Repair and Maintenance of Post-Tensioned Concrete Bridges (2021) / Chapter Skim
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2 Post-Tensioning Basics Post-tensioning (PT) systems are used in bridge applications in superstructure and substructure members to induce a prestressing force to the concrete section; this prestressing force causes a precompression of the concrete, which is beneficial in resisting induced tensile stress.
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Literature Review and Background 3Â Â grout as a filler material. Both types of tendons may experience durability issues associated with construction practice and filler material quality; rarely have internal tendons been identified with evidence of corrosion.
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4 Repair and Maintenance of Post-Tensioned Concrete Bridges Post-tensioning tendons can be either bonded or unbonded -- a categorization based on the tendon's contact with the surrounding concrete section and indicative of stress-strain compatibility between the tendon and the surrounding concrete. Tendons filled with cementitious materials can be either bonded, as in the case of internal tendons, or unbonded, as in the case of external tendons (such as those used in some segmental construction)
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Literature Review and Background 5Â Â Issues are usually identified through the observation of crack formation and growth. A recent example of rapidly occurring structural degradation and "exponentially growing" cracks in a PT segmental structure is the West Seattle High-Rise Bridge.
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6 Repair and Maintenance of Post-Tensioned Concrete Bridges of tendons, the loss of prestress due to corrosion, or the loss of both can lead to serviceability concerns (i.e., excessive deflections) or tendon failure.
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Literature Review and Background 7  Evidence of soft grout -- unset grout with unhydrated cement products as described by Randell et al.
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8 Repair and Maintenance of Post-Tensioned Concrete Bridges Schupack pressure tests are used to examine the effect of temperature and mixing time on bleed water generation. In a Schupack pressure test, the mixed grout is placed in a pressurized vessel and the pressure is incrementally increased (see Figure 3)
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Literature Review and Background 9Â Â through wicking action than increased height with a single strand, suggesting that wicking action occurs between strands as well as between interstitial spaces between individual wires. Increased temperatures during mixing of the grout increased the bleed quantity in most of the tested grouts when performing the Schupack pressure test.
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10 Repair and Maintenance of Post-Tensioned Concrete Bridges are often filled with a cementitious nonshrink grout to provide protection, but some agencies have reported the contractor's use of conventional concrete as pour-back material, or the complete absence of a pour back (Ahmad et al.
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Literature Review and Background 11  water, and expansive admixtures. Grout issues identified in the pre-2000 period were associated with • Presence of excessive bleed water, • Presence of voids in the filler material, • Splitting of the encapsulating polyethylene ducts, and • Water infiltration of the duct (water recharge)
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12 Repair and Maintenance of Post-Tensioned Concrete Bridges Following much consideration, the agency implemented several important changes to improve PT structure durability: (a) adoption of flexible fillers, (b)
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Literature Review and Background 13Â Â Flexible filler materials are not new. Since World War I, flexible filler products have been used for corrosion protection of naval machinery and coastal artillery.
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14 Repair and Maintenance of Post-Tensioned Concrete Bridges • To facilitate remediation during flexible filler injection, it is useful to have the following on hand: – Buckets with water and wet rags to cover small leaks – Heat gun or torch to address clogs in plumbing – Clean-up supplies, such as sand and shovels, in case of flexible filler spillage – Tarp or other covering for areas adjacent to the inlet and outlet points (to keep these areas clean in case of material leaking) – Barrier or other protection at key areas in case of blowout • Ensure that all components of the pumping system are clear prior to injection, including plumbing on the pump and between filler reservoirs.
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Literature Review and Background 15Â Â Source: Fuzier et al.
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16 Repair and Maintenance of Post-Tensioned Concrete Bridges Protection of PT Systems In general, ensuring durability for PT systems is related to the protection of the system against water intrusion. Bulletin 33, the fib guidance for improving durability of PT systems, identifies potential sources of durability concerns and breaks them down into two categories: failure of the external barriers, and failure of the tendon corrosion protection system.
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Literature Review and Background 17Â Â nature. They may also be formulated with corrosion-inhibitor additives to further facilitate resistance to mechanisms of corrosion.
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18 Repair and Maintenance of Post-Tensioned Concrete Bridges while other states (such as Florida) go a step further and have their own Construction Training and Qualification Program in conjunction with recognized certification programs.
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Literature Review and Background 19Â Â stay cables and rock anchors that are already covered by other PTI documents. The protection levels outlined in the M50 guide are the same as those found in fib Bulletin 33 (PTI/ASBI 2012)
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20 Repair and Maintenance of Post-Tensioned Concrete Bridges Project-specific detailing, construction errors, and material selection can affect the ease of inspection. Internal post-tensioning tendons -- and especially grout-filled post-tensioning tendons -- pose a difficulty for inspectors.
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Literature Review and Background 21Â Â Electrochemical Methods Electrochemical methods are inexpensive methods used to monitor active corrosion of strands in concrete structures by measuring the electrical properties of the reinforcing steel. Most electrochemical techniques use the same measurement setup that consists of a reference electrode, a working electrode, a counter electrode, and a voltmeter.
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22 Repair and Maintenance of Post-Tensioned Concrete Bridges to the corrosion rate. The half-cell potential method is practically and widely employed to identify the presence of corrosion.
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Literature Review and Background 23Â Â tool presented accurately detected corrosion in the strand, but the testing apparatus that was used did not lend itself well to in-field examinations. Radiological Methods Using X-rays to assess the condition of internal tendons is not common because of safety concerns and equipment expense, but it has been demonstrated to effectively identify damaged strands.
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24 Repair and Maintenance of Post-Tensioned Concrete Bridges Nondestructive Evaluation for Flexible Filler The effectiveness of many NDE methods for PT inspection is influenced by the concrete cover, duct material, and rebar congestion, all parameters that are affected by the use of flexible filler. For example, the smooth HDPE pipe used with flexible filler is thicker than the typical corrugated duct used for grouting.
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Literature Review and Background 25Â Â Currently, repair recommendations to address duct issues encountered in bridge construction are not available from the Post-Tensioning Institute. However, its repair committee, DC-80, has published recommendations and guidance documents for the repair of sheathing of the more uniform scenarios encountered with the single-strand tendons used in building construction.
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26 Repair and Maintenance of Post-Tensioned Concrete Bridges Advances in Post-Tensioned Bridge Design in the United States Since the introduction of post-tensioned bridge structures to the United States, several advances have improved the durability of these systems. Key changes include the following: • Improved quality control of grout, including mixing and injection procedures • Improved grout materials, including thixotropic prebagged grouts • Improved detailing, particularly at connections • Introduction of grout certification programs • Introduction of flexible filler materials • Enhanced inspection methods (by some states)
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