About the AuthorMichael ButtressVice President of Operations

Michael Buttress, has beenwith Quality Built since itsreorganization in 2010. Inaddition to managingoperations, Michael has alsomanaged all MassInspection Programs atQuality Built.

Michael's constructionmanagement backgroundwith both large scalecommercial projects as wellas residential projects giveshim a unique perspective onthe inspection business. Michael is a graduate of theUniversity of Florida andholds a degree in BuildingConstruction.

You can reach Michael at:mbuttress@qualitybuilt.com

Quality Built® Tech AlertJuly 2015

Volume 2, Issue 9

Fully Encapsulated PostTension SystemsThe saying goes that a chain is only as strong as itsweakest link. This is particularly true for post tensioncable systems. The use of post tension cable systemshas become increasingly popular for meeting manydesign needs; creating solid, stable concretestructures that exceed the capabilities ofconventionally reinforced concrete. Other advantagesand benefits that a post tensioned concrete frameprovide are: a faster construction cycle, minimizedinitial and life-cycle costs and it’s favorablesustainability ratings. However, Post tension cablesystems are more susceptible to failures if anycomponents of the system are not installed properly.

Post tension cable systems are made of 5 maincomponents:

1. A braided tendon that is usually 7 wires woundtogether to make a single cable.

2. A grease-filled plastic sheathing that protects

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the tendon from corrosion or damage.3. Chairs that support the tendon and ensure they

have the appropriate profile.4. Dead-end anchors, which anchor the end of a

tendon that is fixed.5. Live-end (AKA Stressing End) anchors, which

anchor the end of a tendon where the stressingmachine attaches.

Fully encapsulated systems are intended to preventthe metal components from corroding over time;corrosion could lead to failure. The cable ends andanchors are considered the most difficult part tocorrectly install and fully encapsulate. In order to fullyencapsulate a tendon the cable end must be cutprecisely so that a grease filled cap can securely fitover the trimmed cable and attach to the anchor body.When installed correctly this anchor is considered fullyencapsulated, water tight and secure.

Depending on the installation environment, andespecially for elevated structures, the properinstallation of live-end anchors can be difficult, whichhas proven to be the leading source of problems inthese systems. Once stressed these systems areplaced under a lot of force and can be dangerous if afailure occurs. If an anchor fails due to corrosion orimproper installation, the result can be that thestructure no longer has the strength as designed, andin some occurrences cables have been known toshoot out the side of a structure like a rubber bandthat was suddenly released. Other indications ofpotential failure can also be observed from corrosion

staining that is showing through the cladding of abuilding from an anchor end, or grout pockets that aretelegraphing through the cladding system.

Once a concrete slab has been poured and properlycured the post tension cables must be stressed inorder to give the structure its designed strength. Thetension in the cable places portions of the structure incompression and adds tensile strength to the concretewhich is favorable for creating strength. The stressingof a cable is typically done with a hand-held hydraulicstressing machine that fits over the cable end and sitsin the pocket created by the live-end anchor. Thecable is stressed to the specified tension, whichleaves an excess length of cable sticking out the sideof the structure, which will have to be cut. In order fora fully encapsulated system to work correctly thecable end must be precisely cut. In accordance withthe Post-Tensioning Institute (PTI) Standards thecable end must be cut so there is between 1/2 inchand 3/4 inch in length for the grease cap to fit properlyand maintain the minimum allowable tail length.

This is accomplished one of several ways. The cableend can be cut using an acetylene torch, a plasmacutter, a grinder, a saw or can be cut usingmechanical shears. The torch has been the tool ofchoice for cutting cables historically, because of itslow cost and ease of use. Unfortunately the torch isalso the least accurate tool making it difficult to cutwith the necessary precision. Mechanical shears are anice option because when used properly they

consistently produce the same length of the tail. Thisis preferable to make sure you get a good grease capinstallation each time. These devices are often timesbulky and heavier than alternatives, require regularmaintenance and require a significant initialinvestment, but are a good option for many projects.Depending on the size of the structure, cutting cablesmay be done thousands of times, so it must be donequickly and efficiently. In a slab-on-grade scenario, itis easier to physically maneuver and trim the cableends, whereas on elevated decks there are moresafety measures that must be considered whenworking on the end of a slab. Either way, thechallenge is the same, the cable end must betrimmed perfectly.

Once trimmed the grease cap must be fitted andsecured. If the grease cap is not properly seated thereis a good chance the system will not be fullyencapsulated, so it is critical to verify that each cap isattached properly. Finally once the grease cap isinstalled the anchor pocket must be fully grouted topack the hole, protect the cable end, and provide asmooth surface for the cladding. Grouting is done byhand with a non-shrink grout that can be packed intothe pocket and cured in place. It is critical that thepocket be properly cleaned of any debris or residueso that the grout can properly bond to the surroundingsurface. Similar to the cable cutting process, this can

be particularly challenging on elevated decks sincethere are additional safety considerations.

Many projects use the Hayes Industries Posi-Lock®Plus anchor system, which is a fully encapsulatedpost tension anchor. This is a very widely usedsystem, which provides good corrosion resistancewhen properly installed and fully encapsulated. Ultimately the precision and accuracy of trimming thecable, placing the grease cap and packing the grouton each and every cable anchor will determine if youhave a weak link in your post tension cable system. Ifcare is taken and the installation is done properly anddocumented, the chances of problems in the futureare significantly reduced. We recommend that QAinspections occur on a statistical sampling of all posttension cables at times where the cuts can beobserved as well as once the grease caps have beeninstalled.

About Quality Built Quality Built, headquartered in Fort Lauderdale,Florida, is a leading national construction qualityassurance and inspection management company.Quality Built provides third-party quality assuranceservices and a full spectrum of quality and riskmanagement solutions such as property conditionassessments, tainted drywall assessments, buildingevaluations, data collection tools, collateral inspectionservices, reporting and support services on high-quality residential and commercial constructionprojects nationwide. Quality Built is well known for its work in Total Quality

Management and was one of first firms to transitionfrom using a traditional quality assurance approachinto implementing a proven, user-friendly and fullyautomated, online inspection system. Quality Built'sproprietary software is cost efficient, paperless, fullycustomizable and completely scalable to handle thedemands of most inspection and quality assuranceprotocols. Quality Built's Quality Management System is ISO9001:2008 registered

For more information about Quality Built and itsservices, contact Beth R. Michaelis, President, atbmichaelis@qualitybuilt.com.

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