ACI AND PTI REQUIREMENTS FOR …

1 July 2007 | PTIJOURNAL|494949 ACI AND PTI REQUIREMENTS FOR encapsulation SYSTEMSLARRY B. KRAUSEROPEN INTRODUCTIONThis paper will assist the reader in understanding the crit-ical aspects of current American Concrete Institute (ACI)and Post-Tensioning Institute (PTI) documents as theyrelate to unbonded post-tensioning encapsulation ACI document is titled: Specification for UnbondedSingle-Strand Tendons and Commentary. The PTI docu-ment is similar but does not have and Commentary in itstitle even though both documents contain similar com-mentary. There is a significant amount of correlationbetween the ACI Committee 423 and PTI Committee forUnbonded Tendons documents; primarily due to the PTIspecification being the starting point of the ACI specifica-tion. The first edition of the PTI document was publishedin 1993 and the second edition in 2000 whilst the ACI doc-ument was officially published in October both documents are living documents, the author willinterject his recommendations for future changes andimprovements throughout this paper.

2 A living documentis a document that is refined and improved over timethrough various updating procedures. Both documentsoffer areas for enhancement in relation to encapsulationsystems that the author will identify. As new techniques forcomponent manufacture are developed and codes requirehigher quality systems, continual improvement of encapsu-lation systems will be market paper will quote the REQUIREMENTS of ACI and noteany major differences with PTI. It will follow the docu-ments from beginning to end. When quoting the docu-ments the source will be DOCUMENT DEFINITIONSF irstly, encapsulation systems are required for aggressiveenvironments as noted in ACI Scope (and PTI ):The more restrictive material, fabrication, and construc-tion REQUIREMENTS for tendons used in aggressive environ-ments are essential to the long-term durability of tendonsused in such definition of an aggressive environment is given inACI Scope (and PTI ):Structures exposed to aggressive environments include allstructures subjected to direct or indirect applications ofdeicing chemicals, seawater, brackish water, or spray fromthese sources; structures in the immediate vicinity of sea-coasts exposed to salt-laden air; and structures whereanchorage areas are in direct contact with soil.

3 Stressingpockets that are not maintained in a normally dry condi-tion after construction should also be considered exposedto an aggressive environment. Nearly all enclosed buildings(office buildings, apartment buildings, warehouses, manu-facturing facilities) are considered to be non-aggressiveenvironments. The engineer should decide if the structure,or a part of the structure, is exposed to an aggressive envi-ronment. Attention should be paid to such areas as thelocation of stressing-end and intermediate anchors, con-struction joints, locations of planters, balconies and swim-ming similar definition is given in ACI Definitions (andPTI ):Aggressive environment An environment in whichstructures are exposed to direct or indirect applications ofdeicing chemicals, seawater, brackish water, or spray fromthese water sources; and salt-laden air as occurs in thevicinity of seacoasts. Aggressive environments also includestructures where stressing pockets are wetted or are direct-ly in contact with present the ACI document does not apply to tendonsused in ground supported post-tensioned slabs for lightresidential construction.

4 The PTI document in theNovember 2003 Addenda removed this wording from theScope but added similar wording to PTI Journal, V. 5, No. 1, July 2007. Open Forum: the comments and opin-ions expressed herein are those of the contributing author and do not nec-essarily reflect official PTI policy. Please submit your comments and dis-cussions no later than for publication in the January 2008 issue of the | PTIJOURNAL | July 200750 Anchorages. The author believes that both ACI and PTIshould revisit the above wording of aggressive environ-ments as they relate to ground supported post-tensionedslabs for light residential construction . It does not makesense that the post-tensioning in buildings whose anchor-age areas are in direct contact with soil would be in moredanger than that of light residential construction. It is theauthor s opinion that the post-tensioning tendon does notknow whether it is in a residential foundation or multi-level parking structure and will perform the same, so whyshould there be a difference?

5 The author recommends thisdistinction be is a sentence in the commentary of ACI Scope(and PTI ) that the author feels should be revisited: Nearly all enclosed buildings (office buildings, apartmentbuildings, warehouses, manufacturing facilities) are con-sidered to be non-aggressive environments. This seemslike a reasonable statement as many engineers feel that theconstruction phase of a project is short enough that finaldurability criteria are not necessary until project comple-tion provides protection; however, the author notes that insome cases encapsulation of the post-tensioning tendons inthese structures should be considered, such as in locationswhere the tendons may be affected during construction by salt-laden air and any other source of corrosion (rain?)prior to enclosure. Once elements of corrosion have beenintroduced into the post-tensioning system they , the author believes that intermediate anchoragesat construction joints should be encapsulated when encap-sulation systems are specified for stressing-end and fixed-end anchorages even in enclosed definition of an encapsulated tendon is given in Definitions (and PTI ):Encapsulated tendon A tendon that is completelyenclosed in a watertight covering from end to end, includ-ing a protective cap over the tendon tail at each PTI document furthers this with The encapsulationshall be free of voids as possible and shall inhibit corrosionof all tendon elements.

6 The PTI document adds under thecommentary side that Some small bubbles and air spacesare normal and unavoidable in the fabrication and assem-bly process and should not be considered as voids in thecontext of this definition. Similar wording is contained inACI What PTI (and ACI) says is the encapsula-tion system shall be as free of voids as possible but it notesthat there may be small air bubbles. However, neither PTInor ACI has quantified what a small air bubble is in theirrespective documents. The author suggests quantifying small air bubbles as the total volume of all air bubbleswithin the design of the system transition componentsshall not be greater than cubic inch. This would allowfor a certain amount of tolerance within the system forassembly. The author notes that if the design of the transi-tion components (sleeves and seals) creates more airspacethan cubic inch, then per ACI and PTI ,post-tensioning coating (grease) may be used to fill thevoid within the transition components.

7 Small air bubblescreated by the extrusion process will vary by manufacturer,type and speed of equipment, and are virtually impossibleto measure; therefore they cannot be reasonably COMPONENT REQUIREMENTSR equirements for components of anchorages in aggressiveenvironments can be found in ACI (and almostidentical wording in PTI ):Anchorages intended for use in aggressive environmentsshall be protected against corrosion. The design shallrequire a watertight connection of the sheathing to theanchorage and a watertight closing of the wedge cavity andprestressing steel in such a way as to achieve corrosion pro-tection of the anchorage, wedges, and prestressing steel atthe fixed-end, intermediate anchorage, and shall be designed to attain watertight encapsula-tion of prestressing steel and all connections shall havedemonstrated the ability to remain watertight whenarranged in a horizontal position and subject to a uniformhydrostatic pressure of psi ( kPa) for a period of 24 h.

8 Injection molded plastic is typically used to protect theanchor castings; however, the commentary allows epoxy-coated anchors as long as additional inspections are per-formed and damage to the epoxy coating is repaired. Bareanchor castings are not allowed. The author suggests theelimination of epoxy-coated anchors because the epoxycoating is rather brittle and in a job site situation the dam-age goes unnoticed most of the time. Another option maybe to require a thickness for the epoxy coating of 50 milswhich would give the epoxy coating similar durabilitycharacteristics to that of injection molded requirement for testing in a horizontal position doesnot seem essential to the author as long as the minimumhydrostatic pressure is maintained. The author suggestsremoving this requirement and substituting: .. ability toremain watertight when subjected to a minimum uniformhydrostatic pressure of .. The same hydrostatic testing REQUIREMENTS are contained inboth the ACI and PTI documents.

9 The test method wasdeveloped in the late 1980 s and was a great start; however,the author believes these test REQUIREMENTS are obsolete andshould be enhanced. The reason for this is: The manufacturer is allowed to select and assemblesamples. The manufacturer will select the best sam-ples not necessarily those that will be consistentlygoing to the job sites. The manufacturer is intimate-ly familiar with his system and can assemble it pre-cisely this is not necessarily a field condition which is what should be tested. Having the inde-pendent testing agency select and assemble compo-nents would be more representative of how the sys-tem will 2007 | PTIJOURNAL|5151 There are no intervals identified for injection molds wear out and change overtime. Some type of interval should be identified. The test method for detecting the presence of mois-ture can actually hide moisture infiltration. Byadding white pigment to the grease any moistureinfiltration is hidden because emulsification of thegrease cannot be seen (emulsification gives thegrease a milky color white).

10 The colored dye usedin the water is not strong enough to be seen contrast-ing with the emulsified grease. The test method encourages adding grease into thesystem. The test method should test the componentsfor watertightness without grease to cover up poorconnections. There are no test REQUIREMENTS for field conditionsafter cutting of the tendon tails to make sure that thecap will actually perform correctly. Burning of ten-don tails may adversely effect how the cap locks intothe wedge advancements in testing and manufacturing of plasticencapsulation components and to improve system quality,enhanced testing should be required. The author recom-mends that the following be incorporated into new testingrequirements: Hydrostatic testing shall be performed annually byan independent testing agency. Representative samples should be selected andassembled by the independent testing agency fol-lowing assembly instructions provided by themanufacturer.