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Federal Highway Administration Research and Technology
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REPORT |
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Publication Number: FHWA-HRT-13-028 Date: October 2013 |
Publication Number: FHWA-HRT-13-028 Date: October 2013 |
HTML Version of Errata for FHWA-HRT-13-028
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Location |
Incorrect Values |
Corrected Values |
Table 32 on page 90 |
CD3 0.08 < Cl− ≤ 0.20b |
|
Table 33 on page 91 |
CD3 0.02 < Cl− ≤ 0.50 |
Post-tensioned (PT) tendons have been widely utilized in concrete bridges in the United States. The advantages of PT bridges compared to bridges constructed using conventional reinforcement include greater span length, structural efficiency, reduced materials, and a more streamlined appearance. However, PT tendons can be susceptible to corrosion and ultimately failure if physical deficiencies (PDs) or chemical deficiencies (CDs) are present. Examples of PDs include separation, segregation, presence of soft material, and free water, while an example of a CD includes concentrations of chloride that exceed the allowable limit as specified by the American Association of State Highway and Transportation Officials and other specifications. The failure of a few tendons can compromise overall structural integrity.
Inspections of bridge PT tendons have revealed both PDs and CDs as well as strand tendon failures caused by corrosion have been reported. This study was performed to provide bridge owners with a practical protocol for inspecting, sampling, analyzing, evaluating, and responding to bridge grout concerns.
Jorge E. Pagán-Ortiz
Director, Office of Infrastructure
Research and Development
Notice
This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document.
The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names appear in this report only because they are considered essential to the objective of the document.
Quality Assurance Statement
The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.
1. Report No.: FHWA-HRT-13-028 |
2. Government Accession No. | 3. Recipient’s Catalog No. | ||||
4. Title and Subtitle: Guidelines for Sampling, Assessing, and Restoring Defective Grout in Prestressed Concrete Bridge Post-Tensioning Ducts |
5. Report Date: October 2013 |
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6. Performing Organization Code | ||||||
7. Author(s): Teddy S. Theryo, William H. Hartt, and Piotr Paczkowski |
8. Performing Organization Report No. | |||||
9. Performing Organization Name and Address:
William H. Hartt, PhD, PE, F-NACE |
10. Work Unit No. | |||||
11. Contract or Grant No. | ||||||
12. Sponsoring Agency Name and Address: Office of Infrastructure Research and Development Federal Highway Administration 6300 Georgetown Pike McLean, VA 22101-2296 |
13. Type of Report and Period Covered: Final Report |
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14. Sponsoring Agency Code | ||||||
15. Supplementary Notes: The Contracting Officer’s Technical Representatives (COTRs) were Y.P. Virmani and H. Ghasemi, HRDI-60. An expert task group comprised of members from industry, academia, State transportation departments, and the post- tensioning industry provided valuable technical input and guidance. This study was part of the Long-Term Bridge Performance Program. |
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16. Abstract: A significant proportion of the U.S. bridge inventory is based on bonded post-tensioned (PT) concrete construction. An important aspect of maintaining corrosion protection of these PT systems is assuring that tendon ducts are properly grouted with an acceptable material. Grout is a cementitious material typically used to provide corrosion protection to the strands used in PT concrete bridges. However, inspections have revealed fractured strands and, in some cases, failed tendons as a consequence of corrosion, even with the newer prepackaged, preapproved thixotropic grouts. Studies to-date have attributed this corrosion to physical or chemical grout deficiencies (or both), the former consisting of air voids, free water, and unhardened, segregated, or separated grout and the latter of chloride concentration in excess of what is specified by the American Association of State Highway and Transportation Officials and other specifications. Based on collected information and data analysis, State transportation departments can evaluate if grout deficiencies are present in the tendons of their PT bridges and determine the significance of any deficiencies. Durability concerns associated with PT tendons were raised as early as 1999 when tendon failures were seen in some PT bridges as a result of strand corrosion due to the collection of bleed water in grout voids at tendon profile locations like anchorages and crest areas. While the development of prepackaged thixotropic grout was thought to provide a solution to the bleed water problem, corrosion-caused tendon failures on relatively new PT bridges have occurred, and forensic studies have revealed separation and segregation of grout materials as well as the presence of soft material, free water, and high chloride and sulfate content. (1–3) Consequently, it has become important to examine the overall quality of materials and construction for some in-place grouts in existing PT bridges. The purpose of this study is to provide State transportation departments with guidance regarding tendon inspection, grout sampling, data analysis, and interpretation. |
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17. Key Words: Post-tensioned tendons, Bridges, Corrosion, Grout, Chlorides, Grout defects, Inspection, Sampling |
18. Distribution Statement: No restrictions. This document is available to the public through the National Technical Information Service, Springfield, VA 22161. |
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19. Security Classif. (of this report):
Unclassified |
20. Security Classif. (of this page):
Unclassified |
21. No of Pages: 130 |
22. Price |
Form DOT F 1700.7 (8-72) | Reproduction of completed pages authorized |
SI* (MODERN METRIC CONVERSION FACTORS) | ||||
---|---|---|---|---|
APPROXIMATE CONVERSIONS TO SI UNITS | ||||
Symbol | When You Know | Multiply By | To Find | Symbol |
LENGTH | ||||
in | inches | 25.4 | millimeters | mm |
ft | feet | 0.305 | meters | m |
yd | yards | 0.914 | meters | m |
ml | miles | 1.61 | kilometers | km |
AREA | ||||
in2 | square inches | 635.2 | square millimeters | mm2 |
ft2 | square feet | 0.093 | square meters | m2 |
yd2 | square yard | 0.836 | square meters | m2 |
ac2 | acres | 0.405 | hectares | ha |
mi2 | square miles | 2.59 | square kilometers | km2 |
VOLUME | ||||
fl oz | fluid ounces | 29.57 | millimeters | mL |
gal | gallons | 3.785 | liters | L |
ft3 | cubic feet | 0.028 | cubic meters | m3 |
yd | cubic yards | 0.765 | cubic meters | m3 |
Note: volumes greater than 1000 L shall be shown in m3 | ||||
MASS | ||||
oz | ounces | 28.35 | grams | g |
lb | pounds | 0.454 | kilograms | kg |
T | short tons (2000 lb) | 0.907 | megagrams (or "metric ton") | Mg (or "t") |
TEMPERATURE (exact degrees) | ||||
°F | Fahrenheit | 5 (F-32)/9 | Celsius | °C |
ILLUMINATION | ||||
fc | foot-candles | 10.76 | lux | lx |
fl | foot-Lamberts | 3.426 | candela/m2 | cd/m2 |
FORCE and PRESSURE or STRESS | ||||
lbf | poundforce | 4.45 | newtons | N |
lbf/in2 | poundforce per square inch | 6.89 | kilopascals | kPa |
APPROXIMATE CONVERSIONS FROM SI UNITS | ||||
Symbol | When You Know | Multiply By | To Find | Symbol |
LENGTH | ||||
mm | millimeters | 0.039 | inches | in |
m | meters | 3.28 | feet | ft |
m | meters | 1.09 | yards | yd |
km | kilometers | 0.621 | miles | mi |
AREA | ||||
mm2 | square millimeters | 0.0016 | square inches | in2 |
m2 | square meters | 10.764 | square feet | ft2 |
m2 | square meters | 1.195 | square yards | yd2 |
ha | hectares | 2.47 | acres | ac |
km2 | square kilometers | 0.386 | square miles | mi2 |
VOLUME | ||||
mL | millimeters | 0.034 | fluid ounces | fl oz |
L | liters | 0.264 | gallons | gal |
m3 | cubic meters | 35.314 | cubic feet | ft3 |
m3 | cubic meters | 1.307 | cubic yards | yd3 |
MASS | ||||
g | grams | 0.035 | ounces | oz |
kg | kilograms | 2.202 | pounds | lb |
Mg (or "t") | megagrams | 1.103 | short tons (2000 lb) | T |
TEMPERATURE (exact degrees) | ||||
°C | Celsius | 1.8C+32 | Fahrenheit | °F |
ILLUMINATION | ||||
lx | luxx | 0.0929 | foot-candels | fc |
cd/m2 | candela/m2 | 0.2919 | foot-Lamberts | fl |
FORCE and PRESSURE or STRESS | ||||
N | newtons | 0.225 | poundforce | lbf |
kPa | kilopascals | 0.145 | poundforce per square inch | lbf/in2 |
*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. (Revised March 2003) |
Figure 1. Illustration. Basic PT anchorage system
Figure 2. Photo. Old generation of PT anchorage system
Figure 3. Photo. New generation of PT anchorage system
Figure 4. Illustration. Internal tendon
Figure 5. Illustration. External tendon
Figure 6. Photo. External tendons at deviator
Figure 7. Photo. CIP concrete box girder bridge on false works
Figure 8. Illustration. CIP bridge typical tendon layout
Figure 9. Photo. Blister at the top flange
Figure 10. Illustration. Transverse internal tendon in the diaphragm
Figure 11. Photo. CIP PT concrete slab bridge
Figure 12. Illustration. PT slab bridge typical tendon layout
Figure 13. Photo. CIP segmental balanced cantilever bridge
Figure 14. Photo. CIP balanced cantilever bridge during construction using form traveler
Figure 15. Photo. Typical bottom flange blister
Figure 16. Illustration. Typical vertical tendon in the web
Figure 17. Illustration. Typical vertical tendon in the diaphragm
Figure 18. Photo. Precast spliced girder bridge
Figure 19. Photo. Precast spliced girder bridge during erection
Figure 20. Photo. Precast spliced U-girder bridge during erection
Figure 21. Photo. Precast U-girder supported on temporary false work
Figure 22. Photo. Precast segmental balanced cantilever bridge erection using a segment lifter
Figure 23. Photo. Precast segmental span-by-span erection using an overhead gantry
Figure 24. Illustration. Typical precast segmental span-by-span external PT tendon layout
Figure 25. Photo. Precast segmental cable stayed bridge
Figure 26. Illustration. Cross section of precast segmental columns
Figure 27. Photo. PT straddle bent during construction
Figure 28. Photo. Failed tendon cross section
Figure 29. Photo. Longitudinal section of a tendon section with types 1–3 grout identified
Figure 30. Photo. Opened tendon end showing predominantly type 3 grout and strand corrosion products
Figure 31. Photo. Opened tendon end showing types 3 and 1 grout and strand corrosion products
Figure 32. Photo. Opened tendon end revealing type 4 grout (bottom half), type 1 grout (center region), and strand corrosion products (upper region)
Figure 33. Photo. Opened anchorage on the Carbon Plant Road bridge over IH-37
Figure 34. Equation. Reaction of the ferrous ion with water and sulfate ion to yield ferrous hydroxide and sulfuric acid
Figure 35. Flowchart. Inspection options
Figure 36. Flowchart. Grout inspection processes
Figure 37. Graph. Risk matrix
Figure 38. Equation. Overall probability of defect indicator
Figure 39. Equation. Overall consequence of failure indicator
Figure 40. Equation. Risk
Figure 41. Equation. Probability mass function for hypergeometric distribution
Figure 42. Graph. Example of PMF for hypergeometric distribution
Figure 43. Equation. CDF
Figure 44. Graph. Example of CDF for a hypergeometric distribution
Figure 45. Graph. Minimum number of tendons required to detect at least one tendon with deficient grout assuming 5 percent of the samples are defective
Figure 46. Graph. Minimum number of tendons required to detect at least one tendon with deficient grout assuming 10 percent of the samples are defective
Figure 47. Graph. Minimum number of tendons required to detect at least one tendon with deficient grout assuming 20 percent of the samples are defective
Figure 48. Graph. Minimum number of tendons required to detect at least one tendon with deficient grout assuming 30 percent of the samples are defective
Figure 49. Graph. Balanced cantilever bridge—tendon risk categories
Figure 50. Graph. Spliced girder bridge—tendon risk categories
Figure 51. Graph. Span-by-span segmental bridge—tendon risk categories
Figure 52. Graph. PT bridge—tendon risk categories
Figure 53. Photo. Dial gauge used to determine the depth of a grout air void
Figure 54. Photo. Grout sample acquisition at an opened internal tendon end cap by light chipping
Figure 55. Photo. Concrete excavation to expose an internal tendon
Figure 56. Photo. Internal tendon access at an intermediate location by drilling
Figure 57. Photo. Access hole in an internal tendon at an intermediate location revealing strand and grout
Figure 58. Photo. View of a duct interior revealing a channel air void
Figure 59. Illustration. Bridge representation used to identify grout sampling locations
Figure 60. Illustration. Chart used to record the location and information for individual grout samples
Figure 61. Illustration. Chart used to record and present analysis results for individual grout samples
Figure 62. Photo. Removing a permanent grout cap
Figure 63. Photo. Exposed anchor head after grout sampling
Figure 64. Photo. Locating internal tendons in the web wall using GPR
Figure 65. Photo. Partial removal of a grout cap
Figure 66. Illustration. Typical cantilever tendon inspection point locations
Figure 67. Illustration. Typical continuity tendon inspection point locations
Figure 68. Illustration. Typical span-by-span bridge external tendon inspection point locations
Figure 69. Illustration. CIP PT bridge inspection point locations
Figure 70. Illustration. Spliced bulb-tee girder bridge inspection point locations
Figure 71. Photo. Spliced bulb-tee girder bridge CIP joint detail
Figure 72. Illustration. Spliced U-girder bridge inspection locations
Figure 73. Photo. Temporary restoration of an external tendon
Figure 74. Photo. Permanent restoration of an external tendon using heat shrink sleeve
Figure 75. Illustration. Permanent restoration of an internal tendon duct
Figure 76. Photo. Completed restoration of a partially cut grout cap prior to coating
Figure 77. Photo. Applying elastomeric coating over a grout cap
Figure 78. Photo. Completed permanent restoration of a grout cap
Figure 79. Flowchart. Inspection, sampling, evaluation, and actions for levels 1 and 2 inspections for option 1
Figure 80. Flowchart. Inspection, sampling, evaluation, and actions for levels 1 and 2 inspections for option 2
Figure 81. Illustration. Segment layout—part 1
Figure 82. Illustration. Segment layout—part 2
Figure 83. Illustration. Bulkhead details
Figure 84. Illustration. Longitudinal PT layout—part 1
Figure 85. Illustration. Longitudinal PT layout—part 2
Figure 86. Illustration. Longitudinal PT layout—part 3
Figure 87. Illustration. Longitudinal PT layout—part 4
Figure 88. Illustration. Longitudinal PT layout—part 5
Figure 89. Illustration. Longitudinal PT layout—part 6
Figure 90. Illustration. Longitudinal PT layout—part 7
Figure 91. Illustration. Longitudinal PT layout—part 8
Figure 92. Illustration. Plan and elevation—main channel unit
Figure 93. Illustration. Bridge section
Figure 94. Illustration. Pier cap PT details
Figure 95. Illustration. Tendon profile—main channel unit
Figure 96. Illustration. Modified Florida bulb-T78 beam end segment
Figure 97. Illustration. Modified Florida bulb-T78 beam haunch segment
Figure 98. Illustration. Modified Florida bulb-T78 beam drop-in segment
Figure 99. Illustration. Modified Florida bulb-T78 beam end block detail
Figure 100. Illustration. Typical sections—haunch segment
Table 1. Bridge condition—probability of defect indicator
Table 2. Construction and inspection records—probability of defect indicator
Table 3. Visual evaluation—probability of defect indicators
Table 4. Tendon geometry and length—probability of defect indicators
Table 5. Weight factors for probability of defect indicator
Table 6. Consequence of failure indicator versus cost of repair or tendon replacement
Table 7. Consequence of failure indicator versus element/tendon redundancy
Table 8. Consequence of failure indicator versus bridge importance
Table 9. Weight factors for consequence of failure indicator
Table 10. Acceptable fractions of undetected tendons with deficient grout
Table 11. Minimum number of tendons required to detect at least one tendon with deficient grout (75 percent confidence)
Table 12. Minimum number of tendons required to detect at least one tendon with deficient grout (95 percent confidence)
Table 13. Balanced cantilever bridge—probability of defect indicator
Table 14. Balanced cantilever bridge—consequence of failure indicator
Table 15. Balanced cantilever bridge—minimum recommended number of tendons for inspection
Table 16. List of continuity tendons
Table 17. List of external draped tendons
Table 18. List of selected transverse tendons
Table 19. List of cantilever tendons
Table 20. Spliced girder bridge—probability of defect indicator
Table 21. Spliced girder bridge—consequence of failure indicator
Table 22. Spliced girder bridge—minimum recommended number of tendons for inspection
Table 23. List of longitudinal draped tendons
Table 24. List of diaphragm tendons
Table 25. List of pier cap tendons
Table 26. Span-by-span segmental bridge—probability of defect indicator
Table 27. Span-by-span segmental bridge—consequence of failure indicator
Table 28. Span-by-span segmental bridge—minimum recommended number of tendons for inspection
Table 29. PT bridge—probability of defect indicator
Table 30. PT bridge—consequence of failure indicator
Table 31. PT bridge—minimum recommended number of tendons for inspection
Table 32. CD classifications as determined by grout Cl− levels from an option 1 inspection and resultant recommended actions
Table 33. CD and PD classifications as determined by grout Cl− levels and in-place grout structure by an option 2 inspection and resultant recommended actions
The purpose of this report is to provide guidance for grout sampling, testing, analysis, and interpretation of test results. The following topics are presented and discussed: (1) post-tensioned (PT) bridge types, (2) types of grout deficiencies, (3) statistical approach to grout sampling, (4) grout sampling protocol and test methods, (5) locations for sampling, and (6) interpretation of results and determination of courses of action. Consideration is given to the possibility that extraction of a statistically significant number of samples from PT structures may pose a significant challenge for State transportation departments because of the possibility that invasive inspection and sample acquisition methods might compromise long-term bridge durability and structural integrity.
Durability issues for PT tendons in the United States came to the forefront in 1999 when bridge engineers became aware of failures that resulted from grout voids, associated bleed water, and tendon strand corrosion at higher elevations, such as at anchorages and crest areas. To date, 10 States have reported tendon problems that stem from grout deficiencies or excessive chlorides (Cl–). (See references 1–4.) Most grouts used for PT bridge construction prior to 2001 consisted of a mixture of cement, water, and added admixtures and were typically mixed at the project site.
To improve grout performance as a corrosion protection method for tendons, the Post-Tensioning Institute (PTI) and some State transportation departments revised their grout specifications between 2001 and 2002. This resulted in the formulation of prepackaged, preapproved thixotropic grouts to eliminate bleed water and thus improve the level of protection provided to PT tendons. Prepackaged grout is a proprietary product that has been widely used in PT bridges since 2001.
While the development of prepackaged thixotropic grouts was thought to provide a solution to the bleed water problem, corrosion-caused tendon failures on relatively new PT bridges have continued to occur. Limited forensic studies involving these newer grouts have revealed the presence of grout segregation, soft grout, bleed water, and high Cl– and sulfate contents. However, not all prepackaged grouts exhibited the above deficiencies. Consequently, it is important to investigative these newer grouts to examine the overall quality of in-place grouts in existing PT bridges. This report is intended as a guide for State transportation departments in this regard.
The overall objective of this study was to develop a general guide for State transportation departments for sampling grouts from external and internal tendons in existing PT bridges. To accomplish this, protocols for sampling grouts with both physical deficiencies (PDs) and chemical deficiencies (CDs) were developed. This report provides a rational approach to extract statistically significant numbers of grout samples for proper interpretation of the corrosion susceptibility to the enclosed strands. At the same time, the sampling approach is such that there is minimal negative impact on the future durability considering both grout sampling location and number.
Specific issues addressed in this report include the following:
Anchorage systems in PT bridges are a proprietary system. Systems created by VSL International, Dywidag-Systems International, Freyssinet International, BBR VT International Ltd. (BBR), and Schwager Davis Inc. can be found in PT bridges in the United States. PT anchorage systems differ in shape, size, and material depending on use. In general, a basic PT anchorage system is comprised of a bearing plate, trumpet, wedge plate (anchor head), grout cap, and grout ports (see figure 1). Prior to drilling a hole through the grout port for internal trumpet inspection, it is necessary to determine the PT system used since each system has different grout port orientation and geometry to access the trumpet interior. The simplest way to identify the PT system and its detail is to locate the PT shop drawings for the project, if available.
©VSL International
Figure 1. Illustration. Basic PT anchorage system.
In 2003, the Florida Department of Transportation (FDOT) required an additional vertical grout port/vent located above the trumpet to facilitate post-grouting inspection and permanent grout cap in its PT specifications. The differences between the older and newer generations of PT anchorages systems are shown in figure 2 and figure 3. Many other State transportation departments have adopted PT anchorages with requirements similar to the FDOT requirements. For the new anchorages, the inspection access into the trumpet interior is much simpler through the vertical grout port.
©Dywidag-Systems International
Figure 2. Photo. Old generation of PT anchorage system.
©Dywidag-Systems International
Figure 3. Photo. New generation of PT anchorage system.
In general, PT bridges built in the United States consist of grouted internal tendons, grouted external tendons, or a combination of the two. A small number of bridges may also have greased unbonded tendons. This report only focuses on cement grouted tendons, which may be internal or external.
Internal tendons are located inside the structural concrete section, are housed in corrugated metal ducts or corrugated plastic ducts, and are bonded to the structural concrete by means of cementitious grout (see figure 4). The plastic corrugated ducts are made from high-density polyethylene (HDPE) or polypropylene material. The high-strength steel tendon can be strands, wires, or bars.
Source: Parsons Brinckerhoff
Figure 4. Illustration. Internal tendon.
External tendons are typically located outside the perimeter of a concrete section, are housed in HDPE smooth duct, and are filled with cementitious grout. External tendons are not bonded with the concrete structural section (see figure 5 and figure 6).
Source: Parsons Brinckerhoff
Figure 5. Illustration. External tendon.
Source: Parsons Brinckerhoff
Figure 6. Photo. External tendons at deviator.
PT bridges can be grouped into several categories based on the design and construction methods. Typical possible tendon types used for each bridge group are discussed in the following sections.
CIP concrete box girder bridges built on false works consist of single- to multi-cell box girders, as shown in figure 7. Typically, these types of bridges have an internally draped tendon in the webs. In a continuous multispan structure, the PT anchors are anchored in the end diaphragms, and some tendons may be anchored in the intermediate diaphragms. For long-span bridges, additional internal tendons are also provided in the top and bottom flanges and anchored in blisters (see figure 8 and figure 9). The top deck could be either transversely PT or reinforced concrete using mild reinforcement.
PT tendon types include the following:
Source: Parsons Brinckerhoff
Figure 7. Photo. CIP concrete box girder bridge on false works.
Figure 8. Illustration. CIP bridge typical tendon layout.
Source: Parsons Brinckerhoff
Figure 9. Photo. Blister at the top flange.
Source: Parsons Brinckerhoff
Figure 10. Illustration. Transverse internal tendon in the diaphragm.
CIP concrete slab bridges (see figure 11) are very popular for short-span bridges. The CIP PT slab and T-girder bridges are also constructed on false works. Typically, this type of structure has shallow draped longitudinal internal tendons in the deck (see figure 12). In most cases, transverse internal tendons in the deck are also provided. The superstructure may be a single-span or multispan continuous structure from abutment to abutment.
PT tendon types include the following:
Source: Parsons Brinckerhoff
Figure 11. Photo. CIP PT concrete slab bridge.
Figure 12. Illustration. PT slab bridge typical tendon layout.
CIP segmental box girder bridges (see figure 13) are popular for long-span bridges. They are constructed using the balanced cantilever method with a set of form travelers (see figure 14). The segment is cast against the previous PT segment, which is about 15 ft long. This type of structure utilizes internal cantilever tendons in the top flange over the webs in combination with continuity internal tendons in the bottom flange anchored at blisters (see figure 15). Additional externally draped tendons may also supplement the internal tendons. The top deck is typically transversely PT with tendons encased in flat ducts. For long span bridges, it is also common to use vertical PT bars in the webs of segments close to the pier segment.
PT tendon types include the following:
Source: Parsons Brinckerhoff
Figure 13. Photo. CIP segmental balanced cantilever bridge.
Source: Parsons Brinckerhoff
Figure 14. Photo. CIP balanced cantilever bridge during constructing using form traveler.
Source: Parsons Brinckerhoff
Figure 15. Photo. Typical bottom flange blister.
Source: Parsons Brinckerhoff
Figure 16. Illustration. Typical vertical tendon in the web.
Figure 17. Illustration. Typical vertical tendon in the diaphragm.
Precast spliced girder bridges have been gaining popularity within the last decade for medium-span bridges (see figure 18 and figure 19). Several long pieces of pretensioned American Association of State Highway and Transportation Officials (AASHTO) I-girders or bulb-tee girders are PT using draped internal tendons in the web to form a continuous multispan girder from end to end. The joints between the girders are CIP concrete. The diaphragms are typically cast at the splice locations and are reinforced concrete or PT transversely. A temporary support is provided at the CIP joint locations to stabilize the structure until the girders are made continuous. The deck slab is CIP after the first PT stage is applied. The final PT is applied after the CIP deck slab reaches minimum concrete strength.
PT tendon types include the following:
Source: Parsons Brinckerhoff
Figure 18. Photo. Precast spliced girder bridge.
Source: Parsons Brinckerhoff
Figure 19. Photo. Precast spliced girder bridge during erection.
Similar to precast AASHTO I-girder bridges, precast spliced U-girder bridges have also been gaining popularity recently for medium-span bridges, especially horizontally curved bridges (see figure 20 and figure 21) Several long segments of pretensioned or PT U-girders are PT using draped internal tendons in the web to form a continuous multispan girder from end to end. The joints between the girders are CIP concrete. The diaphragms are typically cast at the splice locations and are reinforced concrete or PT transversely. A temporary support is provided at the CIP joint locations to stabilize the structure until the girders are made continuous. The CIP deck slab is placed after the first stage PT is applied and the rest of the PTs are stressed after the CIP deck is hardened.
PT tendon types include the following:
Source: Summit Engineering Group
Figure 20. Photo. Precast spliced U-girder bridge during erection.
Source: Summit Engineering Group
Figure 21. Photo. Precast U-girder supported on temporary false work.
Precast segmental balanced cantilever bridges are erected using the balanced cantilever method either with an overhead gantry, a beam and winch, a segment transporter/lifter, or a ground-based crane (see figure 22). The segments are precast using match cast in short-line or long-line casting yard that is about 10 to 12 ft long. During segment erection, epoxy is applied at the match cast joints and stressed by internal cantilever tendons in the top flange.
PT tendon types include the following:
Source: Parsons Brinckerhoff
Figure 22. Photo. Precast segmental balanced cantilever bridge erection using a segment lifter.
Precast span-by-span bridges consist of precast match cast segments that are 10 to 12 ft long and erected using an under-slung gantry or an overhead gantry as shown in figure 23. The entire span is temporarily supported by overhead or under-slung gantry stressed together using PT bars after epoxy is applied on the match cast joint. The CIP joints are cast between precast segments and the diaphragm segments. Permanent longitudinal external tendons are PT from both diaphragms to complete the span construction. The process is repeated at the next adjacent span.
PT tendon types include the following:
Source: Parsons Brinckerhoff
Figure 23. Photo. Precast segmental span-by-span erection using an overhead gantry.
Source: Parsons Brinckerhoff
Figure 24. Illustration. Typical precast segmental span-by-span external PT tendon layout.
There are three types of concrete cable-supported bridges in the United States as follows:
The CIP cable-stayed bridge concrete deck construction typically utilizes a set form traveler and is constructed with a balanced cantilever construction method. The previously cast segments are supported by stay cables until the CIP deck reaches the mid-span. Next, a closure segment is placed between the two tips of cantilevers. The CIP deck is typically designed in the form of PT transverse floor beams supported on reinforced concrete edge girders where the stay cable anchorages are located. The pylons and pier columns can be CIP or precast elements with vertical PT.
The precast segmental cable-stayed bridge deck consists of precast box girders. The construction method of the precast deck is very similar to a precast segmental balanced cantilever bridge, except the previously erected segments are supported by stay cables. The segments are PT longitudinally with internal grouted tendons, external tendons, and internal transverse tendons, including diaphragm tendons. The pylons and pier columns can be CIP or precast elements with vertical PT.
An extradosed segmental bridge is a hybrid between a balanced cantilever bridge and a cable stayed bridge and has a very low tower height-to-span ratio. The superstructure of an extradosed bridge is very similar to CIP or precast segmental cable stayed bridges.
The pylon cross beams that support the superstructure of the three types of cable-stayed bridges are normally PT with internal tendons.
PT tendon types include the following:
Source: Parsons Brinckerhoff
Figure 25. Photo. Precast segmental cable stayed bridge.
Aside from the previously listed superstructure bridge types, PT substructures such as PT segmental precast piers (see figure 26), pylon or CIP PT straddle bents (see figure 27), C-bents, pier caps, and pile caps are also common. Most of these structures utilize internal tendons except segmental precast piers—internal, external vertical PT, or combined.
PT tendon types include the following:
Source: Parsons Brinckerhoff
Figure 26. Illustration. Cross section of precast segmental columns.
Source: Parsons Brinckerhoff
Figure 27. Photo. PT straddle bent during construction.
The first line of corrosion protection of grouted PT tendon relies on adequate sealing of ducts from external sources of corrodants (i.e., water, air, Cl – , and carbon dioxide), while the second line of corrosion protection relies on encasement and direct contact of strands with a cementitious grout for which a high pH (> 13) is maintained. Thus, if strands are inadequately grout coated, protection may still be feasible if the duct is adequately sealed and there are no internal sources of corrodants. Alternatively, even if the first protection (by duct) is lacking, the grout acts as a secondary protection to the PT tendon. A necessary but not sufficient condition for strand protection is that Cl – , either as a background grout contaminant or from an external source, is maintained below a critical concentration.
PTI, the American Concrete Institute (ACI), and AASHTO all list 0.08 weight (wt) percent cement as an upper acid soluble Cl – ; limit for PT grout or prestressed concrete. (See references 5–8.) The applicable European standard lists this upper limit as 0.10 wt percent cement. (9) In sufficient concentration, Cl – facilitates corrosion of conventional reinforcement and PT strand by transitioning steel from a passive state for which corrosion rate is negligible to an active one where corrosion rate may be unacceptably high and reducing grout electrical resistivity, thus minimizing macrocell activity and a higher corrosion rate than would otherwise develop. The disclosure that grout may have Cl – concentrations greater than the above limit (0.08 wt percent cement) and that approximately 100 PT bridge projects may have utilized this material has prompted immediate concerns regarding the long-term integrity of these bridges and a need for reactive strategies and actions. In the limited PT tendon grout sampling that has been performed to-date on a bridge that utilized this Cl –contaminated grout, Cl –concentrations as high as 5.27 wt percent grout were reported.(1) However, Cl – concentration of this grout may have varied with production variables such that this contaminant was in an acceptable range for some lots but not others. The purpose of this section of this report is to define the deficiencies that are thought to have occurred with the contaminated grout and other PT grouts with emphasis placed on the consequences of Cl –concentration possibly being in excess of the above specified limit of 0.08 wt percent cement.
1Approximately two-thirds of grout is typically composed of cementitious material.The corrosion process in cementitious materials such as PT grout involves two phases: (1) a time to corrosion initiation, Ti, during which the steel is passive and (2) a period of corrosion propagation, Tp, to the point where repair, rehabilitation, or replacement becomes necessary. PT strands in grout that have a Cl –; concentration below the threshold for active corrosion initiation are normally passive and exhibit a negligible corrosion rate. However, if Cl –; is present above a limiting value, passivity is compromised, and active corrosion may occur at an unacceptable rate, provided oxygen and moisture are present. Oxygen and moisture are invariably present in grout pores of atmospherically exposed tendons, although water content may be minimal under extremely dry conditions, and oxygen concentration may be negligible if the grout is water saturated. From the standpoint of strand corrosion, a worst-case scenario arises in situations where the grout is subjected to repetitive wetting (presumably from periodic water infiltration from a source external to the duct) and drying. Such cycling may also be facilitated by temperature variations. Strand corrosion is particularly severe at an air-water interface. Both Ti and Tp are a function of a number of material and exposure variables, which are described in this chapter in conjunction with a discussion of the Cl –concentration threshold.
The Cl –threshold itself, CT (i.e., the concentration of this species required to initiate active corrosion) is understood to be greater than 0.08 wt percent cement for good quality, high alkalinity in-place grout, although a definitive concentration has not been defined. This is because CT is known to conform to a distribution rather than being a distinct value, and there are a number of variables of influence. For example, air voids greater in diameter than about 0.1 inches that intersect the reinforcement facilitate local premature corrosion initiation. (See references 10–17.) Such occurrences, either involving air voids or relatively large air pockets, result from air entrapment because of inadequate duct venting, incomplete duct filling, strands pressing against the duct interior surface, strand congestion, subsidence, or poor consistency with segregation (or a combination of these). These occurrences have been reported within PT ducts irrespective of the advent during the past decade of thixotropic grouts. (See references 10 and 18 – 20.) Also, steel corrosion at air-grout interfaces, which can occur in conjunction with the previously listed causes, has been reported even with Cl –concentrations below the prescribed upper limit of 0.08 wt percent.(21,22) However, the presence of Cl –should enhance this attack because of steel depassivation or reduced grout resistivity (or both).
Other variables that have been reported to influence CT include mix proportions, cement type, tricalcium aluminate content, concentration of blended materials, water/cementitious materials ratio, temperature, relative humidity, and steel surface condition. However, it can be reasoned that CT depends on cement content alone irrespective of whether or not grout, mortar, or concrete is an issue since Cl –predominantly resides in the cement phase.(23) As a result, CT is normally expressed on a cement wt percent basis. Also, it is the cementitious phase that is predominantly contiguous with conventional reinforcement or bonded PT strand. Thus, for conventional reinforcement, ACI reports CT as 0.2 wt percent cement for concrete, whereas Alonso et al. determined CT to be in the range of 0.39 to 1.16 wt percent cement (also for mortar).(24–26) Mean and standard deviations for CT in concrete have been reported as 0.896 and 0.260 wt percent cement, respectively.(27,28) While this seems high compared to the other CT values listed, the preceding values are not mean values but concentrations at which initial corrosion onset occurred. Thus, if two standard deviations are subtracted (0.260 × 2 = 0.520) from the listed mean (0.896) and if it is assumed that the data are normally distributed, then the result (0.376 wt percent cement) indicates that 2.5 percent of embedded steel should be active at this value. This concentration is in the same range as the other results. However, all the CT determinations listed previously are for conventional reinforcing steel embedded in sound cementitious material. Conversely, if steel is exposed in air and if free water is present, then CT is essentially 0 wt percent. Because the corrosion rate of active carbon steel in aqueous solutions and in cementitious materials is normally controlled by oxygen availability, variations in alloy composition or microstructure (or both) generally have little influence. Consequently, corrosion behavior of PT strand is expected to be generally similar to that for conventional reinforcement.
Previous forensic investigations of existing bridge tendons with prepackaged thixotropic grouts have reported the following four distinct grout textures/appearances:
2 Cl − concentration of the grout from this bridge was below the specified 0.08 wt percent cement upper limit.
Figure 28. Photo. Failed tendon cross section.
In figure 28, the strands contact the duct inner surface at the upper-right portion of the tendon, and type 4 grout is apparent in the lower region. Type 3 grout, either outlined or delineated by a somewhat broad white line (labeled “C”), is also apparent at intermediate elevations, and soft, wet grout is seen near the top. As such, in this case, the segregated white grout tended to separate the gray grout from segregated soft, wet grout or air space, which is apparent near the top (labeled “A”). Thus, segregated white grout occurred either as a volume of material embedded in the grout or as an approximately 0.04-inch-thick layer at the top of the gray grout. The type 2 grout (not apparent in figure 28) has been identified as either unmixed or segregated silica fume. Because segregation involved gravimetric causes, the three undesired grout forms have been most pronounced at elevated horizontal locations, near the top of inclines, and at anchorages. Although not necessarily identifiable in figure 28, corrosion products are contiguous to the upper strands. Figure 29 shows a longitudinal overhead view of the strands and grout in figure 28 with the duct top half removed and all three segregated grout types identified. Section A shows type 1 segregated wet, soft grout, B shows type 2 black segregated grout, and C shows type 3 segregated chalky white grout.
Figure 29. Photo. Longitudinal section of a tendon section with types 1–3 grout identified.
Figure 30 and figure 31 show opened tendon anchorages. In these cases, there was no grout contamination by Cl − , and grout filling of the duct was complete. However, regions of varied grout quality and strand corrosion products are apparent. For example, the grout in figure 30 consists predominantly of type 3 grout with an interconnecting network of cracking. Corrosion products at some of the strand ends are also visible. Figure 31 illustrates much the same but with type 1 grout in the central region. Figure 32 is similar to figure 31 but with type 4 grout in the bottom region. Thus, poor grout quality for these ducts is pervasive.
Figure 33 shows an opened anchorage on the Carbon Plant Road bridge over IH-37 in Texas as reported by the Texas Department of Transportation (TxDOT).(1). Type 1 grout is seen in the upper region, while the lower portion consists of type 4 gray grout. Free water flowed from the anchorage upon opening, and corrosion of strands in the type 1 grout and air space was apparent.
Source: Parsons Brinckerhoff
Figure 30. Photo. Opened tendon end showing predominantly type 3 grout and strand corrosion products.
Source: Parsons Brinckerhoff
Figure 31. Photo. Opened tendon end showing types 3 and 1 grout and strand corrosion products.
Source: Parsons Brinckerhoff
Figure 32. Photo. Opened tendon end revealing type 4 grout (bottom half), type 1 grout (center region), and strand corrosion products (upper region).
Figure 33. Photo. Opened anchorage on the Carbon Plant Road bridge over IH-37. (11)
Bertolini and Carsana reported forensic analysis results for a PT bridge for which a tendon failure was disclosed less than 2 years after construction.3(21) The cementitious grout was not identified by the manufacturer or as being thixotropic; however, a water/cement ratio of 0.32 was specified along with a commercial unidentified admixture specific for PT grouts. While most tendons examined were characterized as consisting of type 4 grout, some, including the anchorage area of the failed tendon, had a whitish unhardened plastic paste, which could be a combination of types 1 and 3. Such regions contained small hardened black spots, which could be type 2 grout. Heavy strand corrosion occurred in areas of the whitish grout.
3 The location of this bridge was not provided, but it is assumed to be in Italy.The limited findings suggest the possibility that segregation and as many as four different textures/consistencies can result for in-place grouts with active strand corrosion occurring for segregated grout types 1–3(1,21,22) . These studies identified air voids/pockets as a major problem and indicated that upward water migration through the grout occurred during setting. The TxDOT study identified Cl−, sulfide (S2−), sodium (Na+), and potassium (K+) in type 1 grout, and FDOT found higher levels of Cl−; and calcium (Ca2+) in type 1 in comparison to the other types.(1,22)
Using an ex-situ leaching method, FDOT measured higher Cl− concentrations at upper compared to lower elevations within a given duct cross section, suggesting that upward migration of this species occurred during grout setting. (22) The concentrations measured at elevated positions were relatively low (maximum ~0.04 wt percent), which was consistent with this grout not necessarily being from Cl− contaminated batches. Also, any partitioning trend for sulfates (SO42−) between grout types was less distinct than for Cl− and Ca2+; however, SO42− concentrations as high as 0.9 wt percent were found in the type 1 grout free water. Sulfates are invariably present in cement pore water, and several authors have associated this species with steel depassivation or passive current densities being an order of magnitude or more higher than for SO42− free simulated pore water solutions as well as passivity breakdown events. (See references 29–31 and 22.) Based on anodic polarization and immersion experiments in saturated calcium hydroxide solutions, Gouda reported the critical SO42− concentration for initiation of active steel corrosion as 0.2 percent.(30) One recommendation has been that the grout maximum cement S2− content be limited to 0.01 percent.(19) Schokker and Musselman reported advanced strand corrosion after relatively brief exposure periods in test assemblies that employed a standard commercial gypsum (hydrated calcium sulfate) grout and contiguous air space but no free water for the purpose of simulating a tendon failure in the Varina-Enon bridge in Virginia.(31) Bertolini and Carsana measured elevated SO42−, Na+, and K+ in the tendon grouts they examined and attributed strand corrosion and resultant fracture to the elevated pH range where the soluble hydrous iron oxide (HFeO2−) corrosion product has been reported and low water resistivity was observed (1.64 ohm-ft). (21,31) Although thermodynamically feasible, such a role of HFeO2− has not been previously reported. It is projected that sulfates were responsible. The European standard lists an upper limit of 4.5 wt percent cement for sulfates and 0.01 wt percent cement for sulfides.(9)
In addition, sulfates may cause electrolyte acidification within occluded regions (crevices), such as the lines of contact between adjacent wires or strands where the solution becomes deaerated. The process is normally described in standard corrosion texts as involving Cl− rather than SO42− , since the former is generally more pervasive; however, any hydrolysable ions, including sulfates, can have the same effect.(33) Thus, the reaction at issue is as follows:
Figure 34. Equation. Reaction of the ferrous ion with water and sulfate ion to yield ferrous hydroxide and sulfuric acid.
Where sulfuric acid is a product. Because of the resultant drop in pH, corrosion rate in such circumstances is expected to be much greater than if the solution were near neutral or alkaline.
Based on the findings discussed in this chapter, the following grout deficiencies are of concern:
The finding that corrosion-induced tendon failures have occurred relatively soon after construction compared to the intended service life, even in situations where Cl− concentrations were relatively low and within the prescribed 0.08 wt percent cement limit, strongly indicates that segregation, subsidence, and incomplete duct filling are major issues.(1,21,22) Corrosion that occurs in such situations is likely to be enhanced by elevated Cl− but will still initiate and propagate even if Cl− concentrations are below the 0.08 wt percent limit.
The determination of the number of tendons to be inspected for grout sampling is an important part of this guideline. If not enough samples are collected, the inspection may not provide a good assessment of the actual condition in the bridge. However, if too many samples are removed, it may be too costly and, without proper restoration, may result in future durability issues. It is critical to select a reasonable number of tendons for each tendon type based on practical and logical considerations such as tendon redundancy, function, workmanship complexity, detailing, etc. For instance, the recommended number of samples for a relatively straight short horizontal tendon will be different than for a long-draped tendon or a cantilever tendon. This chapter provides guidance on the determination of a reasonable number of sampling locations for each type of tendon by utilizing a statistical risk-based approach to categorize and rank the elements of the PT system. The recommended numbers can be adjusted on a case-by-case basis.
As previously stated in chapter 4, there are two main sources of grout deficiencies as follows:
Assuming that deficiencies stem from only grout material contamination, relatively few samples may be required for tendons grouted with the material from a single lot, provided the tendon grouting log information is available. In cases where tendons are grouted with multiple grout lots, a greater number of grout samples will be required in order to best assure that as many lots are included in the sampling as possible. However, the grout log information may not be available for every bridge. Therefore, the statistical grout sampling method is a reasonable approach as it is implemented in other fields in the industry by ASTM E141-10, “Standard Practice for Acceptance of Evidence Based on the Results of Probability Sampling.”(36)
Deficiencies caused by poor workmanship may require more samples because the variation in workmanship is likely to be more random in nature, and there may not be any correlation between different grouting locations. The two main sources of deficiencies can occur independently or concurrently at the same location. The combined effects of deficiencies and the uncertain nature of any poor workmanship complicate the formulation of the quantitative basis for an optimal and cost effective grout sampling/inspection program.
In some cases, bridge owners may be concerned with the Cl− contamination only. Therefore, the inspection guidelines provided in this chapter contain the following two options with two different inspection plans, as shown in figure 35:
For owners who are only concerned with the grout Cl− contamination, the sampling procedure for option 1 should be followed. Otherwise, the sampling procedure for option 2 is followed, as shown in figure 35.
Figure 35 . Flowchart. Inspection options.
Option 2 adopts 75 and 95 percent confidence levels for levels 1 and 2, respectively. The 75 percent confidence approach requires fewer samples than the 95 percent confidence, but it should still provide reasonable findings.
Option 1 assumes no grout PDs, voids, or strand corrosion. If grout PDs are found during sampling, it is recommended to perform an option 2 inspection.
Under this approach, either a minimum of three random samples is taken for each lot from the primary tendon types, or, alternatively, one sample is taken randomly from 50 percent of all batches (one sample per batch) depending on whichever of the two is larger (see chapter 7 for recommended locations of grout sampling). If the test results show that one or more samples has a Cl− content higher than the threshold limit (0.08 wt percent cement), then a level 2 inspection is required. If all analysis results are below this limit, then no further sampling is required.
Under this approach, a minimum of one random grout sample for each batch from the primary tendon types should be obtained (see chapter 9 for the interpretation of the test results for any further actions).
Grout sampling for this type of project should follow the procedures based on statistical methods of sampling for all grout deficiencies (option 2).
In general, the inspection process is performed in four steps (see figure 36). In the first step, an engineer should review the as-built plans, PT shop drawings, specifications, and construction records. Then, the engineer should conduct a walk-through visual inspection for the length of the bridge. The main objective of the walk-through inspection is to evaluate the overall condition of the structural system and identify possible defects and signs of deterioration. The visual inspection does not require any specialized equipment. The third step is to obtain grout samples and visually assess and document the grout condition and any signs of tendon defects. Depending on the type of structure/tendon, some specialized equipment/tools might be necessary. At this point, the possibility of causing a “weak spot” or distress in the element (e.g., tendon damage during drilling) and rendering it prone to corrosion in the future if not properly repaired becomes an important factor in selecting the number of tendons for sampling. Therefore, it is important to minimize the number of sampled tendons while still providing an acceptable level of accuracy in representation of the grout chemical composition and condition. Chapter 7 of this report discusses the strategy that should be used to identify and preselect test locations within a tendon that are most likely to have deficient grout.
Figure 36 . Flowchart. Grout inspection processes.
Since there is no quantitative methodology for ranking and identifying deficient areas, the elements of the PT system should be categorized based on the qualitative opinion of experts. The risk-based inspection methodology was adopted, and tendons are prioritized in terms of the risk associated with the potential failure.(38) All tendons in the bridge are assigned to one of the following groups:
Figure 37. Graph. Risk matrix.
The overall condition and previous service performance history of a bridge is an important factor affecting the likelihood of an ongoing corrosion process. Existing serviceability issues such as visible cracks, discoloration of concrete, or poor overall workmanship (or a combination of these) should be considered in assigning this factor. Table 1 lists the probability of defect indicators for the bridge condition categories considered.
Bridge Condition | Description | Value |
---|---|---|
Poor | Very high degradation probability | 5 |
Moderate | High degradation probability | 4 |
Good | Average degradation probability | 3 |
Very Good | Low degradation probability | 2 |
Excellent | Very low degradation probability | 1 |
An important factor affecting the risk associated with the potential tendon failure is the availability of the construction records and previous inspection records. Recommended probability of defect indicators are listed in table 2.
Construction and Inspection Records | Value |
---|---|
No construction and inspection records exist | 5 |
Limited construction and inspection records exist | 4 |
Some construction and inspection records exist | 3 |
Comprehensive construction and inspection records exist | 2 |
Very comprehensive construction and inspection records exist | 1 |
The visual evaluation serves as an important first tool in selecting any problem areas and estimating the likelihood of more serious structural defects. The probability of defect indictors for the visual inspection categories are listed in table 3. The maximum value of the defect indicator should be selected from the list of assigned probability of defect indicators in this category.
Visual Inspection Category | Major Defect | Moderate Defect | Small Defect | Very Small Defect | No Defect |
---|---|---|---|---|---|
Signs of grout leakage | 5 | 4 | 3 | 2 | 1 |
Workmanship | 5 | 4 | 3 | 2 | 1 |
Cracked concrete | 5 | 4 | 3 | 2 | 1 |
Duct condition (external PT) | 5 | 4 | 3 | 2 | 1 |
Voids (external PT) | 5 | 4 | 3 | 2 | 1 |
Signs of water leakage | 5 | 4 | 3 | 2 | 1 |
Corrosion protection | 5 | 4 | 3 | 2 | 1 |
It was observed from prior investigation experiences that it is unlikely to have defective grout in short straight tendons or tendons with small curvature changes. The likelihood of a defect is higher for multispan long tendons with large curvature changes and large distances between lowest and highest points, particularly tall vertical tendons. The probability of defect indicators for the tendon shape categories are listed in table 4.
Tendon Geometry and Length | Value |
---|---|
Long multispan tendons with large curvature changes/large distance between lowest and highest points; tall vertical tendons | 5 |
Short single-span tendons with large curvature changes/large distance between lowest and highest points; short vertical tendons | 4 |
Long tendons with small curvature changes | 3 |
Long straight tendons or short tendons with small curvature changes | 2 |
Short straight/horizontal tendons | 1 |
The overall probability of defect indicator, P, is determined as a weighted sum of the contributing factors as follows:
Figure 38. Equation. Overall probability of defect indicator.
Where:
Pi = Partial probability of defect indicator.
Wi = Appropriate weight factor, as shown in table 5.
Probability of Defect Indicator | Weight |
---|---|
Overall bridge condition | 0.15 |
Construction and inspection records | 0.15 |
Visual evaluations | 0.30 |
Tendon geometry and length | 0.40 |
The consequence of failure indicator is intended to categorize the tendons in terms of the effect that eventual failure due to corrosion resulting from an undetected defect will have on the structure.
The relative cost of repair/replacement accounts for the funding that would be needed to restore the full functionality of the structure after the elements have become damaged or failed. Table 6 presents the consequence of failure indicators in this category.
Rank | Cost of Repair or Tendon Replacement |
---|---|
Very high | 5 |
High | 4 |
Moderate | 3 |
Low | 2 |
Very low | 1 |
Redundancy is generally defined as the extra capacity of a structural system to carry loads after partial damage or failure of its elements. For example, cantilever tendons have higher redundancy than bottom continuity tendons since a portion of the cantilever tendons is required to support the dead loads and erection equipment during free cantilever construction. After the cantilever tips from two adjacent piers are connected, the negative moments demand will be reduced. However, the extra tendons are typically left in place. Diaphragm tendons are also considered to have high redundancy due to the presence of a large amount of ordinary reinforcement. The consequence of failure indicators associated with the element/tendon redundancy are listed in table 7.
Element/Tendon Redundancy | Value |
---|---|
Loss of some tendons will cause a catastrophic failure | 5 |
Loss of some tendons will cause severe distress to the structure—repairable | 4 |
Loss of some tendons will decrease the capacity of the structural system—possible need for posting, no structural distress | 3 |
Sufficient safety reserve exists. Loss of a small number of tendons will slightly impact the capacity, but the system can remain in service without posting | 2 |
Tendon does not contribute to the resistance of the structural system | 1 |
The criticality of the bridge as an element of the transportation system is considered in this category. Major bridges carrying large volumes of traffic should be assigned to a high consequence category, while bridges in a rural area with low traffic volumes should be assigned a low number. Recommended consequence of failure indicators for this category are listed in table 8.
Bridge Importance | Value |
---|---|
Critical bridges | 5 |
Non-critical bridges | 3 |
Less important bridges | 1 |
The overall consequence of failure indicator, C, is determined as a weighted sum of the contributing factors as follows:
Figure 39. Equation. Overall consequence of failure indicator.
Where:
Ci = Partial probability of defect indicator.
Wi = Appropriate weight factor, as shown in table 9.
Consequence of Failure Indicator | Weight |
---|---|
Cost for repair/tendon replacement | 0.4 |
Element redundancy | 0.4 |
Criticality of the bridge | 0.2 |
Figure 37 is used to determine risk level and prioritize/categorize the tendons according to the relationship, as shown in figure 40.
Figure 40. Equation. Risk.
Where:
P = Overall probability of defect indicator.
C = Consequence of failure indicator.
The recommended minimum number of sampled tendons depends on element risk and the possibility of creating distress for the structure throug intrusive inspection. The acceptable number of tendons with undetected deficient grout is lower for elements with high risk. Within a risk category, the acceptable number of tendons with undetected deficient grout decreases as the likelihood of structural distress associated with intrusive sampling increases. Table 10 presents recommendations regarding the acceptable number of tendons with undetected deficient grout to categories of element risk and inspection costs.
Element Risk | Structural Impact Caused by Sampling | ||
---|---|---|---|
High | Medium | Low | |
High | 20 percent deficient | 10 percent deficient | 10 percent deficient |
Medium | 30 percent deficient | 20 percent deficient | 10 percent deficient |
Low | 30 percent deficient | 30 percent deficient | 20 percent deficient |
To determine the minimum number of inspected tendons, tendons identified for testing are arranged in groups within which every tendon has approximately the same likelihood of having defective grout; each group is considered as a separate population. Depending on the total number of tendons in one group, N, as compared to the number of tendons in that group selected for inspection, n, selecting the sample n tendons from population N might significantly change the remainder of the population regardless of how many tendons with defective grout are in the sample. Therefore, it is assumed that the distribution of the number of tendons with defective grout in a random sample of n tendons is an approximately hypergeometric distribution with parameters m, k, and N, where m is the actual number of tendons with defective grout and k is the number of times when the selected tendon is with defective grout. The probability mass function (PMF) of this distribution is expressed as follows:
Figure 41. Equation. Probability mass function for hypergeometric distribution.
Where P(k) is the probability of observing k number of deficiencies when selecting without replacement n samples from the population of size N.(39) Figure 42 presents an example of PMF of the fraction of samples with defective grout calculated for N = 100, m = 25, n = 45, and k = 1…20. Each bar represents the probability that when 45 tendons are randomly inspected from a total population of 100 tendons, k tendons will have a defect. Since in this example 25 percent of tendons have defective grout, the expected and most probable number of successes (detection of tendon with defect) is 25 percent of 45, or 11 tendons.
Figure 42. Graph. Example of PMF for hypergeometric distribution.
The cumulative distribution function (CDF) of the hypergeometric distribution is defined as follows:
Figure 43. Equation. CDF.
Figure 44 shows CDF for PMF for the example PMF presented in figure 42. Based on the properties of CDF, it can be determined that there is (1 – Pd) = 10 percent probability of discovering not more than eight defective tendons and a Pd = 90 percent chance of discovering eight or more defective tendons where Pd is defined as the probability of detecting more than i number of defective tendons in the population of N with m tendons being defective.
Figure 44. Graph. Example of CDF for a hypergeometric distribution.
Calculations were performed to determine the minimum number of samples required to detect at least one tendon with defective grout, assuming different fractions of tendons with defective grout in the total population. For all considered cases, the probability of detection (confidence of detecting at least one) is set equal to 75 percent (level 1 inspection) and 95 percent (level 2 inspection). The results are presented in the table 11 and table 12. Based on the results in table 12, if 100 tendons are identified in one risk group and eight are initially tested with no defects, then there is a 95 percent probability that the fraction of tendons with defects in the selected population is less than 30 percent. By testing an additional five tendons, it can be assured that there is 95 percent probability that the fraction of tendons with defects in the selected population is less than 20 percent.
In some cases, it might be necessary to relax the assumptions by decreasing the probability of detecting at least one defective to minimize the number of tendons required to detect at least one tendon with defective grout. Figure 45 to figure 48 present the minimum required number of sampled tendons for confidence levels equal to 95, 85, and 75 percent assuming 5, 10, 20, and 30 percent of the tendons have defective grout. The figures are intended to assist in making informed decisions regarding the sample size to minimize the sampling effort.
Table 11. Minimum number of tendons required to detect at least one tendon with deficient grout (75% confidence).
Number of Identified Tendons | Percent of Tendons with Deficient Grout | ||
---|---|---|---|
10 Percent | 20 Percent | 30 Percent | |
10 | 8 | 5 | 3 |
20 | 10 | 5 | 4 |
50 | 12 | 6 | 4 |
100 | 12 | 6 | 4 |
150 | 13 | 6 | 4 |
200 | 13 | 6 | 4 |
500 | 13 | 6 | 4 |
> 1,000 | 13 | 6 | 4 |
Table 12. Minimum number of tendons required to detect at least one tendon with deficient grout (95% confidence).
Number of Identified Tendons | Percent of Tendons with Deficient Grout | ||
---|---|---|---|
10 Percent | 20 Percent | 30 Percent | |
10 | 10 | 7 | 6 |
20 | 15 | 10 | 6 |
50 | 22 | 12 | 7 |
100 | 25 | 13 | 8 |
150 | 26 | 13 | 8 |
200 | 26 | 13 | 8 |
500 | 27 | 13 | 8 |
> 1,000 | 28 | 13 | 8 |
Figure 45. Graph. Minimum number of tendons required to detect at least one tendon with deficient grout assuming 5 percent of the samples are defective.
Figure 46. Graph. Minimum number of tendons required to detect at least one tendon with deficient grout assuming 10 percent of the samples are defective.
Figure 47. Graph. Minimum number of tendons required to detect at least one tendon with deficient grout assuming 20 percent of the samples are defective.
Figure 48 . Graph. Minimum number of tendons required to detect at least one tendon with deficient grout assuming 30 percent of the samples are defective.
The procedure to determine the recommended minimum number of sampled tendons to be tested consists of the following steps:
After the minimum number of sampled tendons to be tested is determined, it is necessary to locate the strategic inspection points for each tendon type (see chapter 7).
The selection of a random sample from the list of identified inspection locations involves the following steps:
The procedure for option 2 inspection is presented in this section as an example using an existing precast segmental bridge. The plans for the considered bridge can be found in appendix A. The bridge consists of 10 spans with the total length of 2,256.33 ft. The following four distinct types of tendons are utilized in this bridge:
Category | Weight(table 5) | Tendon Type | |||
---|---|---|---|---|---|
Transverse | Cantilever | Continuity | External | ||
Bridge condition (table 1) | 0.15 | 2 | 2 | 2 | 2 |
Construction and inspection records (table 2 ) | 0.15 | 5 | 5 | 5 | 5 |
Visual inspection (table 3 ) | 0.30 | 1 | 1 | 1 | 1 |
Tendon shape and length (table 4 ) | 0.40 | 1 | 2 | 2 | 4 |
Probability of defect indicator | 2 | 2 | 2 | 3 |
Category | Weight (table 9) | Tendon Type | ||||
---|---|---|---|---|---|---|
Transverse | Cantilever | Continuity | External | |||
Cost of repair or replacement (table 6 ) | 0.40 | 3 | 5 | 4 | 2 | |
Element/tendon redundancy (table 7 ) |
0.40 | 2 | 2 | 4 | 2 | |
Bridge importance (table 8 ) | 0.20 | 5 | 5 | 5 | 5 | |
Consequence of failure indicator | 3 | 4 | 4 | 3 |
Figure 49. Graph. Balanced cantilever bridge—tendon risk categories.
Type of Tendon | Total Number of Tendons | Risk | Structural Impact Caused by Inspection | Acceptable Fraction Of Undetected Defective | Minimum Number of Tendons for Inspection | |
---|---|---|---|---|---|---|
Level 1 | Level 2 | |||||
Transverse tendon | 694 | Medium | High | 30 percent | 4 | 8 |
Cantilever tendon | 242 | Medium | High | 30 percent | 4 | 8 |
Continuity tendon | 112 | Medium | Medium | 20 percent | 6 | 13 |
External tendon | 40 | Medium | Low | 10 percent | 12 | 22 |
Table 16 through table 19 present the summary of all tendons. Each tendon is identified by its label. The first letter and number denotes the span number "S". The second letter denotes the tendon name as shown in the as-built plans. The last number helps identify the tendon in case there are two or more tendons with the same profile at one location. All of the data in tables are sorted with respect to the random number assigned to each tendon. Bold text indicates level 1 inspections, and bold italics indicates level 2 inspections.
# | Random | Tendon ID | # | Random # | Tendon ID | # | Random # | Tendon ID | # | Random # | Tendon ID |
---|---|---|---|---|---|---|---|---|---|---|---|
1 | 0.022 | S7-C4-1 | 29 | 0.237 | S1-C5-2 | 57 | 0.461 | S1-C3-1 | 85 | 0.783 | S8-C1-2 |
2 | 0.028 | S5-T20-2 | 30 | 0.254 | S4-T21-2 | 58 | 0.462 | S2-C3-1 | 86 | 0.788 | S4-C2-2 |
3 | 0.055 | S7-C5-1 | 31 | 0.258 | S4-T20-2 | 59 | 0.467 | S4-C3-1 | 87 | 0.802 | S2-C1-1 |
4 | 0.055 | S7-T20-1 | 32 | 0.270 | S8-C2-1 | 60 | 0.476 | S2-T20-2 | 88 | 0.806 | S5-C4-1 |
5 | 0.056 | S9-T21-2 | 33 | 0.274 | S10-C1-2 | 61 | 0.504 | S3-C2-2 | 89 | 0.812 | S2-C4-1 |
6 | 0.063 | S9-C2-1 | 34 | 0.282 | S2-C2-2 | 62 | 0.504 | S1-T20-2 | 90 | 0.813 | S1-C4-1 |
7 | 0.078 | S5-T20-1 | 35 | 0.287 | S9-C3-1 | 63 | 0.509 | S8-C4-2 | 91 | 0.818 | S6-C2-1 |
8 | 0.088 | S3-C2-1 | 36 | 0.298 | S9-C2-2 | 64 | 0.521 | S7-T20-2 | 92 | 0.818 | S3-T20-2 |
9 | 0.092 | S6-C3-1 | 37 | 0.328 | S7-C2-1 | 65 | 0.527 | S5-C2-1 | 93 | 0.833 | S3-C1-1 |
10 | 0.098 | S5-C3-1 | 38 | 0.359 | S6-C5-2 | 66 | 0.569 | S4-C2-1 | 94 | 0.835 | S3-C6-1 |
11 | 0.112 | S2-C2-1 | 39 | 0.363 | S10-C2-1 | 67 | 0.573 | S1-C2-1 | 95 | 0.843 | S1-C2-2 |
12 | 0.112 | S6-C3-2 | 40 | 0.364 | S6-T20-1 | 68 | 0.578 | S7-C1-1 | 96 | 0.850 | S3-C6-2 |
13 | 0.114 | S5-C5-1 | 41 | 0.372 | S7-C3-2 | 69 | 0.593 | S7-C4-2 | 97 | 0.855 | S6-C1-1 |
14 | 0.134 | S5-C4-2 | 42 | 0.383 | S6-T20-2 | 70 | 0.605 | S1-C4-2 | 98 | 0.861 | S5-C1-2 |
15 | 0.134 | S8-C5-2 | 43 | 0.389 | S7-C1-2 | 71 | 0.609 | S3-C1-2 | 99 | 0.892 | S8-C2-2 |
16 | 0.147 | S7-C3-1 | 44 | 0.399 | S6-C2-2 | 72 | 0.620 | S2-C4-2 | 100 | 0.895 | S10-C2-2 |
17 | 0.161 | S8-T20-2 | 45 | 0.407 | S8-C1-1 | 73 | 0.640 | S1-C5-1 | 101 | 0.907 | S4-T20-1 |
18 | 0.162 | S7-C2-2 | 46 | 0.414 | S5-C3-2 | 74 | 0.646 | S2-C1-2 | 102 | 0.918 | S3-T20-1 |
19 | 0.176 | S4-C1-1 | 47 | 0.416 | S1-T20-1 | 75 | 0.666 | S9-C3-2 | 103 | 0.920 | S4-C3-2 |
20 | 0.184 | S4-C4-2 | 48 | 0.419 | S2-T20-1 | 76 | 0.692 | S8-C3-1 | 104 | 0.931 | S10-T20-2 |
21 | 0.194 | S8-C3-2 | 49 | 0.424 | S1-C3-2 | 77 | 0.704 | S5-C1-1 | 105 | 0.939 | S3-C5-2 |
22 | 0.195 | S3-C3-1 | 50 | 0.428 | S9-T20-1 | 78 | 0.707 | S7-C5-2 | 106 | 0.942 | S1-C1-1 |
23 | 0.199 | S9-C1-1 | 51 | 0.432 | S3-C4-1 | 79 | 0.707 | S5-C2-2 | 107 | 0.943 | S3-C4-2 |
24 | 0.207 | S10-C1-1 | 52 | 0.441 | S6-C4-2 | 80 | 0.714 | S3-C3-2 | 108 | 0.968 | S1-C1-2 |
25 | 0.219 | S9-C1-2 | 53 | 0.450 | S8-C5-1 | 81 | 0.718 | S2-C3-2 | 109 | 0.970 | S8-C4-1 |
26 | 0.230 | S10-T20-1 | 54 | 0.456 | S6-C4-1 | 82 | 0.737 | S4-T21-1 | 110 | 0.976 | S6-C5-1 |
27 | 0.235 | S4-C4-1 | 55 | 0.459 | S4-C1-2 | 83 | 0.752 | S9-T20-2 | 111 | 0.989 | S9-T21-1 |
28 | 0.236 | S6-C1-2 | 56 | 0.460 | S5-C5-2 | 84 | 0.783 | S8-T20-1 | 112 | 0.998 | S3-C5-1 |
# | Random # | Tendon ID | # | Random# | Tendon ID |
---|---|---|---|---|---|
1 | 0.001 | S10-E1-1 | 21 | 0.416 | S9-E1-2 |
2 | 0.040 | S7-E2-1 | 22 | 0.453 | S4-E1-2 |
3 | 0.062 | S4-E1-1 | 23 | 0.457 | S4-E2-2 |
4 | 0.064 | S10-E2-1 | 24 | 0.493 | S3-E1-1 |
5 | 0.074 | S7-E1-1 | 25 | 0.600 | S4-E2-1 |
6 | 0.080 | S10-E2-2 | 26 | 0.631 | S5-E2-1 |
7 | 0.090 | S9-E2-2 | 27 | 0.642 | S3-E2-1 |
8 | 0.116 | S9-E1-1 | 28 | 0.645 | S8-E2-1 |
9 | 0.118 | S3-E1-2 | 29 | 0.685 | S1-E1-1 |
10 | 0.119 | S8-E2-2 | 30 | 0.693 | S8-E1-1 |
11 | 0.129 | S7-E2-2 | 31 | 0.783 | S1-E2-2 |
12 | 0.148 | S2-E1-1 | 32 | 0.792 | S5-E2-2 |
13 | 0.161 | S3-E2-2 | 33 | 0.794 | S2-E2-1 |
14 | 0.201 | S1-E1-2 | 34 | 0.894 | S10-E1-2 |
15 | 0.234 | S2-E2-2 | 35 | 0.897 | S7-E1-2 |
16 | 0.243 | S5-E1-2 | 36 | 0.906 | S5-E1-1 |
17 | 0.274 | S2-E1-2 | 37 | 0.909 | S9-E2-1 |
18 | 0.360 | S6-E2-2 | 38 | 0.932 | S1-E2-1 |
19 | 0.362 | S6-E1-2 | 39 | 0.943 | S8-E1-2 |
20 | 0.372 | S6-E1-1 | 40 | 0.974 | S6-E2-1 |
# | Random # | Tendon ID |
---|---|---|
1 | 0.001 | 249 |
2 | 0.001 | 255 |
3 | 0.004 | 103 |
4 | 0.005 | 388 |
5 | 0.006 | 694 |
6 | 0.009 | 426 |
7 | 0.015 | 456 |
8 | 0.020 | 638 |
# | Random # | Tendon ID | # | Random # | Tendon ID | # | Random # | Tendon ID | # | Random # | Tendon ID |
---|---|---|---|---|---|---|---|---|---|---|---|
1 | 0.016 | P3-T11-2 | 62 | 0.285 | P4-T1-2 | 123 | 0.503 | P4-T2-2 | 184 | 0.741 | P5-T3-1 |
2 | 0.016 | P10-T2-1 | 63 | 0.288 | P7-T10-1 | 124 | 0.504 | P4-T2-1 | 185 | 0.744 | P6-T1-1 |
3 | 0.020 | P8-T5-2 | 64 | 0.291 | P9-T3-1 | 125 | 0.504 | P5-T9-2 | 186 | 0.746 | P8-T14-2 |
4 | 0.022 | P7-T9-1 | 65 | 0.304 | P2-T3-2 | 126 | 0.509 | P5-T3-2 | 187 | 0.759 | P5-T7-1 |
5 | 0.027 | P2-T10-1 | 66 | 0.306 | P4-T8-2 | 127 | 0.511 | P8-T10-1 | 188 | 0.760 | P3-T1-1 |
6 | 0.032 | P3-T7-1 | 67 | 0.307 | P2-T8-2 | 128 | 0.522 | P4-T5-2 | 189 | 0.762 | P5-T6-2 |
7 | 0.038 | P6-T2-1 | 68 | 0.309 | P7-T5-1 | 129 | 0.525 | P7-T7-2 | 190 | 0.763 | P10-T4-2 |
8 | 0.038 | P3-T4-2 | 69 | 0.309 | P4-T10-1 | 130 | 0.528 | P8-T10-2 | 191 | 0.782 | P8-T4-1 |
9 | 0.046 | P6-T4-2 | 70 | 0.309 | P3-T8-2 | 131 | 0.529 | P8-T13-1 | 192 | 0.782 | P7-T6-2 |
10 | 0.049 | P3-T5-2 | 71 | 0.322 | P8-T2-1 | 132 | 0.531 | P6-T14-2 | 193 | 0.789 | P10-T2-2 |
11 | 0.050 | P4-T10-2 | 72 | 0.328 | P6-T5-1 | 133 | 0.533 | P4-T16-2 | 194 | 0.798 | P9-T9-1 |
12 | 0.053 | P2-T8-1 | 73 | 0.343 | P8-T7-1 | 134 | 0.536 | P6-T9-2 | 195 | 0.807 | P9-T4-1 |
13 | 0.055 | P10-T1-1 | 74 | 0.350 | P6-T4-1 | 135 | 0.538 | P9-T6-2 | 196 | 0.808 | P6-T8-1 |
14 | 0.058 | P7-T4-2 | 75 | 0.351 | P9-T1-1 | 136 | 0.540 | P6-T13-1 | 197 | 0.810 | P6-T9-1 |
15 | 0.058 | P10-T5-1 | 76 | 0.353 | P10-T6-2 | 137 | 0.548 | P9-T1-2 | 198 | 0.815 | P9-T7-2 |
16 | 0.059 | P2-T5-1 | 77 | 0.359 | P8-T9-1 | 138 | 0.549 | P4-T11-2 | 199 | 0.836 | P3-T10-2 |
17 | 0.061 | P6-T13-2 | 78 | 0.369 | P5-T11-2 | 139 | 0.552 | P5-T10-2 | 200 | 0.838 | P8-T9-2 |
18 | 0.070 | P3-T14-1 | 79 | 0.377 | P3-T10-1 | 140 | 0.560 | P9-T12-1 | 201 | 0.838 | P2-T7-2 |
19 | 0.074 | P6-T8-2 | 80 | 0.377 | P6-T11-2 | 141 | 0.568 | P9-T14-1 | 202 | 0.844 | P7-T13-1 |
20 | 0.084 | P6-T7-1 | 81 | 0.380 | P5-T1-1 | 142 | 0.568 | P8-T3-2 | 203 | 0.847 | P4-T4-1 |
21 | 0.085 | P8-T12-2 | 82 | 0.380 | P8-T14-1 | 143 | 0.569 | P8-T7-2 | 204 | 0.853 | P5-T7-2 |
22 | 0.089 | P4-T7-1 | 83 | 0.381 | P8-T5-1 | 144 | 0.572 | P5-T10-1 | 205 | 0.858 | P7-T13-2 |
23 | 0.097 | P3-T1-2 | 84 | 0.381 | P9-T16-2 | 145 | 0.578 | P9-T5-2 | 206 | 0.860 | P7-T10-2 |
24 | 0.100 | P3-T6-1 | 85 | 0.389 | P9-T8-2 | 146 | 0.581 | P10-T7-2 | 207 | 0.862 | P7-T4-1 |
25 | 0.104 | P9-T5-1 | 86 | 0.396 | P4-T9-2 | 147 | 0.582 | P3-T14-2 | 208 | 0.865 | P6-T5-2 |
26 | 0.105 | P6-T1-2 | 87 | 0.398 | P4-T4-2 | 148 | 0.586 | P10-T3-2 | 209 | 0.872 | P3-T2-2 |
27 | 0.107 | P2-T1-2 | 88 | 0.398 | P5-T4-1 | 149 | 0.590 | P3-T15-1 | 210 | 0.876 | P6-T14-1 |
28 | 0.109 | P9-T12-2 | 89 | 0.399 | P7-T11-1 | 150 | 0.608 | P5-T1-2 | 211 | 0.878 | P3-T15-2 |
29 | 0.112 | P7-T8-1 | 90 | 0.399 | P3-T5-1 | 151 | 0.609 | P3-T9-2 | 212 | 0.886 | P5-T13-2 |
30 | 0.120 | P4-T7-2 | 91 | 0.400 | P2-T5-2 | 152 | 0.618 | P4-T6-1 | 213 | 0.888 | P3-T12-2 |
31 | 0.134 | P10-T4-1 | 92 | 0.403 | P2-T9-2 | 153 | 0.626 | P7-T12-2 | 214 | 0.889 | P5-T11-1 |
32 | 0.136 | P9-T7-1 | 93 | 0.404 | P8-T3-1 | 154 | 0.627 | P6-T7-2 | 215 | 0.891 | P9-T15-2 |
33 | 0.138 | P7-T8-2 | 94 | 0.405 | P9-T15-1 | 155 | 0.629 | P6-T12-1 | 216 | 0.892 | P3-T4-1 |
34 | 0.146 | P2-T6-1 | 95 | 0.405 | P8-T8-1 | 156 | 0.630 | P9-T11-2 | 217 | 0.894 | P9-T10-1 |
35 | 0.153 | P9-T16-1 | 96 | 0.410 | P4-T13-1 | 157 | 0.630 | P7-T2-2 | 218 | 0.895 | P4-T6-2 |
36 | 0.159 | P4-T8-1 | 97 | 0.412 | P10-T1-2 | 158 | 0.630 | P4-T14-2 | 219 | 0.907 | P6-T3-1 |
37 | 0.162 | P7-T9-2 | 98 | 0.413 | P6-T3-2 | 159 | 0.633 | P6-T12-2 | 220 | 0.925 | P3-T13-2 |
38 | 0.163 | P4-T9-1 | 99 | 0.415 | P9-T6-1 | 160 | 0.633 | P9-T13-1 | 221 | 0.933 | P4-T11-1 |
39 | 0.165 | P3-T13-1 | 100 | 0.416 | P6-T6-1 | 161 | 0.636 | P4-T12-1 | 222 | 0.937 | P2-T7-1 |
40 | 0.165 | P3-T16-1 | 101 | 0.418 | P4-T16-2 | 162 | 0.640 | P8-T6-1 | 223 | 0.940 | P9-T13-2 |
41 | 0.165 | P7-T14-1 | 102 | 0.420 | P3-T2-1 | 163 | 0.644 | P8-T11-1 | 224 | 0.940 | P6-T6-2 |
42 | 0.169 | P3-T16-2 | 103 | 0.435 | P10-T6-1 | 164 | 0.652 | P6-T10-2 | 225 | 0.952 | P7-T14-2 |
43 | 0.182 | P6-T10-1 | 104 | 0.435 | P3-T11-1 | 165 | 0.654 | P2-T10-2 | 226 | 0.962 | P8-T1-2 |
44 | 0.185 | P7-T3-2 | 105 | 0.437 | P7-T1-1 | 166 | 0.659 | P2-T1-1 | 227 | 0.964 | P9-T3-2 |
45 | 0.186 | P4-T15-2 | 106 | 0.441 | P7-T2-1 | 167 | 0.660 | P9-T10-2 | 228 | 0.965 | P5-T5-2 |
46 | 0.189 | P5-T4-2 | 107 | 0.446 | P9-T2-1 | 168 | 0.664 | P7-T12-1 | 229 | 0.967 | P4-T3-1 |
47 | 0.194 | P10-T8-2 | 108 | 0.447 | P4-T3-2 | 169 | 0.668 | P6-T2-2 | 230 | 0.968 | P7-T5-2 |
48 | 0.207 | P3-T3-2 | 109 | 0.449 | P2-T2-1 | 170 | 0.669 | P9-T9-2 | 231 | 0.969 | P8-T4-2 |
49 | 0.210 | P8-T8-2 | 110 | 0.452 | P8-T1-1 | 171 | 0.669 | P10-T8-1 | 232 | 0.971 | P2-T2-2 |
50 | 0.221 | P3-T8-1 | 111 | 0.466 | P3-T3-1 | 172 | 0.671 | P5-T12-1 | 233 | 0.971 | P6-T11-1 |
51 | 0.228 | P9-T11-1 | 112 | 0.471 | P4-T5-1 | 173 | 0.671 | P2-T6-2 | 234 | 0.974 | P5-T5-1 |
52 | 0.234 | P4-T13-2 | 113 | 0.472 | P7-T7-1 | 174 | 0.677 | P5-T2-2 | 235 | 0.975 | P4-T15-2 |
53 | 0.250 | P10-T5-2 | 114 | 0.479 | P3-T7-2 | 175 | 0.699 | P3-T12-1 | 236 | 0.976 | P9-T14-2 |
54 | 0.252 | P5-T9-1 | 115 | 0.480 | P4-T1-1 | 176 | 0.708 | P2-T3-1 | 237 | 0.979 | P8-T13-2 |
55 | 0.254 | P7-T3-1 | 116 | 0.482 | P4-T14-2 | 177 | 0.713 | P8-T12-1 | 238 | 0.979 | P3-T9-1 |
56 | 0.256 | P7-T11-2 | 117 | 0.487 | P5-T12-2 | 178 | 0.720 | P7-T1-2 | 239 | 0.982 | P10-T3-1 |
57 | 0.258 | P5-T6-1 | 118 | 0.488 | P5-T13-1 | 179 | 0.726 | P9-T2-2 | 240 | 0.984 | P8-T2-2 |
58 | 0.261 | P8-T6-2 | 119 | 0.489 | P2-T9-1 | 180 | 0.727 | P2-T4-1 | 241 | 0.990 | P4-T12-2 |
59 | 0.262 | P5-T2-1 | 120 | 0.493 | P9-T4-2 | 181 | 0.728 | P3-T6-2 | 242 | 0.992 | P5-T8-2 |
60 | 0.265 | P10-T7-1 | 121 | 0.495 | P8-T11-2 | 182 | 0.729 | P9-T8-1 | 235 | 0.975 | P4-T15-2 |
61 | 0.277 | P5-T8-1 | 122 | 0.500 | P7-T6-1 | 183 | 0.737 | P2-T4-2 |
In a second example, a typical existing spliced girder bridge was used with a 3-span configuration (main channel unit) as part of a 24-span bridge for a total length of 3,585 ft. The approaches were constructed with eight lines of Florida bulb-T78 precast prestressed concrete girders. The main channel unit is continuous over three spans that are 196.6 × 250 × 196.6 ft. It was constructed with eight lines of modified Florida bulb-T78 girders. Each girder line consists of two haunched pier segments, a main span drop-in segment, and two side span drop-in segments. Each segment is an individually precast pretensioned girder that supports its own weight and handling loads. All segments in one line of girder are continuously PT with four 15-internal draped tendons with 0.6-inch diameter from end to end. The haunch segment diaphragms are transversely PT with three tendons at each diaphragm. The plans can be viewed in appendix B.
The bridge is in good condition but has less than desirable workmanship based on visual inspection. The complete construction and inspection records are not available. The considered structure is a typical highway bridge carrying medium traffic volumes. It is assumed that the owner performed an option 2 inspection. The sampling procedure used in this example is as follows:
Category | Weight (table 5) | Tendon Type | ||
---|---|---|---|---|
Longitudinal Draped | Pier Cap Transverse | Diaphragm Transverse | ||
Bridge condition (table 1) | 0.15 | 3 | 3 | 3 |
Construction and inspection records (table 2 ) | 0.15 | 4 | 4 | 4 |
Visual inspection (table 3 ) | 0.30 | 1 | 1 | 1 |
Tendon shape and length (table 4 ) | 0.40 | 5 | 2 | 1 |
Probability of defect indicator | 3 | 2 | 2 |
Category | Weight(table 9) | Tendon Type | ||
---|---|---|---|---|
Longitudinal Draped | Pier Cap Transverse | Diaphragm Transverse | ||
Cost of repair or replacement (table 6) | 0.40 | 5 | 4 | 3 |
Element/tendon redundancy (table 7) | 0.40 | 3 | 2 | 2 |
Bridge importance (table 8) | 0.20 | 3 | 3 | 3 |
Consequence of failure indicator | 4 | 3 | 3 |
Figure 50. Graph. Spliced girder bridge—tendon risk categories.
Type of Tendon | Total Number of Tendons | Risk | Structural Impact Caused by Inspechion | Acceptable Fraction Of Undetected Defective | Minimum Number of Tendons for Inspection | |
---|---|---|---|---|---|---|
Level 1 | Level 2 | |||||
Longitudinal draped | 32 | Medium | High | 30 percent | 4 | 7 |
Pier cap transverse | 16 | Medium | High | 30 percent | 4 | 6 |
Diaphragm transverse | 6 | Medium | Medium | 20 percent | 5 | 6 |
Table 23 through table 25 present the summary of all tendons. Each tendon is identified by its label. The first letters and number denote the girder number “G,” pier number “P,” and pier diaphragm number “DP.” The second letter denotes the tendon name “T” as shown in the as-built plans. All of the data are sorted with respect to the random number assigned to each tendon. The required number of tendons is selected from the top of the tendon list and is bold for the level 1 inspections and bold italic for the level 2 inspections.
# | Random # | Tendon ID | # | Random # | Tendon ID |
---|---|---|---|---|---|
1 | 0.494 | G1-T4 | 17 | 0.954 | G8-T2 |
2 | 0.876 | G6-T2 | 18 | 0.843 | G4-T2 |
3 | 0.387 | G4-T1 | 19 | 0.883 | G3-T4 |
4 | 0.725 | G3-T3 | 20 | 0.817 | G5-T2 |
5 | 0.396 | G6-T1 | 21 | 0.283 | G2-T3 |
6 | 0.931 | G7-T4 | 22 | 0.930 | G8-T1 |
7 | 0.809 | G1-T1 | 23 | 0.196 | G2-T1 |
8 | 0.582 | G7-T1 | 24 | 0.071 | G8-T3 |
9 | 0.409 | G5-T1 | 25 | 0.866 | G2-T2 |
10 | 0.487 | G4-T4 | 26 | 0.269 | G2-T4 |
11 | 0.847 | G4-T3 | 27 | 0.287 | G1-T2 |
12 | 0.587 | G7-T3 | 28 | 0.284 | G3-T1 |
13 | 0.966 | G6-T3 | 29 | 0.467 | G3-T2 |
14 | 0.913 | G5-T4 | 30 | 0.126 | G1-T3 |
15 | 0.540 | G7-T2 | 31 | 0.095 | G5-T3 |
16 | 0.459 | G6-T4 | 32 | 0.226 | G8-T4 |
# | Random # | Tendon ID |
---|---|---|
1 | 0.068 | DP11-T1 |
2 | 0.098 | DP10-T2 |
3 | 0.414 | DP10-T1 |
4 | 0.431 | DP11-T2 |
5 | 0.699 | DP10-T3 |
6 | 0.884 | DP11-T3 |
# | Random # | Tendon ID |
---|---|---|
1 | 0.049 | P10-T4 |
2 | 0.120 | P10-T2 |
3 | 0.261 | P12-T3 |
4 | 0.321 | P11-T4 |
5 | 0.440 | P11-T3 |
6 | 0.465 | P9-T3 |
7 | 0.554 | P11-T2 |
8 | 0.618 | P9-T1 |
9 | 0.620 | P11-T1 |
10 | 0.725 | P12-T2 |
11 | 0.794 | P9-T4 |
12 | 0.812 | P12-T1 |
13 | 0.945 | P10-T3 |
14 | 0.946 | P10-T1 |
15 | 0.977 | P9-T2 |
16 | 0.997 | P12-T4 |
In the third example, a typical eight-span precast segmental span-by-span bridge construction with a span configuration of 8 × 130 ft is examined. The superstructure consists of a single-cell box girder with 40-ft-wide top deck that is 8 ft deep. There are two types of PT tendons on this bridge as follows:
Category | Weight (table 5) | Tendon Type | |
---|---|---|---|
Transverse | External | ||
Bridge condition (table 1) | 0.15 | 4 | 4 |
Construction and inspection records (table 2) | 0.15 | 1 | 1 |
Visual inspection (table 3) | 0.30 | 5 | 5 |
Tendon shape and length (table 4) | 0.40 | 1 | 4 |
Probability of defect indicator | 3 | 4 |
Category | Weight (table 9 ) | Tendon Type | ||
---|---|---|---|---|
Transverse | External | |||
Cost of repair or replacement (table 6) | 0.40 | 2 | 3 | |
Element/tendon redundancy (table 7 ) | 0.40 | 2 | 4 | |
Bridge importance (table 8) | 0.20 | 5 | 5 | |
Consequence of failure indicator | 3 | 4 |
Figure 51. Graph. Span-by-span segmental bridge—tendon risk categories.
Type of Tendon | Total Number of Tendons | Risk | Structural Impact Caused by Inspection | Acceptable Fraction Defective | Minimum Number of Tendons for Inspection | |
---|---|---|---|---|---|---|
Level 1 | Level 2 | |||||
Transverse | 347 | Medium | High | 30 percent | 4 | 8 |
External | 48 | High | Low | 10 percent | 12 | 22 |
A five-span continuous bridge with a three-cell box girder superstructure was constructed on a 160-ft-long false-work per span. The bridge was PT with 4 19-strand draped internal tendons with 0.6-inch diameter per web from abutment to abutment. In addition, at each pier the box girder was also PT with top internal tendons and anchored at blisters located at the intersection of web and top deck. The top longitudinal tendons over the exterior web are 3 19-strand tendons with 0.6-inch diameter and 6 19-strand tendons with 0.6-inch diameter over the internal web. The top deck is transversely PT with four 0.6-inch internal tendons at 3-ft spacing. The diaphragm is transverse PT with six 31-strand tendons with 0.6-inch diameter. The diaphragm has ample redundancy due to the large amount of ordinary reinforcing bars. Each diaphragm also has 12 vertical internal PT bars with 13/8-inch diameter. The box girder is 7 ft deep. The bridge is in good condition and, it has no defects and average workmanship based on visual inspection. No construction and inspection records are available. The structure carries large traffic volumes. The procedure is as follows:
Category | Tendon Type | |||||
---|---|---|---|---|---|---|
Weight (table 5) | Transverse | Top | Longitudinal | Transverse Diaphram | PT Bars | |
Bridge condition (table 1) | 0.15 | 3 | 3 | 3 | 3 | 3 |
Construction and inspection records (table 2) | 0.15 | 5 | 5 | 5 | 5 | 5 |
Visual inspection (table 3 ) | 0.30 | 3 | 3 | 3 | 3 | 3 |
Tendon shape and length (table 4) | 0.40 | 1 | 1 | 5 | 3 | 1 |
Probability of defect indicator | 3 | 3 | 4 | 3 | 3 |
Category | Tendon Type | ||||||
---|---|---|---|---|---|---|---|
Weight (table 9) | Transverse | Top | Longitudinal | Transverse Diaphragm | PT Bars | ||
Cost of repair or replacement (table 6) | 0.40 | 2 | 2 | 4 | 1 | 1 | |
Element/tendon redundancy (table 7 ) | 0.40 | 2 | 2 | 5 | 1 | 1 | |
Bridge importance (table 8) | 0.20 | 3 | 3 | 3 | 3 | 3 | |
Consequence of failure indicator | 2 | 2 | 4 | 1 | 1 |
Figure 52. Graph. PT bridge—tendon risk categories.
Type of Tendon | Total Number of Tendons | Risk | Structural Impact Caused by Inspection | Acceptable Fraction Defective | Minimum Number of Tendons for Inspection | |
---|---|---|---|---|---|---|
Level 1 | Level 2 | |||||
Transverse tendon | 267 | Medium | High | 30 percent | 4 | 8 |
Top tendon | 72 | Medium | High | 30 percent | 4 | 8 |
Longitudinal tendon | 16 | High | High | 20 percent | 5 | 8 |
Transverse diaphragm tendon | 36 | Low | High | 30 percent | 4 | 7 |
Vertical diaphragm PT bar | 72 | Low | High | 30 percent | 4 | 8 |
Material (grout in the present case) characterization requires that both composition and structure (macro and micro) be determined. Composition is determined by wet chemistry and analytical techniques such as ion chromatography, X-ray florescence, and energy dispersive spectroscopy (EDS), while structure is determined by petrographic methodologies, electron microscopy, and X-ray diffraction.
As an initial step in the case of external tendons, the general appearance of the tendon should be documented. The duct should be inspected for any cracks or connections that could serve as conduits for corrosives from an external source. Access to grout and strands can be accomplished either by end cap removal or by sectioning away duct at an intermediate location along the tendon. In the latter case, duct wall thickness is determined from construction documents, and duct sectioning can be performed using either a plastic cutting wheel or a depth guard that limits grinding depth to that of the duct wall thickness. Caution should be used in duct sectioning so that strands are not impacted. It is important to recognize that these strands may press against the interior duct wall at some circumferential orientation. Upon exposing the grout, its visual appearance and presence of any strand corrosion (or lack thereof) should be documented. Direct access to strands may require removal of some grout cover, which is described later in this chapter. Alternatively, potential measurements can be made to assess the corrosion state of embedded strand. Figure 53 shows a dial depth gauge being used to measure the size of a grout air void at the top of a tendon at a location where access to the underlying grout was made on an external tendon.
Source: Concorr Florida
Figure 53. Photo. Dial gauge used to determine the depth of a grout air void.
Internal tendons represent special challenges because intermediate locations along the length require concrete excavation, and there is greater potential that damage to the tendon may occur compared to external tendons. Similar to external tendons, the investigation should document the initial general appearance of internal tendon end caps. An appropriate number of these end caps (see chapter 5) should be opened, and their condition should be assessed. Figure 54 shows a grout sample being taken at an opened internal tendon end by chipping after the cap was removed. Note the plastic sheet beneath the anchorage to ensure no sample contamination. If the grout condition within these is good and grout Cl− analysis results determine that concentration of this species is within acceptable limits, then no further sampling at intermediate locations should be required. However, if the grout appearance is problematic (i.e., undesirable grout types are encountered, see chapter 4), Cl− contaminated, or otherwise defective, then consideration must be given to inspect and sample additional end caps, intermediate locations, or both. This access should be performed by standard concrete excavation methods but with due diligence being taken to ensure that reinforcement is not cut or otherwise compromised and the tendon itself is not damaged. Figure 55 shows concrete excavation to access an internal tendon subsequent to its being located using ground penetrating radar (GPR). Once the tendon is exposed, then inspection and analysis can be performed the same as for external tendons.
Source: Parsons Brinckerhoff
Figure 54. Photo. Grout sample acquisition at an opened internal tendon end cap by light chipping.
Source: Concorr Florida
Figure 55. Photo. Concrete excavation to expose an internal tendon.
If a grout sample has been acquired from an end cap, then it is probably not necessary to acquire additional samples from the same tendon. However, it may be appropriate to determine the grout quality at intermediate locations, particularly the presence of any air pockets at high points. This can be done for internal tendons with minimal concrete and tendon disruption by drilling with a 1-inch-diameter bit, as shown in figure 56. There are noticeable changes in the drill noise and vibration when a tendon is contacted, which helps ensure that there is no strand damage. If an air pocket is disclosed, it can be inspected using a borescope.
Source: Parsons Brinckerhoff
Figure 56. Photo. Internal tendon access at an intermediate location by drilling.
If it is necessary to acquire grout samples at intermediate locations on internal tendons, relatively small excavations can be used, as shown in figure 57. In the figure, an approximately 6-inch-diameter access was created to reveal the exposed strand and grout.
Source: Parsons Brinckerhoff
Figure 57. Photo. Access hole in an internal tendon at an intermediate location revealing strand and grout.
Prior to opening a tendon end cap or duct at an intermediate location (external or internal), preparations should be made to capture a sample of any free water that might be present and otherwise lost. This involves placing an opened bag or sealable plastic container beneath the location(s) of anticipated runout.
Particular attention should be given to the following:
Source: Parsons Brinckerhoff
Figure 58. Photo. View of a duct interior revealing a channel air void.
The following should be collected and deposited in a clean freezer bag using a clean tool such as a flat blade screw driver, chisel, or chipping or a light duty power tool:
A minimum of 75 g of solid sample (this may consist of more than one piece) of each grout type should be obtained and designated by number/letter according to location. While powder can be used for chemical analyses, a solid sample is required for petrography. If soft, wet grout is found, then at least one sample should be placed in a bag and sealed in such a manner that as much air as possible is expelled and that enough pressure is exerted on the grout such that free water is separated. The grout itself should then be removed and placed in a separate bag, and the free water should be retained in the original bag for compositional analysis. Also, as noted in chapter 4, soluble ions have been reported to migrate upward through the grout as it hardens.(1,22) Consequently, concentration of these may be greater in the upper regions of the grout. This possibility should be taken into account when acquiring samples.
The following techniques are available for compositional determinations:
As a minimum, X-ray fluorescence and wet chemistry analysis should be performed. Components for which determinations should be made are Cl−, SO42-, K+, and Na+, Cl−, and SO42 are determinants of loss of passivity and onset of active corrosion, and K+ and Na+ are determinants of cement alkalinity. Any free water samples should be analyzed by ion chromatography. Estimating grout Cl− concentrations on site in real time can be accomplished using the Germann Instruments (GI) Rapid Chloride Test (RCT) test kit for acid soluble Cl−. The procedure requires a 5-g powdered sample and takes about 10–15 min to perform. A test kit for determining water soluble Cl− concentration (GI RCT water) is also available; however, the test for acid soluble Cl− is recommended.
Petrographic analysis should be performed in accordance with the applicable ASTM standard.(41) Researchers should also include an analysis and explanation of any grout color and consistency distinctions and lack of set. All analyses should be performed by the resident State transportation department or by a transportation department certified laboratory.
Additionally, an option is available to assess corrosion state and rate for strands embedded in grout. Corrosion state is determined by measuring potential. The methodology and data interpretation are described for conventional reinforcing steel in concrete in “Standard Practice for Calculation of Corrosion Rate and Related Information from Electrochemical Measurements.”(42) Corrosion rate is determined by measuring polarization resistance from which corrosion rate can be calculated.(43,44) Both techniques require a standard reference electrode, a high impedance voltmeter, access to grout in the vicinity where the strands of interest are embedded, and an electrical connection to one or more strands. In addition, polarization resistance measurements require an external (counter) electrode in contact with the grout and a means for imposing small potential changes on the strands via the counter electrode. Because surface area of the strands is likely not known, corrosion rate determinations are qualitative in nature. Lau et al. reported results using both measurement procedures on opened PT tendons.(22) The procedures should only be employed by people familiar with the technologies, equipment, and methods.
The following lists contain needed information, equipment, and instrumentation for the respective categories.
Background information (as available) includes the following:
It is recommended that a common data collection and reporting format be employed by the transportation departments conducting grout sampling programs. Figure 59 shows a simplified bridge schematic where three horizontal PT tendons are in place on each side of the box segments. This allows for the identification and representation of grout sampling locations. Likewise, figure 60 provides a format for documenting individual grout samples, and figure 61 illustrates a tabular form for presenting data and analysis results.
Figure 59. Illustration. Bridge representation used to identify grout sampling locations.
Figure 60. Illustration. Chart used to record the location and information for individual grout samples.
Figure 61. Illustration. Chart used to record and present analysis results for individual grout samples.
After the number of sampled tendons are determined based on the methodology presented in chapter 5, it is important to strategically locate the sampling areas in a certain type of tendon after the target tendons are selected. The target tendons should be randomly selected from the bridge being inspected. It is not always possible to remove grout samples from a cut window in the duct away from the anchorages of an internal tendon due to the strand configuration in the duct. If the tendon has a permanent grout cap over the anchor head, the simplest way to collect grout samples is from the cap internal area as shown in figure 62 and figure 63. Extracting grout samples from the grout cap should be used as the first option.
Source: Parsons Brinckerhoff
Figure 62. Photo. Removing a permanent grout cap.
Source: Parsons Brinckerhoff
Figure 63 . Photo. Exposed anchor head after grout sampling.
Grout voids are typically formed by bleed water or trapped air during pumping of the grout. Bleed water tends to move upward in a tendon while transporting chemical compounds such as Cl−, which is a similar trend with trapped air. However, trapped air can be anywhere along the tendon. Therefore, typical grout CDs and PDs can be found at the high elevation of the tendon.
This chapter provides a guideline on locating the strategic locations along each type of tendon, also known as the inspection point. In terms of internal tendons, prior to opening a hole in the tendon, the tendon should be located using GPR as shown in figure 64. It is not recommended to use as-built plans to locate internal tendons because tendon locations might change during construction.
For both options 1 and 2 inspections, at least one grout sample per tendon should be selected from the preselected tendons determined in chapter 5.
Source: Parsons Brinckerhoff
Figure 64. Photo. Locating internal tendons in the web wall using GPR.
If a permanent grout cap is accessible or exists, the cap should be removed from the blister (see figure 62 and figure 63). Alternatively, the cap can be partially removed using a core drill about 3–4 inches in diameter toward the upper location, as shown in figure 65. The cored section should be saved for later for grout cap repair. The grout sample should be removed as required in chapter 6 for further testing, and its physical condition should be inspected, including the exposed strands in the anchor head and the presence of voids. If the grout cap is not available/accessible or if a void is present in the grout cap, the trumpet interior should be inspected by drilling through a grout port over the trumpet area. This drilling has to be done with extreme care to avoid damage to any strand in the trumpet. If a void is present, a videoscope should be used to inspect the condition of the internal void area. For each continuity tendon selected randomly, both ends of the anchorage should be inspected as shown in figure 66 and figure 67. If severe corrosion is discovered, the entire grout cap and grout over the anchor head should be removed for further investigation.
Source: Parsons Brinckerhoff
Figure 65. Photo. Partial removal of a grout cap.
Source: Parsons Brinckerhoff
Figure 66. Illustration. Typical cantilever tendon inspection point locations.
Source: Parsons Brinckerhoff
Figure 67. Illustration. Typical continuity tendon inspection point locations.
After grout sampling and inspection are complete, the grout cap should be restored according to the procedure provided in chapter 8.
In some bridges, no permanent grout cap was installed, and the anchor heads are protected by concrete pour-back. As a result, grout samples cannot be obtained from the anchor head areas. It is recommended to remove grout samples from internal ducts at the end of the blister by chipping concrete over the duct. If grout cap inspection cannot be done, workers should drill through the trumpet and check the condition of the grout in the anchorage.
The simplest way to access the cantilever tendon is from the top deck. Because maintenance of traffic is required, it is recommended to access this tendon at night. After the tendon is identified, a chipping gun should be used to remove concrete in the PT block-out a minimum 1 × 2 ft in plan view or larger if necessary. If permanent grout cap is present, a core drill should be used to remove about 3- to 4-inch-diameter specimen of grout cap front face toward an upper location. For a cantilever tendon, at least two inspection points are required, as shown in figure 66 and figure 67.
The majority of span-by-span segmental bridge construction consists of external tendons as shown in figure 68. The typical external tendon profile is designed as inclined tendon anchored at both diaphragms of a particular span and draped down at one or two deviators in the bottom flange. For each tendon, a minimum of three inspection points are recommended for grout sampling and investigation. The first point is at the grout cap at the diaphragm, the second point is at the top duct coupler adjacent to the diaphragm, and the third point is at the lower duct coupler close to a deviator.
Source: Parsons Brinckerhoff
Figure 68. Illustration. Typical span-by-span bridge external tendon inspection point locations.
The CIP box girder bridge typically has draped internal tendons in the webs (see figure 69). It is not always possible to access the end anchorages in the end diaphragms. However, it is feasible to probe the tendon from inside the box girder close to the anchorages. For a typical two-span continuous bridge, at least three inspection points should be selected—the first point adjacent to the anchorages, the second point at a high point, and the third point at the lowest point of the tendon. For multiple continuous span bridges with more than two spans, additional inspection points may be required.
Figure 69. Illustration. CIP PT bridge inspection point locations.
A PT spliced girder bridge typically has three to four internal tendons from end to end of a multispan continuous unit as shown in figure 70, figure 71 for bulb-tee girders, and figure 72 for a U-girder bridge. Similar to a CIP PT bridge, it is almost impossible to access the anchorage area due to a lack of sufficient clearance at the bridge ends. For a three-span continuous bridge, it is recommended to have a minimum of three inspection points—the first adjacent to the anchorages, the second at the CIP closure joint near the pier, and the third at the highest area over the pier (see figure 70 and figure 72).
Source: Parsons Brinckerhoff
Figure 70. Illustration. Spliced bulb-tee girder bridge inspection point locations.
Source: Parsons Brinckerhoff
Figure 71 . Photo. Spliced bulb-tee girder bridge CIP joint detail.
Source: Summit Engineering Group
Figure 72. Illustration. Spliced U-girder bridge inspection locations.
Restoration of the tendon damage as a result of inspection and grout sample collection is one of the most important activities in grout sampling. Improper repair/restoration will provide a future path for corrosive agents to the PT system and can compromise long-term durability. Two types of restorations can be performed after inspection and grout sample collection are completed: temporary restoration and permanent restoration.
Temporary restoration should be performed for the following reasons:
An exposed open tendon should not be left for more than 4 h without proper temporary protection. Each day before leaving the project site, all areas of exposed tendons should receive a temporary protection prior to applying temporary restoration or permanent restoration. At minimum, waterproof tapes and plastic covers should be applied. If possible, permanent restoration of the corrosion protection should be performed on the same day as the inspection. The inspection team should provide a consistent and visible color marking at the inspection points based on the approved work plan (prior to the inspection) so that the client/owner of the bridge can keep track of what has been done for future maintenance activities. The industry repair standard practice should be adopted with owners’ approval, including the material ingredients, mixture proportions, mixing, placing, and curing method. The restoration material and methods should be included in the work plan and approved by the owners prior to onsite construction restoration.
The procedure for temporary restoration of external tendons is as follows:
Source: Concorr Florida
Figure 73. Photo. Temporary restoration of an external tendon.
The procedure for temporary restoration of internal tendons is as follows:
The procedure for permanent restoration of external tendons is as follows:
Source: Concorr Florida
Figure 74. Photo. Permanent restoration of an external tendon using heat shrink sleeve.
The procedure for permanent restoration of internal tendons is as follows:
Figure 75 . Illustration. Permanent restoration of an internal tendon duct.
The procedure for permanent restoration of grout caps is as follows:
Source: Parsons Brinckerhoff
Figure 76. Photo. Completed restoration of a partially cut grout cap prior to coating.
Source: Parsons Brinckerhoff
Figure 77. Photo. Applying elastomeric coating over a grout cap.
Source: Parsons Brinckerhoff
Figure 78. Photo. Completed permanent restoration of a grout cap.
It is recommended that approved material from the bridge owners be used in the restoration of tendons after invasive testing. For grout restoration, after the existing grout is removed for a test sample, the grout should be patched with an approved prepackaged grout mortar/hydraulic cement grout mortar applied to both internal and external tendons.
Basic materials used for external tendon restoration include the following:
Grout issues fall into the following categories:
Table 32 lists CD classifications and recommended actions, respectively, for option 1 inspections according to the determined condition of individual tendons. This considers that strands are embedded in sound grout and only CDs, as expressed in terms of four Cl−, are at issue. Conversely, option 2 inspections consider that the same CD classifications for option 1 are an issue as well as PDs (grout structure and presence of any air voids, strand corrosion, or strand fractures), as listed in table 33. Based on findings for either inspection option, individual tendons are assigned a grade from 1 to 10—the higher the grade, the more problematic the tendon condition.
For option 2, assigning a tendon grade based on the determined CDs and PDs and projecting any resultant action requires multiple considerations. For example, if it is determined that Cl− is less than or equal to 0.08 wt percent cement (CD1) and a grout air pocket is noted (PD1), then a grade of 2 is assigned, and no action (A1) is recommended (see table 33). If the air pocket is long and larger than 0.5 inches, action A6 may be considered by regrouting the void. However, if strands also exhibit surface corrosion but no section loss (PD4), then the grade 6 is assigned and actions A3 and A6 result. In other words, the highest PD determines the grade and recommended action. The term “section loss” refers to any reduced cross section for all strands in a particular tendon, as affected by fractures. For example, if a tendon has 22 strands and 1 has fractured but the others remain load bearing, then section loss is 4.5 percent (PD5 in table 33) in combination with other deficiencies. If grade 8 CD3 is selected, actions A2, A4, A5, and A6 are recommended. Conversely, if there are 18 strands, then 1 fracture translates to 5.6 percent section loss (PD6), which is a grade of 10, and actions A2 and A4 through A8 should be taken.
Table 32. CD classifications as determined by grout Cl− levels from an option 1 inspection and resultant recommended actions.
CDa | |||||
CD1 | Cl− ≤ 0.08 | X | |||
CD2 | 0.08 < Cl− ≤ 0.20 | X | |||
CD3 | 0.2 < Cl− ≤ 0.50b | X | |||
CD4 | Cl− > 0.50b | X | |||
Action | Grade | ||||
---|---|---|---|---|---|
1 | 5 | 7 | 9 | ||
A1 | None | X | |||
A2 | Expand sampling | X | X | ||
A3 | Reinspect in 5 years | X | |||
A4 | Reinspect in 2 years | X | X | ||
A5 | Tendon monitoring | X | X | ||
a Chloride concentration units are wt percent cement. b If strand corrosion or fracture(s) are found (PD5 or PD6 under option 2 in table 33 ), then grade 9 or 10 should be assigned as appropriate per option 2 actions. |
AMENDED January 7, 2014
Table 33. CD and PD classifications as determined by grout Cl− levels and in-place grout structure by an option 2 inspection and resultant recommended actions.
CDa | |||||||||||
CD1 | Cl− ≤ 0.08 | X | X | X | X | ||||||
CD2 | 0.08 < Cl− ≤ 0.20 | X | X | ||||||||
CD3 | 0.2 < Cl− ≤ 0.50 | X | X | ||||||||
CD4 | Cl− > 0.50 | X | X | ||||||||
PD | |||||||||||
PD0 | Sound grout | X | |||||||||
PD1 | Grout air pocket | X | X | X | X | X | X | X | X | X | |
PD2 | Exposed strand/tendon | X | X | X | X | X | X | X | X | ||
PD3 | Soft or segregated grout | X | X | X | X | X | X | X | |||
PD4 | Tendon surface corrosion (no section loss) | X | X | X | X | X | |||||
PD5 | Tendon surface corrosion (< 5 percent section loss) | X | X | X | |||||||
PD6 | Tendon with partial or full fracture (≥ 5 percent section loss) | X | |||||||||
Action | Grade | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | ||
A1 | None | X | X | ||||||||
A2 | Expand sampling | X | X | X | X | ||||||
A3 | Reinspect in 5 years | X | X | X | |||||||
A4 | Reinspect in 2 years | X | X | X | X | ||||||
A5 | Tendon monitoring | X | X | X | X | ||||||
A6 | Consider repairing deficiency as necessaryb | X | X | X | X | X | X | X | X | X | |
A7 | Structural evaluation/load rating | X | |||||||||
A8 | Tendon replacement | X | |||||||||
a Chloride concentration units are wt percent cement. b This applies to PD1, PD2, and PD3. |
AMENDED January 7, 2014
The protocol is simpler in the option 1 inspection case in that the determined Cl− translates directly to a respective grade as indicated in table 32 and table 33. However, if strand corrosion and/or fractures are observed as part of option 1 grout sampling activities, then, as indicated, this reverts the action recommendation from table 32 to that in table 33. Figure 79 and figure 80 summarize these decision processes for options 1 and 2, respectively.
Figure 79. Flowchart. Inspection, sampling, evaluation, and actions for levels 1 and 2 inspections for option 1.
Figure 80. Flowchart. Inspection, sampling, evaluation, and actions for levels 1 and 2 inspections for option 2.
Evaluation of test and inspection results should be performed by treating tendons of a common type as a group and assigning a grade to individual tendons of that group according to the protocol provided in table 32 for option 1 inspection and table 33 for option 2. An option 1 inspection may be either level 1 with no further sampling required or expanded to level 2, which involves additional sampling depending on the table 32 grading. Typically, for an option 2 level 2 inspection, all tendons of the types considered are inspected.
ExamplesPreliminary information is as follows:
Inspection Results:
Inspection results are as follows:
Recommended Course of Action:
Table 32 should be used as follows:
Conclusions:
Discontinue inspection and reinspect all tendon types in group 1 in 5 years.
Preliminary information is as follows:
Inspection Results:
Inspection results are as follows:
Recommended Course of Action:
Table 33 should be used as follows:
Conclusions:
Based on the examples, the following procedure was created:
Reinspections, Non-Destructive Testing (NDT), and Monitoring
It is intended that the term “reinspect” in table 33 refers to further inspection and sampling, as described in chapters 5 through 8 of this report, in order to either increase reliability of the findings or determine if any deterioration has progressed since an earlier inspection and sampling. In addition, consideration can be given to adapting one or more of the following NDT technologies that are available:
Monitoring tendons for wire and strand breaks is an option for assuring that any fractures are detected in a timely manner. Passive acoustic emission instrumentation with remote monitoring is an option in this regard, and commercial systems are available for implementation.
Figure 81. Illustration. Segment layout—part 1.
Figure 82. Illustration. Segment layout—part 2
Figure 83. Illustration. Bulkhead details.
Figure 84. Illustration. Longitudinal PT layout—part 1.
Figure 85. Illustration. Longitudinal PT layout—part 2.
Figure 86. Illustration. Longitudinal PT layout—part 3.
Figure 87. Illustration. Longitudinal PT layout—part 4.
Figure 88. Illustration. Longitudinal PT layout—part 5.
Figure 89. Illustration. Longitudinal PT layout—part 6.
Figure 90. Illustration. Longitudinal PT layout—part 7.
Figure 91. Illustration. Longitudinal PT layout—part 8.
Figure 92. Illustration. Plan and elevation—main channel unit.
Figure 93. Illustration. Bridge section.
Figure 94. Illustration. Pier cap PT details.
Figure 95. Illustration. Tendon profile—main channel unit.
Figure 96. Illustration. Modified Florida bulb-T78 beam end segment.
Figure 97. Illustration. Modified Florida bulb-T78 beam haunch segment.
Figure 98. Illustration. Modified Florida bulb-T78 beam drop-in segment.
Figure 99. Illustration. Modified Florida bulb-T78 beam end block detail.
Figure 100. Illustration. Typical sections—haunch segment.