon ducts was completed in 1997 on a 14-span, precast segmental bridged The superstructure consisted of precast grouted post-tensioned cantilever beams connected by drop-in precast, pretensioned AASHTO sections (see Figures 2-3 and 2-4)
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Figure 2-4 Post-Tensioned Cantilever Beam: (a) Elevation of One-Half of a Cantilever Beam, Showing the Parabolic Profile of the Tendon Duct Tested; (b)
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Figure 2-5 Exposed, Pully-~outed Duct Flare 2-6 Palely Routed Tendon Duct Found by ImpacLEcho 17
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It has had limited success, and some have concluded that its best application may be the location of ducts/tendons for further testing or invasive inspection.30 One disadvantage is that impulse radar signal interpretation must be made by a skilled operator, and signal interpretation becomes difficult in areas with closely spaced reinforcement. Closely spaced reinforcement is often found in anchorage zones in segmental bridge structures.
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From page 19... ...
More work on evaluating fully grouted tendons needs to be conducted.35 Although acoustic monitoring shows great potential as a management tool for continuing corrosion damage, it cannot provide information on existing damage to the post-tension~ng of a structure. In tendon systems utilizing metal ducts, half-cell corrosion p.otentials36 and corrosion rates using the 3LP linear polarization method37 may be used to determine if corrosion is occurring and its relative rate.
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From page 20... ...
The lack of a continuous duct across segment joints was identified as a potential "weak link" in the corrosion protection of internal tendons in precast segmental bridges.
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, a L 4.5in. -l End View Figure 2-S Typical Test Schematic for Segmental loins 21 r 12 in.
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From page 22... ...
with signs of corrosion have dry segment joints. However, corrosion rate calculations indicate that the overall magnitude of corrosion of all the specimens in the study is very low to negligible.39 Additionally, some ofthe data indicates that some specimens are experiencing "reverse macrocell" corrosion; that is, the embedded mild steel is corroding preferentially to the strand.
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From page 23... ...
. Figure 2-10 Strand in the Vicinity of the Segment Joint Showing No Corrosion (White Line Indicates Location of Segment Joint)
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From page 24... ...
However, the "post-tensioning industry standard consisting of a bare anchorage, galvanized steel duct with duct taped joints, bare prestressing strand and normal cementitious grout adequately protected the encased bare strand from corrosion when embedded under only about 1 inch of concrete cover.~341 A comprehensive study was conducted at the University of Texas at Austin on improving the durability of bridge decks using transverse prestressing.42 In general, grouted galvanized duct was found to adequately protect the tendons between the anchorages. Thin concrete covers at anchorages were found to provide inadequate corrosion protection.
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