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1. Report No. 2. Government Accession No.FHWA/TX-84/32+248-l

4. Title and SubtitleEXPLORATORY STUDY OF SHEAR STRENGTH OF JOINTSFOR PRECAST SEGMENTAL BRIDGES

7. Author/.)K. Koseki and J . E. Breen9. Performing Organization Name and Addre . .

TECHNICAL REPORT STANDARD TITLE PAGE3. Recipient's Catalog No.

5. Report Dot.September 1983

6. Performing Organi zation Code

8. Performing Organi zation Report No.Research Report 248-1

10. W o r ~ Unit No.

II . Contract or Grant No.Research Study 3-5-80-248

Center for Transportation ResearchThe University of Texas at AustinAustin, Texas 78712-1075 13. Type 0/ Report and Period Coyered~ ~ ~ ~ ~ ~ ~12. Sponsoring Agency Nome and AddreTexas State Department of Highways and PublicTransportation; Transportation Planning DivisionP. O. Box 5051

Interim1

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EXPLORATORY STUDY OF SHEAR STRENGTH OF JOINTSFOR PRECAST SEGMENT L BRIDGES

byK Koseki and J. E Breen

Research Report No 248 1Research Project No 3 5 80 248

Reevaluation of SHTO Shear and Torsion Provisions forReinforced and Prestressed Concrete

Conducted for

TexasState Department of Highways and Public Transportation

In Cooperation with theU.S. Department of TransportationFederal Highway Administration

byCENTER FOR TRANSPORTATION RESEARCH

BURE U OF ENGINEERING RESEARCHTHE UNIVERSITY OF TEXAS T AUSTIN

September 1983

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The conten ts of t h i s repor t r e f l ec t the views of the authors whoare responsible for the facts nd accuracy of the data presented herein.The conten ts do not necess r i ly r e f l ec t the views or pol ic ies of theFederal Highway Adminis tra t ion. This repor t does not c ons t i t u t estandard specif ica t ion or regulat ion.

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P R E F ACEThis report presents the resul ts of an exploratory study which

considered the behavior and cr i ter i for design of shear keys forsegmental prestressed concrete box girder bridges. This study forms apart of a larger study which is reevaluating the basic AASHTO shear andtorsion provisions for reinforced and prestressed concrete and stemsdirect ly from review comments wherein FHWA asked the researchers toconsider the cr i t e r i for design of such shear keys in the overal lstudy. The objective of the program reported herein was to reviewexist ing data and to conduct a l imited scope experimental program todetermine re l t ive shear t ransfer strength across different types ofjoints between adjacent segments typical of precast segmental bridges.The types of joints considered included single large key multiple lugkeys and joints with no keys. Both dry and epoxy joints were studied.

The work was sponsored by the Texas State Department of Highwaysand Public Transportat ion and the Federal Highway Administration andadministered by the Center for Transportation Research at The Universityof Texas at Austin. Close l iaison with the State Department of Highwaysand Public Transportat ion has been maintained through Mr. Warren AGrasso and Mr Dean W Van Landuyt who served as contact representativesduring the project and with Mr T E Strock of the Federal HighwayAdministration.

The project was conducted in the Phil M Ferguson StructuralEngineering Laboratory located a t the Balcones Research Center of TheUniversi ty of Texas a t Austin. The authors would l ike to acknowledgethe assistance of Figg and Muller Incorporated of Tallahassee Floridawho provided detai led information about the mul t ip le key jointconfiguration used in the study and Kajima Corporation of Tokyo Japanwho provided financial support for Mr Koseki throughout his study. Theauthors were particularly indebted to Mr Gorham W Hinckley LaboratoryTechnician at the Ferguson Laboratory who greatly helped in carryingout the laboratory work involved.

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S U M M A R YThe joints between the precast segments are of cr i t ica l importance

n segmental bridge construction. They are cr i t ica l in the developmentof s t ruc tura l capacity by ensur ing the t ransfer of shear across thejo in t s and often p l a ya key ro le in ensur ing durab i l i ty by pro tec t ingthe tendons against corrosion. However construction of the jo ints mustbe simple and economical. A number of types of jo in t conf igura t ionshave been used in various precas t segmental bridges in the UnitedStates although re l a t ive ly l i t t l e information is avai lab le on thebehavior and des ign of such jo in t s . This study repor ts on a modestscope experimental inves t igat ion to determine the re l a t ive sheart ransfer s t rength across di f f e r en t types of jo in t s typ ica l ly usedbetween adjacent segments of precast segmental bridges. The types ofjoints considered included no keys single large keys and mUltiple lugkeys. Both dry and epoxy jo in t s were t es ted . The t e s t r esu l t sindicated substantial differences in the strength at a given sl ip in thevarious types of the dry jo ints but indicated that al l types of jo intswith epoxy essent ia l ly developed the full strength of a monolithicallycas t joint .

v

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I M P L MEN T A T I O NThe r esu l t s of th i s study wi l l a s s i s t the designer to assess the

mer i ts of various types of j o in t s proposed for th i s popular type ofconstruction. Current design specif icat ions do not address this problemand these results while representing only a l imited exploratory studydo ind ica te important t rends and allow the br idge designer to be t te runderstand the t rade of fs that are being made in the choice of jo in ttype.

vi i

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CON T E N T S

Chapter

1

2

INTRODUCTION1.1 Precast Segmental Bridge Construction1.2 Objective and Scope

1.2.1 Shear keys and epoxy bonding agent1.2.2 Objective 1.2.3 Shear t s t

1.3 Previous Related Studies1.3.1 Shear fr ic t ion1.3.2 Shear tes ts on joints with no keys betweenprecast post tensioned units 1.3.3 Shear tests on joints with single key

between post tensioned precast segments1.3.4 Studies on shear strength of multiple keys

TEST SPECIMEN 2.1 Specimen Dimensions2.2 Materials

2.2.1 Micro concrete2.2.2 Reinforcement.2.2.3 Prestressing tendon 2.2.4 Epoxy

2.3 Fabrication2.3.1 Reinforcing cages and forms2.3.2 Casting procedure2.3.3 Concrete strength

ix

Page

113367772

77

2323262626262632323235

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x

Chapter Page

3. TEST PROCEDURE ND RESULTS 393.1 Preparation 393.2 Joining and Post-Tensioning 39

3.2.1 Match-cast joint surfaces 393.2.2 Epoxying 393.2.3 Prestressing 403.2.4 Prestressing forces 40

3.3 Shear Test Arrangement 423.4 Test Results 42

3.4.1 Stresses due to prestressing 423.4.2 Load vs slip 463.4.3 Concrete strength prestressing force andmaximum load 53.4.4 Crack pattern a t failure 52

4. DISCUSSION OF THE RESULTS 634.1 Capacity of the Specimen 63

4.1.1 Flexural capacity 634.1.2 Section shear capacity 654.1.3 Bearing capacity 66

4.2 Shear Strength of the Joints 664.2.1 Shear fr ict ion 664.2.2 Single key joint 74.2.3 Multiple key joint 784.2.4 Effect of epoxy 82

4.3 Appraisal of Types of Joint 85

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xi

hapter age

5 CONCLUSIONS 87

REFERENCES 9

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T B L E STable Page1.1 Test Specimens 81.2 Examples of Multiple Key Configuration 192.1 Design Mix Proportion 272.2 Concrete Strength a t 7 Days 37

3.1 Stresses Due to Prestressing 443.2 Concrete Strength Prestressing Force and1a.x i mt lm Load 5

4.1 Capacity of the Test Specimens i f Monolithic 644.2 Shear Capacity of the Test Specimens 69

xi i i

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F I G U R SFigure Page1.1 Typical Precast Segmental Bridge Construction 21.2 Examples of Shear Keys 51.3 Web Model for the Tests 91.4 Shear Test of Joint . 101.5 Model Precast Segments and Test Specimens. 111.6 Shear Test on Joints of Precast Segmental Beamsby Franz 131.7 Shear Tests on Joints between Precast Units byJones 141.8 Unbonded Push off Test by Gaston Kriz . 161.9 Effect of Shear Key Height to Depth Ratio 91.10 Concrete Prism Specimen 21.11 Segmental Girder Specimen 22

2.1 Profile of Model Precast Segment 242.2 Configuration of Joints . . . 252.3 Stress Strain Relationship of 6 Reinforcing Bars 282.4 Stress Strain Relationship of 10 Gage Wire 292.5 Development of Strength of Epoxy 302.6 End Forms 332.7 Match Casting of Segments . 342.8 Separation of Segments af ter Form Removal 36

3.1 Post Tensioning Arrangement . 413.2 Shear Test Arrangement 433.3 Stresses a t Joint . . . 45

xv

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xvi

Figure3.4 Load vs Slip t Joints without Epoxying 3.5 Load vs Slip t Joints with Epoxying3.6 Comparison of the Results with and without Epoxying 3.7 Comparison of f F and Pc p m x3.S Crack Pattern t Failure (Single Key Joint w/o

Epo'XY'ing) . . 3.9 Crack Pattern t Failure (Single Key Joint w/oEpoxying) - Photo - 3.10 Crack Pattern t Failure (Multiple Key Joint w oEpoxying) . . 3.11 Sheared-off Joint after the Test (Multiple KeyJoint w o Epoxying) . 3.12 Crack Patterns t Failure Specimens w Epoxying) .3.13 Crack Pattern at Failure (Epoxied, No-Key Joint)3.14 Crack Patterns t Failure (No-Joint)3.15 Crack Pattern at Failure (Monolithic, No-Joint)

4.1 Correction of Load vs Slip Curve of Single KeyJoint w o Epoxying . . . . 4.2 Forces Acting on Single Key (Without Epoxy) 4.3 Forces Acting on Multiple Keys (Without Epoxy)4.4 Comparison of Behavior of Joints

Page4748495

54

55

56

5859606162

7274s

84

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C H A P T E R

I NTRODUCT ION

1 1 Precast Segmental Bridge ConstructionIn precas t segmental bridge cons t ruc t ion the s t ruc ture i s

constructed by pos t- tens ioning together precas t segments which areusual ly manufactured as shor t longitudinal sect ions of the box girdercross section. Balanced cant i lever erect ion (see Fig. 1.1 was theearly predom inant method of constructing segmental bridges. In a numberof recent applicat ions, the span-by-span method with segments assembledon a falsework t russ has seen wide use.

The technology of precast segmental construction was an extensionof cast-in-place segmental prestressed construction which was developedby Ulrich Fins terwalder and the firm of Dyckerhoff Widmann A G(Dywidag) in West Germany in the 1950 s [1,2] . The f i r s t majorapplication of precast segmental construction was in the Choisy-le-RoiBridge in 962 [1,2]. The structure was designed by Jean Muller and thefi rm of Entrepr ises Campenon Bernard in France. Thereaf te r thetechniques of precasting segments and assembling them in the s t ructurehave been continually refined.

Precast segmental construction was introduced to the United Statesin the early 1970s. The JFK Memorial Causeway in Corpus Christi , Texas,was the f i rs t applicat ion of the method and was completed in 973 [1,2].Since 1975, t h i s technique of cons t ruc t ing br idges has gained rapidacceptance, and there are present ly over 80 such br idges e i the rcompleted, under cons t ruc t ion or in design in North America [3 ] .During the i n i t i a l development of segmental cons t ruc t ion the br idgeswere const ruc ted by the balanced can t i l ever method. Curren t ly suchtechniques as span-by-span cons t ruc t ion incremental launching, andprogress ive placing are also being ut i l i zed . The Long Key Bridge inFlorida, the Wabash River Bridge in Indiana, and the Linn Cove projectin North Caro l ina are examples o f each of these p roceduresrespecti vel y.

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Precast segments

a) Balanced cantilever erection withlaunching gantry

Pier

Fig. 1.1 Typical precast segmental bridge construction

b) alanced cantilevererection with crane

f>,

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A l a r ge number of p r ecas t segmenta l br idges use an epoxy res in

jo in t ing mater ia l between precast segments. The th ickness of the epoxyj o in t i s on the order o f 1 32 in . The use o f an epoxy j o in t r equ i r e s aperfect f i t between the ends of adjacent segments. This i s achieved bycast ing each segment against the end face of the preceding one Cmatchcas t ing , and then erect ing the segments in the same order in which theywere cast.

While numerous examples o f success fu l pr o jec t s wi th such j o i n t se x i s t t he r e are a lso a number of poss ib le d isadvan tages in pr ecas tsegmental const ruc t ion:

- - N e c e s s i t y fo r a high degree o f geomet r y c o n t r o l dur ingfabr icat ion and erec t ion of segments.

- -Potent ial jo in t weakness due to lack of mild s tee l reinforcementacross the j o in t .

--Temperature and weather l imi tat ions regarding mixing and placingepoxy jo in t ing mater ia l .

- - F r equen t loading and unloading of segments , with the r i s k o fdamage.

The la rge number of successful projects in Europe, North America,and other par ts of the world suggest tha t these obs tac les wil l not curbthe rapid growth in the use of precas t segmental br idge cons t ruc t ion .Epoxy jo in ts grouted tendons, and shear keys have reduced dependence onthe bonded mild s tee l jo in t reinforcement, while the ve r sa t i l i ty of thematch-casting procedure in numerous major pro jec t s invo lv ing complexhorizonta l and ver t i ca l alignment has shown tha t the precas t procedurescan deal with geometr ical problems. A number of recent projec ts havebeen bu i l t with mult ip le key dry jo in t s to el iminate epoxy coat ings andthei r at tendant problems.

1.2 Objective and Scope1.2.1 Shear Keys and Epoxy Bonding Agent. The jo in ts between the

pr ecas t segments a re o f c r i t i c a l impor tance in segmental br idgecons t ruc t ion . They must have high s t r eng th to t r ans f e r shea r . I ftendons pass through the jo in t s then they must have assured durabi l i tyin ord er to pro tec t the t endons agains t cor ros ion . In ad d t o n,

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cons t ruc t ion o f the j o in t s must be reasonably easy and not over lysensi t ive to atmospheric conditions.

In match-cast segments single or multiple keys are almost alwaysused in the webs with epoxy r es ins of ten used to coat the contactsurfaces between adjacent segments.

The web keys serve two funct ions. The f i r s t i s to al ign thesegments during erec t ion . The second i s to t ransfer the shear forcebetween segments during that period while the epoxy applied to the jointi s s t i l l plas t i c and acts only as a lubr ican t . At tha t t ime the webshear keys alone must be relied upon to transfer the shear force acrossthe jo int since the fluid epoxy bonding agent minimizes the coeff icientof fr ict ion between the segments. Examples of single and multiple keysare shown in Fig. 1.2. A t yp ica l mult ip le key j o in t has a se r i e s ofsmall interlocking keys over the entire web height. This arrangementwas developed to relieve the cured epoxy bonding agent of any structuralfunct ion [4 5J. Use of in ternal s t i f fene rs on the webs provides ananchor zone for the permanent pres t ress ing tendons thus moving themfrom the face of the web and permitting use of the multiple key design.

In some br idges however shear keys have not been used. In thePasco-Kennewick In terc i ty Bridge the match-cast joint contains no shearkeys [6 7J. o fac i l i ta te alignment 2-in. diameter steel pint les wereused in the top slab at each joint . The segments were bonded togetherwi th epoxy. The shear force acting on the joint is al ways smaller than5 of the longitudinal force because of the multi-cable stay system.

The major function of epoxies used in segmental jo ints i s fourfold:In the l iquid s ta te

o act as a lubricant which faci l i ta tes joint ing.o even out minor i r regular i t ies between the mating surfaces and

to perfectly match the adjoining segments.In the cured s ta te

o provide water t igh tness and durab i l i ty a t the j o in t and toprotect the post-tensioning tendons running through the jo intfrom corrosion.

o t ransfer shear forces and to contr ibu te to the s t ruc tura lr ig id i ty .

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5

a) J K Memorial Causeway, Corpus Christi (Single key)

.25

2896 (h) dIs 3/1243 d/h .21

Web Key (Detail)

b) Long Key Bridge (Multiple keys, dry joint)

3-7/8 Number 9(h) of Keysh/H .60

R= 1 dIs 2/1read/h .32

Web Keys (Detail)

Fig. 1.2 Examples of shear keys

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Although in the ear ly development of segmental construction, themain purpose of the epoxy bonding agent was to transfer shear stressesbetween adjacent segments, proponents of the use of mult iple shear keysclaim tha t th i s i s no longer necessary. In fact some recent bridgeshave not used the epoxy bonding materials . For example, in the Long KeyBridge in Flor ida the use o f epoxy was omit ted [8 ] . Dry j o in t s wereused with span-by-span erection, internal location of the tendons, nonfreeze-thaw climate, and use of multiple shear keys.

Epoxy joints with a single large key in each web have been widelyused on ear ly pro jects in the U S Most appl icat ions have beensuccess fu l . However, a ser ious jo in t f a i lu re occurred on a bridgeacross the Kishwaukee River in I l l i n o i s [9]. The epoxy fa i led toharden, lubr ica ted the j o in t and caused cracks and spal l ing in a webwhere a s ing ly keyed j o in t was used. The fa i lure was a t t r ibu ted toimproper blending and i nsuf f i c i en t mixing of the epoxy res in andhardener. This ind icates that improper use or choice of the epoxy canbe cr i t ica l with respect to shear strength of the jo in t .

1 2 2 Objective. ue to the rapidly growing number of segmentallycons t ruc ted br i dge s t he r e i s a need to b e t t e r unders tand thecharacteris t ics of the different shear key configurations and the roleof the epoxy bonding agent in shear t r ans fe r . The overal l success o fprecast segmental construction will depend heavily on the behavior ofthe jo in ts .

The object ive of t h i s explora tory study was to determine there l a t ive shear t ransfer s t rength across di f f e r en t types of j o in t scommonl y used in precast segmental bridges. Types of joints consideredincluded s ing le large key, mult ip le lug-keys and no-key jo in t s . Bothdry and epoxied jo in ts were stud ied

Currently, shear fr ict ion concepts are often used in the designo f j o in t s . Thus, the dry jo in t with no-keys was included to al lowr esu l t s to be re la ted to shear f r i c t ion theory. All specimens werecompared to the behavior of monolithically cas t specimens to indicatere la t ive eff ic iencies . The t es t ser ies was envisioned as an exploratoryse r ie s and did not i nves t iga te e i the r the wide range of formulat ions

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7available for epoxies or consider the wide range of loading conditionspossible in service.

1 2 3 Shear Test. To accomplish the above-stated objective, sevenjointing conditions for the test specimens were determined, as shown inTable 1.1.

Since the shear force act ing on a box girder sec t ion i s carr iedprimarily by the webs, simple rectangUlar web model sections were usedin the t e s t s . Dimensions of the smal l - sca le model specimens are thesame as those of the 1 4 sca le model used by Stone for the s tudy o fpost-tensioned anchorage zone tensile s tresses [10]. Figure 1 3 showsthe relationship between typical box girder sections, a prototype websection, and the model web section used.

In order to obtain the re la t ive shear t ransfer strength across thejo in ts the specimens were subjected to a predominantly shear t es t usingthe loading scheme shown in Fig. 1.4. This corresponds to a low aidra t io . Since dis t r ibu ted load appl icat ions provide smaller maximummoment and less bearing stresses than concentrated ones, while givingthe same amount o f shear force at the j o i n t the load was appl ied inthat manner as shown in Fig. 1.4 b) .

Since time and resource requirements res tr ic ted the magnitude oft h i s explora tory study, only one t e s t specimen was made for eachjoint ing cond i t ion. Figure 1 5 i l lus t ra tes the fabricat ion sequence ofthe model precast segments and the t e s t specimens made out o f thosemodel segments. Fabr ica t ion methods for the model segments and thede t a i l s of the t e s t wil l be described in the fol lowing two chapters .The test results will be discussed in Chapter 4.

1.3 Previous Related StudiesSeveral research papers which deal directly or indirect ly with the

subjec t studied herein have been published to date . Some of them arereviewed below, and the re su l t s obtained from those s tud ies wil l bereferred to la ter in Chapter 4.

1.3.1 Shear Fr ic t ion. Shear fr ic t ion theory applic at ions forprecas t connect ions are based on the work done by Birkeland andBirkeland [11] and Mast [12] a t B M Engineers, Inc. , and Concrete

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8

Specimen

12

3

5

6

7

T BLE 1.1 TEST SPECIMENS

Type of Epoxy Shear CarryingJoint Key MechanismNo-Key No FrictionSingle No Friction + KeyKeyMultiple No Friction + KeysKeysNo-Key Yes Friction + EpoxyBondingSingle Yes Friction + Key + EpoxyKey BondingMultiple Yes Friction + Keys +Keys Epoxy Bonding

MonolithicSpecimen V +VNo-Joint) c s

Concrete StrengthFixed Condition Amount of Prestress

Type of Epoxy

PracticalApplicationNoNo

Yes

Yes

Yes

Yes

*Pasco-Kennewick Bridge -Prestressed concrete cable-stayed bridgewith multiple stays-) has no shear keys n the web, but has steelpins n the upper slab.

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80

Typical web reinforcement,Longitudinal: 13 or 14 10 -12\Transverse: 7 or 18 @ 12 -15lllillIflf Shear area 7 - 9 typo

70 - 9 5 ' J 1 I ' : ~ I . . . _typo 12 -15 typo

(a) Typical box girder section

12

@ 12

(c) 1/4 web model(b) Web prototype

Fig. 1.3 Web model for the tests

9

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Load P

IIVJ01n tIIII

P/2 lp/2.

Moment dueto load

Shear forcedue to load

\

1: p = P)

II~ o i n tIIIII

a) Concentrated load appl icat ion b) Distr ibuted load appl icat ionFig. 1.4 Shear t es t of jo in t

t-o

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I,IIII

,I I 1I I II I I II I 1I I 1

(a) Fabrication sequence of 7 model precast segments

No key

Multiplekeys

Single key

Without epoxying With epoxying(b) Test specUnens

Fig. 1.5 Model precast segments and t s t spectmens

MaCu

IIII

Solid

tch-cast jointst t ing l ines

I 203 Section

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2

Technology Corp., Tacoma, Washington. The ACI Building Code [13,14]adopted the theory, referr ing to other re la ted s tudies [15,16] byMattock and Hawkins. The AASHTO Specif ications [17] are la rgelypatterned after the ACI recommendations.

Shear frict ion theory is an explanation of interface shear transferwhich states that the shearing force across a potential crack or planeof weakness is resisted by virtue of frict ion along the crack or plane,i f there is a normal compressive force across the crack.

oeff ic ien t of f r i c t ion values given by Mast, the AASHTOSpecifications, the ACI Building Code, and the PCI Design Handbook areas follows:

Concrete placed monolithicallyConcrete placed against

hardened concrete withroughened interface

Concrete cast against steelConcrete cast against smooth

concrete

Mast1.4-1. 7

1.40.7-1.0

0.7-1.0

AASHTOand ACI318-77

1 .4

1 .00.7

PCIHandbook1.4

1.00.6

0.4

1.3.2 Shear Tests on Joints ~ i t h No-keys e t ~ ~ Precast Post tensioned Units. A number of such tes ts have been reported.

1.3.2.1 Test.Q1. Franz at the Karlsruhe Technical ~ o l l e g e in ~ e s tGermany [18]. The test ing method used by Franz is schematically shownin Fig. 1.6. The t es t resul t obtained from the specimens without anyepoxy, any mortar or indentat ion in the jo int surfaces indicated tha tthe coeff icient of frict ion is practically independent of the amount ofthe normal force, and the value is around 0.70. I t was also independentof the eccentrici ty of the normal force existence of bending moment .Applied normal stress at the centroid was 230 to 930 psi.

1.3.2.2 Test by Jones at e ~ e n t and Concrete Associat ion inEngland [19]. The test ing method used by Jones is schematically shownin Fig. 1.7. The surfaces of the ends of the specimen butted together

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N

Prestressing force

1 N[-1 ~J ____ t

I > > 7 177>>

20 mI ..

Section

Pas t tens ioningons

d 20 m

Fig. 1.6 Shear test on joints of precast segmental beams by Franz Ref. 18

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(a) Plain but t joints

-'-~ 1 - -- --

I L 27r(b) Precracked units

. - -

~ p l i e d load

3 Ij-I} 1-1/2

-

t 21

-

/Applied load

-

lJ

Prestressingforce

~ aSectionIi a

Fig. 1.7 Shear tests on joints between precast units by Jones (Ref.

t

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5

were obtained by casting against stee l bulkheads, thus resul t ing in avery smooth finish. Failure occurred by slipping of surfaces. Theminimum value of the coefficient of fr ic t ion of the surfaces a t theplain butt joint was found to be 0.39 and tha t of the surfaces at amortar jo int 0.65. Up to 3000 psi of prestress, the coeff ic ient offriction is constant.

1.3.2.3 Test by Gaston and Kriz at Portland Cement Association[20]. The testing method used by Gaston and Kriz is schematically shownin Fig. 1.8. The nature of the contact surface, the contact area of thejoint, and the normal stress on the contact area were variables. Halfof the specimens were assembled with no bonding medium between thesurfaces, and half had a 1-in. layer of mortar between the concreteblocks. The slip between the contact surfaces increased slowly untilthe maximum load was reached and a sudden, large s l ip occurred. Novisible damage to the contact surfaces of ei ther the bonded or theunbonded specimens was detected. They reported that the coefficient offriction may be pred icted as

F 43 x AJ = N = 0.78 N ] for unbond ed specimenAj = contact surface area in. 2N = normal force lb)

This ind icates that increases slightl y as the contact area increasesor as the normal force decreases.

1.3.2.4 Test by Moustafa at Concrete Technology Corp., Washington[21]. The performance of a segmentally constructed prestressed concreteI-beam bridge was investigated by Moustafa. In order to test the jointi tse lf without any help from shear keys or alignment pins, the segmentswere cast with f l a t smooth ends. Epoxy was applied on each of themating surfaces. To determine the shear strength of epoxy joints, smalltes t beams made from 6-in. cubes were prepared in the same way as thesegmental girders. The loading was applied in such a way as to force afailure in pure shear at the joints Failure always occurred in theconcrete layer adjacent to the epoxy. The shear strength increased from

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6

~ ~ ~ ~ rI iI...:t

I I~ iI

I- f)-II iI

S 1

8 / ~ Z I I '7/1-SI I I :I I I II I II I I III I , II I I ,I II I II I I II I I III I I , II I I II II I I II I I II I I IL I II I I ,II I I II I I II II I I II I I It t

'

118/1 L

r ;

N

..be

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71130 to 1900 psi when the normal prestress introduced by post-tensioningwas increased from 0 to 400 psi . From these resu l t s i t was inferredthat the shear s t rength of the jo in ts i s not c r i t i ca l in precastsegmental beam girders when epoxy is applied to the joints .

1 3 3 Shear Tests on Joints with Single Key between Post-tensionedPrecast Segments. Comprehensive studies were made by Kashima and Breenat The University of Texas a t Austin [22] related to the JFK MemorialCauseway Bridge in Corpus Christi , Texas As a part of the studies, anultimate shear test was carried out, using a 1/6-scale model specimenwith whole box girder sect ion and jo in ts having s ingle large keys.Epoxy was applied to the joints. The test specimen essent ial ly developed the theoret ical shear for a monoli thical ly cast box girder . Theresu l t s of the t es t can be taken as a conclusive indicat ion of theefficiency of properly applied epoxy joints in segmental construction.Provision of these joints did not significantly lower the shear strengthof the unit .

Design of the key in single key jo in ts may be considered to besomewhat analogous to that of corbels. Among the references related tothe corbel design are Refs. 1 3 1 5 1 6 2 3 2 4 2 5 2 6 and 27.

1.3.4 Studies Shear Strength of Multiple Keys1.3.4.1 MIT Inves t igat ion. A comprehensive l i tera ture review of

the past s tudies on jo in ts in large panel precast concrete s t ructureswas conducted by Zech a t Massachusetts Ins t i tu te of Technology [28].Several parameters inf luence re inforced concrete jo in t s t rength andbehavior. Among the most important of these are the geometry of paneledges, bond between jo in t and panel concrete , and existence of normalforces simultaneously acting with shear.

a) Geometry of panel. The geometry of panel edges will determinethe amount of mechanical interlock at the joint. In increasing order ofstrength, these jo in ts may be plain, grooved, or keyed Oigh t ly -heavily). Under monotonic load, keyed joints may be as much as 3 to 4 5times stronger in ultimate strength than plain ones when the joints areotherwise ident ical ly constructed. Strength is dependent not only onthe presence of keys but on thei r shape and size. Tests of sinusoidaland t r iangular keys have shown less shear s t rength than the typical

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18trapezoidal forms. I t has further been shown that the slope of the keyfaces should be greater than 55 to 6 for grea test strength. I t isrecommended tha t the depth of keys be no less than 0.4 in. and thedepth- to- length ra t io d/h be greater than 0.125 see Fig. 1.9). Forcomparison, examples of multiple key configuration used in three recentsegmental bridges are shown in Table 1.2.

The reason for the depth-to-length ratio l imit is that extremelylong keys will fa i l by shearing off or crushing of only one corner,ra ther than the whole key, with a resul t ing smaller fa i lure load. Ingeneral , the larger the proportion of key area to panel edge area, thestronger the joint will be. This relationship will only be true up to acertain point. Beyond this , failure will commence in the overall panelrather than in the jo int keys.

b) Bond e t ~ ~ jo in t and panel concrete. Until the bond isbroken, the behavior of a jo in t panel assembly i s approximatelymonolithic. Unreinforced jo in ts will fa i l by s l ip at the contactsurface as soon as bond is broken. This i s a dis t inc t ly br i t t l efailure. In one case, shear capacity of unbonded castellated joints wasfound to be higher. This was thought to be due to the more uniformshear distribution along the joint .

c) Existence of normal compressive forces. The effect of normalcompression is cumulative with that of the shear friction steel acrossthe jo in t so that the important parameter was found to be N Avfy.This is reasonable since the effect of either is to increase fr ict ionalresistance. The effect of normal compression is the same as that of theclamping act ion of the reinforcement, except tha t s l ip is required tomobilize resistance of the shear friction reinforcement. Post-tensionedjo in ts provide an added shear f r ic t ion resistance beyond tha t in anotherwise identical nonprestressed joint .

1.3.4.2 Tes t by Kup fe r li c h . 2 . ~ : . . s . e r ~ s . Da . ~ : . a t heTechnical Universi ty , Munich [29]. A very in teres t ing ser ies ofexperimental invest igat ions to determine the s t ructural behavior ofsegmental precast prestressed girders with cement mortar jo in t s andepoxy bonded jo in ts was undertaken by Kupfer e t al . In prel iminarytests both the use of a modified cement mortar by application to very

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TABLE 1 2 EXAMPLES OF MULTIPLE KEY CONFIGURATION

1dI s[J-/ _ h

s

Failureload

o

BridgeNumberof Keys

/

h/HdIs

dlh

.125

Long RedKey River

9 7-3160 73

2/1

32 31

25dlh

LinnCove2

70

1 25/1

.36

Fig 1 9 Effect of shear key height to depth ratio Ref. 28)

9

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20thin jo in ts of only a few mill imeters wide closed jo in ts and thedevelopment of the strength green strength) of grouted joints of a fewcentimeters wide were investigated. Concrete prisms with jo in tsoriented obliquely to the direction of compression see Fig. 1.10 maybe regarded as sections of a compressing s t ru t in the jo in t area of asegmental precast girder. Compression tests on such prisms with cementmortar jo in ts provided with multiple keys and a large angle (cr= 50 0 yielded a jo in t strength amounting to 91 of that of comparablemonolithic concrete prisms in the case of the closed jo in t and 18 inthe case of the grouted j o in t . The structural behavior cracking) andthe load capacity especially concerning transverse shear) were furtherinvestigated by load t e s t s on two segmentally constructed prestressedgirders, about 9 m long and 10 cm deep see Fig. 1.11 . The girder withmultiple key joints of modified cement mortar showed the same favorablebehavior as the girder with epoxy bonded multiple key j o n t ~ The webfai l ing in incl ined compression indicates that in both cases the loadcapacity due to shear was not impaired by the presence of the multiplekey joints .

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e,)oD

F

B

Cemeni /ortor /

S = O 7 m m ~50

Section B

J 20cm JFig. 1.10 Concrete prism specimen from Ref. 29)

21

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Section CC10= CJSiO to= ti lID e IUie IT lS I I i i ~ c m ;-JC:

O0j4.F

F.0 6.F 1MA= 175 F2 =- 05 F,

-- , r-- ' 75em156 lei 56 All dimensions in em.

Shear Diaora m

A=F

Moment DiaQram

FI ls.O....

Fig. 1.11 Segmental girder specimenRef. 29)

NN

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C HAP T E R 2

TEST SPECIMEN

2.1 Specimen DimensionsFigure 2.1 shows the general profile of the model precast segment.

Dimensions of the segments were 3 in. width) x 20 in. depth) x 48 in.length), except for the f i rs t segment whose length was 20 in.

For s t i r rups 6 1/4 in.) deformed bars were used at 3 in.spacing. Ten gage wire was also used at 3 in. spacing for supplementarylongitudinal reinforcement. The reinforcement cage of th is 1/4 scalemodel segment corresponds to typical prototype web reinforcement of 8stirrups and 4 longitudinal bars both at 12 in. spacing see Fig. 1.3).However, th is correspondence of model reinforcement to that of someprototypes has relatively l i t t l e importance since the joint shear tes tshown in Fig. 1.4b) was chosen so that the maximum shear force occurs

only in the jo in t v icin i ty or at the mirror image of that jo in t .However, the web reinforcement is important in the t e s t of themonoli thic model with no joint used as the baseline for comparison ofthe various jointing methods.

Two tendon ducts one at the top and the other at the bottom) wereplaced in each specimen with 8 in. of eccentr ic i ty . Prestressingtendons were later inserted to provide normal force and bending momentresistance. The duct location caused minimal disturbance in the centerportion of the jo in t surface and the tendons prevented s ignif icantflexural tensile stresses in the specimen.

The three types of joint configurations examined are i l lustrated inJ:ig. 2.2. In the s ingle key specimens, both male and female keys werereinforced with 10 gage wire. On the other hand, in the mul t ip le keysegments, no reinforcement in the keys was provided as is the practicein usual multiple key joint construction. A trapezoidal shape withoutany intentional rounding off of the corners was used for the multiplekey configuration. The characterist ics of the single and multiple keyswere meant to be similar to those of the JFK Memorial Causeway Bridgeand the Long Key Bridge, respectively.

23

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(unit - lnchea)]/4 . tendon ductConduit)

--::Jr: :"

CUUlnBl tne\

- - I I - - -u- 11 _ __ _

10 _ e vhe6 deformed bar

1/2 )

II II 1 I . I :I . I \D- L..I

All

cC i

N[ ::jJ===11==1I===l= ~ = = = ~ = = I I = : I I ~ ~ = = ~ = = ~ = = = ~ ~ = ~ = = I =1 1 ~l 15 @4: 45 J I J I 1SectioD. A-A :

Fi 2.1 Profile of model precast segment

Nj

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~ ~ l ... . . . . . . . ~ . -/ ~ ' :-> :- :~t - - - ' I I ~ I I -11-1 I

II

1 l11 l l l

-11 II

' - ' / ~ : : : : t z L _ ~ ~ b- - ; ' ; ~ ~ f = 2 1 / / ~ ~ E : : : f V. jJ l j

N

00

00

N

,,------tt1l-10 gage

Key area/web area = .3dIs = 3 /1 d/h = .25)

wire

a) No-key b) Single keyFig. 2.2 Configura t ion of jo in ts

Unit : inches)

I..:: =::H:::::::l : II a

- I f ' ; ; X ~ i ( ~ ; : ~ ~ ' f . I}':'-

h/H = .63, dIs = 2/1,d/h = .4)c) Mult iple keys

I

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262.2 Materials

2.2.1 Microconcrete. Microconcrete was used as a model concrete.Mix proport ions of the microconcrete were determined based upon thatused by Stone [10]. The basic design mix proportion used is shown inTable 2.1. Design 7-day compressive strength of the concrete was 4000psi. Twelve 3 in. diameter x 6 in. cylinders were made at each castingof concrete. Cylinder specimens were tested at 7 days and at the timeof each shear test . Concrete strengths at time of testing are shown inTable 3.2. Prototype for the concrete mix proportion was Type Hspecified by the Texas State Department of of Highways and Public Transportation [10].

2.2.2 Reinforcement. Six (1/4 in.) Grade 60 deformed bars and10 gage plain annealed wire were used as reinforcement. Figure 2.3shows a s t ress -s t ra in re la t ionship of 6 mm rebar. The mechanicalproperties of the bars are also shown in the figure. The stress-strainrelationship of the annealed wire is shown in Fig. 2.4. Yield strengthof the wire was much lower than expected. The wire was not deformed,but several studies verif ied that the bond s t rength of the wire tomicroconcrete would be adequate.

Al though, as mentioned in Section 2.1, reinforcement of thespecimens might not have a significant meaning in the shear tes t exceptfor the baseline monolithic specimen, such conventional prac t ice inmaking structural model tes t specimens was followed in all specimens.

2.2.3 Prestressing Tendon. Two-hundred seventy ksi 1/2 in.diameter seven-wire pre s t re s s ing s t rand (fpu 270 ksi Aps0.153 in.2 was used in the test . The tendon sheath was flexible metalconduit with 3/4 in. inner diameter.

2.2.4 Epoxy. The epoxy bonding agent was a standard concretepatching adhesive formulated by the Texas State Department of Highwaysand Publ ic Transpor ta t ion (Epoxy Adhesive A103 . The s t rengthdevelopment characterist ics of the epoxy are shown in Fig. 2.5. Thesestrength development curves were obtained from a simple bond tes t usinga metal-to-metal lap shear joint, which was carried out using 2 in. widemetal sheet s t r ips . This preliminary t e s t was conducted with twodi f fe ren t lap areas, i .e. one with 2 in. x 1/2 in. lapping and the

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TABLE 2.1 DESIGN MIX PROPORTION per f t 3

3/B aggregateblast sand

Ottawa sil ica sandType III cementWaterAdmixture ASTM C494 Type B)

Water cement ratioCement ratio

41.0 lbs33.B lbs36.B lbs21.25 lbsl4.B lbs0.51 f l oz.

0.706.4 sacks/yd3

27

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8

7

c ~ c h a n i c a l PropertiesAverage of 3 Specimens)61.1 kaiy (3 )

0.00200Y (5 )E 30,500 ks(5 )

o 0.002 0.004Strain in. l in.)

Fig. 2.3 Stress-strain relationship of 6mmreinforcing bars

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10

ksi)50

40

10 gage wire(0. 135 in. q,

Pt] 20Omm(Demec gage length)

p: 30 , ,- I1 1. j. j

I .f.l

20

10 I Origin for strain measurement(Demec gage) oStrain

= 33.6 ksif 1

Y= 45.2 ksiu t

(Average of two spectmens)

1,000< 10-6

2,000

Fig. 2.4 Stress-strain relationship of 10 gage wire

N\

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t:l I'CI

r IbO

-.-1.lII:ctlQJ1 1a I

lb)

1,000

500

P - -

oo

- Ppoxy - - lapP 2 widemetal sheet

Epo P0 P >

. a r; JIo2 widemetal sheet

o 5 10Age of epoxy

Fig. 2.5 Development of strength of epoxy

Shear tes t ages foreoncrete specimens wepoxying

15 (Days)

w

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31other with 2 in. x 2 in. As seen in Fig. 2.5, both specimens gavesimi lar development curves of epoxy strength. The shear t e s t for theepoxied concrete specimens was planned to be carried out when the epoxybonding agent was considered to have developed i t s s t reng thsuff ic ient l y. The epoxy bonding agent was exposed to the samelaboratory curing condit ions (mainly, temperature) , both in theprel iminary t e s t on the metal s t r ip specimen and in the shear t e s t ofthe concrete web model specimens.

The compressive strength of the epoxy was approximately 5000 psi at24 hours according to a rough compression test . I t was assumed that thestrength would reach more than 6000 psi at the time of the shear tes t .

The following is an extract from Texas Highway Department SpecialSpecif ica t ion Item 2131 Epoxy Bonding Agent, which is based upon thework done by Kashima and Breen [20] at The Universi ty of Texas a tAustin:

The epoxy mater ial shal l be of two components, a resin and ahardener, meeting the following requirements:a.b.c.d.e.f.

Pot l i feCompressive strengthTensile strengthSpecific gravityViscosity at 68FCoefficient of thermal

expansion

90 minutes min. at 68F6000 psi min.2000 psi min.70 to 120 lbs/cu. f t .10,000to 50,000 cpsWithin 10S of that for concrete

The jo in t mater ial shal l be able to develop 95 of the f lexuraltensile strength and 70S of the shear strength of a monolithic testspecimen.

The Precast Segmental Box Girder Bridge Manual [2] specifies sevenepoxy bonding agent t e s t s which are (1) sag flow, (2) gel t ime, 3)open time of mixed epoxy bonding agent, (4) three-point tensi le bendingtes t (5) compression s t rength of cured epoxy bonding agent, (6)temperature deflection of epoxy bonding agent, and 7) compression andshear strength of cured epoxy bonding agent.

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3These t es t s on the epoxy were not performed in t h i s study s ince

t h i s epoxy had been examined thoroughly in connect ion with otherprojects [22,30].

2.3 Fabrication2.3. Reinforcing Cages and Forms. The cages were t i ed by hand

and shear key reinforcement was added for segments with single keys.Plywood forms were used. n order to firmly posit ion the preceding

segment in match-casting, an adjacent bottom form panel was provided.Three types of end forms were fabricated. These were for the segmentswith no-key, with s ing le key, and with mult ip le keys, respect ively .Single and multiple key configurations were produced using metal sheet,as shown in Fig. 2.6.

2.3.2 Cast ing Procedure. Concrete segments were care fu l lyfabricated. Special procedures included:

Preparation--Lining of the end face of the previously cast segment with aluminum

foi l for bonding breaking purposes. (This was done successfu l ly .There was no t rouble in l in ing even mult ip le shear keys. Thealuminum foil was also painted with lacquer and oi l so as to avoidchemical reaction between the aluminum and the high alkaline cementpaste so lu t ion . In ac tua l match-cast ing of precas t segments, aplain vegetable soap, sometimes mixed with talcum powder, i s oftenused as a bond breaker between segments. I t i s r insed of f withwater after str ipping, since the face of the segments must be cleanfor applicat ion of the epoxy. Gallaway [31] reports tha t to aid instr ipping and provide a small clearance during erection, the femaleportion of the previously cast shear key can be l ined with plast ictape. The use of aluminum foi l el iminated the r ins ing ou toperation of the vegetable soap.)

--Assemblage of the form panels in match casting position (Fig. 2.7) .Inse r t ion of temporary s tee l rods in tendon sheaths to keep thesheaths s t ra igh t .

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33

I .

a) Single key

l .

- ., f .- _ \ _ ~ I _ . ~ . . .

b) Multiple keysFig 2.6 End forms

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Wooden form made of 3/4 plywood . 2 x 4 , studs

1 3 [ = : : : = : ' h ~ : = = = = = ~ J :i j\Match-cast /001

48

20 [:

Old segment New segment

Fig. 2.7 Hatch-cast ing of segments

w: -

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35

Casting and Curing--Microconcrete was mixed in a 2 cu. f t . laboratory mixer.- -Concrete was compacted in te rna l ly with 6 mm s t ee l rods, and

externally with a concrete vibrator .--Forms were removed 24 hours af ter casting.--Separation of the segments Fig. 2.8).

Cutt ing and grinding of sheath a t end face with a hand saw and ahand grinder.

Movement of the segment into the next posit ion for match-casting.--Curing of segment concrete and cylinders with plast ic sheet cover

and occasional water supply for approximately 1-2 months.2.3.3 Concrete Streng th l:.1. 3 in. x 6 in. cyl inder specimens

were capped with sulfur capping material . Three cylinders were testedfor each concrete a t 1 days a f t e r cast ing for the purpose of qual i tycont ro l . The t e s t resul t s for the 1 day compressive s t rength o f theconcrete are summarized in Table 2.2. Overall average of f61 was 4410psi and the coefficient of variance of the data was 8 . This resul t wasconsidered to be sa t isfactory .

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~ t c h c a s t joint48 481250 lhs 250 lhs

t ie

Old segment \ ew segment

Fig. 2.8 Separation of segments af ter form removal

.......,

120

W

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37

TABLE 2.2 CONCRETE STRENGTH AT 7 DAYS

7 ps i)Casting Coefficient of variance)

SequenceSegment

Each(Ave. of 3 - ~ 3 ' ~ Total

1 D 4,670(7 )2 I 5,120(4 )3 ( 4,080(5 )4 { I 4,240 4,470(5 70) (8 )5 I 4,360(7 )6 ( 4,340(5 )

] 4,480(1 )

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"#$% &'() *)&+',)% '- $-.)-.$/-'++0 1+'-2 &'() $- .#) /*$($-'+3

44 5"6 7$1*'*0 8$($.$9'.$/- ")':

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C H A P T E R 3

TEST PROCEDURE ND RESULTS

3.1 PreparationEach model segment was cut into two parts , as i l lus t ra ted in Fig.

1.5 using a concrete saw. The cut sur faces were the exposed end facesof the test specimens and were not to be joined.

3.2 Joining and Post-Tensioning3.2.1 Match-cas t Jo in t Surfaces . For match-cast j o in t s the

surface including the formed keys should be even and smooth to avoidpoint contact and surface crushing or chipping off of edges during posttensioning. I t is part icularly important before applying epoxy tha t theadjacent surfaces are sol id clean, and free of dust and greasymaterials. s in any adhesive bonding, the preparation of the surfacewill quite often determine the success of the jo int .

Tiny pi t s were observed in some of the match-cas t keyed jo in tsurfaces . However, they were l e f t as they were, s ince none of themseemed to be harmful even for the specimens tested without epoxy. Suchpits can be found in the surfaces of actual match-cast precast segments.

The j o in t sur faces were cleaned with a wire brush and wiped withacetone. In actual precast segmental construction, i t is recommendedthat l igh t sandblasting be used for preparation of the concrete surfacesfor good bonding.

3.2.2 Epoxying. Components of the epoxy mix resin and hardener)were propor t ioned and mixed thoroughly unt i l a uniform color wasobtained, following the instructions of the manufacturer.

The concrete sur faces to be bonded were kept dry. The epoxyadhesive was applied by hand, using protective gloves, immediately af termixing. Both mating surfaces were coated and brought together while theepoxy was s t i l l viscous.

The Prestressed Concrete Inst i tute [2,32] specifies tha t a minimumcompression of 30 psi shal l be provided by means of temporary pos t-

39

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40tensioning over the entire joint area during the open time period of thebonding agent until the permanent tendons are stressed. Uniform posttensioning stress of about 70 psi was applied in this ser ies . Duringthe provisional post-tensioning, excess epoxy was squeezed out of thejoint. Care was taken to prevent epoxy from entering the tendon ducts.The temporary clamping force was provided using the post- tensioningsystem described in Sec. 3.2.3. The load was controlled by calibratedload cel ls and pressure indicators for the f i rs t specimen. Afterobtaining the relationship between load cells and pressure indicators,only the la t ter were used. The epoxy joints were air-cured inside thelaboratory for 2 to 4 days before the specimens were tested.

3.2.3 Prestressing. The post-tensioning arrangement is shown inFig. 3.1. Special provisions in the post-tensioning procedure included:

Preparation - Large end plates were used to distribute the bearingstresses to prevent end surfaces from bursting or chipping off.Prestressing - As seen in Fig. 3.1 anchoring chucks with nuts and

anchor plates with s l i t s were used for the purpose of detensioning andremoval of the strands after the test .

- Stepwise loading of prestress so that significant tensile stresswould not appear in any part of the specimen concrete. Top and bottomstrands were tensioned alternately. The loads were monitored by loadcells along with pressure indicators. Relatively large seating loss wasexperienced. This was partly because the length of the specimens, whichwas 48 in., was short. The amount of prestress was determined asdescribed in Sec. 3.2.4.

3.2.4 Prestressing Forces. According to the ACI Building Code[13], maximum permissible tensile stress in prestressing tendons due tojacking force is 0.8fpu. In the t e s t the bottom strand was tensionedup to 0.7fpu which corresponds to 28.9 kips. o attempts were made toincrease this value. As mentioned above significant amounts of seatingloss were observed af ter force t ransfer . The force in the top tendonwas adjusted to maintain the condition that the s tress from prestressingalone at the top fiber would be zero.

As wil l be shown la ter the average prest ressing forces in thebot tom and top tendons just before the shear test were 18.7 kips and 6.7

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... ______ Anchoring chuck with nut (112 )

.........-.

1/2 7-wirestrands

Anchor pla te___ 50 ton load ce l l

~ ) Anchor pla te with a l i t

Woodpla te

~ = = = : = = = : : : ~ ~ : = ~ = = : : : : : = : ~ ~ ~ ~ ~ ~t 6.7 K (Avg.) I8 V oint

II

Anchoring chuck (1/2 t1 am chair

30 ton centerhole ramTemporary chuck

~ To hydraul ic pump8 I_ : b _ : : ~ : ? _ K _ ~ ~ ~ ~ I _

To s t r a in \ind ica torEnd anchorpla te

It rydros tonebeeween concrete and end plateea t spec imen .Fig. 3.1 Post-tencioning arrangement

t

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42kips, respectively, with a to tal of 25 4 kips. The average compressives t r e s s in concre te due to the pres t ress ing was 424 psi with thecoeff icient of variance of

3.3 Shear Test ArrangementFigure 3.2 i l l us t ra t e s the shear t e s t arrangement. The load was

applied using a calibrated S TEC tes t ing machine. In order to achieve adistributed load application, three sets of bearing plates were used, asseen in Fig. 3 2 Hydrostone was appl ied between the top load ing plateand the top sur face of the specimen. Since bottom surfaces o f thespecimens were as f la t as the bottom form panel, no hydrostone was usedthere . Instead, the bear ing s t re ss was lowered by inc reas ing thebearing area of the plate . This worked well and no damages wereobserved a t the bottom surface of the specimen after the t e s t .

The t es t was conducted immediately a f t e r the appl icat ion of thefinal prestressing. Tendons were not grouted. Our ing the tes t forcechanges in pres t ress ing s t rands were measured using load ce l l s . Nos ign i f ican t changes in pres t r ess ing forces were observed unt i l majors l ip took place.

Slip a t the j o in t s was determined visual ly and by use of a dia lgage. The dial gage was attached to the specimen with a clamp, so thati t could indicate a re la t ive displacement between the points across thejo int . t should be noted tha t the or ig ina l purpose of the dia l gagewas j us t to de tec t the s l ip occurrence and, t he re fo re the setup wassimple rather than sophisticated.

3.4 Test Results3.4.1 Stresses Due to Pres t ress ing . S t resses in each specimen

introduced by pre s t ress ing are sum mar i zed in Table 3.1. Over a l laverages of pres t res s ing s t r es ses a t top, a t cent ro id and a t bottomwere 56 psi tension), 424 psi compression), and 903 psi compression),respectively. l though sl ight flexural tensi le stresses were computedin the top por t ion of the segment sect ion t h i s was not observed tocause any t roubles in the t es t . Figure 3.3 shows t ha t the t e s tspecimens had only minor f lexural tensi le s t r es ses a t the j o i n t s

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HydrostoneRam* *SATEC tes ting machine

~ ,---==- ::.. ::.. ::.. ::..---I 6 8 . : ~ Joint>11

7 _ 1 ~ 7 , , 13 ~ 3 . , l 4 n J3 P

- 4Welding- ~

Loading table*

Fig. 3.2 Shear t s t arrangement

Dial gage

1/2 7-wire posttensioningstrands

+v

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44

TABLE 3.1 STRESSES UE TO PRESTRESSING

N M e Stresses*or or or NIA MIs psi)Specimen Ft Fb 8 Fb - Ft ) MINkips) k-in) in) Top BottomNo-Key 25.3 92.0 3.64 38 -882w O

E Single Key 26.0 86.4 3.32 1 -865P Multiple Keys 24.8 86.4 3.48 19 -8450X Average 25.4 88.3 19 -864Y

Standard deviation) - - 19) 19)Coef. of Variance) (2%) (4%) - -wI No-Key 25.7 101.6 3.95 80 -936E Single Key 25.1 98.4 3.92 74 -910p0 Multiple Keys 25.9 103.2 3.98 84 -948XY No-Joint D 25.0 94.4 3.78 55 -889orS No-Joint @ 25.6 104.0 4.06 93 -9470 Average 25.5 100.3 77 -926LI Standard Deviation) 14) 26)-D Coef. of Variance) (2%) (4%) - -

Total Average 25.4 95.8 3.77 56 -903Standard Deviation) 0.4)Coef. of Variance) (2%)

k6.7 Avg.)Ft8- - ~ 8 ~ t t ~ ~ ~ ~ ~ ~ ~ ~ N ~ ~ M

Fb18. 7k Avg.)

7.1) 0.26) 33)(7%) (7%) -* -: Compression+: Tension

39)-

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(a) Stresses due to prestressing

q5.8 k- in .------...._--- '1-25.4 :- IVJOintIII(b) Stresses due to load P

Moment atjoint3= iP k-in.

(c) Stresses at testing

(a) (b)

P k

45

@3 56 psi

903 psi

tf75 ps 1,013 ps

9 ~828 psiP = 10k

110 psP ::: 13Sk

Fig. 3.3 Stresses at jOint

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46throughout the t e s t even for the highest level of load applied in anyspecimen P = 135k).NOTE: Since the specimen with the epoxied multiple key joint showed no

damage in one ent i re half after the shear t e s t tha t undamagedportion was subjected to another shear tes t to obtain backup datafor the monolithic specimen. This extra specimen is designated asNo-Joint 2, while the original monolithic specimen is called NoJoint 1.3.4.2 Load Slip. Figures 3.4 to 3.6 show the re la t ionship

between the applied load and the joint slip observed in the test .(a) Specimens i th nonepoxied jo in ts - Figure 3.4 shows the data

for the specimens with nonepoxied joints. As expected, each specimenslipped and failed at the joint. But, each type of joint configurationshowed a definitely different load vs s l ip relationship.

The specimen with the no-key jo in t sl ipped at a load of 28 kips,which corresponds to a coefficient of friction of 0.55. The specimen,however, continued to carry load up to more than 70 kips with increasings l ip a t the jo in t . No damage was observed in the specimen. Jointsurfaces were checked af te r the t e s t and they seemed to be in tact inappearance.

In the specimen with a s ingle large key, the data from the dialgage indicated that s l ipping occurred from the beginning of the loadapplication. However, i t was f e l t that the s l ipping a t such an ear lystage was unlikely and i t is believed that something might be wrong withthe displacement measuring system for this specimen. This subject willbe discussed later in Chapter 4 in conjunction with the performance ofjo in t configurations.

The crude data for the specimen with the single key joint indicatethat 1) the inclination of the load vs s l ip curve is almost constant upto 34 kips, (2) from 34 to 84 kips, the slope is again constant but thevalue is less than that for the load range of 0-34 kips, and 3) from 84kips to the major failure load, the slope of the curve i s dramaticallydecreased showing sl ight resistance against slip. I t is interesting tonote that the slope of the load vs s l ip curve for the no-key joint takes

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150

100

Loadp

50

(kips)

Multiple keys

. -. .-;. ttI

p7 17

3 t

~ 2 0

--- _ ...~ i n g l e :ey

. . -0/0/ ------.. 0 -- ....---~ . . ..... _ ....-.-,. ....-------- L . .o r l No key

Jlt -o 10

Slip at joint was observed

20 30 40 50 60 70 80 90Relative displacement between points across the jo in t

Fig. 3.4 Load vs sl ip at joints without epoxying

(XlO-3 in)100 110

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(kips)I150

100

Loadp

(a) Original dataM u t i p ~ keys

o ke\+

f

p+

?Single

t

key

50 I o sl ip a t joint wasobserved up to Pmaxi/

. fl ?Ii fI J if

fliJ---o / (X10-3in. )w:;:.. =r J I ,o o 10 20 30 40Relative displacement betweenpoints across the joints

(kips)15a- (b) After correction

100

Loadp

50

oo 10

key

Multiple keys

-3(x10 in . )I20 30 40Relative displacement betweenpoints across the jo intsFig. 3.5 Load vs sl ip at jo ints with epoxying

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(kips)

150

100

Loadp

50

No keyw/p..poxySingle keyw epoxy

r~~~Multiple keysw/epoxy

Multiple keysw o epoxy

p Joint 310~ _ . . . . .- -, r ~ S i n g l e key w o epoxy. ~ ~; ~//. ,

. .. --~ . .....-- ---_.-- No key w o epoxy........ ,IIII;l -3 )(x10 in.o rL-____ ____ _______ J_ _ _ _ _ _ _ _ _ _ _ __ J_ _ _ _ _ _ _ _ _ _ _ _ i _ _ _ _ __ _______L _______L _______ _____o 10 2 30 40 50 60 7 80 90 100 110Relative displacement between points across the jOint

Fig. 3.6 Comparison of the resul ts with and without epoxying

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50an in te rmedia te value between those mentioned in s tages 2) and 3) .The major fai lure occurred in the single key portion.

The s l ip a t the jo in t of the specimen with the multiple key jo in toccurred gradually as the load increased, showing substant ia l ly largerjo in t s t if fness a t higher load levels than the other nonepoxied joints .The slope of the load vs s l i p curve decreases cont inuously un t i l thef a i lu re load i s reached. This phenomenon i s c ha r a c t e r i s t i c of themult ip le key j o in t . Direct shear f a i lu re took place in the mul t ip lekeys.

The re la t ive loads a t given s l ips were substant ia l ly higher for themultiple key jo in t .

b) Specimens with Epoxied Jo in t s The r e l a t ionsh ip o f appl iedload and r e l a t i ve j o i n t displacement for the specimens with epoxiedj o i n t s i s shown in Fig. 3.5. The f igure inc ludes two se t s o f load vsre l a t ive displacement curves: Fig. 3.5 a) shows plo ts with or ig ina ldata ; and Fig. 3.5 b) shows p lo t s with corrected data . In the t e s t , nos l ip a t the jo in ts was observed visual ly up to the fai lure load. On theother hand, the uncorrected data o f the re l a t ive displacement showedla rge s l i p s a t a very ear ly loading s tage. Since t was thought t ha tthese early stage s l ips were highly unlikely and represent instrumenter ro r , the data were cor rec ted using overa l l slopes o f the plo t tedcurves. The corrected data plotted in Fig. 3.5 b) indicate no sl ips andagree with the visua l observat ion. Again, t was concluded tha tsomething was wrong, for some unknown reasons , with the r e l a t i vedisplacement measuring system, which worked well for the f i r s t twospecimens; tha t i s , the specimens with nonepoxied no-key and mult iplekey jo in t s .

s seen in Fig. 3.5 b), a l l three specimens with epoxy jo in ts gavealmost the same load vs re la t ive displacement curves.

The r e su l t s o f a l l jo in ted specimens are compared in Fig. 3.6.These will be discussed in Chapter 4.

3.4.3 Concre te Strength , Pres t r ess ing Force, and Maximum Load.Concrete s t rength of the contro l cy l inders a t the t ime of the appl iedshear t es t , prest ressing forces jus t before the t es t and a t the maximumloads , and the maximum appl ied loads are summarized in Table 3.2 and

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TABLE 3.2 roNCRE l E STRENGTH, PRESTRESSING FORCE AND MAXIMUM LOADWithout Epoxying With Epoxying

Coacrete Prestressing Maximulll Load Concrete Prestressing a x ~ LoadType of Strength Force Pmax Strength Force Pmax:Jointfc Before At Test Predicted f Before At Test PredictedTest Pmax

c Test Pmax(psi) (Ups) (Ups) (kips) (Ups) (psi) (Ups) (kips) (kips) (kips)~ 6.9 6.7 @ 6.5 6.5 7.450 73 - 6.Sl0Jle-Key 18.4 18.4 19.2 19.4 116 109 6.720 (28)* (20-35)* 6.200(f;) 25.3 25.1 25.7 25.9

7.6 7.3 @ 6.4 6.5 6.790 89 40-60 6.270Sin sle < > 18.4 18.9 @ 18.7 18.8 134 108Key 6.630 (34)* (20-36)* 5.940(f;) 26.0 26.2 ( ;) 25.1 25.3~ 7.0 6.8 C 6.5 6.5 7.450 96 44-68 @ 6,460Hultiple ~ 17.8 18.8 19.4 19.4 l24 109Keys 7.000 (60-80)* (-) * 6.440I ~ 24.8 25.6 I ~ 25.9 25.9

tc - Average of 3 c y l i n d e r ~ Slipping load.tc Prestressing It) P u I.)(psi) Betore At Pmax Test Predicted

< 6.6 6.6I

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52Fig. 3.7. The shear forces on the j o in t s are one-ha lf the appliedloads. Table 3.2 also includes predic ted maximum loads , for whichdeta i ls are presented in Chapter 4.

As shown in Fig. 3.7, overal l average of the concre te s t rength a tthe ages of tes t ing was 6550 psi and the coeff icient of variance was 7 .

I t was confirmed tha t the top and bottom pres t ress ing s t randspicked up the moment due to the loading during the t e s t , but the forcechanges in the tendons were very small. The average prestressing forcewas 25.4 kips and the cent ro idal concrete s t re ss was 424 psi with thecoeff ic ient of var iance of 1.7 .

Slip loads for the th ree specimens with the nonepoxied no-key,s ing le key, and mul t ip le key jo in t s were 28 kips, 34 kips , . and 60-80kips, respectively. In the same order, the maximum loads were 73 kips,89 kips, and 96 kips for the th ree specimens with nonepoxied j o in t s .All three specimens with epoxied j o in t s , including the no-key j o in t ,seemed to a t t a in much higher f a i lu re loads and were very s imilar tothose for a monolithic segment. The maximum loads for the epoxy jo in tand monolithic specimens ranged from 116 kips to 134 kips. Thus, a veryimpressive increase in fai lure load was real ized by uti l izing an epoxybond i ng ag ent

3.4.4 Crack Pattern a t Failure(a) Specimen with Nonepoxied No-key Joint - As mentioned before, nodamage was observed.(b) Specimen with Nonepoxied Single Key Joint - Figures 3.8 and 3.9

show the crack pattern of th i s specimen. Major cracking occurred a t thetop end of the male key. The cracks extended in to the specimen with adownward angle o f about 45 The cracks were also observed along theshear plane of the male key. After the tes t , segments were separated.Large and wide cracks were observed in the p lanes o f the keyreinforcement. Some of the concrete was spalled off .

c) Specimen with Nonepoxied Multiple Key Joint - Figure 3.10 showsthe failure pattern of the specimen. Diagonal cracks were observed ineach key. Generally speaking, cracks were localized in the neighborhood

.Obtained from the corrected load vs sl ip curve see Chapter 4 .

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a) Concrete strength f )c Xi S.psi)

10,000

_ l.X 1-Average of 3 cylindersi xi - Sif c 5,000

0 ~ - w ~ o - L E ~ * ~ W ~ , / 2 E - - - - - w ~ , / T O ~ E - - ~ w ~ / E - - - - w - / 7 0 ~ E - - - w ~ / ~ E - - - - - - ~ C 9 l ~ - ? ~ 2 ~ - -No key Single key Multiple keys

b) Prestressing force F ) and Maximum load P )P ~

FP

kips)150

and 100Pmax

50F -

T II II I ,I II II II II II II

No joint

pave .. ---. --- - _. --- - - --- - - -- -25.4k1. 7 )o F Ppw/o E

No key

F PPw/EFp P Fp Pw/o E w/ESingle key

Fp Pw/o E

F PPw/EMultiple keys

*w/o E = without epoxy; w/E = with epoxy

Fig. 3.7 Comparison of f , F and Pc p max

F P F Pb tNo joint

53

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Slip at jOint

a)

Spalledoff

b) Joint surface fter test

Slip t

c)Fig. 3.8 Crack pattern t fa ilure single key jo in t w o epoxying)

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I. .

Fig. 3.9 Crack pattern a t fa i lure single keyjoin w o epoxying)

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Sheared-off

Slip t joint Slip t joint

a) b) Joint f ter tes t c)Fig. 3.10 Crack pattern t fai lure multiple key jo in t w o epoxying)

Spalled-off

U

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7

of the keys. Keys on one side of the segments were completely shearedoff as shown in Fig. 3.11. The failure pattern was regarded as di rec tshear failure.

d) Specimen with Epoxied No-key Jo in t Figure 3.12 a) and Fig.3.13 show the crack pat tern of the specimen. The major shear crack runsdown along the j o in t from the top to the midheight where the crackleaves the j o in t and goes toward a support . Bearing f a i lu re wasobserved underneath the loading plate . This f a i lu re pat tern may beregarded as combination of shear and bearing f a i lu res . There was nos l ip at the jo in t .

e) Specimen with Epoxied Single Key Joint Figure 3.12 b) showsthe crack pattern at failure for the specimen. Cracks appeared in theupper ha l f o f the specimen propagat ing from the bottom of the loadingplate . This i s considered to be a bear ing f a i lu re . The j o in t wasin tac t .

f) Specimen with Epoxied Multiple Key Joint Figure 3.12 c) showsthe crack pattern at failure for the specimen. This is essent ia l ly thesame as that of the specimen with the epoxied single key joint . Bearingfailure occurred, and the joint was intact .

g) Specimens with No Jo in t s Monol i thic) Figures 3.14 and 3.15show the crack pa t t e rns of the monol i th ic specimens. As seen in Fig.3.14, crack pa t t e rns o f the two specimens were almost i de n t i c a l . Thespecimens had a combination fa i lure of shear and bearing.

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58

Fig. 3.11 Sheared-off joint f ter the t es tmultiple key jo in t w o epoxying)

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59

a) No-key

o

ot

b) Single lo

0

t tc) Multiple keys t

' IIIzzza eZlIzt t t

Fig. 3.12 Crack patterns at failure specimens w/epoxying)

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6

Fig . 3.13 Crack pat tern t fa i lu reepoxied, nokey jo in t

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6

a) D

o

o

b) D

o

Y VllZlZ JtFig. 3.14 Crack patterns at failure no-joint)

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. l n ' . . ek , , , , f f l l ~ n_1111> . ..... o )

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C H A P T E R 4

DISCUSSION OF THE RESULTS

4.1 Capacity of the SpecimenFlexural, shear, and bearing capaci t ies of the tes t specimens, i

assumed to be monolithic specimens, were calculated as described in th i ssec t ion . The ca lcu la ted maximum loads Pmax for each type o f loading ,which could be carr ied by the specimen, assuming monolithic action, aresummarized in Table 4.1. In these calculat ions, the capacity reductionfactor if was taken as uni ty . In Table 4.1 the maximum t e s t loads andthe type of fai lure obtained in the tes ts are also shown for comparison.

4.1.1 Flexura l Capaci ty . In accord ance with CI 318-77, Sect ion18.7.2, the f lexura l s t rength for each specimen, i monol i th ic , wasdetermined using an effect ive ult imate prest ressing force based on

fcf ps = f se 10 100 Pp (ksi)This equation should s t r i c t ly only be applied to members with unbondedprestressing tendons and with fse O.5fpu In the t es t s , fse was onlyO.45fpu for the bottom tendons,but it was f e l t adequate to use t h i sequation with measured pres tress ing forces and concrete strength. Thel imi t ing applied load P for a f lexural fai lure i s then obtained from thefol lowing formulas, assuming a uniformly d i s t r ibu ted t es t ing loadappl icat ion.

Pp = Aps = 0.153d 3 x 18 0.00283

T = A fps psTa = 0.85f bc

M = T (d -n63

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64

T B L ~ 4.1 CAPACITY OF THE TEST SPECIMENS IF MONOLITHICIn terms of maximum applied load, P )max

Calculated Values Maximum* Issuming MonolithicAction TestType of Joint Flexure Shear Bearing Loadkips) kips) kips) kips)78

No key 166 111 152:> 28)x00. 89. .J Single key 163 110 139 34):J0.c. .J 96t3: Multiple keys 162 110 152 60 - 8 0

No key II 167 109 133 116:>x0 134. Single key 163 108 128.c. .J i;3 Multiple keys 169 109 132 124

D 118 Ionolithic 156 108 127 INo Joint) D I168 109 131 134 I*, **Values in parentheses show slipping loads.

NatureofFailureSlipalongJointShearFailure inKeyShearFailure inKeysCombinedShear andBearing Fa i lureBearingFailureBearingFailureCombinedShear andBearing Fa i lureCombinedShear andBearing Fai lure

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Mmax 5= p2

2:.p = 5 Mn

65

As seen in Table 4.1, the f lexural capaci ty of the specimens wasdesigned to be relat ively high as compared to the shear capacity so thata premature f lexural fa i lu re would not mask the jo in t shear capaci ty .No flexural failures occurred in the test ser ies.

4.1.2 Section Shear Capacity. The calcu la ted shear s t rength ofeach tes t specimen, assuming monolithic action and ignoring the jointeffec t , was calcu la ted following ACI 318-77, Sections 11.4 and 11.5,which were adopted by AASHTO as Art ic le 1.6.13 in the 1980 Inter imchanges.

-Shear strength prov ided by concrete VcV.

f lexural shear) V . = 0.61fT b d + Vd + ----M MC1 C W crmax(web shear) V = 3.51fT + 0.3f )b d + Vcw c pc w p

Since the shear span of the specimens i s very shor t , web sheargoverns. Therefore, Vc = Vcw The actual shear capacity should besomewhat higher than the Vcw estimate due to the short shear span.-Shear Strength provided by shear reinforcement Vs

Vs d= A fv y s

The nominal shear s t rength Vn = V c + Vs ) was calcu la ted usingmeasured fb and fpc for each specimen. Other values are given below.bw = 3 in .d = 18 in.Vp = 0

T (6)2 0.0877 in2Av = 2 x 7; x 25.4 =

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fy = 61.1 ksis = 3 in.p 2 x Vn

This calculated monolithic shear strength was not developed in any ofthe jointed t es t specimens without epoxy but was fu l ly developed in al lmonolithically cast and al l epoxy jointed t es t specimens.

4.1.3 Bearing Capacity. The ca lcu la ted bear ing s t rength of eachtes t specimen assuming monolithic action was determined following ACI318-71 Section 10.16 and AASHTO Art ic le 1.5.36 using the fol lowingequations:

p = 8 5 f ~ A 1A1 = 3 x 8 = 24 in . 2

The measured a t the t ime of the shear t e s t was used in thecalculat ion for each specimen.

As seen in Table 4.1 the bear ing capacity o f the specimens wasdesigned to be higher than the shear capaci ty . The two epoxy jo in tedspecimens with keys which underwent bearing-type fai lures both fai ledwithin 6 of the predicted bearing capacity.

4.2 Shear Strength of the JointsThe method for determining the shear strength of jo in ts in precast

segmental cons t ruc t ion has not been standardized. There are a widerange of approaches which have been suggested. The shear s t rength ofeach specimen has been computed by var ious appl icab le theor ies asde ta i l ed in t h i s sect ion The resu l t s of these ca lcu la t ions aresummari zed in Table 4.2. For comparison both the calcu la ted shearstrength assuming monolithic action and the measured shear strengthand the major sl ipping loads are also given.

4 2 1 Shear Friction. The shear strength of the nonepoxied jo in twithout keys was calculated using shear fr ict ion concepts. Values ofthe coef f i c i en t 0 f f r i c t ion given by codes or obta ined from t e s t forconcrete-to-concrete smooth interfaces are as follows:

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67f J

CI 318-77 [13] and SHTO [17] 0.7-1.0PCI Design Handbook [26] 0.4Mast [12] 0.7-1.0Franz [18] 0.7Jones [19] 0.4 43AGaston and Kriz [20] 0.78 1 = 0.88

N

In many cases such as CI 318-77 and SHTO the case of rejoining matchcas t uni t s i s not d i r e c t l y covered. Values of = 1.0 are suggestedfor concrete placed against hardened concrete while values of = 0 7are used for concrete placed against as-rolled structural s teel . Thiscase may be in between.

In the shear f r ic t ion method of calculation in reinforced concretedesign i t i s assumed tha t a l l the shear resistance i s due to fr ict ionacross the crack f aces . The ACI Bui ld ing Code and SHTOSpeci f ica t ions therefore use a r t i f i c i a l l y high values of thecoeff icient of fr ict ion in order to compensate for the neglect of dowelact ion of the re inforcement cross ing the crack and r es i s t ance to theshearing off of protrusions on the crack faces.

For precast construct ion i t has been reported [16 25 28] tha t anexternally applied compressive s tress acting t ransversely to the shearplane i s additive to the reinforcement parameter pf y in calculations ofthe shear t ransfer s t rength of both i n i t i a l l y cracked and uncrackedconcrete.

t has also been repor ted [18 19 25] tha t the coef f ic ien t offr ict ion and the shear transfer strength are not signif icantly affectedby the presence of moment in the crack or jo int providing the appliedmoment is less than or equal to the flexural capacity of the section.

The shear s t rength of the j o in t s for the t e s t specimens withnonepoxied joints was computed in accordance with the CI Building Code[13] the PCI Design Handbook [26] and an a l te rna te shear t ransferdesign method proposed by Mattock [25]. The r e su l t s are shown below.In the ca lcu la t ion = 1.0 was used s ince the mater ia l s t rength andspecimen dimensions were accurately known

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68

(a) CI 318-77, Section 11.7Vn J..,J.N, P 2 Vmax n

No-keySingle keyMultiple keys

Vn (kips)J..,J. 0.4) J..,J.

10.110.49.9

0.7 1-1 1. 0)

17.7 25.318.2 26.017.4 24.8

s can be seen from Table 4.2, the values based on e i the r fl = 0.7 or 1.0for concre te placed aga ins t hardened concrete are conserva t ivepred ic tors o f the maximum load values but underes t imate the value a twhich signif icant s l ip occurred.(b) PCI Handbook, Section 5.6

In the applicat ion to precast concrete connections, the use of aneffective shear f r ic t ion coeff ic ient , J.. J. i s recommended.e

Vn = Avff fl Vn = NflY e elOOO cr 1.I NlOOO A 1.I. . V crl e V n Vn n

V IlOOOA N 1.In crwhere Acr = area of contact surface.

For concrete- to -concrete connect ions with a smooth interface surface,PCI recommends t ha t a value of 0.4 be used for 1-1. This gives thefollowing values for Vn

No-keySingle keyMul t ip le keys

Vn (kips)1-1 0.4)24.625.024.4

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Type of ACI 318Joint AASHTOi f monol i hic)

No key 111>M0p..

r..:I Single key 1101.1;:0.c:1.1 r tul t iple keys 110:i

No key 109>M0p..r..:I.c: Single key 1081.1 1:x Mul t iple keys 109

C 108No Joint 109

TABLE 4.2 SHEAR CAPACITY OF THE TEST SPECIMENSIn terms of maximum applied load, Pmax )

Calculated Values for Shear Strength of Joint kips)In terms of maximum applied load, Pmax = 2 Vn)

Shear Mod. ACIFriction Shear Shear Corbel PCIACI 318-77 Fric- Fric- Theory ACI Hand-.Sec. 11.7 tion t ion using Corbel bookPCI Mthd Shear Theory Corbel11= IJ= IJ.= Hand- Ref. Fric- Sec. Sec.0.7 1.0 1.4 book 25 tion 11. 9 5. 11

35 51 49 8436 52 50 85 50 55 35

35 50 I 49 83II 72

707370

72

- -* Values in parentheses show slipping loads.

Nom.ShearKeyVn=vbh Max.*v=_ Test6/f " ~ o d8 kips)

7828)44 89I 48

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70

These values are very close to the ACI AASHTO values based on IJ 1.0 inthe ACI procedures and are again conservat ive pred ic tors o f u l t imateload, but are signif icantly greater than the load at which s l ip began.c) Modified Shear Fric t ion Method of Mattock [25]

Vu = 400 psi 0.8PfyVn = 400 bwd 0.8N

No-keySingle keyMul t ip le keys

Vn kips)41.842.441.4

This method overestimates the in i t i a l sl ip load of the keyless and thes ing le key specimens and also overes t imates the u l t imate load on thekeyless specimen.

As described in Sec. 3.4.2 and shown in Fig. 3.4, the jo in tedspecimen with no epoxy and no keys continued to carry load a f t e r thei n i t i a l s l ip a t the j o in t and reached a capac i ty of 7 kips . The loadvs s l i p r e la t ionsh ip was bi l inea r . t i s supposed tha t the increasedload was car r ied by r es i s t ance to the shear ing off of the f inepro trus ions on the f la t jo in t surfaces , and by dowel ac t ion of theunbonded prest ressing tendons. I t i s unlikely that any design credenceshould be given to the increased load capacity af ter marked s l ip .

4.2.2 Single Key Joint4.2.2.1 Behavior of the t es t specimen with nonepoxied single key

jo int . The crack pat tern of t h i s specimen Figs. 3.8 and 3.9) showedth ree types o f major cracking charac te r i s t i c s : 1 ) f lexural cracksstar t ing a t the junction of the top face of the male single key and theend face of the segment, 2) shear cracks in the shear plane of the malekey, and 0) sp l i t t ing cracks in the shear key reinforcement planes ofboth adjacent segments which were accompanied by spa l l ing-of f in themale key region. The f lexura l cracks 1) were f i r s t observed j u s tbefore the major shear fa i l ure 2).

As described in Sec. 3.4.2 and shown in Fig. 3.4, the load vs s l i pcurve for the specimen with the nonepoxied single key jo in t implied that

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7

s l ip occurred at the j o in t from the very beginning of the loadapplicat ion. A s l i p of 0.013 in. was recorded a t P = 34 kips . Slip a tthe j o in t was also being observed visua l ly by watching the re l a t ivedisplacement of stra ight l ines drawn on the sides of the specimen acrossthe jo int . No visual s l ip was observed a t the load level of 34 kips forthe specimen. The load vs s l ip relat ionship was essentia l ly t r i l inea r .

Consider ing the above-mentioned facts and the t es t resul ts for thespecimen with the dry no-key jo in t i t was concluded that some type ofseat ing error occurred in the s l i p gage and the load vs s l i p data werecorrected, as shown in Fig. 4.1. The corrections were made so tha t thef i r s t l inear portion of the original t r i l inear curve for the dry singlekey jo int would agree with the in i t ia l slope of the load vs s l ip curvesfor the other specimens. The curve was sh i f ted in propor t ion to themagnitude of the applied load. Since shear forces are f i r s t t ransferredby the fr ic t ion in the contac t sur fac es wi th pres t r ess i ng forc esproviding ac t ive r es i s tance i t was considered reasonable tha t thein i t ia l portions of the load vs sl ip curves be assumed ident ical to eachother.

Figure 4.1 i nd ica tes t ha t considerable s l i p must take place inorder for a s ing le la rge key to act and to develop high shear ingstresses. In other words, contr ibutions of the fr ic t ion and the key toini t ia l s l ip loads are not additive. The corrected apparent s l ip loadfor the specimen wi th dry single key joint was 34 kips 1-1= 0.61), whilethat for the dry no-key jo int was 28 kips u 0.55 as mentioned in theprev ious chapter.

Figure 3.6 shows tha t the ult imate shear transfer strength of thedry jo int was somewhat improved by the existence of a key from 73 to 89kips). Even af ter the flexural fa i lure of the key, the jo int continuedto carry the shearing load, but the slope of the load vs s l ip curve wasmuch f l a t t e r than in the case of the dry no-key j o in t . I t i s supposedthat the reinforcement in the vic ini ty of the key helped to maintain theload-carrying capacity. After the shear failure of the male key, whichmight have been accompanied by the spl i t t ing cracks in the key, the loaddropped rapidly.

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Single key with corrected datal oI100 r Correction ; ) / Key fai lure/Load

50

o

I Key fai lure ~ . ; ; : = = = . . _in f lexure ./ \

/ . / L Single key withuncorrected data/0/ ./0/ /} -I 1Slipping

-3x10 in.Io 10 20 30 40 50 60 70 80 90 100 110

Relative displacement a t joint

Fig. 4.1 Correction of load vs s l ip curve of Bingle key jointw/o epoxying

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74.2.2.2 Forces ac t ing on s ing le key. Figure 4.2 i l l u s t r a t e s

forces act ing on a key of the specimen with the nonepoxied s ing le keyjo in t .

In Fig. 4.2(a), the probable forces on the s ing le key j u s t befores l ipp ing are shown. The resu l t an t force on the key was obtainedassuming uniform d i s t r i bu t i on of normal compressive s t r es ses due toprestressing and no contact between top or bottom faces of the male andfemale keys, result ing in no forces act ing on the top or bottom face ofthe male key. Maximum shear f r i c t ion s t r es ses before s l ipp ing wasapproximately 280 ps i which corresponds to a coeff ic ient of f r ic t ion of0.61.

The probable forces on the key jus t before key fai lure in flexureare shown in Fig. 4.2(b). The resu l t an t force was ca lcu la ted usingpres t ress ing force shear f r i c t ion force and forces ac t ing on the topface of the key. The shear fr ic t ion force and forces act ing on the topface of the key were determined using the load vs s l i p curves for theno-key and s ing le key jo in t s . t i s assumed t ha t the shear f r i c t ionforce would be the same as t ha t o f the no-key j o in t a t an i den t i ca lamount of s l ip . Uniform d i s t r i bu t i on o f the force ac t ing on the topface of the key i s a lso assumed.calculated as follows:

The s t r esses in the key wer e

-Average shear s tress in the key base jus t before fa i lure

v ave = ~A = (18 + 5.9) 10 3 = 1 3303 x 6 x PS1Thi s correspond s to 0.20 fb or 16 Jfb > sJrb-Flexural t ens i le s tress a t the top reentrant corner

3M = 18 x 4 + 5.9 3x =2 22.4 k-inM Nf t = S - A =

322.4 x 10 _ 46018

= 780 psi

This corresponds to O J f ~ or9.6,Jf 6 > 7 . ~ ) .

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26k ~lSk

7.9k3,800 psi

Resultant forceon key)

t t9 k ~ 15k\\.. 26k 26k

\ 460 psi (prestress)260 psiShear frict ion, = 0.55)

.rJ -'- t0k

18k

24.9kResultant force

I n key)S 9 k ~J t 26k16 9 k ~ /

22k

71

460 psi390 psi prestress)Shearfrict ion, = 0.85)a) Just before sl ipping (p 30 kips). b) Just before key fai lure p 80 kips)

Fig. 4.2 Forces acting on single key without epoxy)

-..j:

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75From this simple calculation, the relat ive magnitude of the shear

force contr ibution by various components can be visualized. Maximumbearing stress on the key faces was approximately 0 6 f ~ 0.85f6.4.2.2.3 Corbel analogy. Single key joints might be considered tobe analogous to corbels. Hence, the shear strength of the dry singlekey joint was calculated using the design methods for corbels.

The ACI Building Code permits the use of the shear fr ic t ionprovisions for the design of corbels in which the shear span-to-depthrat io id is one-half or less, providing l imitations on the quantity andspacing of reinforcement in corbels. ACI 318-77, Section 11.9, governsthe design of corbels with a shear span-to-depth rat io id of unity orless. Provisions of Section 5.11 of t