PTI JournalCase StudiesREPAIR AND REPLACEMENT OF PARTIALLYGROUTED EMBEDDED EXTERNAL TENDONSByHAYAT TAZIR, HUSAM NAJM, AND DAVID GRIFFITHAuthorized reprint from: August 2015 issue of the PTI JournalCopyrighted 2015, Post-Tensioning InstituteAll rights reserved.
CASE STUDIESREPAIR AND REPLACEMENT OF PARTIALLYGROUTED EMBEDDED EXTERNAL TENDONSBY HAYAT TAZIR, HUSAM NAJM, AND DAVID GRIFFITHThis paper discusses the removal and replacement of twopartially grouted embedded external tendons in a two-span220 ft (67 m) long viaduct. Embedded external tendons wereused for cast-in-place viaducts for the Terminal Area RoadwaysProject (TAR) at Boston Logan Airport in Massachusetts.The project comprised more than 200,000 ft2 (18,580 m2) ofviaduct. More than 70% of the viaducts were designed usingexternal tendons partially embedded in the bottom slab ofthe box girder near midspan. The remaining viaducts hadinternal tendons inside the webs due to their tight curvature.The quality and integrity of grout is of prime importancefor the structural integrity and long-term durability of posttensioned concrete structures. Grout is one of several layersof protection against corrosion. It prevents corrosion of thestrands by completely encasing them, it prevents water fromcollecting and freezing in the ducts by eliminating all voids,and it provides effective bond between prestressing steel andconcrete. Improperly grouted tendons do not meet specifications or performance requirements and must be repaired orreplaced. This paper discussed repair alternatives andsuccessful replacement of two partially embedded externaltendons that were improperly grouted. These tendons will bereferred to as “partially grouted” hereafter.KEYWORDSbent pipes; deviation saddles; polyethylene ducts;repair, strands; tendon replacement; unbonded tendons.INTRODUCTIONEarly use of external prestressing dates back to the late1920s when external tendons were first used in prestressedconcrete bridges in Germany.1,2 By the late 1960s, externalprestressing was applied in some bridges in Belgium,France, and England. However, because of insufficientcorrosion protection of these tendons in these early applications, many of the external tendons corroded.There are multiple levels of protection for posttensioned tendons. According to FHWA,3 six possibleprotection levels can be provided depending on the posttensioning (PT) system used and the surrounding environment. These include exterior protection to concretemember, concrete cover, PT duct, grout, sheathing/coating of strands, and corrosion resistance of the strandsthemselves. According to the FHWA recommendations,3 agood practice requires at least three of these levels be properly applied from anchorage to anchorage.Grout is a mixture of cementitious materials and waterwith or without mineral additives, admixtures, or fineaggregate, proportioned to produce a pumpable consistency without segregation of the constituents, injectedinto the duct to fill the space around the tendons.4,5 Theuse of grouted PT tendons as a viable construction technique began in the 1950s. Since then, many post-tensionedsystems have been used and grout and grouting techniques have improved. During the 1990s and early 2000s,prepackaged grout gained popularity in the United States.The grout material, testing, design, and application werefirst specified in 2001 in the first edition of PTI M55.1,“Specification for Grouting of Post-Tensioned Structures.”4Those specifications helped improve grouting practices.The quality and integrity of grout is of prime importance for the durability of PT concrete structures. Groutprevents corrosion of the strands by completely encasingthem, prevents water from collecting and freezing in theducts by eliminating all voids, provides effective bondbetween prestressing steel and concrete, and completesthe concrete cross section. Potential problems in groutmay arise from the quality of grout, application methods,joints and sealants in PT ducts, and vibration at early age.6Another factor that may contribute to inadequate strandprotection is the draping of tendons. If bleeding occurs inthe grout, voids may appear at the tendon high points.PTI JOURNAL August 2015 35
CASE STUDIESAccording to the specifications for grouting of posttensioned structures published by PTI,7 the followingfactors can influence the quality of grout: 1) cementhydration rate, which affects working time and set time;2) grout fluidity as a function of time and temperature;3) volume control; 4) permeability; 5) strength; 6) bleedstability characteristics; 7) level of corrosion protectionrequired; and 8) segregation of materials during mixingand placement. There are several ASTM specifications thatcontrol these factors, such as ASTM C953,8 which specifies initial minimum and maximum set time at 3 hours and6 hours, respectively, and ASTM C1202,9 which specifiesmaximum permeability at 2500 coulomb. In addition, PTIrequires certification for grouting and PT personnel in thefield. The Virginia Transportation Center for Innovationand Research (VTIR) has made several recommendations10 concerning grouting operations. These includedesign recommendations as well construction and materialrecommendations. VTIR10 recommends mockup tests formajor PT projects and, for the most critical tendon locations, to identify potential grouting problems prior togrouting operations.Among the other issues addressed in the third editionof PTI M55.1-127 are the chloride content and segregation of grout. Some of the practices that were allowed inthe 2001 specification4 were tried to remedy the groutingproblems in this project. These practices are no longeracceptable in the 2012 PTI specifications.7 The 2012edition does not allow flushing of PT ducts to clean theducts from debris or dust prior to grouting operations. Italso does not allow flushing of ducts to remove grout incase of problems in grout materials or grouting operations.The 2012 specification7 also recognized the impact of workerqualifications and skills on grouting quality and operations.The 2012 specification7 requires grouting operations beperformed and supervised by qualified personnel. PTIM50.3-1211 requires the supervisory personnel of posttensioning operations and the foreman of each installationand stressing crew to be certified as PTI Level 2 BondedPT Field Specialist; and the foreman of each grouting crewto be certified as PTI Level 2 Bonded PT Field Specialistand ASBI Certified Grouting Technician. Also, at least25% of each crew is to be certified in PTI Level 1 BondedPT – Field Installation.This paper discusses repair alternatives and replacementof two deficient partially grouted embedded external tendons.36 August 2015 PTI JOURNALGROUTING OPERATIONSGrouting operations were specified in the TAR projectspecifications and special provisions.12 These specifications primarily followed the 2001 PTI specification4 andthe Central Artery/Tunnel Project Specifications,13 whichwere used for many cast-in-place (CIP) and segmentalviaducts in Boston between 1990 and 2000.PT ducts should be sealed from debris and intrusionsprior to PT operations and grouting should start as soon asPT of the strands is complete. Before starting the groutingoperations, the tendon ducts as well as all inlets and outletsmust be checked for obstruction. This is typically donewith oil-free compressed air. The ducts on this projectwere flushed with water to remove corrosion protectioncompounds or to clear debris and blockages consistentwith the prevailing. This practice is no longer allowed byPTI.7 In this case, the flush water should meet the samerequirements of water used in the grout and any waterleft in the duct must be blown out with compressed air.When blowing out the water, any already-installed groutcaps must be removed. Ducts, particularly those of thinmetal, are often rendered non-tight by corrosion in transit,by tearing in handling, or when placing adjoining reinforcing steel. Duct joints may accidentally be pulled apart.Ducts may be inadvertently compromised by drilling holesfor form ties or by rough use of internal vibrator. Suchdefects cause the grout to leak, resulting in unacceptablevoids in the grout. All leaks must be filled to ensure propergrouting. Ducts may be sealed or repaired by several wraps ofwaterproof tape or even, more positively, by heat-shrinksleeve. When holes or gaps are larger sleeve than 1/4 in.(6 mm), they should be sealed by a metal strip taped inplace over the hole.When a blockage occurs during grouting, every effortmust be made to ensure that the tendon duct will be groutedproperly. A blocked duct should not be grouted from theother end because air or water would be trapped inside andthe corrosion protection for the prestressing steel couldno longer be guaranteed. If the tendon ducts cannot beproperly grouted, the injected grout should be immediately flushed out with water from the opposite end untilclear flush water emerges from the grouting point. Oncethe tendon duct has been flushed clear and compressedair blown through, grouting operations are repeated witha fresh mixture. Standby water-flushing equipment with apressure capability of 200 to 300 psi (1.4 to 2.1 MPa),powered by a separate power source, should be availableduring all tendon grouting.
CASE STUDIESThe project specifications12 state that: the exposureinterval between strand installation and grouting shall belimited to 20 days for moderate atmospheric conditions(humidity between 40 and 70%), 40 days for very dry conditions (humidity less than 40%), and 10 days for humid conditions (humidity more than 70%), unless temporary corrosionprotection measures are taken.DESCRIPTION OF PROJECTThe Terminal Area Roadways (TAR) project12,14 iscomprised of a series of viaducts connecting the terminalsat Boston Logan Airport to the Sumner Tunnel to the cityof Boston, to Route 1A north, and to the Ted WilliamsTunnel and I-90. The project consists of approximately200,000 ft2 (18,500 m2) of post-tensioned cast-in-placebox-girder viaducts with span lengths varying from 90 to160 ft (27.5 to 49 m). The substructure consists of singlecolumn piers and straddle bents supported on drilled shafts.The column diameter varied from 5.5 to 7 ft (1.68 to 2.13 m).The foundations are 7 and 8 ft (2.13 and 2.44 m) diametersingle-drilled shafts stepped at approximately 80 ft (24.5 m)below ground level. The superstructure is made of 6.5 ft(2.0 m) single and multiple-cell box girders. The roadwaywidth is variable and ranges from 24 to 70 ft (7.3 to 21.5m). The design of the cast-in-place box girders includedboth external and internal tendons. The external tendonswere embedded in the bottom slab near midspan. Theconstraints imposed by the project site and the construction staging requirements required the design to considerseveral stages of construction, which increased the duration of construction.GROUTING OF TENDONS T1 AND T2 IN SPANSEE11 AND EE12Spans EE11 and EE12 are two equal spans each119 ft (36.3 m) long simply supported at Bents EE12 andEE10 and continuous over Bent EE11, as shown in Fig. 1.The two-span structure is a three-cell box girder 6.5 ft(2.0 m) deep and 56 ft (17.1 m) wide. The box girder ispost-tensioned with 12 tendons (T1 through T12), eachhaving twenty-seven 0.5 in. (13 mm) diameter strandswhose profile is shown in the elevation in Fig. 1. Each spanhas two deviation saddles (Fig. 2) and the tendons arepartially embedded in the bottom slab. The anchorage locations at each end were 4.5 ft (1.4 m) from the soffit of the6.5 ft (2.0 m) two-cell box girder. At midspan, the tendonswere located 6 in. (152 mm) from the soffit of the box andthe intermediate support (Bent EE11); the high point waslocated 5.75 ft (1.8 m) from the soffit. The connectiondetails for the ducts at the deviation saddle locations areshown in Fig. 3. Strands were installed in Tendons T1 andT2 and the two tendons were stressed following the installation and stressing procedures specified on the contractdrawings and in the project specifications. While groutingtendon T1, the Tendon developed multiple leaks. Leakswere identified at the rubber coupling connection betweenthe galvanized deviation pipes and the high-density polyethylene (HDPE) exposed ducts. Leaks also occurred at aspalled location in the bottom slab. Once the leaks becameevident, the grouting operations stopped and groutingremediation was attempted.The tendon was not flushed while the repairs werebeing made. As it became clear that the repairs would notwork, the tendon was flushed so that grouting operationscould be resumed the next day. The grout was flushed fromanchorage location EE12 through Bent EE11 and fromanchorage location EE10 to Deviation Saddle DV4. Thesection from Bent EE11 to Deviation Saddle DV4 couldnot be flushed because grout had already set at the timeflushing was attempted.Fig. 1—External tendon layout for Tendons T1 and T2 in SpansEE11 and EE12.Fig. 2—Deviation saddle and bottom slab reinforcement.PTI JOURNAL August 2015 37
CASE STUDIESTendon T2 also developed leaks during the groutingprocess. The grout was flushed from Anchorage EE10 toDeviation Saddle DV4 and from Anchorage EE12 to Deviation saddle DV3. The section of duct embedded in theconcrete slab between Deviation Saddles DV4 and DV3was flushed with good water flow but it was uncertain ifthe grout was completely removed.Fig. 3—Duct details at deviation saddles.PROPOSED REMEDIAL ACTION FOR PARTIALLYGROUTED TENDONSIt was initially proposed that for Tendon T1, theconnection between the HDPE duct and the galvanizedpipes on each side of Deviation Saddles DV4 and DV3 andthe Span EE11 side of Bent EE12 be opened and visuallyinspected to confirm the completeness of the groutingof this section. If no voids were to be found, the connection would be closed and an additional grout vent wouldbe installed on the EE10 side of Deviation Saddle DV4.This initial remediation proposal also required that theportion of the duct in the concrete slab between DeviationSaddles DV3 and DV4 be chipped away in small locationsto verify that complete grouting was achieved; however,this may not be necessary if the galvanized pipes are filledwith grout at each end of the bottom slab. If voids werefound between Deviation Saddles DV3 and DV4, then thelocations of the end of the void would be determined andthe concrete would be chipped to expose the tendon near theend of the void. An additional vent would be installed at thislocation, and the excavated area would be sealed and patched.For the two partially grouted tendons (TendonsT1 and T2), a majority of the length of the strands hadbeen exposed without corrosion protection for periodsexceeding the project-specified limits. In addition, theremay have been standing water in the duct left behind afterit was partially flushed out when grouting operations werestopped. As a result, it was likely that the steel strands inthese ducts had developed an unacceptable level of corrosion. A region of Tendon T1 was found not completelygrouted at Location C near Saddle DV4 in Fig. 4 (also referto Fig. 1), so it was possible that other voids existed in thegrout as well. There is no satisfactory method of verifyingthat a tendon is completely grouted without performingsome destructive examination. However, because of theprobability of existing corrosion of the steel strands, at thispoint, there seemed to be no acceptable remedial actionrepair but to remove and replace the two tendons.REPAIR ALTERNATIVESFig. 4—Repair locations in elevation and plan in thickened slab inSpan EE11.38 August 2015 PTI JOURNALRemoving and replacing partially grouted tendonshas been successfully completed on other projects.15 Theprocedure requires removal of the HDPE ducts; clampingthe strands; chipping the grout; cutting the strands oneby one, detensioning and removing the strands; andremoving the wedge plates. Sudden detensioning of a largeforce should be avoided for safety reasons; it may causedamage in the deviation saddles and trigger unbalanced
CASE STUDIESloading that could compromise the integrity of the structure. Extra care should be taken to avoid sudden releaseof large forces and individual strands should be removedusing monostrand jack by breaking the bond betweenthe strand and the grout. This may require cycling detensioning and tensioning of the strands to avoid breakingof the strand before breaking of the bond with the grout.After all strands have been removed, the remaining groutmust be removed from the duct. The ducts were cleanedat the deviation saddles. The tendon was then re-installedusing new strands and wedges. The tendons should notbe removed by jack-hammering the concrete elements inwhich the tendons are encased. Several methods for thegradual release of the prestressing force (slackening) inpartially grouted Tendons T1 and T2 in this project wereconsidered.12 The following methods were evaluated takinginto account time considerations, safety, cost, and specificproject requirements:1. Heating of tendons to release as much as possibleof the prestressing force to avoid sudden release offorce when the strands are cut. This method requiresremoving the PE duct and as much grout as possiblearound the tendon. The tendon is then heated alongits entire length to release as much PT force as practical. However, although this method will save time,it requires significant heat to release the tendonforce and the heating will not be uniform alongthe tendon. Also, to heat the portion of the tendonembedded in the bottom slab, the top concrete coverover that portion of the tendon would have to beremoved. The bottom portion of the tendon wouldstill not be fully exposed, as the heat would only beon the top of the tendon. In addition, inducing suchhigh temperatures inside a confined space of a boxgirder may be hazardous and unsafe.2. Removal of concrete around anchorages, andburning wedges to remove tendons. Although thismethod could have saved time, it was thought to beunsafe.3. Controlled release of prestressing force (slackeningof strands) and removal of individual strands afterchipping as much grout as possible. This method,although time-consuming, involved less risk thanthe other methods, used successfully on otherprojects15 and was proposed for the removal ofpartially grouted tendons T1 and T2; it is describedin detail in the tendon removal section that follows.REMOVAL AND REPLACEMENT OF TENDONST1 AND T2Removal of partially grouted Tendons T1 and T2 inSpans EE11 and EE12 by slackening the strand alternativelyon each side of the deviation saddle was recommended.This procedure is similar to the one used for the removalof corroded external tendons of the Mid-Bay Bridge inFlorida in 2001.15 The difference between the tendons inthe Mid-Bay Bridge and those in Spans EE11 and EE12of this project was the embedded portion of the tendonsin the bottom slab near midspan. This portion was approximately 45 ft (13.7 m) long in each span between deviationsaddles and it was difficult to remove it.14 The removal andreplacement of this portion was time consuming. Portionsof the slab had to be chipped away at various locations toallow for the tendon removal and the contractor was ableto do that without removing any portions of the deviationsaddles or causing damage in the saddle. In Span EE11,approximately 30% of the of the bottom slab had to bechipped away to allow for the removal of Tendon T1 and8% for the removal of Tendon T2, as shown in Fig. 4. InFig. 5—Repair locations in elevation and plan in thickened slab inSpan EE12.PTI JOURNAL August 2015 39
CASE STUDIESSpan EE12, approximately 16% of the of the bottom slabhad to be chipped away to allow for the removal of TendonT1 and only minor chipping was needed for the removal ofT2, as shown in Fig. 5. The procedure used for the removaland replacement of Tendons T1 and T2 is described in thefollowing sections.TENDON REM
BY HAYAT TAZIR, HUSAM NAJM, AND DAVID GRIFFITH This paper discusses the removal and replacement of two partially grouted embedded external tendons in a two-span 220 ft (67 m) long viaduct. Embedded external tendons were used for cast-in-place viaducts for the Terminal Area Roadway
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