recommended practice for segmental construction in ... · recommended practice for segmental...

RecommendedPractice for

Segmental Constructionin

Prestressed ConcreteReported by

PCI Committee on Segmental Construction

JOHN M. HANSON(Chairman)

HERBERT A. BRAUNER MORRIS SCHUPACKTHOMAS J. D'ARCY WILLIAM. M. SLATER*HEINZ P. KORETZKY MARIO G. SUAREZVILAS MUJUMDAR H. E. WESTENBERGBERNARD O'BRIEN Y. C. YANGGUY PETRILLOWALTER PODOLNY, JR. PAUL ZIAPETER REINHARDT ZENON A. ZIELINSKI*Chairman of this committee at the time that work was begun on this report.

This PCI committee report presents recommendationsfor the design and construction of precastprestressed concrete structures which are composedof individual segments that are tiedtogether using post-tensioning. Primary emphasisis placed on bridge construction.The recommendations cover the fabrication,transportation, and erection of the precast componentsegments, the construction of joints, details oftendons and anchors, and design considerationsapplicable to segmental construction.It should be emphasized that this report is publishedas a guide on segmental construction and should notbe considered a specification.

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CONTENTSChapter1—Introduction ........................... 24

1.1—General1.2—Scope1.3—Definitions

Chapter 2—Fabrication of Precast Segments ......... 252.1—General Considerations2.2—Methods of Casting2.3—Formwork2.4—Concrete2.5—Joint Surfaces2.6—Bearing Areas

Chapter 3—Transportation and Erection ofPrecastSegments ...................... 31

3. —P:ant Handling and Transportation3.2 Methods of Erection3.3 Erectio.i To'erances

Chapter4—Join ts ................................ 334.1 Cast-in-place Joints4.2 Dry-Packed Joints4.3 Grouted Joints4.4 Epoxy-Bonded Joints4. —Metal Joints

Chapter 5—Post -Tensioning Tendons ............... 355.1 Sheathing5.2 Anchor Plates5.3—Tendon Layout5.4 Placement and Stressing of Tendons5.5 Grouting5.6 U^bonded Tendons

Chapter 6—Design Considerations .................. 386.1 General6.2 Design of Segments for Box Girder Bridges6.3 Design of Segments for Decked I-Beams6. —Force Transfer Between Segments6.5 Supplemental Reinforcement6.6 Bearing and Anchorage6.7 Couplers

References ....................................... 41

PCI JOURNAL/March-April 1975 23

CHAPTER 1-INTRODUCTION

1.1—GeneralSegmental construction is defined asa method of construction forbridges, buildings, tanks, tunnels,and other structures in which pri-mary load carrying members arecomposed of individual segmentspost-tensioned together.

This report is a recommended prac-tice on segmental construction, not abuilding code or specification.

It is presented as a guide to the de-sign and construction of structuresusing segmental elements to insuresafety and serviceability. Profession-al engineers must add engineeringjudgment to applications of the rec-ommendations.

Throughout the report, major em-phasis is on bridge construction.Very recent developments applyingsegmental construction to otherstructures such as stadiums and cool-ing towers are not considered. Thecommittee intends to revise and up-date the report, to more fully consid-er future developments in segmen-tal construction of buildings as wellas bridges.

1.2—ScopeThese recommendations are intend-ed to cover those conditions whichaffect segmentally constructed mem-bers differently from non-segmental-ly constructed members. They con-sider the fabrication, transportation,and erection of the precast compo-nent segments, the construction ofjoints, details of tendons and an-chors, and design considerations ap-plicable to segmental construction.

1.3—Definitions

Adhesive—A bonding material usedin joints.

Anchorage—The means by whichthe prestressing force is transmittedfrom the prestressing steel to theconcrete.

Coating—Material used to protectagainst corrosion and/or lubricatethe prestressing steel.

Couplers—Hardware to transmit theprestressing force from one partial-length prestressing tendon to an-other.

Grout—A mixture of cement, sand,and water with or without admix-tures.

Joint—The region of the structurebetween interfaces of precast seg-ments, with or without adhesive orgrouting materials and with or with-out reinforcement.

Prestressing steel—The part of apost-tensioning tendon which iselongated and anchored to providethe necessary permanent prestress-ing force.

Segment—A precast element madeout of concrete, with or without rein-forcement, prestressed or nonpre-stressed.

Sheathing—Enclosure around theprestressing steel.

Tendon—The complete assembly forpost-tensioning, consisting of an-chorages, couplers, and prestress-ing steel with sheathing.

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CHAPTER 2-FABRICATION OF PRECAST SEGMENTS

2.1—General ConsiderationsDuring design of a segmental struc-ture, consideration should be givento the formwork necessary to achieveeconomy and to obtain efficiency inproduction. It is generally preferableto use as few units as possible, con-sistent with economic shipping anderection.In the case of girder segments, econ-omy and speed of production may beincreased by:(a) Keeping the length of the seg-

ments equal and keeping themstraight, even for curved struc-tures.

(b) Proportioning the segments orparts of them, such as keys, insuch a way that easy strippingof the forms is possible.

(c) Maintaining a constant webthickness in the longitudinal di-rection.

(d) Maintaining a constant thick-ness of the top flange in the lon-gitudinal direction.

(e) Keeping the dimensions of theconnection between webs andthe top flange constant.

(f) Beveling corners to facilitatecasting.

(g) Avoiding interruptions of thesurfaces of webs and flangescaused by protruding parts foranchorages, inserts, etc.

(h) Using a repetitive pattern, ifpractical, for tendon and an-chorage locations.

(i) Minimizing the number of dia-phragms and stiffeners.

(j) Avoiding dowels which have topass through the forms.

Variation of the cross section of gir-

der segments is generally limited tochanging the depth and width of thewebs and the thickness of the bottomflange. Curves in the vertical andhorizontal direction and twisting ofthe structure are easily accommodat-ed if longitudinal features of the seg-ment remain at the same horizontaldistance from the centerline.Segmental construction is distin-guished by the type of joint betweenelements. The following types havebeen used:(a) Wide (broad) joints.(b) Match-cast joints.(c) Wide joints made to match in a

later state.The sharpness of line of segments as-sembled with wide joints dependsmainly on the accuracy of the manu-facturing of the joints during erec-tion and less on the accuracy of thesegments. Curvature and twisting ofthe structure may be obtained with-in the joint.The principle of the match-cast jointis that the connecting surfaces fit toeach other very accurately, so thatonly a thin layer of filling material isneeded in the joint. Each segment iscast against its neighbor. Use of abond breaking agent allows the seg-ments to be taken apart. The sharp-ness of line of the assembled con-struction depends mainly on the ac-curacy of the manufacture of thesegments.

2.2—Methods of CastingSegments to be erected with widejoints may be cast separately. Match-cast joint members are cast by the"long-line" or "short-line" method.


2.2.1—The long-line method

Principle—All of the segments are cast,in correct relative position, on a longline. One or more formwork units movealong this line. The formwork units areguided by a preadjusted soffit. An ex-ample of this method is shown in Figs.la through lc.

Advantages—A long line is easy to setup and to maintain control over theproduction of the segments. After strip-ping the forms it is not necessary totake away the segments immediately.

Disadvantages—Substantial space maybe required for the long line. The mini-mum length is normally slightly morethan half the length of the longest spanof the structure. It must be constructedon a firm foundation which will not set-tle or deflect under the weight of thesegments. In case the structure iscurved, the long line must be designedto accommodate the curvature. Be-cause the forms are mobile, equipmentfor casting, curing, etc., has to movefrom place to place.

2.2.2—The short-line method

Principle—The segments are cast at thesame place in stationary forms andagainst a neighboring element. Aftercasting, the neighboring element istaken away and the last element isshifted to the place of the neighbor-ing element, clearing the space to castthe next element. A horizontal castingoperation is illustrated in Figs. 2athrough 2c. Segments intended to beused horizontally may also be cast ver-tically.

Advantage—The space needed for theshort-line method is small in compari-son to the long-line method, approxi-mately three times the length of a seg-ment. The entire process is centralized.Horizontal and vertical curves andtwisting of the structure is obtained byadjusting the position of the neighbor-ing segment.

Disadvantage—To obtain the desiredstructural configuration, the neighbor-ing segments must be accurately posi-tioned. Concrete at the top of segments,which are intended for horizontal use,but which are cast vertically, may be oflower quality and has to be finishedproperly.

2.3—Formwork

2.3.1—General

Formwork must be designed to safelysupport all loads that might be appliedwithout undesired deformations or set-tlements. Soil stabilization of the foun-dation may be required, or the form-work may be designed so that adjust-ments can be made to compensate forsettlement.Since production of segments is basedon reusing the forms as much as possi-ble, the formwork must be sturdy andspecial attention must be given to con-struction details. Forms must also beeasy to handle. An example of a formused to construct I-shaped girder seg-ments is shown in Fig. 3.Paste leakage through formwork jointsmust be prevented. This can normallybe achieved by using a flexible sealingmaterial. Special attention must begiven to the junction of tendon sheath-ing with the forms.The forms may need to be flexible inorder to accommodate slight differencesof dimensions with the previously castsegment. They must be designed insuch a way that the necessary adjust-ments for the desired camber, curva-ture and twisting can be achieved accu-rately and easily.Special consideration must be given tothose parts of the forms that have tochange in dimensions. To facilitatealignment or adjustment, special equip-ment such as wedges, screws or hy-draulic jacks should be provided.Anchorages of the tendons and insertsmust be designed in such a way thattheir position is rigid during casting.

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Formwork

Fig. Ia. Cross section of formwork using long-line method.

Outside Formwork

Inside Formwork

........... ^L7

ELEVATION

PLAN

Fig. lb. Start of casting (long-line method).

ELEVATION

PLAN

Fig. 1c. After casting several segments (long-line method).


Fig. 2a. Formwork for short-line method.

Inside Formwork -Bulkhead

SEGMENT] OLD ] ' OLDER

Soffit

Carriage

Fig. 2b. Just before separation of segments (short-line method).

Fig. 2c. Just before casting next segment (short-line method).

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IVL.-1y,I.1.;

Fig. 3. Form to construct I-shaped girder segments. (Note: Plane ofdeck may be varied with respect to web of I-beam).

Fittings must not interfere with strip-ping of the forms.If accelerated steam curing using tem-perature in excess of approximately160 F is foreseen, the influences of thedeformations of the forms caused byheating and cooling must be consid-ered, in order to avoid development ofcracks in the concrete.External vibrators must be attached atlocations that will achieve maximumconsolidation and permit easy exchangeduring the casting operations. Internalvibration may also be required.

2.3.2—Provisions for tendons

Holes for prestressing tendons may beformed by:(a) Rubber hoses which are pulled out

after hardening of the concrete.(b) Sheathing which remains after

hardening of the concrete. Flexi-ble sheathing made out of spirallywound metal is usually stiffenedfrom the inside by means of dum-my cables, rubber or plastic hoses,etc. during the casting operation.

(c) Rigid sheathing with smooth or cor-rugated wall may be used that will

not deform excessively under thepressure of wet vibrated concreteand for which there is no dangerof perforation.

(d) Movable mandrels.Holes must be accurately positioned,particularly when a large number ofholes is required. Further informationon forming holes for tendons and ten-don anchorages is presented in Chapter5.

2.3.3—Provisions for supplementalreinforcement

Supplemental reinforcement is used toresist forces either not covered or notfully covered by prestressing. Require-ments for supplemental reinforcementare listed in Chapter 6.

2.3.4—Tolerances

Formwork that produces typical boxgirder segments within the followingtolerances is considered good workman-ship.

Width of web ..........± % in.Depth of bottom slab .... + Y2 to 0 in.Depth of top slab ....... ±- 1/a in.Overall depth of segment . L 1/4 in.


Overall width of segment . -!- r/4 in.Length of match-cast

segment ± 1/4 in.Diaphragm dimensions . ± 1/z in.Grade of roadway and

soffit ................ ± 3/s in.

Depending upon the detail at bridgepiers, the tolerances for the soffit of asegment may need to be limited to± ?pis in. The tolerance of a segmentshould be determined immediatelyafter removing the forms. If specifiedtolerances are exceeded, acceptance orrejection should be based on the effectof the over-tolerance on final alignmentand on whether the effect can be cor-rected in later segments.In match-cast construction, a perfect fitis established between segments. Lim-its for smoothness and out-of-square-ness of the joint should be established.Where wide joints are used, the toler-ance in out-of-squareness may be thatwhich can be accommodated in thejoint thickness.

2.4—ConcreteUniform quality of concrete is essen-tial for segmental construction. Pro-cedures for obtaining high qualityconcrete are covered in PCI andACI publications.1.2Both normal weight and structurallightweight concrete can be madeconsistent and uniform with propermix proportioning and productioncontrols. Ideal concrete for segmen-tal construction will have as near aspractical zero slump and 28-daystrength greater than the strengthspecified by structural design. It isrecommended that statistical meth-ods be used to evaluate concretemixes.The methods and procedures used toobtain the characteristics of concreterequired by the design may varysomewhat depending on whether the

segments are cast in the field or in aplant. The results will be affected bycuring temperature and type of cur-ing. Liquid or steam curing or elec-tric heat curing may be used.A sufficient number of trial mixesmust be made to assure uniformityof strength and modulus of elasticityat all significant load stages. Carefulselection of aggregates, cement, ad-mixtures and water will improvestrength and modulus of elasticityand will also reduce shrinkage andcreep. Soft aggregates and poorsands must be avoided. Creep andshrinkage data for the aggregatesand/or concrete mixes should beavailable or should be determinedby tests.Corrosive admixtures such as cal-cium chloride should not be used.Water-reducing admixtures and alsoair-entraining admixtures which im-prove concrete resistance to environ-mental effects such as deicing saltsand freeze and thaw actions arehighly desirable. However, their usemust be rigidly controlled in ordernot to increase undesirable varia-tions in strength and modulus ofelasticity of concrete.The cement, fine aggregate, coarseaggregate, water, and admixtureshould be combined to produce ahomogeneous concrete mixture of aquality that will conform to the mini-mum field test and structural designrequirements. Care is necessary inproportioning concrete mixes to in-sure that they meet specified criteria.Reliable data on the potential of themix in terms of strength gain andcreep and shrinkage performanceshould be developed for the basis ofimproved design parameters.Proper vibration should be usedto afford use of lowest slump con-

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crete and to allow for the optimumconsolidation of the concrete.Consideration should be given to de-laying erection of segments untiltheir internal moisture is reduced toa level that subsequent drying willnot produce excessive deformation.

2.5—Joint SurfacesRequirements concerning surfacequality must be stricter for match-cast joints than for wide joints filledwith mortar or concrete.

2.5.1—Orientation

Surfaces should be oriented perpendic-ular to the main post-tensioning ten-dons, to minimize shearing forces anddislocation in the plane of the joint dur-ing post-tensioning. Inclination with re-spect to a plane perpendicular to thelongitudinal axis is permitted for jointswith assured friction resistance. The in-clination should generally not exceed20 deg. Larger inclination, but notmore than approximately 30 deg, maybe permitted if the inclined surfacearea is located close to the neutral axisand does not exceed 25 percent of thetotal joint's surface area.

2.5.2—Quality

For match-cast joints, the surface, in-cluding formed keys, should be evenand smooth, to avoid point contact and

surface crushing or chipping off ofedges during post-tensioning.For wide joints, rough surfaces arepreferable, as they produce better bondbetween segment and filling material.Since it is difficult in normal practice toproduce perfect sharp edges, it is ad-visable to make joint surface edgesslightly rounded or chamfered. Al-though this will tend to make joints vis-ible, it will also reduce the contrast ifneighboring segments have slight colorvariation. Rounding or chamfering ofedges should not decrease the jointsurface area by more than approxi-mately 2.5 percent.

2.5.3—Holes for tendons and couplers

Holes or sheathing for tendons must belocated very precisely when producingsegments joined by post-tensioning.Care is required to prevent leakage orpenetration of joint-filling materials in-to the duct, blocking passage of thetendons.

2.6—Bearing AreasBearing areas at reactions should beeven, without ridges, grooves,honeycomb, etc., to assure uniformdistribution of bearing forces. It maybe desirable to place bearing ele-ments like pads or steel plates in theforms before casting. Otherwise, ce-ment mortar or epoxy may be re-quired on contact surfaces.

CHAPTER 3-TRANSPORTATION AND ERECTION OFPRECAST SEGMENTS

3.1—Plant Handling andTransportationSegments should be handled care-fully, without impact, in a mannerthat limits stresses to values compati-ble with the strength and age of theconcrete. It should be verified that

the segment weights are less thanthe capacity of the lifting equip-ment. Highway and site transporta-tion may produce dynamic stresseswhich need to be considered. Specialcare of cantilever projections is oftenneeded to prevent cracking. Loca-


tion of lifting hooks and insertsshould be determined carefully toavoid excessive stresses in the seg-ment during handling, and theyshould have an adequate safety fac-tor.Storage of units at the site should bearranged to minimize damage, de-flection, twist, and discoloration ofthe units. Stockpiling should belimited to avoid excessive direct oreccentric forces. Special precautionsmay be required to avoid settlement.Inserts, anchorages and other em-bedded items may need to be pro-tected from corrosion and frompenetration of water or snow duringcold weather.Segments of decked I-beams, with-out pretensioning, may be transport-ed and assembled up to about 100 ft.

3.2—Methods of Erection3.2.1—Assembly on ground

Settlement of the segments during theirassembly must be prevented. Handlingand lifting equipment should be re-moved from the segments at the com-pletion of each operation. The segmentsshould be stabilized and braced to re-sist lateral forces, wind, earthquake,and impact. Potential difficulty in gain-ing access for handling the final mem-ber should be considered.Before delivery to the site or beforeerection, sheathing should be cleared ofall obstructions. Restraint- of the seg-ments against shortening must be pre-vented during post-tensioning opera-tions. Consideration of subsequentchange in forces at supports due tocamber and redistribution of dead loadmay be required. Stability due to pre-stressing should be checked.

3.2.2—Assembly on scafffoIding

Settlement and shortening of scaffold-ing due to dead load of segments as

well as construction loads must be con-sidered. Segments need to be carefullyaligned and leveled before formingjoints and post-tensioning.Shortening of the segments and jointingmaterials due to temperature, settle-ment, or change in loading conditionsshould be checked before post-tension-ing of the structure. If joints separatedue to the above causes, the post-ten-sioning may cause uneven distributionof stresses, grout leakage, etc.

3.2.3—Balanced cantilever constructionBy this method, segments are erectedsuccessively on each side of a centralpier. Mobile cranes, winch and beammechanisms or launching gantries areused to raise the segments. Temporarysupports or struts may be needed. Caremust be exercised to maintain compres-sion, by means of temporary post-ten-sioning, over the entire joint area untilthe permanent tendons are stressed.The magnitude of stresses must beevaluated at all stages of construction.

3.3—Erection TolerancesMaximum differential between out-side faces of adjacent units in theerected position should not exceed?/4 in.The most important item of toler-ance or acceptance is the final geom-etry of the erected superstructure.The elevation of the deck surface ofeach segment used in the cantileverportions of the bridge superstruc-ture, measured after closure sectionsare in place, must not vary from thetheoretical profile grade elevation bymore than that specified for theproject. The gradient of the decksurface of each segment should notvary from the theoretical profilegradient by more than 0.3 percent.More liberal tolerances may be ac-ceptable if the design incorporates awearing surface.

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CHAPTER 4-JOINTS

Depending on their width, jointsmay be cast-in-place with concrete,dry packed with mortar, or grouted.The joint surfaces must be clean,free from grease and oil, etc. andpreferably wire brushed or sand-blasted. Prior to construction ofthe joint, the adjacent concrete sur-face should be kept thoroughly wetfor approximately 6 hours or a bond-ing agent should be applied. Match-making joints are normally bondedwith epoxy.

4.1—Cast-in-place JointsThe design width of a joint must al-low access for coupling of conduits,welding of reinforcement, and thor-ough vibration of concrete. Typicaljoint widths are twice the thicknessof the web or one-half the flangewidth, but not less than 4 in.The compressive strength of the con-crete at a specified age should beequal to the strength of the concretein the adjacent precast segments.High early strength portland cementmay be used. Aggregate size shouldbe selected to ensure maximum com-paction.The height of each concrete place-ment or lift must be limited so thatthe concrete can be properly consoli-dated. Ports are normally providedfor inspection.Formwork must prevent leakage ofconcrete during and after its place-ment. Adequate curing is necessaryto reach the design strength of theconcrete.

4.2—Dry-Packed JointsThe concrete mortar must have acompressive strength equal to the

concrete in adjacent segments, or atleast 4000 psi. Good mortar shouldbe thoroughly mixed and have zeroslump. Maximum aggregate size nor-mally does not exceed 3As in. Mortarshould be rammed into place using aheavy hammer and a wood ram.The width of dry-packed jointsshould not exceed approximately 21fzin. Mortar should be introduced intothe joint in small quantities orbatches not exceeding 10 lbs byweight. Each batch must be thor-oughly tamped and packed beforethe next batch is placed. Contain-ment may be necessary, particularlyat the bottom of the joint.

4.3—Grouted JointsThe width of grouted joints shouldnot exceed approximately 2 in.Grouted joints may be made witheither gravity or pressure methods.In either case, provisions must bemade to contain the grout.The compressive strength of thegrout at a specified age should equalthat of the concrete in adjacent seg-ments, or be at least 4000 psi. Insome cases it many be difficult toattain a grout strength equal to thatof the adjacent concrete. Nonmetal-lic grout mixes should be used whenthe grout will be exposed to weather.To assure filling of the joint whengravity grouting is used, rodding ortamping should be used to consoli-date the grout. Practical problemsmay be encountered with gravitymethods, particularly on thin hori-zontal joints. For this purpose thegrout should consist of approximate-ly one part of cement, one-third part


of clean sand passing a # 16 meshscreen, and a water-reducing agentwhich preferably minimizes sedi-mentation. The total water contentshould be kept as low as practical.Pressure grouting of joints requiresequipment in good condition andadequate preparations. The jointmust be tightly sealed in order tosustain the pressure. Pressure grout-ing is particularly effective whenforms or gaskets can be easily used.Formwork must be vented at thetop. At the conclusion of the grout-ing operation, the vent should beclosed, and the pressure increasedto a minimum of 15 psi at the vent.Twenty-four hours after grouting,the vent should be opened and filledif sedimentation has occurred.

4.4—Epoxy-Bonded JointsIt is particularly important that theadjacent surfaces are solid, clean,and free of dust and grease. All rem-nants of formwork oil, grease, soaptalcum, powder, etc. must be takenaway by proper washing or sand-blasting. Sandblasting should be re-quired when joints will be subjectedto tension.

4.4.1—Selection of epoxy

The compressive strength of the epoxyshould equal that of the concrete in ad-jacent segments under any environmen-tal condition that may be encounteredduring the life of the structure.Only highly cross-linked adhesivesshould be used that have been thor-oughly tested in accordance with cur-rent ASTM and industry standardswhenever applicable. Informationwhich should be obtained from themanufacturer, or be determined by test,includes:(a) Pot life and "open time" at various

temperatures.

(b) Compressive strength and modulusof elasticity at various tempera-tures after curing for at least 7days at 68 F (20C).

(c) Direct shear strength of bondedconcrete prisms at various temper-atures, with notation of whetherfailure occurs in the joint or in theconcrete.

Data should be available on the resin/hardener system used in formulatingthe adhesive, including creep behaviorand performance of bonded concreteprisms.

4.4.2—Application

The adhesive should be supplied inpreweighted packs or cans of resin andhardener component. Resin and hard-ener should have different colors.Components of the epoxy mix must beproportioned and mixed thoroughly un-til a uniform color is obtained, followingthe instructions of the manufacturer.Each batch should be assigned a num-ber. The adhesive must be applied im-mediately after mixing, and the jointsurfaces brought together before thepot life expires. A record of the locationof the joint where the batch of epoxyis used, weather, temperature, and ob-served pot life should be maintained.The concrete surfaces which are to bebonded should not be wet; a damp butnot dark, shiny surface is permissible.To get rid of a wet surface, the con-crete may be dried with hot air just be-fore applying the adhesive.The adhesive should be applied in auniform thickness to both surfaces.Care must taken that no epoxy mix en-ters the ducts for the tendons. Afterjoining the segments the tubes must bechecked to be sure they are not blockedby the adhesive. Some post-tensioningmust be applied within the "open time"of the adhesive. If the correct amountof adhesive has been used, a little willextrude from the joint when pressure isapplied. It is recommended that a mini-

34

mum of 30 psi (2 kg/cm2) be appliedto the joint.In case of unforseen interruptions, the"open time" may have expired beforethe segments are fully joined. Then theepoxy has to be removed with a spatulaand the remainder washed off with sol-vents according to the instructions ofthe manufacturer. Sandblasting will benecessary. Particular care is required incold weather.In case corrections in the alignment areanticipated, it is recommended thatwire mesh be embedded in the epoxylayer.

4.4.3—Testing on site

If desired, test specimens can be madeat the site to later verify the propertiesof the epoxy. These may include prismsof adhesive to test the compressivestrength and diagonally cut concretecylinders or prisms bonded together

with the epoxy to indirectly test shearstrength by compressive loading.

If necessary, concrete cores may bedrilled through the bond line and testedin compression.A record of the testing should be com-piled.

4.5—Metal JointsMetal joints in the form of matingpairs of steel plates which are weld-ed together, or pin connected metalhinges, etc. may be used. However,they are classified more as connec-tions than as typical joints betweenprecast elements in segmental struc-tures. Special attention is requiredfor welded joints to prevent crackingor spalling of the concrete and toprevent staining of adjacent sur-

faces.

CHAPTER 5-POST-TENSIONING TENDONS

Background information on variouspost-tensioning systems and their ap-plications is given in the PCI Post-tensioning Manual.3

5.1—SheathingSheathing is used to form the holesor enclose the space in which theprestressing steel is located. It maybe located inside or outside of theconcrete section. Sheathing materi-als are listed in Section 2.3.2.The prestressing steel can be placedwith the sheathing, or installed afterthe concrete is placed. The cross sec-tion of the sheathing must be ade-quate to allow proper installation ofthe prestressing steel and to provideenough passage area for filling thetube with grout subsequent to stress-

ing. The ratio of the net area of theprestressing steel to sheathing cross-sectional area should comply withthe PCI Guide Specification for Post-Tensioning Materials in Reference 3.Sheathing must have sufficient grout-ing inlets, vent pipes for escape ofair, shut-off valves, and drains to al-low proper grouting and to avoid ac-cumulation of water during storage,Inlet and/or vent pipes should belocated at all high points of the ten-don profile and, if necessary, at lowpoints. The distance between inletand/or vent pipes should not bemore than approximately 100 ft.Sheathing placed inside the concretemust be resistant to irreparable dam-age caused during installation andplacing of the concrete. It must pre-


vent leakage of concrete into thevoid, and must be secured againstbuoyancy.When semi-rigid sheaths are used,they must bend sufficiently to followthe required tendon profile. Flexiblesheaths need to be tied securely toauxiliary reinforcement to minimizewobble of the tendon.In certain applications, the sheathsare arranged outside the concretesection, either in voids of the sectionor along the outside of the concretecross section. In addition to the re-quirements listed above, they mustbe protected against corrosion.Special attention must be given tothe alignment of the sheathing. Atthe joints, accurate placing is man-datory. Sharp curvature must beavoided or difficulty may be encoun-tered in threading tendons throughthe sheathing.Sheathing may be connected atjoints by:(a) Telescopic sleeves pushed over

the protruding ducts.(b) Screw-on type sleeves.(c) Rubber or plastic sleeves.(d) Gaskets.For joint types (a), (b), and (c),leak tightness must be assured toavoid the entrance of jointing mate-rials into the sheaths, causing possi-ble blockage. Also, gasket jointsmust offer the same leak tightness asthe rest of the sheathing.

5.2—Anchor PlatesThe anchor plate is the componentof a post-tensioning system whichtransmits the prestressing force fromthe tendon anchoring device direct-ly to the concrete. Its function is todistribute the concentrated forcefrom the anchoring device over alarger bearing area to the concrete.

The anchor plate must be of suchshape and dimensions as to limit thebearing stresses to those specified inthe PCI Post-Tensioning Manual s orin applicable codes.

5.2.1—Cast into precast segment

This is the most commonly used pro-cedure. The concrete must be thor-oughly vibrated behind the bearingplate to avoid honeycomb and toachieve proper strength.

5.2.2—Placed against precast surface

When the bearing plate is placedagainst the hardened surface of the pre-cast element, dry-packed mortar shouldbe used between the plate and the con-crete. The strength of the mortarshould not be less than the concretestrength on which it bears, and thethickness of the joint should be limitedto 2 in.The reinforcement of the concrete un-derneath the dry-packed mortar shouldbe the same as if the plate was cast intothe precast segment.

5.2.3—Placed against cushioningmaterials

Cushioning material may be used undera bearing plate when special circum-stances require a highly uniform stressdistribution between bearing plate andconcrete. The cushioning material mustnot creep after final load. Very carefulattention to details is required.The cushioning material must be chem-ically stable and must not change physi-cal properties with time and/or underload. It must also be compatible withthe concrete and the bearing plate ma-terial. When metals are used as cush-ioning material, special attention mustbe given to the possibility of corrosiondue to electrolysis.

5.2.4—Surface under anchor plate

The preparation of the surface underthe bearing plate will depend on the

36

type of plate and placing procedureused.If the bearing plate is not cast into theconcrete, the bearing surface must beapproximately perpendicular to the pre-stressing steel to minimize shearstresses. The surface must be clean andfree from foreign matter. The surfaceshould be wire brushed to remove alllaitance and loose concrete chips. Ifthe bearing plate is to be grouted, thesurface may be roughened by chiseling.

5.3—Tendon LayoutAttention must be given to the ten-don layout, to make it compatiblewith:(a) Sequence of the segmental con-

struction.(b) Progressive and subsequent

load conditions which the seg-mental structure will undergowhile being erected.

It is important to anticipate the sec-ondary effects (stresses due to re-strained deformations) which post-tensioning may have on the struc-ture, and how these effects can be in-fluenced by slight changes in thetendon profile.Couplers and splices of post-tension-ing tendons should preferably not belocated in areas where yielding mayoccur under ultimate load condi-tions.

5.4—Placement and Stressingof TendonsWhen tendons are installed in thesegments before casting, they areusually subsequently coupled to-gether at each joint. This construc-tion method permits stressing of partof a tendon, after installing one ormore segment, before the full lengthis completely installed.Tendons may also be installed after

casting and erection in partiallengths and coupled together at spe-cial openings. Usually, however,these tendons are installed fulllength. It may be noted, though, thatthis procedure permits intermediatestressing of portions of the structure,by using tendons of variable lengths,stressing the short ones first and longones later.Special attention must be given tothe corrosion protection of the post-tensioning steel which, at any stageof construction, has to be unbonded.If a tendon which contains couplersis to be stressed over the full length,the couplers must be able to move.

5.5—GroutingThe purpose of grouting is to pro-vide corrosion protection to the pre-stressing steel, and to develop bondbetween the prestressing steel andthe surrounding concrete. To accom-plish this, the grout has to fill all thevoids in and around the post-ten-sioning tendon for its entire length.Grouting procedures should followthe Recommended Practice forGrouting of Post-Tensioned Pre-stressed Concrete4 wherever applica-ble.Grouting is preferably done from thelowest points of the tendon, towardthe higher points, where vent pipesmust be installed. The grouting mustbe done gently and slowly to assurethe displacement of the air. Highlypressurized grout may mix with theair, creating entrapped air bubbles.The tendon can be flushed with wa-ter before grouting, to avoid block-age when injecting the grout. Thewater can be blown out with pres-surized air, or displaced by thegrout. Sufficient purging must be al-


lowed, but not less than 1 minute pertendon.Grouting must not be done if thetemperature in the duct is less than35 F or if the surrounding concretetemperature is less than 32 F.With vertical tendons, segregation isgreater than with horizontal ten-dons.Precautions must be taken to elimi-nate the danger of blockage of groutdue to the couplers. Couplers aregenerally housed in a special, en-larged encasement, which should:

(a) Have the same open cross-sec-tional area for passage of groutas the rest of the tendon.

(b) Be provided with a grout inletand/or vent pipe.

5.6—Unbonded TendonsUnbonded tendons may be used insegmental construction, provided thestructural requirements of the post-tensioning steel are also met by thetendon anchorage. In unbondedpost-tensioning, a corrosion protec-tion system must be provided, whichmust assure at least the same degreeof corrosion protection as grout.This can be achieved by using aprotective coating on the prestress-ing steel, and enclosing the coatedtendon in an encasement to protectthe coating during handling, instal-lation and stressing of the tendon.If unbonded and unprotected ten-dons are used, construction detailsmust prevent accumulation of waterin the ducts.

CHAPTER 6-DESIGN CONSIDERATIONS

6.1—GeneralAnalysis and design of segmentalstructures is essentially the same asfor monolithic prestressed concretestructures. Design should be basedon two main considerations: accept-able behavior of the structure at allstages of construction and under ser-vice load, and adequate ulti-mate strength. Special considera-tion should be given to redistribu-tion of forces in structures erectedby the balanced cantilever method.Provisions of the ACI BuildingCodes and the AASHTO Specifica-tions6 intended for non-segmentalprestressed construction are general-ly applicable. Additional useful in-formation is contained in the PCIDesign Handbook.' Unless specialconsideration is given to crack con-

trol and to prevention of corrosion, itis recommended that flexural orprincipal tension not exceed the al-lowable tensile strength of the con-crete under service loads. Use of awaterproof membrane on the topsurface may assist in preventing cor-rosion or deterioration.

6.2—Design of Segments forBox Girder Bridges

For constant depth segmentalbridges, span-to-depth ratios from18:1 to 25:1 are currently consideredpractical and economical. Variabledepth bridges may have span-to-depth ratios of 40 to 50 based on thedepth at the center of the span.It is recommended that single cellbox girders, including cantilevers,not exceed 40 ft in width or a width-to-depth ratio of 6:1 unless special

38

attention is given to the design. Forwide segments, multiple cells are de-sirable and the webs should be lo-cated to reduce moments in the topslab and to minimize transversebending of the webs. Generally,these moments are computed withsufficient accuracy by consideringthe top slab and webs as a rigidframe with fixed column bases.Fillets should be used in the interiorcorners of box sections to improvethe flow of stresses. They also pro-vide additional room for tendons.Vertical diaphragms are normally re-quired at abutments, piers, andpoints of articulation.

6.3—Design of Segments forDecked I-Beams

The spanning capability of typical I-shaped sections is extended whenthe beam and deck are precast inte-grally. Reference 8 describes meth-ods of design for segmentalAASHTO Type III and Type IV sec-tions for spans up to 180 ft.

6.4—Force Transfer BetweenSegments

The joints between segments shouldbe capable of transferring the com-pressive, shearing, and torsionalforces required by the design. Nor-mally, tension is not permitted be-tween segments under any stage oferection or service loading. How-ever, if an epoxy or grout is used inthe joint that has a tensile strength atleast equal to that of the concrete,the allowable tensile stress underservice loading may be the same asfor nonsegmental construction.

6.4.1—Keys

Keys in the webs of segments are use-ful to facilitate alignment and to trans-fer shear during erection. It is recom-

mended that the keys be proportionedfor erection loads so that the bearingstress does not exceed 28 3t,.' and theshear stress on the gross section of thekey does not exceed 2/f7.

6.4.2—Compression

As indicated in Sections 4.1 through4.4, the compressive strength of ma-terial in a joint should be at least equalto that of the concrete in adjacent seg-ments. If a dry joint is used, considera-tion should be given to the preventionof spalling around the edges of thecontact area. It is recommended thatdry joints be used only when verifiedby previous acceptable performancewith regard to transfer of prestressingforce and long-term weathering.

6.4.3—Shear

Shear in a joint is normally consideredto be resisted by friction. Consideringthe magnitude of the resultant com-pressive force across the joint and aconservative estimate of the coefficientof friction between the surfaces, theavailable shear resistance should be atleast twice the applied shear at serviceloads. At ultimate load, the availableshear resistance should be at least equalto the applied shear. Diagonal web ten-dons across a joint may be used to in-crease shear capacity.If the shear is not considered to be re-sisted by friction, simple or multiplekeys9 are required.Wide grouted joints should containnonprestressed reinforcement equiva-lent to that in adjacent segments.

6.5—Supplemental ReinforcementSupplemental reinforcement to atleast meet minimum requirements isnormally required in segments for:(a) Transverse bending moments.(b) Shear.(c) Torsion due to curvature or ec-

centric loading.


(d) Concentrated forces at reac-tions and due to tendon anchor-ages.

(e) Thermal and volume changeforces.

Additional supplemental reinforce-ment may be required for:(a) Longitudinal and lateral forces

on the superstructure.(b) Temporary forces imposed dur-

ing fabrication, transportation,or erection.

(c) Local forces due to changes inshape of cross section, additionof curbs and parapets, use ofdiaphragms, connections for util-ities, etc.

(d) Severe environmental exposure.To decrease the possibility of dam-age to segments during post-tension-ing due to accidental poor quality oruneven surfaces, or during handlingof segments, supplemental reinforce-ment may be desirable immediatelyadjacent to the joint surface. This re-inforcement typically consists of 1/4

in. diameter wires spaced in two di-rections at 2 to 3-in, intervals or ofcomparable mesh.Horizontal and vertical reinforce-ment may also be placed betweensegments in wide joints filled withmortar or concrete when the trans-verse thickness of the joint is morethan 2 in.Since too many reinforcing bars maycause difficulty in placing concrete,the arrangement of reinforcement incongested areas should receive spe-cial attention.Special consideration must also begiven to the effects of severe envi-ronmental exposure. For example, itmay be essential to increase cover ofsteel, use coatings such as epoxy onreinforcement, or provide other pro-tection.

6.6—Bearing and Anchorage

Bearing areas may be subjected tolarge concentrated forces; they mayalso need to accommodate substan-tial movement due to volumechanges and thermal effects.Post-tensioning tendon anchors arelocated in end block or anchorageareas. Their purpose is to safelytransfer anchorage forces into thestructure during all loading stages,including time of initial post-tension-ing. End blocks may not be requiredif the stresses in the anchorage areaare not excessive.When all tendons extend the fulllength of the structure, the endblocks are located in end segmentsof segmental structures, which, in asimply supported beam system, willalso be the bearing or the supportsegments. In some cases, however,anchorage areas may be located inintermediate segments (continuousbeam or cantilever structures withsegmental tendons distributed ac-cording to the moment or sheardiagram) or in non-bearing end seg-ments (cantilever structures).Special attention should be given toproper reinforcement of bearingsand anchorages. They should beprovided with minimum reinforce-ment. In particular, anchorage zonesshould contain sufficient horizontaland vertical stirrups or grill rein-forcement placed in the plane par-allel to the end surface. Closed stir-rups or ties should be used, or pre-ference may be given to grills orwelded wire mesh.Generally three different areas of di-agonal splitting and cracking can beidentified in anchorage areas whichrequire reinforcement:(a) Under end surfaces, not more

40

than 3 in. deep, to control pos-sible surface cracking aroundanchorages.

(b) Internally, to prevent splitting ofseparate anchorages. Size andlocation of this area and of themagnitude of splitting (bursting)force depends on the type ofanchorage and the force in thepost-tensioning tendon, andshould be investigated individ-ually for each type of anchorage.

(c) Internally, to prevent tearingbetween groups of anchorages(not distributed on bearing sur-face uniformly). For each case,the tearing force and necessaryreinforcement should be definedseparately for the vertical andhorizontal directions.

Concrete which is placed aroundanchorages, after post-tensioning,should be provided with reinforce-ment to assure that it will not spall.

6.7—CouplersCouplers should be designed to de-velop the full ultimate strength ofthe tendons they connect. Adjacentto the coupler, the tendon should bestraight for a minimum length of 12times the diameter of the coupler.Adequate provisions should be madeto assure that couplers can moveduring prestressing. The void areasaround a coupler should be deductedfrom gross section areas when com-puting stresses at the time of pre-stressing.

REFERENCES

1. Manual for Quality Control forPlants and Production of PrecastPrestressed Concrete Products,MNL-116-70, Prestressed ConcreteInstitute, Chicago, Illinois, 1970.

2. ACI Manual of Concrete Practice,Part 1-1973, American Concrete In-stitute, Detroit, Michigan.

3. PCI Post-Tensioning Manual, Pre-stressed Concrete Institute, Chicago,Illinois, 1972.

4. PCI Committee on Post-Tensioning,"Recommended Practice for Grout-ing of Post-Tensioned PrestressedConcrete," PCI JOURNAL, Vol. 17,No. 6, Nov.-Dec. 1972.

5. ACI Committee 318, Building CodeRequirements for Reinforced Con-

Discussion of this report is invited. Pleaseforward your discussion to PCI Headquar-ters by August 1, 1975.

crete (ACI 318-71), American Con-crete Institute, Detroit, Michigan.

6. Standard Specifications for HighwayBridges, Eleventh Edition, AmericanAssociation of State Highway andTransportation Officials, Washing-ton, D.C., 1973.

7. PCI Design Handbook, PrestressedConcrete Institute, Chicago, Illinois,1971.

8. Anderson, Arthur R., "Stretched-Out AASHO-PCI Beams Types IIIand IV for Longer Span HighwayBridges," PCI JOURNAL, Vol. 18,no. 5, Sept.-Oct. 1973, pp. 32-49.

9. Muller, Jean, "Ten Years of Experi-ence in Precast Segmental Construc-tion," PCI JOURNAL, Vol. 20, No.1, Jan.-Feb. 1975, pp. 28-61.


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