1、Report on Corrosion and Repair of Unbonded Single-Strand TendonsReported by Joint ACI-ASCE Committee 423ACI 423.4R-14First PrintingNovember 2014ISBN: 978-0-87031-953-2Report on Corrosion and Repair of Unbonded Single-Strand TendonsCopyright by the American Concrete Institute, Farmington Hills, MI. A
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11、he annually revised ACI Manual of Concrete Practice (MCP).American Concrete Institute38800 Country Club DriveFarmington Hills, MI 48331Phone: +1.248.848.3700Fax: +1.248.848.3701www.concrete.orgThis report gives general information regarding evaluating corro-sion damage in structures reinforced with
12、unbonded single-strand post-tensioning tendons. Historical development of those parts of the building code dealing with durability and corrosion protection is explained. Evolution of the types and components of unbonded tendons is described. Specific aspects of corrosion in unbonded single-strand te
13、ndons are described, and common problems in structures reinforced with these tendons are discussed. Methods are presented for repairing, replacing, and supplementing tendons.Keywords: allowable stresses; anchorage; carbonation; concrete construc-tion; corrosion; corrosion protection; cover; durabili
14、ty; embrittlement; external post-tensioning; post-tensioned concrete; prestressed concrete; sheathing; single-strand tendons; unbonded post-tensioning.CONTENTSCHAPTER 1INTRODUCTION, p. 21.1General, p. 21.2Background, p. 21.3Scope, p. 31.4Limitations, p. 3CHAPTER 2DEFINITIONS, p. 32.1Definitions, p.
15、3CHAPTER 3REVIEW OF CODE REQUIREMENTS AND CHANGES, p. 33.1General, p. 33.2Cover requirements for unbonded tendons, p. 33.3Allowable tensile stresses in concrete, p. 43.4Protection of unbonded tendons, p. 4CHAPTER 4UNBONDED TENDONS, p. 64.1Historical development of unbonded tendons, p. 64.2Sheathing
16、problems, p. 74.3Detailing practices, p. 74.4Storage, handling, and construction problems, p. 9H. R. Trey Hamilton III, Chair Carin L. Roberts-Wollman, SecretaryACI 423.4R-14Report on Corrosion and Repair of Unbonded Single-Strand TendonsReported by Joint ACI-ASCE Committee 423Theresa M. AhlbornRobe
17、rt W. BarnesFlorian G. BarthRoger J. BeckerKenneth B. Bondy*Charles W. DolanJames P. Donnelly*Martin J. Fradua*William L. GambleHarry A. GleichShawn P. GrossPawan R. GuptaWilliam M. HaleMohammad IqbalL. S. Paul JohalDonald P. KlineLarry B. KrauserJason J. KrohnMark E. MooreTheodore L. NeffSami H. Ri
18、zkallaJames RogersBruce W. RussellDavid H. SandersThomas C. SchaefferAndrea J. SchokkerMorris Schupack*Richard W. StoneAmy M. R. TrygestadMiroslav F. VejvodaJeffery S. VolzH. Carl WalkerZuming XiaPaul ZiaConsulting MembersRobert N. Bruce Jr.Ned H. BurnsChunsheng “Steve” CaiSteven R. CloseHenry J. Cr
19、onin Jr.Ward N. Marianos Jr.Hani MelhemAntoine E. NaamanThomas E. NehilChair of subcommittee responsible for preparation of report.*Member of subcommittee respon-sible for preparation of report.ACI Committee Reports, Guides, and Commentaries are intended for guidance in planning, designing, executin
20、g, and inspecting construction. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute d
21、isclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the cont
22、ract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.ACI 423.4R-14 supersedes ACI 423.4R-98 and was adopted and published November 2014.Copyright 2014. American Concrete Institute.All rights reserved including rights of reproduction and use in any
23、form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the cop
24、yright proprietors.14.5Deterioration mechanisms, p. 104.6Performance record, p. 11CHAPTER 5CONDITION ASSESSMENT AND TESTING, p. 125.1General, p. 125.2Condition surveys of concrete, p. 125.3Condition surveys of tendons, p. 135.4Nondestructive testing, p. 135.5Exploratory openings, p. 145.6Strand remo
25、val, p. 155.7Other testing and investigative procedures, p. 16CHAPTER 6REPAIR METHODS, p. 166.1General, p. 166.2Tendons with observable corrosion, p. 166.3Spliced repair of strand, p. 176.4Strand replacement, p. 186.5Tendon replacement, p. 186.6External post-tensioning, p. 186.7Continuous acoustic m
26、onitoring, p. 196.8External nonprestressed reinforcement or support, p. 196.9Total demolition and replacement, p. 20CHAPTER 7SUMMARY, p. 21CHAPTER 8REFERENCES, p. 21Authored documents, p. 22CHAPTER 1INTRODUCTION1.1GeneralThis report provides general information on the evalua-tion of known or suspect
27、ed corrosion problems in unbonded single-strand tendons, historical information on the types and components of unbonded tendons and on the durability and corrosion protection provisions in the building code, and describes typical repair methods in use today. Expertise in design, construction, evalua
28、tion, and repair of structures using single-strand unbonded tendons is strongly recom-mended for a team undertaking evaluation and repair of corrosion problems.Historically, there have been corrosion problems with other types of pre- and post-tensioning systems (Nehil 1991); however, certain aspects
29、 of corrosion of unbonded single-strand tendons are unique. The causes and effects of corrosion of unbonded single-strand tendons are, in several respects, different from those of bonded conventional reinforcing or other post-tensioning (PT) systems, so the methods for evaluating and repairing corro
30、sion of single-strand tendons are also different. For example, because the tendons are largely isolated from the surrounding concrete, they may not be affected by deleterious materials such as chlorides and moisture in the concrete. The surrounding concrete does not stop corrosion alone because, if
31、water gains access to the inside of the sheathing, PT coating is rendered ineffective. Measures taken to repair and protect the surrounding concrete may not repair or reduce deteriora-tion of the prestressing steel where corrosion has been initi-ated. The tendons usually require separate evaluation
32、and repair.1.2BackgroundUnbonded PT systems were introduced to North America in the 1950s. At that time, there were no accepted standards for design or material specifications for prestressing steels. Guidance came in the form of tentative recommendations from Joint ACI-ASCE Committee 323 (1958), Bu
33、reau of Public Roads (1954), or Structural Engineers Association of Northern California (SEA/NC) (1959). Unbonded tendons in the early systems used bundles of wires or strands, some-times inaccurately called cables, of various diameters, and were assumed to be protected by PT coating and paper sheat
34、hing that were sometimes applied by hand (Nehil 1991; Schupack 1991a).The use of unbonded tendons became more common during the late 1950s and early 1960s, as progress was made in establishing design and materials standards. Acceptance of the concept was regional at first and was largely the result
35、of sales efforts and design tutoring by tendon suppliers. The use of post-tensioning increased rapidly during the late 1960s and 1970s as advantages of the system were demon-strated. For many types of structures, these advantages included shorter construction time, reduced structural depth, increase
36、d stiffness, and savings in overall cost. In addition to their use in enclosed buildings, unbonded PT systems were used in parking structures and slabs-on-ground, and bonded post-tensioning was used on water tanks, bridges, dams, and soil tie-back systems. Unbonded multi-wire and multi-strand tendon
37、s have been used extensively in nuclear power structures.Incidents of corrosion of unbonded single-strand tendons began to surface during the 1970s. It was believed that corrosion protection would be provided by the PT coating during shipping, handling, and installation, and by the concrete thereaft
38、er. The early PT coatings, however, often did not provide the corrosion-inhibiting characteristics that are presently required in ACI 423.7 or PTI M10.2. In the early 1980s, the Post-Tensioning Institute (PTI) recognized the structural implications of corrosion and began imple-menting measures to in
39、crease the durability of unbonded PT systems (Post-Tensioning Institute 1985). Relying on the experience and practice of the nuclear industrys use of corrosion-inhibiting hydrophobic grease, similar performance standards for grease were adopted. In 2000, PTI published the second edition of its “Spec
40、ification for Unbonded Single Strand Tendons (PTI M10.2-00).” In ACI 318-89, measures were incorporated that related the required protection of the tendons and the quality of the concrete to the environmental conditions that could promote corrosion of the post-tensioning.In 2007, ACI published ACI 4
41、23.7, which provides detailed specifications for PT coating. With the adoption of this specification, the term “grease” was replaced with the more generic term “PT coating” to address other mate-American Concrete Institute Copyrighted Material www.concrete.org2 REPORT ON CORROSION AND REPAIR OF UNBO
42、NDED SINGLE-STRAND TENDONS (ACI 423.4R-14)rials used as protection systems inside the sheathing. These improved coatings were introduced by PTI M10.2 long before the publication of ACI 423.7. Structures built before the adoption of these standards, especially those in aggres-sive environments, are m
43、ore likely to experience corrosion of the PT system than those designed and built since then.Tendons that are broken or damaged by corrosion can be repaired or supplemented by several methods. The more difficult task is determining the extent of corrosion damage and degree to which tendon repairs ar
44、e needed. This report provides guidance on the evaluation of suspected or known corrosion problems and describes repair methods currently in use.1.3ScopeThis report reviews the following:a) Codes and code changes affecting unbonded PT tendonsb) Past and present corrosion protection systems and how t
45、hey have changed to enhance corrosion protectionc) Types of corrosion damage found in prestressing steeld) Methods for evaluating structures that are suspected of, or known to have, corrosion damage in the PT systeme) Basic repair options1.4LimitationsThis report presents a summary of typical proble
46、ms expe-rienced with unbonded PT systems and includes general guidelines for evaluating and repairing single-strand tendons. While the methods presented are general in nature, they are not universally applicable. Standard specifications and details are not included because each structure is unique a
47、nd should be analyzed accordingly.This report is not intended to be included as a part of spec-ifications for investigations and repairs. Presently, there is no practical method to determine the total extent of damage to a PT system. The unpredictable nature of tendon failures exhibited by inadequat
48、ely protected, corroding strand makes estimating tendon life uncertain.A wide variation exists in the durability and rate of dete-rioration of older PT systems. This is due, in part, to the composition of the parts of the tendon: strand, anchors, PT coating, and sheathing, and in part to the quality
49、 of the surrounding concrete, environmental exposure, and type of maintenance performed on the structure. The investigator should rely on available information on the historical perfor-mance of similar structures and should be experienced in interpreting external evidence that may give an indication of latent internal problems.CHAPTER 2DEFINITIONS2.1DefinitionsACI provides a comprehensive list of definitions through an online resource, “ACI Concrete Terminology,” http:/www.concrete.org/Tools/ConcreteTerminology.aspx. Defi-nitions provi
