Durability Design of Post-Tensioned Bridge Substructure Elements

Document Sample
Durability Design of Post-Tensioned Bridge Substructure Elements Powered By Docstoc
					                                                   CENTER FOR TRANSPORTATION RESEARCH
                                                     THE UNIVERSITY OF TEXAS AT AUSTIN


                                                                  Project Summary Report 0-1405-S
    Center for                                    Durability Design of Post-Tensioned Bridge Substructure Elements
    Transportation Research
          The University of Texas at Austin          Authors: Ruben M. Salas, Andrea J. Schokker, Jeffrey S. West,
                                                                   John E. Breen, and Michael E. Kreger
                                                     Center for Transportation Research, The University of Texas at Austin
                                                                     September 2004
                                              Durability Design of Post-Tensioned Bridge
                                                 Substructure Elements: Summary
                     Corrosion protection for               when using advanced techniques        superstructures. This information
                bonded internal tendons can be              to assess localized voids or active   allowed the scope of the
                very effective. Within the                  corrosion, since in these cases       experimental portion of the project
REPORT




                elements, internal tendons can be           the analysis is limited to specific   to be narrowed.
                well protected by the multilayer            areas in selected bridge elements,         Component 2 was developed
                protection system— including a              overlooking grout voids or even       and implemented under TxDOT
                sound design taking away the                corrosion of prestressing steel in    Project 0-1264 and transferred to
                surface water, surface treatments,          other member locations.               TxDOT Project 0-1405 for long-
                high quality concrete, plastic or                                                 term testing. A total of thirty-
                galvanized duct, sound cement               What We Did...                        eight macrocell specimens were
                grout, coatings and other internal                                                used to investigate the corrosion
                barriers in the prestressing steel,              After an extensive literature    protection of internal tendons at
                and good anchorage protection               review, the project was divided       segmental joints. See Figure 1(a).
                measures. However, potential                into the following main               Half of the specimens were
SUMMARY




                weak links exist when, among                components:                           autopsied after four and a half
                other conditions, the following             1. Literature Review and Survey       years of highly aggressive
                hold true: (1) the concrete has                of Existing Bridge Substructure    exposure. The second half of the
                high permeability; (2) the concrete            Inspection Reports                 specimens were autopsied with
                has cracking; (3) the ducts are not            (BRINSAP),                         over eight years of very
                adequately spliced or adequately            2. Investigation of Corrosion         aggressive exposure. The
                protected by impermeable                       Protection for Internal            variables included: joint type,
                concrete; (4) the Portland cement              Prestressing Tendons in            duct type, grout type, and level of
                grout has voids, bleed water, and              Precast Segmental Bridges,         joint compression.
                cracks; and/or (5) the prestressing         3. Development of Improved                 Component 3 consisted in
                steel is not adequately protected              Grouts for Post-Tensioning,        testing numerous grouts in three
                and handled during construction.            4. Long-Term Corrosion Tests          phases to develop a high
                     One of the major problems                 with Large-Scale Post-             performance grout for corrosion
                that agencies face today is the                Tensioned Beam and Column          protection. The testing phases
PROJECT




                difficulty of providing good                   Elements, and                      included fresh property tests,
                monitoring and inspection                   5. Development of                     accelerated corrosion tests, and a
                techniques for bonded post-                    Recommendations and Design         large-scale clear draped parabolic
                tensioned structures. Condition                Guidelines for Durable Bonded      duct test that allowed observation
                surveys are often limited to visual            Post-Tensioned Bridges.            of the grout under simulated field
                inspections for signs of cracking,                                                conditions. See Figures 1(b) and
                spalling and rust staining, which                Component 1 included             1(c). Two grout mixes were
                can often overlook the                      literature review of a substantial    recommended and have already
                deterioration of prestressing steel         amount of relevant information        become widely used in practice.
                and fail to detect the potential for        that could be applied to the               Component 4 included
                very severe and sudden                      durability of post-tensioned          testing of twenty-seven large-
                collapses. Similar limitations exist        bridge substructures and              scale beam specimens, which



                 Project Summary Report 0-1405-S                                 –1–
were constructed in two phases. See                                    years of exposure). Partial autopsies                               Figure 1(e). Variables included column
Figure 1(d). Phase I beams (16                                         were performed on two Phase I beams.                                to foundation connection, loading,
specimens) were used to investigate                                    Full autopsies for the remaining                                    concrete type, prestressing bar
the effect of prestressing levels, crack                               specimens will be performed at a                                    coatings, and post-tensioning ducts.
widths, and high performance grout.                                    future date under TxDOT Project                                     Full autopsies were performed at the
Phase II beams (11 specimens) were                                     0-4562. Component 4 also included                                   end of testing, after six and a half
used to investigate duct splices, grout                                testing of five non-prestressed and                                 years.
type, concrete type, strand type, duct                                 five post-tensioned column specimens
type, and end anchorage protection.                                    to investigate corrosion mechanisms
                                                                       and chloride ion transport (“wicking
                                                                                                                                           What We Found...
Full autopsies were performed on six
                                                                       effect”) in various column connection                               High-Performance Grouts and
Phase I beams (after four and a half
years of exposure) and also on six                                     configurations and to evaluate                                      Grouting Procedures
Phase II beams (after three and a half                                 corrosion protection measures. See                                  • A 30% fly ash grout with a 0.35 w/c
                                                                                                                                               (water content) had excellent
                                                                                                                              ERmeas
                                                                                                                                               performance in horizontal
                                                                                                                     +
                                                                                                                          V
                                                                                                                                   CE
                                                                                                                                               applications.
                                                                                                                     100 Ω
                                                                                                                                   WE      •   A 2% anti-bleed grout with 0.33 w/c
                                                                                                                     V
                                                                                                                      +
                                                                                                                          Ecorr
                                                                                                                                   RE
                                                                                                                                               had excellent performance in vertical
                                                                                                                                               applications.
                                                                                                                                           •   The standard TxDOT grout had
                                                                                                             CE - Counter Electrode
                                                                                                             WE - Working Electrode
                                                                                                             RE - Reference Electrode
                                                                                                                                               below average performance.
                                                                                                                  (SCE)
                                                                                                                                           •   Grout voids, due to entrapped air,
                                                                                                                                               bleed water, incomplete grout filling
                                                                                                                                               or lack of grout fluidity were found
                                   (a)                                                             (b)                                         to be detrimental not only to the
            Vent                                                           Vent
                   2.4 m                              Vent                                         2.9 m                                       prestressing strand, but also to the
        X                                                             X           X            X
                                                                                                                                               galvanized duct, as shown in
        A           X                                     X                       G
                    B              C          X            E
                                                                       F                       H                 I            J    0.9 m       Figure 2.
                                   X              D                                                             X             X            •   Grout is not prestressed and grout
                                                  0.9 m        0.9 m        0.9 m     0.9 m
            1.2 m          1.2 m                                                                                      0.6 m                    cracks were often found under
                                                               9.8 m
                                                                                                                                   Inlet       service loading.
                                                                 (c)
                                                                                                                                           •   Calcium Nitrite corrosion inhibitor
                                                            15’ 2”
                                                       4.62 m (15' 2")
                Tube
                Section                16 mm PT Bar                                                       Cross Section
                                                                                                                                               added to the grout did seem to
                                        5/8 in. PT Bar                    Ponded Salt                    Cross Section:
               Spring
                                       (5/8 in.)                          Solution                        18
                                                                                                         457xx24 in.mm
                                                                                                                610                            provide some enhanced long-term
                                                                                                           (18 x 24 in.)
                                                                                                                                               strand corrosion protection.
               Channel
               Section                                    Specimen

                                                          Reaction                                                                         Ducts for Internal Post-Tensioning
                                                           Beam
                                                                                                                                           • Galvanized steel ducts performed
                                                                                                                                             poorly (see Figure 2). Plastic ducts
                                       1.37ft
                                        4.5 m                  1.37 ft
                                                                4.5 m                4.5 m
                                                                                    1.37 ft
                                       (4.5 ft)                (4.5 ft)             (4.5 ft)                                                 were superior.
                                                                 (d)
                                                                                                                                           • The use of completely filled epoxy
                                                     Column
                                                  reinforcement
                                                                                                                                             joints with unspliced plastic ducts
                                                      at joint
                                                                                                                                             showed very good protection.
                                                                                        bearing plate
                                                                                        and nut


                                                                                        4 PT bars
                                                                                                                                           Cracking and Joints
                                                                                        5/8“ diameter
                                            6 dowels:           rubber
                                                                                                                                           • Transverse cracking due to loading
                                            #6 bars
                                                                                                                                               had a definite effect on corrosion
                        2 in.
                        cover                                   gasket                  coupler

                                                                                                                                               damage. Larger crack widths and
                                                                                                                                               crack density were the cause of very
                         Dow eled Joint                        Post-Tensioned Joint                     No Dowel                               severe localized and uniform
                                                                  (e)                                                                          reinforcement corrosion activity.
Figure 1. (a) Macrocell Specimen, (b) Schematic of ACTM Station, (c) Large-
Scale Clear Draped Parabolic Duct Test, (d) Beam Test Setup, (e) Column-
Foundation Joint

Project Summary Report 0-1405-S                                                                    –2–
                                                                                  • Duct gaskets in epoxy joints
                                                                                    should be avoided.
                                                                                  • Mixed reinforcement members
                                                                                    should be used in aggressive
                                                                                    exposures only if special
                                                                                    provisions are made to seal
                                                                                    cracks and to prevent concrete
         Figure 2. Bleed Water Voids and Galvanized Duct Corrosion
                                                                                    exposure to chlorides.
• Longitudinal or splitting cracks      Post-Tensioning Bars or Strands           • Fully prestressed members are
  always indicated very severe          • PT bar coatings showed                    recommended in aggressive
  corrosion within the member.            enhanced general corrosion                environments.
• Dry joints and incompletely filled      protection, in comparison to plain
                                            .                                     • High performance concrete
  epoxy joints in the macrocell           PT bars. However, under very              (w/c=0.29) is recommended in
  specimens showed very poor              severe localized attack, as in a          aggressive environments.
  performance.                            crack or joint location, corrosion      • Fly ash (Class C) concrete
                                          activity was severe.                      (w/c=0.44) may also be
Levels of Post-Tensioning                                                           considered when the environment
• As the level of post-tensioning or    Exposure Testing Methods                    is less aggressive.
  concrete precompression               • After using half-cell potential         • Development of better duct
  increased, the corrosion                readings, chloride content                splicing systems should be a high
  protection increased. Lower             determinations and corrosion              priority.
  permeability due to increased           current readings, only the first        • Inspection of in-service bridges
  precompression also provided            two showed some degree of                 should identify potential
  better resistance to wicking            correlation with forensic                 reinforcement corrosion from any
  effects.                                examination results.                      noted longitudinal or splitting
                                                                                    crack.
Concrete Type                           The Researchers                           • A minimum of 2-inch cover to
• High performance concrete                                                         any reinforcement should be used
  appears to be effective in
                                        Recommend...
                                        • For PT Tendons with small rises           in any post-tensioning design.
  minimizing the chloride                                                         • Fully plastic chairs to ensure
  penetration through concrete.             under severe exposure conditions,
                                            the following grout should be           proper concrete cover are
                                            used: 0.35 w/c, 30% fly ash             recommended to eliminate
Concrete Cover
                                            (Class C) replacement and               corrosion damage, instead of
• Small concrete cover was clearly
                                            4 ml/kg superplasticizer.               chairs made out of steel.
  shown to be detrimental to
                                        •   For PT Tendons with large rises       • Column elements should be
  reinforcement performance.
                                            under severe exposure conditions,       prestressed to improve spiral and
Galvanized Duct Splices                     the following grout should be           rebar corrosion protection in very
• Neither the industry standard             used: 0.33 w/c and 2% anti-bleed        aggressive environments.
  splice (duct taped) nor the heat-         admixture.                            • Galvanized steel bars or epoxy
  shrink splices appear to be           •   Plastic ducts should be used in all     coated are susceptible to severe
  satisfactory to prevent moisture          situations where even moderate          localized corrosion.
  and chloride ingress.                     aggressive exposure may occur.        • Bar coatings can be used when
                                        •   Epoxy joints should always be           positive sealing of cracks or joints
Gaskets for Post-Tensioning                 used with internal prestressing         is attained.
• The use of gaskets in the joints to       tendons.                              • Half-cell potential readings and
  avoid epoxy filling of the ducts in   •   Stringent inspection and                chloride content determinations
  segmental construction, or the            construction practices should be        could be used to assess to some
  use of rubber gaskets to seal the         enforced to guarantee complete          degree the service condition of
  duct ends at column joints, were          grouting and good epoxy filling at      the specimens. Better corrosion
  detrimental to the performance of         the joints in segmental                 assessment methods should be
  the specimens.                            construction.                           developed and evaluated.



Project Summary Report 0-1405-S                          –3–
       For More Details...
       Research Supervisor: John E. Breen, P.E., (512) 471-4578, jbreen@mail.utexas.edu
       TxDOT Project Director: Bryan Hodges (TYL), (903) 510-9127, bhodges@dot.state.tx.us.
           The research is documented in the following reports:
                    − Research Report 0-1405-1, State of the Art Durability of Post-Tensioned Bridge Substructures, October 1999.
                    − Research Report 0-1405-2, Development of High-Performance Grouts for Bonded Post-Tensioned Structures, October 1999.
                    − Research Report 0-1405-3, Long-Term Post-Tensioned Beam and Column Exposure Test Specimens: Experimental
                      Program, October 1999.
                    − Research Report 0-1405-4, Corrosion Protection for Bonded Internal Tendons in Precast Segmental Construction, October 1999.
                    − Research Report 0-1405-5, Interim Conclusions, Recommendations and Design Guidelines for Durability of Post-
                      Tensioned Bridge Substructures, October 1999.
                    − Research Report 0-1405-6, Final Evaluation of Corrosion Protection for Bonded Internal Tendons in Precast Segmental
                      Construction, October 2002.
                    − Research Report 0-1405-7, Long-Term Post-Tensioned Beam Exposure Test Specimens: Final Evaluation, August 2003.
                    − Research Report 0-1405-8, Long-Term Post-Tensioned Column Exposure Test Specimens: Final Evaluation, August 2003.
                    − Research Report 0-1405-9, Conclusions, Recommendations and Design Guidelines for Corrosion Protection of Post-
                      Tensioned Bridges, August 2003.
           To obtain copies of the above reports, contact the Center for Transportation Research, The University of
       Texas at Austin, (512) 232-3126, ctrlib@uts.cc.utexas.edu.



                                                TxDOT Implementation Status
                                                       October 2004
          Results from research project 0-1405 have substantially impacted the design and construction of post-tensioned
     structural elements for bridges, and have been implemented both within Texas and on a national level. For example,
     the TxDOT specifications for grout materials were changed based on this research, and prepackaged grouts are now
     commercially available. A national grouting certification program has been implemented through the American
     Segmental Bridge Institute for construction personnel, and galvanized metal ducts are no longer specified for post-
     tensioning applications in corrosive environments in Texas. Epoxied segment joints are now an industry standard.
     Post-tensioning bar and prestressing strand coatings (epoxy, galvanized, and other) show enhanced corrosion
     protection and are being conclusively evaluated under ongoing TxDOT research project 0-4562.
         For more information please contact Tom Yarbrough, P.E., RTI Research Engineer, at (512) 465-7403 or email at
     tyarbro@dot.state.tx.us.

                                                   Your Involvement Is Welcome!

                                                                      Disclaimer
     This research was performed in cooperation with the Texas Department of Transportation and the U. S. Department of
Transportation, Federal Highway Administration. The contents of this report reflect the views of the authors, who are responsible
for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official view or policies of the
FHWA or TxDOT. This report does not constitute a standard, specification, or regulation, nor is it intended for construction,
bidding, or permit purposes. Trade names were used solely for information and not for product endorsement. The engineers in
charge were John E. Breen, P.E. (Texas No. 18479), and Michael E. Kreger, P.E. (Texas No. 65541).



  Center for
  Transportation Research
       The University of Texas at Austin


The University of Texas at Austin
Center for Transportion Research
3208 Red River, Suite #200
Austin, TX 78705-2650

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:16
posted:10/16/2011
language:English
pages:4