423.4R-98 Corrosion and Repair of Unbonded Single Strand ...civilwares.free.fr/ACI/MCP04/4234r_98.pdf · 3.3—Detailing practices 3.4—Storage, ... rounding concrete may not repair

ACI Committee Reports, Guides, Standard Practices, aCommentaries are intended for guidance in planning, desiging, executing, and inspecting construction. This documentis intended for the use of individuals who are competeto evaluate the significance and limitations of its contenand recommendations and who will accept responsibilityfor the application of the material it contains. The AmericanConcrete Institute disclaims any and all responsibility for thstated principles. The Institute shall not be liable for any loor damage arising therefrom.

Reference to this document shall not be made in contrdocuments. If items found in this document are desired the Architect/Engineer to be a part of the contract docments, they shall be restated in mandatory language forcorporation by the Architect/Engineer.

423.4

This report gives general information regarding the evaluation of corrosiondamage in structures reinforced with unbonded single strand post-tension-ing tendons. Historical development of those parts of the building codedealing with durability and corrosion protection is explained. Evolution ofthe types and components of unbonded tendons is described. Spficaspects of corrosion in unbonded single strand tendons are described, andcommon problems in structures reinforced with these tendons are discussed.Methods are presented for repairing, replacing, and supplementing tendons.

Keywords: allowable stresses; anchorage; carbonation; concrete con-struction; corrosion; corrosion protection; cover; durability; embrittle-ment; external post-tensioning; grease; inspection; post-tensionedconcrete; pre-stressed concrete; repair ; sheathing; single strand tendons;surveys; tendons; tests; unbonded post-tensioning.

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Corrosion and Repair of Unbonded Single Strand Tendons

ACI 423.4R-98

Reported by ACI/ASCE Committee 423

Charles W. DolanChairman

Henry J. Cronin Jr.Secretary

Kenneth B. Bondy† Catherine W. French Gerard J. McGuire† David H. Sanders

Robert N. Bruce Jr. Clifford Freyermuth Mark Moore† Thomas C. Schaeffer

Dale C. Buckner William L. Gamble Antoine E. Naaman Morris Schupack†

Ned H. Burns† Hans R. Ganz Kenneth Napior Kenneth Shushkewich

Gregory P. Chacos* Mohammad Iqbal Thomas E. Nehil* Khaled S. Soubra

Jack Christiansen Francis J. Jaques Mrutyunjaya Pani Patrick J. Sullivan

Todd Christopherson Daniel P. Jenny Kent H. Preston Luc R. Taerwe

Steven R. Close L.S. Paul Johal Denis C. Pu Carl H. Walker

Thomas E. Cousins Susan N. Lane Julio A. Ramirez Jim J. Zhao

Apostolos Fafitis Ward N. Marianos Jr. Ken B. Rear Paul Zia

Mark W. Fantozzi Leslie D. Martin David M. Rogowsky

Martin J. Fradua† Alan H. Mattock Bruce W. Russel

*Co-chairmen of subcommittee responsible for preparation of report†Member of subcommittee responsible for preparation of report.

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CONTENTSChapter 1—Int roduction, p. 423.4R-2

1.1—General 1.2—Background1.3—Scope1.4—Limitations

Chapter 2— Review of code requirements and changes, p. 423.4R-3

2.1—General2.2—Cover requirements for unbonded tendons2.3—Allowable tensile stresses in concrete2.4—Protection of unbonded tendons

Chapter 3—Unbonded tendons, p. 423.4R-53.1—Evolution of unbonded tendons3.2—Sheathing problems 3.3—Detailing practices3.4—Storage, handling, and construction problem3.5—Deterioration mechanisms3.6—Performance record

ACI 423.4R-98 became effective February 23, 1998.Copyright 1998, American Concrete Institute.All rights reserved including rights of reproduction and use in any form or by any

means, including the making of copies by any photo process, or by electronic ormechanical 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 inwriting is obtained from the copyright proprietors.

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Chapter 4— Evaluating cor rosion dam age, p. 423.4R-10

4.1—General4.2—Condition surveys of concrete 4.3—Condition surveys of tendons4.4—Nondestructive testing4.5—Exploratory concrete removal4.6—Exposing tendons4.7—Strand removal4.8—Other testing and investigative procedures

Chapter 5—Repair schemes and methods, p. 423.4R-14

5.1—General5.2—Existing tendon repair5.3—Strand replacement5.4—Tendon replacement5.5—External post-tensioning5.6—Continuous acoustic monitoring5.7—External non-prestressed reinforcement or support5.8—Total demolition and replacement

Chapter 6—Summa ry, p. 423.4R-18

Chapter 7—References, p. 423.4R-187.1—Recommended references7.2—Cited references

CHAPTER 1—INTRODUCTION1.1—General

This report is intended to provide historical and generaformation on the evaluation of known or suspected corrosproblems in unbonded single strand tendons, and to destypical repair methods currently in use. It has been prepto describe the state of knowledge as perceived by the cmittee. It is not a standard or recommended practice. Extise in design, construction, evaluation, and repair structures utilizing single strand unbonded tendons is stroly recommended for a team undertaking evaluation andpair of corrosion problems.

There have been corrosion problems with other typepre- and post-tensioning systems.1 However, certain aspectof corrosion of unbonded single strand tendons are uniq

The causes and effects of corrosion of unbonded sistrand tendons are, in several respects, different from tof bonded conventional reinforcing or other post-tensionsystems. Thus, the methods for evaluating and repairingrosion of single strand tendons are also different in somespects. For example, since the tendons are largely isofrom the surrounding concrete, they may not be affecteddeleterious materials such as chlorides and moisture inconcrete. However, they also are not passivated by therounding concrete, and can corrode if water gains access tinside of the sheathing or anchorage and the grease proteis inadequate. Measures taken to repair and protect therounding concrete may not repair or reduce deterioration

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the prestressing steel where corrosion has been initiated. Thetendons usually require separate evaluation and repair.

1.2—Background Commercially viable unbonded post-tensioning syste

were introduced to North America in the 1950s. At that tithere were no accepted standards for design nor maspecifications for prestressing steels, and guidance camthe form of tentative recommendations from a joint commtee of the American Concrete Institute (ACI) and the Amican Society of Civil Engineers (ASCE), from thPrestressed Concrete Institute (PCI), or from the BureaPublic Roads, United States Department of Commerce.bonded tendons in the early systems used bundles of wirstrands, sometimes inaccurately called “cables,” of varidiameters, and protected by grease and paper sheathinwere sometimes applied by hand.1,2

The use of unbonded tendons became more commoning the late 1950s and early 1960s as progress was maestablishing design and materials standards. Acceptanthe concept was regional at first, and was largely the resusales efforts and design tutoring by tendon suppliers. Theof post-tensioning increased rapidly during the late 19and 1970s as the advantages of the system were demoned. For many types of structures, these advantages inclshorter construction time, reduced structural depth, creased stiffness, and savings in overall cost. In additiotheir use in enclosed buildings, unbonded post-tensiosystems were used in parking structures and slabs on gand bonded post-tensioning was used on water tanks, bes, dams, and soil tie-back systems. Unbonded multiwiremultistrand tendons have been used extensively in nucpower structures.

Incidents of corrosion of unbonded single strand tendbegan to surface during the 1970s. It had been believesome that corrosion protection would be provided by grease during shipping, handling and installation, and byconcrete thereafter. However, the early greases often diprovide the corrosion-inhibiting characteristics that are quired in the current Post-Tensioning Institute (PTI) “Spefications for Unbonded Single Strand Tendons.” In the e1980s, PTI recognized the structural implications of corsion and began to implement measures to increase the bility of unbonded post-tensioning systems. In 1985, Ppublished the first performance standard for single strtendons.3 Relying on experience and practice in the nuclindustry using corrosion-inhibiting hydrophobic greasimilar performance standards for grease were incorporaIn the 1989 edition of ACI 318, “Building Code Requirments for Reinforced Concrete,” changes were made tocorporate measures that related the required protection otendons and the quality of the concrete to the environmeconditions that could promote corrosion of the post-tensing. Structures built prior to the adoption of these new sdards, especially those in aggressive environments, are likely to experience corrosion of the post-tensioning systhan those designed and built since.

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Tendons that are broken, or are known to be damagecorrosion, can be repaired or supplemented by any of semethods. The more difficult task is to determine the extencorrosion damage and the degree to which tendon repaineeded. This report is intended to provide guidance inevaluation of suspected or known corrosion problems andescribe repair methods currently in use.

1.3—ScopeThis report includes a review of the following:

• Codes and code changes affecting unbonded post-tsioning tendons;

• Past and present corrosion protection systems and those systems have changed to enhance corrosion tection;

• Types of corrosion damage found in prestressing st• Methods for evaluating structures which are suspec

of, or known to have, corrosion damage in the post-sioning system; and

• Basic repair options currently in use.

1.4—LimitationsThis report presents a summary of typical problems e

rienced with unbonded post-tensioning systems and inclgeneral guidelines for evaluating and repairing single sttendons. While the methods presented herein are genenature, they are not universally applicable. Standard specations and details are not included since each structuunique and must be analyzed accordingly.

This report is not intended to be included as part of spfications for investigations and repairs. Presently, there ipractical method to ascertain the total extent of damagepost-tensioning system. The unpredictable nature of tenfailures exhibited by inadequately protected, corrodstrand makes estimating tendon life expectancy uncerta

A wide variation exists in the durability and rate of deteoration of older post-tensioning systems. This is due in to the composition of the parts of the tendon (strand, ancgrease, and sheathing), and in part to the quality of therounding concrete, the environmental exposure and theof maintenance performed on the structure. The investigmust rely on a knowledge of historical performance of silar structures and must be experienced in interpreting enal evidence which may give an indication of latent interproblems.

CHAPTER 2—REVIEW OF CODE REQUIREMENTS AND CHANGES

2.1—GeneralWhen evaluating corrosion damage in post-tensio

structures with unbonded tendons, the investigator mustsider the age of the structure and the standards of praavailable to the designer and contractor at the time of struction. Although ACI published building regulations freinforced concrete as early as 1920, ACI 318-47 wasfirst to acknowledge the significance of environmental exsure. The early Codes (ACI 318-47, ACI 318-51, and A318-56) also recognized the importance of clear cover

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concrete quality in providing adequate corrosion protectito the non-prestressed, bonded reinforcement.

In 1958, ACI-ASCE Joint Committee 323 publishe“Tentative Recommendations for Prestressed Concrete” aaddressed the protection of prestressing steel in three aof recommended practice: concrete cover, allowable tensstresses, and, for unbonded systems, protection of the stor wire with grease and a sheathing material. Since 19provisions for prestressed concrete have included requments for corrosion protection. The grease and sheathwere viewed, by most, primarily as a lubricant and bonbreaker, and secondarily as a corrosion deterrent durshipping, handling, and placing. Long-term corrosion prtection was viewed by some as being provided by the ucracked concrete cover.

In 1963, prestressed concrete was first included in A318 with provisions for concrete cover, allowable tensistresses, and strand protection. These items have been mified from time to time, but the substantive change came1989 when durability was emphasized in ACI 318.

2.2—Cover requirements for unbonded tendonsACI 318-63 required the following under prestressed co

crete-concrete cover:a) The following minimum thickness of concrete cove

shall be provided for prestressing steel, ducts and non-pstressed steel.

b) In extremely corrosive atmosphere or other severe posures, the amount of protection shall be suitably increas

Eight years later, in ACI 318-71, the cover requiremenwere increased from 2 in. (50 mm) to 3 in. (75 mm) for prstressed members cast against and permanently exposeearth. In addition, a new requirement for concrete protectifor reinforcement was introduced and required that cover quirements be increased 50 percent in members with alloable concrete tensile stresses above 6 psi (0.5 MPa). The provisions for concrete cover in the 1963 Cowere also revised to require that density and non-porosityaddition to cover) of the concrete be considered when creasing concrete protection. The Code provisions for coof unbonded prestressing strand did not change in the 191983 and 1989 Code revisions. In the 1983 Commentarydiscussion was added as follows:

R7.7.5—Corrosive environmentsWhen concrete will be exposed to external sources ochlorides in service, such as deicing salts, brackish wa-ter, seawater, or spray from these sources, concrete musbe proportioned to satisfy the special exposure require-ments of Code Section 4.5. These include minimum

Cover, in. (mm)

Concrete surface in contact with ground 2 (50)Beams and girders:Prestressing steel and main reinforcing bars 1½ (40)Stirrups and ties 1(25)Slabs and joists exposed to weather 1 (25)Slabs and joists not exposed to weather ¾ (20)

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content, maximum water-cement ratio (or minimustrength of lightweight concrete), maximum chlorideion in concrete, and cement type. Additionally, for cor-rosion protection, a minimum concrete cover for reiforcement of 2 in. (50 mm) for walls and slabs and in. (60 mm) for other members is recommended. precast concrete manufactured under plant control con-ditions, a minimum cover of 1½ and 2 in. (40 and mm), respectively, is recommended.

This discussion is important as it calls the designer’s atten-tion to the importance of air content, maximum water-cment ratio, maximum chloride-ion content, and cement typewhen designing for corrosion protection.

Finally, a distinction between plant-cast members and er prestressed concrete members was made in the 1971with the addition of distinct cover requirements for placast members.

2.3—Allowable tensile stresses in concrete In aggressive environments, designing to minimize crack

was used to improve durability by reducing ingress of corroelements. Though a properly greased tendon in an intact shing may not be affected at first by a crack in the surroundconcrete, corrosion of nearby conventional reinforcing cause spalling which may expose the tendon to physical dage which may then lead to corrosion of the strand.

For consideration of long-term durability and corrosionprotection, the maximum allowable tensile stresses inconcrete at service loads, after allowance for all prestresslosses, are of most interest. The initial recommendationspre-sented in 1958 by ACI-ASCE Committee 323 limited allowable tensile stresses in pretensioned members to 6 psi(0.5 MPa) where not exposed to weather or corrosivenvironments. In post-tensioned members not exposed toweather or corrosive environments, tensile stresses wereited to 3 psi (0.25 MPa). No mention was made allowable stresses in unbonded systems.

In ACI 318-63, allowable tensile stresses in concrete wlimited to 6 psi (0.5 MPa) for members not exposedto freez ing tempera tu re o r to a co rros ive env ironm en t, prov idedthe m em bers con ta ined bonded re in fo rcem en t (prestressed o rnon -prestressed ) to con tro l c rack ing. F o r a ll o ther m em bers, notensile s tresses a t serv ice load leve ls w ere a llow ed .

The 1971 Code, “Prestressed Concrete—PermissStresses,” required that tensile stresses in concrete be limto 6 psi (0.5 MPa) for most members, but allowup to 12 psi (1.0 MPa) in tension in the concreprovided computations were made using the cracked transformed section and a bi-linear moment-deflection relatiship to confirm that long-term deflection of the membesatisfied “Strength and Serviceability Requirements—Con-trol of Deflections.” In addition, the allowable tension limcould be exceeded provided that experimental and analyticalwork could show that performance would not be impaired.

In the 1977 Code, for an allowable tensile stress up to12 psi (1.0 MPa), a provision was added requiringthat the concrete cover for prestressed and non-prestressed

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steel be increased for prestressed members exposed to earthweather or corrosive environments. “Concrete protection forreinforcement” required a 50 percent increase in cover fomembers exposed to weather, earth, or corrosive envirments and with a tensile stress greater than 6 psi(0.5 MPa). The 1995 Code retains the allowable tensstresses as outlined in the 1977 Code.

2.4—Protection of unbonded tendonsThe protection of unbonded prestressing strands was

tially described by ACI-ASCE Committee 323 to consista grease- or asphalt-impregnated material enclosed sheath. Specification and approval of the method of protion was left to the engineer. In ACI 318-63, Section 2620dicated that “unbonded steel shall be permanently protefrom corrosion.” However, no specific information was pvided relative to the sheath material and the required cosion protection. As noted previously, concrete cover sometimes interpreted as providing the permanent protefrom corrosion.

The 1971 Code included a section entitled “Corrosion Ptection for Unbonded Tendons.” It stipulated that “unbontendons shall be completely coated with suitable materiensure corrosion protection.” This section also requiredwrapping should be continuous over the entire unbonzone of tendons in order to prevent bonding with surrouing concrete and loss of the coating material during concplacement.

In 1972, the Post-Tensioning Institute (PTI) publishedfirst edition of the “Post-Tensioning Manual,” which inclued a specification for post-tensioning materials. Subsequly, this specification was revised and published as a gspecification in the second and third editions of the Man(1976 and 1981, respectively). In 1985 PTI published “Spification for Unbonded Single Strand Tendons,” the first tailed specification to be based on experience. ACI referenced this document in 1989 under “Corrosion Protion for Unbonded Prestressing Tendons.”

Significant new provisions of the 1985 PTI specificatiare discussed below.• Definition of exposure conditions: The 1985 specific

tions addressed tendons in “normal (non-corrosive) environments,” and tendons in “aggressive (corrosivenvironments.” Normal environments were defined athose present in nearly all enclosed buildings with dinteriors, and exposed structures in areas with very lor no snow. Aggressive environments were defined those that would expose the structure to direct or inrect applications of deicer chemicals, seawater, bracish water, or spray from these sources.

• Anchorages and couplings: Anchorages were requirto have watertight connection to the sheathing and awatertight closing of the wedge cavity. Couplings werequired to be protectively coated the same as the strand, since they become part of the strand.

• Sheathing: Sheathing material thickness was specifaccording to exposure, and was given as 0.025 in. (

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mm) for normal environment and 0.030 in. (0.8 mmfor aggressive environment. The inside diameter offinished sheathing was required to be at least 0.010(0.3 mm) greater than the diameter of the strand.

• Corrosion-preventive coating: (Hereafter, the term “grease” will be used to describe the corrosion-prottive coating, defined in the PTI specification as “...aorganic coating with appropriate polar, moisture-displacing, and corrosion-preventive additives.”) The mimum weight of grease coverage of the strand was given, as was a list of test requirements for the greaitself. The corrosion protection performance was baon ASTM B-117 for the time until Rust Grade 7 devoped and was set as 720 hr minimum for normal enronment and 1000 hr for aggressive environment.

PTI reissued this specification in 1993 after substantial rvisions were made to all chapters. Major changes are summarized below:• Definition of exposure conditions: The terms used i

previous specifications “normal (non-corrosive)” and“aggressive (corrosive),” were changed to “normal” and “aggressive.” The definitions of normal and aggsive remained similar to the 1985 specification excethat exposed structures in areas with very little or nsnow were no longer mentioned as being in a normenvironment. The designer was advised to “evaluateconditions carefully to determine if the environmentwhich a structure is located is considered aggressivany way.”

• Definition of exposure conditions: A stipulation wasadded that stressing pockets not maintained in a drcondition after construction should be considered exposed to an aggressive environment.

• Prestressing steel: Protection requirements were adfor packaging and identification. A criterion was addthat limited surface rust to pits no more than 0.002 (0.05 mm) diameter or length. (This type of rust canremoved with fine steel wool and might not be felt wthe fingernail.)

• Anchorages and couplers (formerly “Couplings”): Static test criteria were clarified and linked to ACI 31and dynamic test requirements were added. Design cri-teria were added for bearing stresses on concrete. A stipulation was added that required anchorages intended for use in aggressive environments to be fully protected against corrosion. Encapsulation of the anchorage, the connection of the sheathing to the anchorage encapsulation, and the seal of the wedge cavity were required to sustain a hydrostatic water pres-sure of 1.25 psi (0.0086 MPa) for 24 hr.

• Sheathing: The thinner sheathing previously allowedtendons to be exposed to normal environments was removed; the minimum thickness of sheathing was sified as 0.040 in. (1.0 mm) for both environments. Thsize of the annular space between the outside of thestrand and the inside of the sheathing was increased0.010 in. (0.3 mm) to 0.030 in. (0.8 mm). Complete

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encapsulation of tendons to be used in aggressive eronments was specified with the same watertightnesrequirements given for anchorages. A statement waadded that calls for the designer to specify the amouof unsheathed strand permitted at the anchorages fotendons exposed to normal environments.

• Corrosion-inhibiting coating (formerly “Corrosion-preventive coating”) (commonly referred to as grease): Atendons are to use grease that meets the ASTM B11Rust Grade 7 criteria after 1000 hr. (The shorter testtime previously permitted for tendons to be used in nmal environments was dropped.)

• Installation requirements: The minimum cover requirment for anchorages was added and was specified tat least 1.50 in. (40 mm) for normal environments andin. (50 mm) for aggressive environments.

• Tendon finishing: Permissible length of strand projection from the face of the wedges was reduced. The pvious projection allowed was 0.75 in. (20 mm) minimum and 1.25 in. (30 mm) maximum; the reviseprojection limits are 0.50 in. (12 mm) minimum and 0.75 in. (20 mm) maximum.

CHAPTER 3—UNBONDED TENDONS3.1—Evolution of unbonded tendons

The first building structures to use unbonded tendonNorth America were lift slabs built during the mid-1950s4

The early, unbonded tendons were greased and helicwrapped with paper. They used high-strength wires, genely 0.25 in. (6 mm) diameter, with an ultimate strength of 2ksi (1650 MPa), with button-head type anchorages, as shby Fig. 3.1. This system had large cumbersome anchora

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large trumpet transitions and a fixed distance betweentons at each end of the tendon. Systems also evolved dthis period that used single and multiple strands.

In the early 1960s the convenience of placing unbonsingle strand tendons was realized and the number of suers increased. The marketplace found the competitivenethis system favorable, and the use of unbonded single stendons increased significantly. Anchorage hardware vaconsiderably and included high-strength spirals, barrelcastings with wedges, and fittings that were swedged chanically attached) to the prestressing steel. The anchoprevailed was a casting that contained a recess to hotwo-piece wedge for use with a single strand. This is curly the predominant system and represents about 60 peof all post-tensioning tonnage.5

By the late 1960s, plastic began replacing paper sheatThree different processes have been used (Fig. 3.2 and

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1. The strand is covered by preformed push-through platubes; 2. The strand is wrapped longitudinally withheat-sealed strip; and (most recently) 3. The greased sis covered by molten plastic that is continuously extrudaround it.6 Thickness and composition of the sheathing mterial were left to the supplier and were not uniform in thedustry. The first effort to regulate these items was by the in their 1985 specification.

423.4R-6 MANUAL OF CONCRETE PRACTICE

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Fig 3.1—(a) Live end anchorage assembly for button-headed wire post-tensioning system(b) intermediate anchorage assembly with threaded coupler rod; and (c) dead end anchoage assembly; dimensions “A” and “B” were usually 4 and 6 in. (100 and 150 mm), respectively, for a typical 7-wire slab tendon (reprinted from Reference 1).

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Initially, there were no corrosion protection standardsthe grease (except in the nuclear industry, refer to ACI 374, “Code for Concrete Reactor Vessels and Containmenso the tendon manufacturers used the grease of their chCorrosion-resisting properties of the grease were not sfied, nor were there standards for the uniformity and amo

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of grease to be applied to the prestressing steel. Deteriorof the grease was not expected, but as problems beknown they were addressed by the PTI in their recommenspecifications.

3.2—Sheathing problemsThe push-through system required that the sheathin

sufficiently oversized to allow the greased strand to beserted without too much difficulty. This resulted in a tendwith many air voids inside the sheathing and allowed intration of water during storage, shipping, and installatiand in service.

With the heat-sealed system, the sheathing was provin rolls of flat plastic tape that was usually 20 to 40 mils (to 1.0 mm) thick. During tendon fabrication, the strawould be taken from the pack and passed through a grextrusion head. The tape was then folded over the grestrand and the lapped seam welded shut with a flame. method also formed a slightly oversized sheathing that chave air voids. The seam weld was interrupted every tthere was a pause in the process, and sometimes the sseam pulled apart during handling or installation. This stem is frequently found to have gaps from one cause orother that expose the greased strand to contact withconcrete.

Seamless extruded sheathing first appeared in the 1970s.7 The extruded sheathing minimized the problems perienced with the push-through and heat-sealed systemproviding a snug, seamless sheathing around the grestrand.

3.3—Detailing practicesCertain details that were initially considered accepta

are insufficient to provide the degree of durability requi.

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It was common practice in both detailing and installato allow the sheathing to stop short of the anchorageshown by Fig. 3.4 (the grease can get wiped away from

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sections of strand during the installation of anchoragestressing and nonstressing ends). During stressing otions, the strand at the stressing end moves about 8 in100 ft (200 mm per 30 m) of length. Direct contact betw

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the unsheathed strand and the hardened concrete is distuat the stressing end during this elongation, thus providinpath for water to find its way to the strand and into tsheathing.2

Since the dead end anchor did not have to move dustressing, it was considered acceptable for the strand neadead end to be exposed to the concrete. However, this allowed the end of the tendon to be exposed to dirt and wduring storage, handling, and placement, prior to placingprotective concrete. The end of the sheathing was, for ptical purposes, open, and the voids inside the tendon sheing provided a means for any available water to gain accto the tendon. Even if the storage time at the site was relatly brief (which was not always the case), there was ampleportunity for water to get into the tendons due to any snand rain that might occur while the tendons were storedthe ground or on the formwork. For those portions of tstrand not adequately protected by grease, water and oxcould cause corrosion and then could be further exacerbby the presence of chloride-ions. Some typical defects ctributing to corrosion in the end anchor region are shownFig. 3.4.

The stressing side of the live-end anchorage was proteby filling the stressing pocket in which the anchorage wascessed with a protective cementitious grout (Fig. 3.4). Tcasting, wedges, and strand tail were not coated. Ofshrinkage of the grout plug in the stressing pocket causespace between the side of the stressing pocket and the that allowed water to gain access to the tendon anchorCorrosion of the bearing plate casting is often found but hasnot been known to cause failures. On tendons with baand wedges sitting on bearing plates, heat-treated ba

423.4R-8 MANUAL OF CONCRETE PRACTICE

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have been found to suffer from brittle failure. The primaproblem has been either corrosion of the wedges or thegration of water down the tendon in the void betweenwires or around the outside edge of the strand. Corrofailures of the strand have occurred in the unsheathed lein front of the anchor, at low points in the profile where wacollects, and at points where sheathing was damagedpermitted direct access of corrosive materials.

At intermediate anchorages, features of both dead-live-end anchorages are found. The sheathing may not bequately sealed to the anchorage on the “first pour” sStrand and anchorages may be in direct contact with crete on the “second pour” side of the anchorage. Wcould gain access to the tendon during shipping, storageinstallation, or after construction has been completed. Plems have developed at the intermediate anchorages the construction joint over the anchorages was not seaprevent leaking of water (sometimes containing chloridthrough the construction joint in elements exposed toweather, such as in parking structures and balconies. Sicant chloride contamination of concrete has occurredresult of leakage through unsealed construction joCorrosion of the upper surface of the anchorage castinand of backer bars, can cause corrosion-induced spallian early age. Such spalls then act as small reservoirs foter, further accelerating the deterioration process. Corroof the backer bars, bearing plates or anchorages, aoccasion the strand, is promoted by this frequent supply water and oxygen adjacent to the joint. With the earlier bing plate and intermediate barrel anchorage hardwarebearing plate was occasionally located outside the buhead of the slab, meaning that the construction joint wbe located directly over the plate and allow water to cointo contact with strand end anchorage.

The designer’s choice of locations for anchoragessometimes led to corrosion problems. Locating anchorin gutter lines or expansion joints of parking structures oexposed edges of slabs in commercial and residential bings (i.e., balconies) has resulted in water infiltration intochorage areas and caused corrosion failures of the sWithout special waterproofing, construction joints at becolumn connections have, on occasion, allowed water tter tendon sheathing through the anchorages in joints inumns. These joints are seldom sealed, even though thefrequently in the drainage flow path, and can be exposerain and melting snow. Water running down the face of aumn can gain access to the tendon anchorage if consolidof the concrete at the construction joint is poor. Honcombed, permeable concrete, ponding areas, leakingstruction joints, misplaced conduits, and other construcdefects can all contribute to water access to tendons.

3.4—Storage, handling, and construction problemsThough the tendon sheathing is fairly tough, the wear

tear to which it is sometimes subjected during shipping, age, handling, and concrete placement can be severe.

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can damage the corrosion protection provided by the greand sheathing.

Typical problem areas have included:• Tearing and/or cutting of the sheathing by wire or me

bands that hold the tendons in coils for shipping;• Tearing and/or cutting of the sheathing by unpadded

slings used to lift the bundles of tendons from the truto the storage area, and from the storage area to theformwork where they are to be placed;

• Unprotected storage, such as leaving the tendons indirect contact with the ground, leaving them exposedsnow or rain, or placing them where they will be damaged by construction traffic;

• Rough handling during placement of tendons in formwork, causing splits and tears in the sheathing;

• Incomplete repair of damage to tendon sheathing;• Leaving the unsealed tendon ends exposed to the

weather, either before concrete placement or after stressing;

• Inadequate cover over the cut-off strand tail at live-estressing pockets;

• Inadequate concrete cover to protect the tendon at hand low points of drape, and at the anchorages;

• Voids inside sleeves or trumpets, where water may clect;

• Improper grouting of the stressing pockets; and • Using grout material that contains chlorides or other

chemicals that will accelerate corrosion of the strandSome current encapsulation systems used in aggressive

environments incorporate oversize sleeves or trumpets to as-sist in sealing the tendon at the transition from the sheathgto the anchorage. Since these systems rely on friction ratherthan on a mechanical connection between the anchor ansleeve, these sleeves have been seen to work loose anpullaway from the anchorage prior to or during concrete place-ment. Final inspection with reattachment as necessary hasbeen required to achieve the intent of the PTI 1985 and 19“Specification for Unbonded Single Strand Tendons” forprotection of unbonded tendons.

Damage to the protective grease and sheathing or expoto moisture during these periods of the construction coadversely affect the future performance of the post-tensing, but neither responsibility for protection nor specimeasures for achieving protection were defined in an indtry-wide specification or procedure. These issues are npartially addressed in “Field Procedures Manual,” 2nd etion. As of 1997, the PTI “Specification for Unbonded Sigle Strand Tendon” is being reviewed, rewritten, aincorporated through a standardization process by AASCE Committee 423, “Prestressed Concrete.” Concerngarding shipping and handling are being considered.

3.5—Deterioration mechanismsIn most cases, the corrosion mechanism requires that

ter and oxygen be present. If a corrosion-inhibiting grecompletely covers the strand, and the grease is not affeby water, corrosion generally will not occur. There is alwa

423.4R-9SINGLE STRAND TENDONS

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the possibility that grease will be discontinuous, so cosion can begin if water and oxygen are available. Corrocan be accelerated if chlorides are present in the watethe wire(s) corrodes and the cross-sectional area decrthe stress in the remaining section rises past its ultimatesile strength, and the wire(s) fails. Failure by embrittlemof the strand wires can also occur if other aggressive mals, such as nitrates and sulfides, are present.2,8,9 Embrittle-ment failures can occur without significant loss of crosection in the strand. It is not uncommon for the failure faces of one or two wires at a strand break to be jaggedical of embrittlement failures, while the remaining wihave cup-cone configurations that are typical of ductile ures. It is not necessary, or likely, that all of the wires given cross section will be similarly affected at the satime (Fig. 3.5, 3.6, and 3.7).

Fig. 3.5—Scanning electron micrograph showing fracturesurface exhibited by one wire in a failed strand. Fracturesurface is brittle and irregular as is characteristic of strescorrosion cracking.

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Fig. 3.6—Photomicrograph showing irregular, transgranu-lar crack path characteristic of stress-corrosion cracking (hydrogen-induced cracking) in cold-drawn, high-strengthwire.

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Fig. 3.7—Brittle fractures in failed strand. Note mixture ofwater, grease and corrosion by-product on wires of strandfreshly extracted from structure.

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A strand can burst from the concrete when it fails if cover is small, usually less than about 3/4 in. (20 mm) (Fig.4.1 and 4.2). Whether this happens depends on the dra

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the tendon, the type of sheathing, and the presence ofpendicular reinforcement between the broken tendon andsurface of the concrete. Occasionally the strand will beleased by the anchor and project past the edge of the sture.2,10

Tendons can be subjected to vehicular damage if the crete cover spalls off or is abraded away. This most cmonly happens where the concrete cover is less than 3/4 in.

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(20 mm) and can be the result of misplacement of the tenor poor screeding and finishing of the concrete. The sheing is not intended to resist direct contact of in-service traand is easily breached under these circumstances. Therethe grease quickly disappears and contaminated water isto enter the sheathing and cause corrosion.

3.6—Performance recordIn general, tendons with extruded type plastic sheath

provide superior corrosion protection when compared wtendons from other unbonded systems and with other tyof sheathing. The improvement in performance, howevmay also be due to improvements in the quality and appltion methods of the grease that occurred at about the stime that most fabricators changed to the extruded plasheathing system. The extruded type sheath now prednates in the industry, probably as a result of the 1985 “Specification for Unbonded Single Strand Tendons.” Frorandom corrosion incidents, it is clear that care must be tato ensure that no aggressive materials, including water, ethrough the sheath or anchorage.

423.4R-10 MANUAL OF CONCRETE PRACTICE

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There are no recorded incidents of sudden collapsstructures using unbonded tendons while the structure service. Demolition of these structures has shown that possess greater reserve of strength than is shown by stures that are not post-tensioned. One characteristic of stures that use unbonded tendons is their propensity to developsignificant catenary action even while the main parts of the sand beams are being pulverized or broken away.11,12 Whilethere is no guarantee against a sudden partial collapselikely that a post-tensioned structure will continue to perfoin a ductile manner even when a significant number oftendons have failed.

CHAPTER 4—EVALUATING CORROSION DAMAGE4.1—General

Almost any structure can have its useful life extendedthe use of appropriate maintenance procedures and surepairs, whether the structure is of post-tensioned concreof some other material. The cost of repairs can usually betimated to a reasonable degree of accuracy and a judgmade as to the feasibility of that investment. For concstructures that do not use unbonded tendons, damage cby corrosion is estimated by examination of rust-stainedspalled areas. It is assumed, based on spot checking apast performance of similar conditions, that the reinforment is serviceable in areas where the concrete is not staspalled, or delaminated. That same logic applies to evation of deterioration of bonded reinforcing bars in structuusing unbonded tendons, but it is of less help in evaluathe tendons themselves.

An appropriate evaluation of the condition of the structshould be performed to minimize the risk of overlookisomething significant. Ultimately, decisions will have to made about the types of repairs to be made and the neaugment the reinforcing system. These decisions are geally based on an engineering evaluation of the data colleand should be made by an experienced investigator whoderstands that all latent deficiencies have probably not bidentified. As in any repair, the objective is to fix the obvio

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defects, eliminate the causes of deterioration where pracslow continued deterioration, and determine requiremfor monitoring and future maintenance.

4.2—Condition surveys of concreteMany references in Chapter 7 provide specific informa

about the generally accepted methods for evaluation ofcrete structures. Typical procedures are reviewed here.

The evaluation should include a careful and well-domented inspection to identify deterioration and distress,to identify their causes. If available, the original desdrawings and shop drawings should be reviewed to ideproblems that might be attributed to the design, detailinmaterial selection. The drawings should be compared the findings of the condition survey to assess the in-seperformance and determine whether suspected problemlocal or might be widespread. It is normal for the as-bconditions to differ from the drawings to some degree. Rto ACI 364.1R, “Guide for Evaluation of Concrete Strutures Prior to Rehabilitation,” and ACI 437, “Strength Evuation of Existing Concrete Structures,” for additioguidelines.

A crack survey is useful since cracks can be causestructural distress, insufficient cover to reinforcing barstendons, incipient delaminations due to corrosion of rforcement or embedded conduit, or restrained shrinkNotes should be made to document the width and lengthe cracks, as well as the presence of leakage, efflorescand rust stains.

A survey should be performed to locate delaminationslabs and other structural members, using methods thaappropriate to the conditions. The delamination surveybe used to estimate the extent and distribution of corrodamage in the reinforcing system.

Typical materials testing to be performed may include• Chloride-ion content testing to determine the depth

intensity of chloride penetration, and to estimate thechloride content of the original concrete. Chloride cotamination can promote corrosion of portions of thepost-tensioning system in contact with concrete as w

423.4R-11SINGLE STRAND TENDONS

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as conventional reinforcing and hardware whose cosion may damage the concrete protection of the postensioning.

• Evaluation of depth of carbonation into the concreteThis reaction decreases the pH of the concrete, thusmaking corrosion more likely in the presence of moiture and oxygen.

• Obtaining measurements of concrete cover over tendons and reinforcing steel and, if appropriate, correling these measurements with the chloride contamination profiles or depth of carbonation.

• Copper-copper sulfate half-cell testing to determine whether corrosion activity is presently occurring in threinforcing system. (These tests may not provide meingful information on tendons because of the lack ofelectrical continuity but may indicate likelihood of corosion activity on anchorage hardware and reinforcinwhere electrical continuity of steel and contact with concrete are present.)

• Extracting concrete cores for compressive strength ting and for petrographic examination to include air vosystem analysis. (Tendons should be located beforeing begins.)

4.3—Condition surveys of tendonsIt is possible to obtain some information about the lik

condition of the post-tensioning system by means of thesual inspection. The visual inspection can help to idenspecific external evidence of possible internal distress. visual inspection can also help identify whether there pattern to the distress, and may point to the possible extea problem.

Cracking of concrete in post-tensioned structures cathe result of problems with the post-tensioning system,all cracks should be evaluated for their impact on futurerability. Following are some examples of cracking patterns may provide information about the post-tensioning system• Cracks that are vertical in beams, and cracks that a

perpendicular to the direction of span in slabs, may indicate a loss of post-tensioning force. A structural analysis taking into account possible losses of post-sioning precompression due to built-in restraint can provide an indication of whether such cracking couldbe anticipated. If a large discrepancy is found betwethe calculated service load stresses and the actual observed extent of cracking, then a loss of post-tensing may be indicated.

• Longitudinal cracks in beams, either on the sides oracross the bottom in line with the tendons, may be aindication of water infiltration into the tendons. As thsheathing fills with water and freezes during winter months in colder climates, expansion due to formatiof ice can cause splitting of the concrete cover over tendons.

• Leaking or leaching of moisture through the cracks ian indication of water infiltration. Cracking due to water infiltration may follow the tendons a considerab

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distance from midspan toward the supports. • An unintended reversal in the curvature of the tendo

at any point in the beam can cause splitting of the bedue to local internal tension in the concrete around treversal, produced when the tendons are tensioned.These cracks can form without affecting the conditioof the post-tensioning tendons but can be a warning potential blowout.

• Cracks in the area of beam-column connections may be due to structural action, to a deficiency of reinforce-ment intended to control bursting stresses in the tendonanchorage zone, to restrained shrinkage, or to a combi-nation of these effects. Water that enters these crackhas easy access to the ends of the tendons and can cafurther deterioration.13

Stains on the surface of the concrete can also provideternal evidence of possible internal distress due to corroof the post-tensioning system. Grease stains on the soffislabs, especially at low points of tendon profiles, can beindication of unrepaired damage to the tendon sheath asas shallow concrete cover over the tendons. Such grstaining may be accompanied by water stains or evidencleaching, indicating water infiltration into the slab tendonRust staining in the vicinity may be the result of corrosionthe post-tensioning steel or perhaps only the supporting sters. A compromise in the corrosion protection of the potensioning is indicated and would warrant further testing exploratory concrete removal, as discussed in Section 4

Visual inspection of exposed end anchorage grout pocshould be performed, especially where exposure to moisis evident, and correlated with any signs of distress sucdescribed above. Evidence of shrinkage, cracking, deboing, freeze-thaw damage or rust staining coming from grout pocket may indicate a potential breach in corrosprotection of the anchorage and post-tensioning tendon.

The most obvious external evidence of corrosion damis the presence of loops of strand sticking out of the struc(Fig. 4.1 and 4.2). Such loops result when the strand breaand the elastic energy is released suddenly. The strtypically will erupt from the slab at high points or low poinin the tendon profile where concrete cover may be shallbut occasionally only a single wire will burst through the sface of the concrete. Loops formed by this phenomenonbe anywhere from 1 in. (25 mm) to 2 ft (600 mm) high. Raer than bursting from the structure at some point midwaytween anchorages, the tendon may also protrude out ostructure a distance of several inches or several feet.

Strand breakage can occur without visible disturbancthe concrete, so the absence of strand loops or projectionot to be taken as an absence of broken tendons. Most tensioned structures use higher strength concrete (with her cracking strength) and/or may incorporate (perhaps utentionally) a significant degree of restraint or redundan(i.e., below grade construction or two-way slab constrtion), so it is possible to have as many as 50 percent otendons broken in a beam or in an area of slab without oous distress.

423.4R-12 MANUAL OF CONCRETE PRACTICE

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The location at which the tendon or wire has eruptedof the structure is usually some distance away from the tion of actual failure. Exploratory concrete removals cbined with the removal of the broken tendon will be requto identify the location, nature, and the possible causes failure.

The visual review of the structure can serve to deterthe distribution of such eruptions in the structure, whichthen be correlated to detailing problems and inadequatrosion protection to help determine the possible approprepair and protection measures. It may be useful to atto determine the distribution of such eruptions over timwell to see if any trends are apparent in the frequency odon failures with the passage of time. Unfortunately, deing repairs to monitor tendon failures may peracceleration in the rate of deterioration. Usually it is bettbegin repairs as soon as a reasonable condition studbeen completed rather than to wait for months or yeagather more information. In a few cases, the rate of corractivity has been observed to increase over time, indicthat the rate of tendon failures could increase with time

4.4—Nondestructive testingCurrently there are no dependable nondestructive te

methods for evaluating the existing condition of tendonside a structure. The main shortcomings of present test ods include inability to provide information about severitlocation of pitting, gross section loss, location of pre-exiscracks or breaks, or breaks and section loss of indivwires in a strand. Several common procedures are revbelow with comments about their limitations for applicato unbonded tendons. • X-ray techniques are often used to locate embedde

reinforcing in concrete, usually for a very limited aredue to their expense. X-ray cannot be used to obtainformation about pitting, breaks, or loss of cross setion of individual wires. The images obtained are fuand do not lend themselves to fine interpretation. Aother reinforcement can shield the tendon being checked. It is usually impractical to use this methodalong the length of a tendon to find strand breaks.

• Radar (ASTM D 4748) can also be used to locate embedded reinforcing in concrete. Radar has the slimitations as X-ray but is more adaptable to scannlarge areas to locate reinforcement, including tendo

• Acoustic emission (not to be confused with ContinuAcoustic Monitoring discussed in Section 5.6) is a technique for locating corrosion based on detectionhigh-frequency sounds emitted at sites of active cosion, and has been used successfully on steel strucsuch as storage tanks. For detection of corrosion dage in post-tensioning, acoustic emission is an expmental procedure from which has come no correlatbetween test results and the condition of tendons. Wacoustic emission testing, it is not possible to determthe nature of the corrosion damage (surface attacksus pitting), the location of the damage (strand vers

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anchorage, or where on the strand), or whether wiresthe entire strand are broken.

• Ultrasonic pulse velocity (ASTM C 597) is useful in determining certain properties of concrete, but it hasnot proven to be adaptable to determining the integriof tendons.

• Half-cell potential (ASTM C 876) can be used to detemine the presence of corrosion activity in bonded reiforcing bars and in anchors and strands that are in contact with concrete. Half-cell testing does not reli-ably locate corrosion along the length of a tendon sinthe grease and sheathing will electrically insulate thestrand. This method is redundant where visible evi-dence of corrosion is present.

• Pachometer devices will locate greased and sheathetendons as well as bonded reinforcement. They are uful in estimating the thickness of concrete cover overanchorages and along strands, and the thickness of grout protection of strand stubs at stressing pockets.Pachometer readings for anchorages and stubs as was along strands may require calibration by comparinagainst actual physical measurements of concrete cothickness.

• Vacuum testing of grouted stressing pockets can assthe quality of the grout and its bond to the sides of the block-out.2 Such testing can help quantify the effective-ness of the grout plug in preventing water from infiltratin the post-tensioning tendon.

• Impact-echo testing can be used to supplement a delamination survey to more accurately determine thdelaminated areas. It can also be used to determine crack depths, voids, and honeycombing. Impact-echoprovides no information regarding the strand conditio

4.5—Exploratory concrete removalUsually it is necessary to chip into the concrete to insp

the strand, grease, sheathing and anchorages, and to athe extent of corrosion activity of the bonded reinforceme

This procedure is slow and is limited to parts of the struture that are accessible. For example, anchorages for btendons are frequently embedded in columns and arereadily exposed. Occupied structures present special plems since concrete removal is noisy and dirty. The numof inspection opportunities may be small compared to thetal number and lineal footage of tendons in a structure, soinformation obtained may not be statistically meaningfuExploratory concrete removal can be used to calibrateconfirm results from nondestructive tests.

Repair of concrete removals should restore grease, sheing and concrete protection. Refer to ACI 546, “Guide to Rpair of Concrete.”

4.6—Exposing tendonsExposing parts of tendons is most convenient where c

crete cover is shallowest, such as at low points or high poin the tendon profile. In most structures, it is preferable to pose the strand at low points in the profile for several reasons:

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first, removal of the upper surface often will be more disrutive to the use of the building; second, the upper surfacoften more exposed to sources of water or other aggresagents, and achieving good quality and timely repairs toestablish protection may be difficult at the time exploratowork is being performed, or it may be desirable to leave test areas open for some period for repeated viewing; third, if water is present in the tendons, it tends to colleclow points in the profile and is more likely to be detectthere, especially for loosely sheathed strand. Care mustaken in all cases not to damage the tendons since theusually fully stressed.

It must be kept in mind that an unblemished, greasy struncovered in a convenient area can be seriously corrodfew feet away without showing distress. Once broken, tendon has lost its structural value for its entire length, sis important to take as many observations as possible anlocate them in the areas of suspected corrosion. Engineejudgment must be applied to estimate the current condiand future performance of the structure.

As previously discussed, grease stains or water stainthe soffit of the slabs and beams can help identify locatiwhere exploratory removals may be desirable. Locations appear to be free from distress should also be examineprovide a more representative picture of the general cotion of the tendons.

Exposing selected tendons for a length of at least 12(300 mm), for as much of their circumference as practiccan provide the following information:• The type of sheathing (paper-wrapped, push-through

heat-sealed or extruded), its tightness to the strand, its thickness;

• Unrepaired damage to the sheathing; • Water within the sheathing;• The condition of the grease; • Corrosion on the strand;• Verification of stand size; and• Loose strand or wire indicating prestress loss.

Loose strand or wire is commonly checked by meanspry bars or screwdrivers. If the strand can be displaced significant amount with a pry bar, it is likely there is little ono tension remaining in the strand. This testing is subjectsince the length of strand exposed affects the amount ofplacement which can be obtained. The screw driver penetion test can be used to identify loose or broken wires. Tinspector attempts to drive a standard (flat blade) screwder between the wires of the strand at an inspection port. etration of the screwdriver indicates little or no force in twire(s). If sufficient length of the strand is exposed, the tcan be performed between each of the six outer wires, identifying individual wire breaks. The test is also subjectand will not necessarily detect all wire breaks, especiallthey are at some distance from the inspection location. Inmediate anchorages and bonding of the tendon by buildof corrosion product may maintain sufficient force in the tedon to mask strand or wire breaks.

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Exploratory concrete removals undertaken in the areaintermediate anchorages are useful, particularly where leage has occurred through construction joints. The stresside of the joint is the side that can be exposed without cpromising the integrity of the tendon; the concrete on other side is under compression and should not be distur

Exploratory concrete removals should not be started wout a detailed plan of action. A specification should be ppared that outlines safety precautions and acceptprocedures for demolition, encapsulation repairs, and ccrete patching. Individuals performing the chipping musteducated about tendons so that they understand what thelooking for and the consequences of breaking a strand. Tshould use slow, careful means of concrete removal, andtools that are appropriate to the task. Refer to ACI 546.“Guide to Repair.” As previously discussed, broken stracan erupt vertically through the top or bottom surfaces ofconcrete, or horizontally through the anchors.

4.7—Strand removal Strands can be removed for inspection, but it is unusua

cut a live tendon for this purpose. It is more common to tract part of a strand that is known to be broken, usually that has erupted from the concrete. It is usually possiblremove the strand between the eruption and the breakconsiderable effort is sometimes required. Laying the strpiece on the deck is a simple way to find the location of break. Once removed, the strand can be examined to dmine the condition of the grease, the frequency and sevof corrosion, and the type of failure of the wires.

Signs of corrosion are sometimes subtle and look mlike stains than rust, so it is important that the strand beamined closely by someone experienced in this type of euation. The failed end of a strand can be submitted tmetallurgical testing laboratory for an analysis of the brokwires. Microscopic examination of the external and interwires of the strand can be performed to look for crackipitting, and corrosion.

Chemical residue present on the wires at the failure lotion can sometimes be identified and related to the failuTesting of physical and mechanical properties of the strcan be performed on strand taken some distance frombreak to determine tensile strength, elongation, and btesting.

4.8—Other testing and investigative proceduresLift-off testing (pulling the strand loose from an anchor

measure its effective force) can be used to check tensiothe strands, but is difficult to do with single strand tendona completed structure. Since the strand has been cut clothe wedges, special tools are required to grip the strandsufficiently to pull on the strand and loosen the wedges. Crosion increases the locking force of the wedges and wens the stub, making the process difficult to execuWelding an extension onto the strand is not feasible becof the likelihood of permanently damaging the anchoraand the strand. When barrel anchorages have been us

423.4R-14 MANUAL OF CONCRETE PRACTICE

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may be possible to clamp onto the barrel and perform aoff of the tendon in this manner. However, care shouldtaken since barrels are generally heat-treated, making thard, difficult to grip, and non-weldable.

Lift-off tests will not provide any information about thamount or distribution of corrosion on the strand, nor wthey give an indication of the loss of ultimate strength. It wonly provide knowledge that the strand had the ability to stain the force held by the jack after the wedges came loTypically, lift-off tests do not exceed the tension originaused to set the wedges. Sometime the strand will breakafter the wedges come loose, indicating that strength wasbefore the test. The logical next step is to pull the strandof the sheathing for inspection.

An alternative to lift-off testing is to check for tension the strand using equipment that analyzes the vibration cacteristics of the strand under tension. This equipmenadapted from that used to check tendons in cable-stabridges and in similar applications.

A portion of the strand is exposed, typically about 2 ft (6mm), and the ends of the exposed length are chocked tovide node points. An accelerometer is attached to theposed strand and the strand excited by means of tappingvibration is transmitted from the accelerometer to computequipment to determine the tension in the strand as a ftion of the frequency of vibration. As with lift-off testingchecking tension in this manner does not provide detailedformation about corrosion on the strand.

Load testing of slabs and beams in accordance with 318 under “Strength Evaluation of Existing Structures” pvides no detailed information about the condition of the dividual tendons. A significant number of tendons couhave failed without being detected by a load test. The peof time subsequent to the testing for which the test resultsvalid is subject to a great deal of uncertainty, and ACI 3provides no guidance for estimating the remaining servlife. Also, load testing is expensive and disruptive. Thefore, although a load test can confirm that the tested pathe structure has adequate strength at the time of the tesmethod has limitations when applied to structures with spected or known corrosion damage to unbonded tendon

CHAPTER 5—REPAIR SCHEMES AND METHODS5.1—General

Prior to launching a repair effort of any kind, it is prudeto determine the causes of corrosion damage and pospreventative measures which may be taken. Incorporameasures that address causes of deterioration can makepair program more effective for extending the useful lifethe structure. Whether the need for tendon repair is critshould be assessed by analyzing the structure to determ ifsufficient reserve capacity exists to adequately supportdesign loads after a few tendons are lost. In some casmight not be necessary to replace every broken or damtendon, or it might be reasonable to use smaller diamstrand to replace the strands that were removed. The smdiameter tendon may be of a higher strength if required

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maintain adequate precompression or ultimate streWhere the type and condition of the strand and sheapermit, it is possible to replace the old strand with one osame diameter. If analysis indicates that a reduction incarrying capacity below code limits, and especially beservice load levels, has occurred, then evacuating or shthe affected portions of the structure may be required invance of repairs.

When tendon corrosion has progressed to the point wthe strength of any part of a structure has been reducedunacceptable level, repairs should be considered to cothe deficiency. The type of repair will depend on the ex(or assumed extent) of the corrosion damage and the ring loss of strength. If the damage is known to be minor,its location is easily identified, the remedy may consissimply repairing and protecting the existing tendons. Mprobably a variety of conditions will be encountered, soof which might require the removal and replacement oftire tendons. Engineering assessment of the proposed methods and sequence is needed to determine how theprocess may further affect the integrity of the structure.

When the actual or assumed extent of damage makcalized repairs impractical, the structure may need tstrengthened with an externally applied structural sysconsisting of post-tensioned tendons, structural steeltions, or concrete members. External post-tensioning tems have been designed to completely replace the ortendons in some structures. These various approachediscussed in the following sections. When external strenening is not feasible, partial or complete demolition andplacement may be necessary.

5.2—Existing tendon repairVisible evidence of tendon corrosion, such as that w

may be seen on exposed tendon anchorages where theing pocket has not been grouted, should be repaired as qly as possible. The exposed part of the anchorage shoucleaned by abrasive blasting or equivalent, coated wrust-preventive paint or other corrosion protection, andstressing pocket properly filled in accordance with “Specification for Unbonded Single Strand Tendons.”

In a structure where tendon corrosion has been diagnappropriate means of stopping or slowing the rate of cosion in the existing tendons should be applied. Eliminawater intrusion is of primary importance, so concrete repshould be made and cracks should be sealed. Random can be routed and sealed, but consideration should be to the application of a waterproofing membrane, possiblcorporating a wearing surface as appropriate, if extencracking is present or if there is widespread deficiencprotective concrete cover throughout the structure or ation of the structure. Refer to ACI 224.1R, “Causes, Evation and Repair of Cracks in Concrete Structures.”

5.3—Strand replacement When a strand has been inadvertently cut or damage

when corrosion damage is known or believed to be local

423.4R-15SINGLE STRAND TENDONS

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repairs are often made by replacement of part of the stbetween anchorages. The old anchors are reused, and twedges are never unlocked. The damaged section of sis cut away and a new piece of strand spliced onto the of the original strand using couplers. The simplest coupare short tubes with two sets of wedges, while others athe strand to be stressed at the splice location (Fig. 5.1and 5.3). The original anchorages do not need to be exp

Fig. 5.1(a)—Typical barrel coupler, partially disassmbled show wedges inside each end that grip as tendon is restressed.

Fig. 5.1(b)—Barrel coupler installed and under tension, bbefore encapsulation.

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Fig. 5.2—Combination of barrel couplers and center-pullcouplers used to repair and restress failed tendons.

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when an in-line stressing coupler is used, and the stresprocess is a limited form of proof test. Some designers sify that for this type of repair the strand must be stresseusual, to 80 percent of ultimate before lock-off. Others knowledge that long-term losses have already occurredrequire that a lower initial tension be applied; the repatendon stressed in this manner will have approximatelysame effective stress as the original tendons.

Replacing a strand for its full length and using the origianchors is also possible, but dislodging the old wedgesometimes difficult and the anchors can be damaged inprocess. It is usually advisable to replace the anchors new ones since this gives the opportunity to improve the tem’s durability. Once free of its anchorage, strand exttion is normally not difficult. In some cases a jack canused to pull the strand out, but this method, while reliable

seh--

slow. Usually the loose strand is pulled out by hand or wthe assistance of a come-along or a vehicle. If binding problem and jacking is required, the applied stress shouldexceed 0.80 fpu. If the tendon cannot be moved, then tcause of the binding has to be located (usually by usingachieved elongation to calculate the distance back tobound location) and removed.

423.4R-16 MANUAL OF CONCRETE PRACTICE

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Prior to inserting the new strand in the old sheathingmaining in the structure, moisture inside the sheathshould be removed (i.e., pulling rags through the sheatuntil dry). A quantity of grease should be pumped into sheath ahead of the new strand. The goal of this proceor any alternative procedure, should be to fill all voids insthe sheathing so that water cannot again enter into the shing and start a new cycle of corrosion. The new strand can be either bare, or pre-greased and encased in a psheathing from which it is pulled out to be inserted. In tendon, only one type of grease should be used. In genpre-greased strand, preferably one covered with an extrsheathing, is desirable for use as replacement strandgrease on a factory-greased strand is applied under preduring fabrication so the coverage and penetration betwthe wires is better than can be achieved if the grease iplied in the field to a bare strand. The sheathing should bsealed at all inspection and repair locations to provencapsulation of the strand.

Epoxy-coated strand meeting ASTM A 822/A may be csidered for strand replacement. A smaller diameter stmust be used to accommodate the thickness of the co(30 to 40 mils, or 0.7 to 1.0 mm). Special anchorageswedges are required for use with epoxy-coated strand, sisting anchorages have to be replaced. There have beenlems in the past with slipping of strand through anchoradue to inadequate bite of the chucks through the coating

New strand is either pulled into the existing sheath, usuby first threading through a smaller diameter strand thatbeen welded to the end of the new strand, or pushed by mof a jack that has been reversed and tied off to providenecessary reactive force. The strand buckles more readthe jack as the embedment friction increases, so shstrokes become necessary, but most strands can be reby this means.

Replacing strands is relatively expensive, intrusive, time-consuming. However, it can be an effective remwhen architectural considerations are important becthere is no change in the appearance of the structure. This

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should be performed by experienced contractors who unstand all aspects of proper corrosion protection for tendons

5.4—Tendon replacementIn some structures or portions of structures where m

strands are damaged in many places, it may be more cofective to abandon some or all of the existing tendons and install new ones, rather than to replace strand and anchorThis is generally practical only for slabs or wide beams sit requires that either intermittent or full-length slots be cuthe concrete. The procedure, termed “trenching,” as shin Fig. 5.4 and 5.5, is a patented process. Detailing at new

Fig. 5.4—Trenching method of internal tendon replacemea patented process (reprinted from Reference 1).

, Fig. 5.5—Parking structure slab with trenches cut prior toinstallation of new unbonded single strand tendons (reprinted from Reference 1).

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chorages and along the length of the new tendon must ethat the compression, uplift, and downward forces are trferred from the new concrete, in which the new tendonsencased, to the existing concrete.

5.5—External post-tensioningExternally applied post-tensioning systems can be effec

tively used to strengthen large portions of existing structures.External post-tensioning systems have found application inthe retrofit of a wide variety of framing systems, includingtwo-way flat plates, flat slabs, one-way slabs, beams, angirders.

Typically these systems consist of adding straight tendalong the tensile zone of existing slabs or beams, or buila tendon truss under or alongside the existing slab or bThe lower (tension) chord of the truss is the post-tensiotendon or tendon group. The vertical truss member is usua structural steel shape (normally a tube) with a bearing pwhich bears on the soffit of the existing structure and appthe beneficial upward force. The top (compression) chorthe truss is the existing structure itself, to which the post-sioning anchors are attached (Fig. 5.6 and 5.7).

ek

Advantages of external post-tensioned retrofits includ• The ability to apply large upward forces to the exist

structure with minimal headroom requirements; • Little or no interference with existing utilities;• Minimal disruption of the existing function of the

423.4R-17SINGLE STRAND TENDONS

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building; and• Possible cost savings over methods such as tendon

replacement. One possible disadvantage of externally applied post-

sioning is that it is visible and in some cases (when it ishidden by a ceiling or other feature) can change the archtural appearance of the structure. Fire protection requments must be in accordance with the governing buildcode. Corrosion protection is also required.

5.6—Continuous acoustic monitoringA proprietary acoustic monitoring system has been rec

ly developed that provides continuous monitoring for potensioned structures.14 When a strand or wire fails, there issudden release of energy. Sensors mounted at varioustions on the structure detect the acoustic response tranted from the location of the break. Information collectedthe sensors is recorded by terminals at the site and dloaded to a central computer where the information is alyzed to determine the probable location of the break.

The monitoring system does not determine the condiof the post-tensioning system at the outset of installationrather provides a record of the number and location of stbreaks in the structure subsequent to installation. The syis intended to assist with managing evaluation and repastructures with known or suspected tendon corrosion plems. The goal is to minimize repair expenditures and ensure structural integrity.

5.7 — External non-prestressed reinforcement or support

Strengthening can be accomplished with non-prestreelements such as structural steel or reinforced concbeams and columns. If architectural requirements perthe most cost-effective method to strengthen an exisstructure may be to add a permanent vertical support sua column. Most structures, however, cannot accommoadditional columns, and require some form of flexural refit (beams and girders, or bonded external reinforcemthat carry loads to existing columns. If non-prestres

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retrofits are designed to share loads with the existing flosystem (that is, the retrofit is designed only for the strendeficiency, not as a total replacement for the flexural ashear strength of the floor framing), then analysis and desmust be performed based on deflection compatibility to cfirm that stresses in the existing concrete remain within lowable limits and/or that deflections remain withiacceptable limits. Pre-jacking the retrofit system against existing structure can cause it to share in supporting exisdead loads as well as applied live loads, and can reduceflections in the system. Composite action between the refit and existing structure can also be used to promote lsharing.

Installation of external non-prestressed reinforcementsupports can involve major disruption in use of the buildiand may require rerouting of existing utilities, depending the type and details of the external reinforcing system.

5.8—Total demolition and replacementDemolition procedures for post-tensioned buildings

their entirety) are similar to those for non-prestressed ccrete buildings, but the collapse mechanisms are differThe tendons cause significant catenary action to devethis can be helpful or detrimental, depending on the produres and equipment used.15

Generally, it is neither necessary nor advisable to cut tdons before demolition begins, since the tendons canmade to assist in controlling the collapses. Selective remoof concrete from parts of slabs and beams is usually neeas is the weakening of walls, stair towers and columns. Pcautions are sometimes needed to prevent broken tenfrom exiting the edges of the structure and damaging acent property, but some studies have shown this to be anlikely event for single strand systems.10 The amount ofprojection will depend on many factors, such as the typesheathing, condition and quantity of grease, amount of draand so on. The potential for tendons exiting the structshould be evaluated for each project.

423.4R-18 MANUAL OF CONCRETE PRACTICE

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CHAPTER 6—SUMMARYDuring the past 40 years, unbonded tendons have bec

a significant form of reinforcement for concrete buildistructures. Tendon types have changed in response to dustry-wide recognition of the vulnerability of the earlitendons to corrosion damage. As the causes of tendon csion became known, specifications by PTI and ACI wchanged to implement requirements for the materials, facation, installation, and design details to protect the tendfrom deterioration. Adoption of requirements to improve durability of unbonded tendons began only 10 years when PTI published its first comprehensive specificatiand it took a few years for this document to be recognizethe design and construction community. Structures usingle strand unbonded tendons that had poor quality greinadequate grease filling, water in the sheath, and poorrosion protection details are more likely to suffer strand fures. Concern for the evaluation and repair of thunprotected and deteriorating structures was the primarypetus for preparing this document.

The most common type of unbonded tendon to be usebuildings during the past 30 years has a single 7-wire stwith cast anchors and two-piece wedges. Techniques fopairing these tendons are known by many contractorsengineers, as is the cost of doing this work. While theremany methods available to evaluate the condition of ccrete and bonded reinforcement in a post-tensioned sture, there is no method available to determine the amthat an unbonded tendon has been weakened by corros

Not every building can be economically repaired by placing all, or parts of, the tendons. Sometimes it is bedisregard the existing tendons and replace them with anternally applied system. This method is adaptable to mcircumstances and can be used for beams as well as for s

Total demolition is the last resort for a deteriorated strture of any type. The decision to demolish a post-tensiostructure should be made after a realistic assessmenbeen made of the condition of the concrete and the boreinforcement, and not just on an estimate of the conditiothe tendons.

CHAPTER 7—REFERENCES7.1—Recommended references

The documents of the various standards-producing onizations referred to in this document are listed with theirrial designation. Since some of these documents are refrequently, the user of this report should check for the mrecent version.

American Concrete Institute 201.1R Guide for Making a Condition

Survey of Concrete in Service201.2R Guide to Durable Concrete222R Corrosion of Metals in Concrete224R Control of Cracking in Concrete

Structures224.1R Causes, Evaluation and Repair o

me

in-

ro-ei-ns

o,ygse,r-

-e-

ornde-ndre-c-ntn. -tox-ybs. -das

edof

a--edst

Cracks in Concrete Structures318 Building Code Requirements for

Reinforced Concrete362 State-of-the Art Report on Parking

Structures362.1R Guide for the Design of

Durable Parking Structures364.1R Guide for Evaluation of

Concrete Structures Prior to 423.3 Recommendations for Concrete

Members Prestressedwith Unbonded Tendons

437 Strength Evaluation ofExisting Concrete Structures

546.1R Guide for Repair of Concrete BridgeSuperstructures

ASTMB 117 Standard Test Method of Salt Spra

(Fog) TestingC 597 Standard Test Method for Pulse

Velocity Through ConcreteC 876 Standard Test Method for Half-Cel

Potentials of Uncoated Reinforcing Steel in Concrete

D 4748 Standard Test Method for Determining the Thicknessof Bound Pavement Layers Using Short Pulse Radar

Post-Tensioning Institute Field Procedures ManualManual for Certification of Plants Producing Unbonde

Single Strand TendonsPost-Tensioning ManualPTI Committee for Development of the Field Procedur

Manual for Unbonded Single Strand Tendons Specifications for Unbonded Single Strand Tendons

Prestressed Concrete Institute PCI Post-Tensioning Manual (1st ed.)

Transportation Research BoardNCHRP Syntheses No.140 Durability of

Prestressed Concrete Highway Structures

NCHRB Report 313 Corrosion Protection of Prestressing Systems in Concrete Bridges

Other PublicationsAalami, B. O., and Barth, F. G. (1989). “Controlled Demolition of a

Unbonded Post-Tensioned Concrete Slab.” Post-Tensioning InstituPhoenix, Ariz.

Aalami, B. O., and Swanson, D. T. (Feb. 1988). “Innovative Rehabilittion of a Parking Structure,” Concrete International, V. 10, No. 2, 6 pp.

Andrew, A. E. (1982). Durability of Unbonded Tendons, FederationInternationale de la Precontrainte, England.

423.4R-19SINGLE STRAND TENDONS

c-

en-

of

h.el

he

on-

on-atory

s of

tion

ded

e forte

sion Pre-nd. of

en-

n ofnale

Cor-ited

amsne atton

on-

ning

en-

ield

rfor-

vice

t of-

ete-

adsern-

n ofaled'ersity

ic

or-

ed

.,

edisco,

rom

Mak-

ittle-

edriz.,

Crane, A. P. (1983). Corrosion of Reinforcement in Concrete Constrution, Ellis Horwood Limited, Chichester.

Elices, M. (1982). “Durability of Prestressing Steel,” A Report of theThird Symposium on Stress Corrosion Cracking, Federation Internationalede la Precontrainte, England.

Etienne, C. F. et al. (1981). “Corrosion Protection of Unbonded Tdons.” Heron, The Netherlands, CURVB.

Falconer, D. W., and Wilson, P. W. (Feb. 1988). “Inspection Unbonded Tendons,” Concrete International, V. 10, No. 2.

Gibala, R., and Heheman, R. F., ed. (1984). Hydrogen Embrittlementand Stress Corrosion Cracking, American Society for Metals, Metal Park,Ohio.

Greenhaus, S. (Nov. 1996). “Parking Lot Corrosion Cure,” Civil Engi-neering.

Halvorsen, G. T., and Burns, N. H., eds. (1989). Cracking in PrestressedConcrete Structures, American Concrete Institute, Farmington Hills, Mic

Kreijger, P. C., et al. (1977). Stress Corrosion in Prestressing Ste,Heron, The Netherlands.

McGuire, G. J. (1989). Unbonded Post-Tensioned Construction tCanadian Perspective, Post-Tensioning Institute, Phoenix, Ariz.

Novokshchenov,V. (Sept. 1989). “Condition Survey of Prestressed Ccrete Bridges,” Concrete International, V. 11, No. 9, 9 pp.

O'Neil, E. F. (Feb. 1977). “Durability and Behavior of Prestressed Ccrete Beams; Post-Tensioned Concrete Beam Investigation with LaborTests from June 1961 to September 1974,” Technical Report No. 6-570,Report 4, U.S. Army Engineer Waterways Experiment Station, CorpEngineers, Vicksburg, Miss. (NTIS AD A038 444).

Peterson, C. A. (Mar. 1980). “Survey of Parking Structure Deterioraand Distress,” Concrete International, V. 2, No. 3, 9 pp.

Poston, R. W. and Dalrymple, G. A. (Apr.-May 1993) “Repairing UnbonPost-Tensioned Concrete,” Concrete Repair Digest, V. 4, No. 2, 4 pp.

Precast/Prestressed Concrete Institute (Jul.-Aug. 1993), “Guidelinthe Use of Epoxy Coated Strand,” Precast/Prestressed Concrete InstituJournal, V. 38, No. 4.

Rehm, G. (1982). “Relationship between Results of Stress CorroTests and Practical Circumstances with Regard to the Sensitivity ofstressing Steels,” Federation Internationale de la Precontrainte, Engla

Schupack, M. (Oct. 1978). “A Survey of the Durability PerformancePost-Tensioning Tendons,” ACI JOURNAL, Proceedings V. 75, No. 10, 12 pp.

Schupack, M. (Feb. 1991). “Corrosion Protection for Unbonded Tdons,” Concrete International, V. 13, No. 2, 8 pp.

Schupack, M. (1983). “Prevention of Failures Related to CorrosioPost-Tensioning Tendon Systems in Concrete,” Federation Internatiode la Precontrainte, England.

Schupack, M., and Suarez, M. G. (Mar.-Apr. 1982). “Some Recent rosion Embrittlement Failures of Prestressing Systems in the UnStates,” PCI Journal.

Schupack, M. (Aug. 1980). “Behavior of 20 Post-tensioning Test BeSubject to up to 2200 Cycles of Freezing and Thawing in the Tidal ZoTreat Island, Maine,” SP-65, American Concrete Institute, FarmingHills, Mich.

Schupack, M. (Dec. 1982). “Protecting Post-tensioning Tendons In Ccrete Structures,” Civil Engineering-ASCE.

Schupack, M. (July 1988). “Unbonded Single Strand Post-TensioTendon Details: How to Avoid Performance Problems,” Concrete Con-struction.

Schupack, M. (Oct. 1989). “Unbonded Performance,” Civil Engineering.Schupack, M. (Oct. 1991). “Evaluating Buildings with Unbonded T

dons,” Concrete International.Schupack, M., and Schupack D. (Mar. 1992). “Non-Destructive F

Test for Concrete Leak Tightness,” Concrete International.Schupack, M. (Dec. 1994). “Unbonded Tendons—Evolution and Pe

mance,” Concrete International.Suarez, M. G., and Poston R. W. (1990). Evaluation of the Condition of

a Post-tensioned Concrete Parking Structure after 15 Years of Ser,Post-Tensioning Institute, Phoenix, Ariz.

Tanaksa, Y., et al. (1988). Ten Year Marine Atmosphere Exposure TesUnbonded Prestressed Concrete Prisms, Post-Tensioning Institute, Phoenix, Ariz.

Tracy, R.; Crower, S.; and Zeort, K. (June 1991). “Evaluation of a Driorated Post-Tensioned One-Way Slab,” Concrete International, V. 13,

No. 6, 6 pp.United States Department of Commerce, Bureau of Public Ro

(1955). “Criteria for Prestressed Concrete Bridges,” United States Govment Printing Office, Washington, D.C.

Vander Velde, H. (June 1994). “Corrosion Testing and RehabilitatioUnbonded Post-Tension Cables in 'Push-Through' or 'Heat SeSheaths,” Prepared for Seminar on Parking Garages, Technical Univof Nova Scotia, Toronto, Canada.

Wagh, V. P. (Oct. 1986). “Bridge-Beam Repair,” Concrete International,V. 8, No. 10, 8 pp.

Williams, M. S., and Waldron, P. (Nov.-Dec., 1989). “DynamResponse of Unbonded Prestressing Tendons Cut during Demolition,”ACIStructural Journal, 9 pp.

Referenced publications are available from the following ganizations:

American Concrete InstituteP.O. Box 9094Farmington Hills, MI 48333-9094

ASTM100 Bar Harbor DriveWest Conshohocken, PA 19428

Post-Tensioning Institute1717 West Northern AvenueSuite No. 218Phoenix, AZ 85021

Precast/Prestressed Concrete Institute175 West Jackson BoulevardChicago, IL 60604

Transportation Research BoardNational Research Council2010 Constitution Avenue, NWWashington DC 20418

7.2—Cited references1. Nehil, T. E., “Rehabilitating Parking Structures with Corrosion-Damag

Button-Headed Post-Tensioning Tendons,” Concrete International, Oct. 1991.2. Schupack, M., “Corrosion Protection for Unbonded Tendons,” Con-

crete International, Feb. 1991.3. “Specification for Unbonded Single Strand Tendons,” Journal of the

Prestressed Concrete Institute, Post-Tensioning Institute, Phoenix, ArizMar.-Apr. 1985.

4. Peterson, C., and Braunfield, A. H., “Our Experience with PrestressLift Slabs,” World Conference on Prestressed Concrete, San FrancCalif., July 29-Aug. 3, 1957.

5. “Summaries of Post-Tensioning Industry Tonnage Statistics f1965 through 1992." Post-Tensioning Institute, Phoenix, Ariz., 1992

6. Schupack, M., “Evaluating Buildings with Unbonded Tendons,” Con-crete International, Oct., 1991.

7. Lang, F. A., “Tendons for Prestressed Concrete and Process for ing Such Tendons,” U.S. Patent No. 3,646,748, Mar. 1972.

8. Schupack, M., and Suarez, M. G., “Some Recent Corrosion Embrment Failure of Prestressing Systems in the United States,” PCI Journal,Mar.-Apr. 1982.

9. Schupack, M., “Evaluating Buildings with Unbonded Tendons,” Con-crete International, Oct. 1991.

10. Aalami, B. O., and Barth, F. G., “Controlled Demolition of an UnbondPost-Tensioned Concrete Slab.” Post-Tensioning Institute, Phoenix, A1989.

423.4R-20 MANUAL OF CONCRETE PRACTICE

ed

e olazix,

rag

ls,

with Unbonded Tendons,” Concrete International, Mar. 1989.

12. Suarez, M. G., and Schupack, M., “Evaluation of the PerformancSingle Strand Unbonded Tendons in the Collapse of the L’Ambiance PLift Slab Building,” Post-Tensioning Institute Project Report, PhoenAriz., April 1988.

13. Nawy, E. G., “Stresses, Strains, and Bursting Cracks in Ancho

fa

e

Zones of Post-Tensioned Beams,” Cracking in Prestressed ConcreteStructures, SP-113, American Concrete Institute, Farmington HilMich., 1989.

14. Elliott, J. F., “Monitoring Prestressed Structures,” Civil Engineering, Jul.1996.

15. Chacos, G., “Demolishing a Post-Tensioned Parking Garage,” Con-crete International, V. 13, No. 10, Oct. 1991, pp. 44-46.