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STRUCTURE magazine July 20098

Structural Strengthening Using ExternalPost-Tensioning SystemsBy Tarek Alkhrdaji, Ph.D., P.E. and Jay Thomas

The primary reasons for the strength-ening of concrete structures are typicallyto increase existing elements capacity tocarry new loads or to resolve an existingdeficiency. Several strengthening techniquessuch as section enlargement, externallybonded fiber reinforced polymer (FRP) re-inforcement, supplemental steel elements,and post-tensioning can be employed toincrease the load carrying capacity andimprove serviceability of existing structures.However, there are many technical factorsthat should be considered when selectinga strengthening system. In addition totechnical concerns such as serviceability,strength, durability, appearance, and firerating, one should consider non-technicalfactors such as constructability, aestheticsand cost.Strengthening systems can be classified

into two categories:passive systemsand ac-tive systems. Passive strengthening systemsdo not introduce forces to the structureor its components. Passive systems, suchas steel reinforced concrete enlargementand the addition of structural steel ele-ments, contribute to load sharing and theoverall resistance of the member when itdeforms under external loads. As such,the effectiveness and load sharing of pas-sive systems significantly affects their axialand bending stiffness. Other examples ofpassive strengthening systems include theuse of bonded steel plates and externallybonded FRP.Active strengthening systems, such asexternal post-tensioning (PT), involvethe introduction of external forces tothe structural elements that would offsetpart or all of the effects of external loads.For example, new post-tensioned mono-strands can be profiled along the spanof an existing beam to produce upliftat mid-span that can be utilized to off-set existing strength deficiency or allow

the member to support additional load.Active systems are usually engaged in loadsharing immediately after installation,and can provide strength increase andinstantaneously improve the service per-formance such as by reducing tensilestresses (or cracking) and deflections.Strengthening with PT is particularlyeffective and economical for long-spanbeams and cantilevered members, andhas been employed with great success toincrease the bending and shear resistanceand correct excessive deflections.

This article presents a general de-scription and design considerationsfor the use of external post-tensionedsystem to restore or strengthen ex-isting concrete structural members.The use of this technique is furtherdemonstrated through a case studyfor two prestressed beams, support-ing the top level of a parking structure,that were repaired using external PT.

How Does It Work?Post-tensioning is the introduction of

external forces to the structural memberusing high strength cables, strands orbars. The PT reinforcement is connectedto the existing member at anchor points,typically located at the ends of the member,

and profiled along the span at strategi-cally located high and low points. Whenstressed, the tendons will produce upwardforces (at low points) or downward forces(at high points) to create reverse loadingon the member.PT strengthening systems can be clas-

sified into two categories, external andinternal. External PT involves exposedcables or steel bars that are anchoreddirectly to the structure. Because the rein-forcement is located outside the member,its use can be limited by fire rating and

durability requirements. To improve du-rability, systems consisting of sheathedcables and coated anchors can be used.

Alternately, the external cables can beplaced inside plastic ducts and then filled

with cementitious grout.An internal PT system, on the otherhand, consists of PT reinforcement thatis placed inside the original member. Itcan be achieved by placing the new PTreinforcement in cores made along the axisof the member, trenches chipped alongthe span of the member, or by placing

the reinforcement in a new concrenlargement bonded to the existmember. The latter is typically meffective where a relatively large increin strength and/or stiffness is requir

An internal PT system can also addresfire rating and durability concerns. Tfollowing sections will discuss the desconsideration of PT strengthening systplaced in a new concrete jacket.

What Do We Know AbouThe Structure?

As with any repair or strengthenproject, the first step in developing a solution is to establish a good understaing of the existing structure and condition through a structural contion assessment. Once the assessmis completed, analysis can be performand used to determine the existing residual) load capacity of various structelements. The capacity results are th

compared with new demands, and uto establish the type and magnitude of isting deficiencies for each member and required level of upgrade to resolve them

Its All About AnchorageTendons have an anchor at each end t

consists of a bearing plate to transfer tendon force to the concrete, as well astress the tendon. The end at which thereinforcement is stressed is referred to aslive end, while the other end is kno

as the dead end. Stressing of PT cableusually achieved using hydraulic jacks.The assembly of anchors from mult

cables, typically located at an end of member, is referred to as the anchor zoThe force in stressed tendons is transferfrom the anchors to the new conc

jacket through bearing, and from new concrete to the existing structumember through shear friction. The signer should provide the proper amoof mild steel reinforcement to addressbearing and bursting forces created in

Failed PT beam due to snow overload.

Completing surface preparation, steel/dowel placemand PT placement.

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anchor zones. Concrete surface profiling andsteel dowels should be designed and detailed totransfer horizontal shear forces at the interface ofthe existing concrete to the new concrete, there-fore produce the desired composite behavior.In general, most existing theories and guide-

lines for the detailing of anchor zones in newconstruction can be utilized for anchor zonesin concrete enlargements. However, soundengineering principles should be used when

modifying current standards to accommodatethe case of PT enlargement.

Key Considerations forPT Strengthening Design

There are several design, detailing, durability,and construction issues that need to be consid-ered when designing a PT strengthening system.Whether internal or external, the effectsof the new forces being introduced to thestructural element must be carefully evaluated.Anchor zones should be carefully designedand detailed to ensure effective transfer offorces from the post-tensioning system to thestructure without failure. A combination ofsurface preparation, concrete placement, andmechanical or adhesive bonded anchors canbe used to achieve the required horizontalshear transfer. The designer must evaluate theeffects of any existing reinforcement (mild and

prestressed) that may be interrupted or severedwhile installing the new strengthening system.Also, modification to existing member geom-etry and reinforcement must be accountedfor in the design of the strengthening system.

X-ray or ground penetration radar (GPR) arenon-destructive techniques that can be used tomap the existing reinforcement before finaliz-ing the layout of the PT system.Any existing damage, deterioration, or cor-

rosion of the existing member should alsobe addressed prior to the installation of thePT system. Most available repair guidelinesrequire that existing large cracks be epoxy-injected, steel corrosion treated, and concretespalls properly repaired to ensure that the newpost-tensioning force is uniformly distributedon the enlarged section of the member.

Strength orService Improvement?

There are three main factors that can influ-ence the final design of the PT system: increasein strength, service stress limits, and the abilityto transfer the PT force to the member. Whilein most cases the system is designed to achievea target strength increase, care should be takennot to overstress the member in use. For exam-ple, excessive uplift can produce high tensilestresses and can cause cracking, especially

when no live load is acting on the memIncreasing the amount of the PT force can make it difficult to transfer the force to member without using a complex anchorsystem. In these cases, the combination of and mild steel reinforcement can be usedachieve a balance between the desired strenincrease and service stresses, while reducthe PT force to an acceptable limit. Tdesigner generally will have to try differcombinations of PT force, profile, size of tion enlargement, and amount of mild steeachieve the final solution.

New post-tensioned enlargement on a bridge pier.

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Fire ConsiderationsPerformance of the strengthened element

under fire must be evaluated and the systemmust be detailed to provide the code-specifiedfire rating. Structures strengthened with PTto carry significantly higher loads may requiremore stringent fire rating. Compared withexternal PT systems, internal systems havethe inherent benefit of full encapsulation inconcrete. A proper fire rating can therefore

be easily achieved by selecting the appropriateconcrete cover thickness.

Form-and-Pump! Do Not Cast!Form-and-pump is a concrete placement

technique that consists of a multi-step processof surface preparation, formwork installation,and pumping of repair material into the cavityconfined by formwork and existing concretesurfaces. The forms are typically pressurizedto 10-12 psi which would force the newconcrete into the pores of the existing concretesurfaces, therefore enhancing the chemicaland mechanical bonds. Form pressurizationwill also consolidate the repair material andprovide full encapsulation of exposed and newreinforcement. Without full bond of the newconcrete jacket to the existing concrete, nocomposite action will be achieved and the newsection will not perform as designed. To achievethe right bond, the surface must be profiled toproduce -inch amplitude and the concretepores should be opened using sandblasting orpressure washing. The use of self-consolidatingconcrete (SCC) is preferred for this techniquesince the high flowability of SCC allows for

better concrete consolidation and flow intothe steel congested areas, especially the anchorzones. While constructability of the repairmaterials requires good pumpability and flow-ability, other characteristics such as low dryingshrinkage (which can cause cracking, delami-nation, and reduced durability and load transfer)should be considered.

Case StudyDuring the spring of 2007, external PT was

used to strengthen two damaged PT beams at

a government facility parking structure in theWashington D.C. area. The garage is a six-story,freestanding, post-tensioned structure withapproximately 500,000 square feet of park-ing area. The typical deck consists of one-wayPT slabs supported on long span PT beams.Failure of two beams was caused by the snowremoval contractor who piled a significantamount of snow on a small area of the roofdeck above the two beams. An approximateeight-foot pile of snow was accumulated ontop of the beams, which corresponds to ap-proximately 200 pounds per square foot. The

existing PT beams are 12 inches (305 mm)wide and 36.5 inches (927 mm) deep, and

support a 5.5-inch (138 mm) thick PT slab.The excessive damage of the beams made it

impossible to estimate their residual capacity.As such, initially, the design team was onlyconsidering replacement of the damaged beams.Full replacement created numerous challenges,as it would require de-tensioning of the slababove and installation of shoring on multiplelevels. This would severely disrupt the parkingsystem in the garage. Structural PreservationSystems, a repair contractor who completedsimilar repair projects, recommended a strength-ening option that consisted of external PT

placed in new concrete jacket that was sixinches (150 mm) wide on each side and fourinches (100 mm) thick on the bottom. Theproposed PT solution provided adequate repairat half the cost of the removal option, withminimal disruption to garage operations.The external PT design assumed that the

original reinforcement would no longer con-tribute to the strength of the beams. Theexternal PT system consisted of ten grade 270ksi, -inch, seven-wire low relaxation strandscoated with corrosion preventive grease andencased in a continuously extruded polyethylene

plastic sheathing. Longitudinal mild steel rein-forcement consisted of additional #7 top bars,#9 bottom bars, and #5 skin reinforcementthat extended the full span of the memberand installed with 90-degree hooks at the endsof bars.The existing concrete surface was roughened

to create a profile with -inch amplitude,and then cleaned using high-pressure waterblasting to produce porous, open substrate.Two rows of six #5 dowels were installed

immediately behind the anchors to withstandbursting forces. Four #5 dowels were installed

along the entire span at 18 inches on centeimprove the composite behavior of the jac

with the existing member. #4 stirrups wplaced in the concrete jacket at 18-inches center and doweled to the underside of the susing epoxy adhesive. Additional dowels astirrups were installed at the cables deviatpoints to allow for proper confinement force transfer.The formwork was designed for a minim

of 14 psi (100 kPa) and detailed to fit tighagainst existing concrete surfaces. Aftersteel dowels, mild steel and PT reinforcem

were installed, the beams were formed aself consolidating concrete (SCC) pumpinto the forms. When the concrete strenreached 3,500 psi, the tendons were stresby means of hydraulic jack to approxima80% of their ultimate strength. All gapressures and cable elongations were recorand reviewed for final approval.The newly repaired beams appeared v

similar to the original beams, with concrenlargements that have the look of an origicast-in-place concrete element.

ConclusionStrengthening of existing structures invo

one of the more complex design and constrtion processes. Strengthening work is typicmore challenging than new construction wdue to complex logistics and the extensive ordination required to deal with occupied, sometimes fully functional, structures. Exterpost-tensioning systems provide many beneincluding improved strength, improved ser

performance, and improved durability dueprecompression of the concrete encasementThe importance of addressing all des

issues, and proper detailing of PT strengening systems, cannot be overemphasizInadequate detailing has led to past failurePT strengthening systems. As such, it is generrecommended that the design and detailingpost-tensioning strengthening systems be signed by experienced engineers with a foon structural strengthening and use of ptensioning for structures.

Tarek Alkhrdaji, Ph.D., P.E. is anEngineering Manager with StructuralGroup. He is involved with the structuralupgrade and rehabilitation of numerousconcrete structures using conventionalmethods, advanced FRP composites and fulscale load testing.

Jay Thomas is a Vice President for StructuraPreservation Systems. He is a regular speakeat the World of Concrete.

Form-and-pump formwork set for SCC pumping.