NSF/ENG-85071

PB87-148177

PMI:SEL REPORT NO. 85-3 DECEMBER 1985

BEHAVIOR OF STRENGTHENED AND REPAIRED REINFORCED CONCRETE COtLUMNS UNDER

CYCLIC DEFORMA'rIONS

By

Bart J. Bett

Richard E. Klingner

James O. Jirsa

Report on a Research Proiect Sponsored by

National Science Founda1 ion Grant No. CEE.820120S

'PHIL M. FERGUSON STRUCTURAL ENGINEERING LABORATORY Department of Civil Engineering / Bureau of Engineering Research

The University of Texas at Austin

REPRODUCED BY U.S. DEPARTMENTOF COMMERCE

NATIONAL TECHNICAL INFORMATION SERVIGE SPRINGFIELD, VA 22161

BEHAVIOR OF STRENGTHENED AND REPAIRED REINFORCED CONCRETE

COLUMNS UNDER CYCLIC DEFORMATIONS

by

Bart J. Bett

Richard E. Klingner

and

James O. Jirsa

Report on a Research Project Sponsored by

National Science Foundation Grant No. CEE-820120~)

Phi 1 M. Ferguson Structural Engineering La')oratory Department of Civil Engineering BUREAU OF ENGINEERING RESEARCH

THE UNIVERSITY OF TEXAS AT AUSTIN

December 1985

.r "

, , I

(

The contents of this report reflect the views of the authors w~o are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect t~e view or policies of the National Science Foundation. This report dc.es not constitute a standard, specification, or regulation.

ii

A C K NOW LED G MEN T S

The research reported was conducted as part (If the Master of SCience program of Bart J. Bett under the direction of Dr. R. E. Klingner. The advice and guidance of Dr. R. L. Carra::quillo of The University of Texas and Loring Wyllie, Jr., Chris Poland, and Roy Moreno of H. J. Degenkolb Associates, Inc., San Francisco, California were most helpful throughout the test program and are gratefully acknowledged.

This project was funded under National Science Foundation Grant No. CEE-8201205. H. J. Degenkolb Associates, Inc. is participating in the project under a separate NSF Grant (CEE-8201 1'87).

The experimental work was conducted in the Fepguson Struct ural Engineering Laboratory at Balcones Research Center of The Uni versi ty of Texas at Austin. The techni cal and admi nistrati ve staff of the Laboratory provi ded the necessary support for' the proj ect and wi thout their cooperation and dedication the project could not have been done. The many graduate assistants who helped with various tasks also added much to the success of the research.

iv

Chapter

2

CON TEN T S

INTRODUCTION .............................................. 1 . 1 Background ••..................••..•.••••••••••.•.••.• 1 .2 Obj ecti yes and Scope .........•..•.....•••.....••...••

Page

1.3 Short Columns in Structures •••.•..•..••••.•.•.••••••• 2 1.4 Short-Column Repair/Strengthening Techniques ••••.•••• 2

EXPERIMENTAL PROGRAM ••.....•..••.••.•.•..•.••..•.•.••••••• 7

2.1 Introduction......................................... 7 2.2 Overall Research Program ••••••••••••••••••••••••••••• 7 2.3 Original Test Specimen 1-1 ........................... 7

2.3.1 Design Requirements •.••..•.•.•••.••.•••••••••• 7 2.3.2 Details of Specimen .•...•.......•.•.......... ~ 10 2. 3 • 3 Cal c ul ate d S t r en gt hs •.•••.•••••••••••••.•.••.• 1 0 2.3.4 Specimen Fabrication .•.....••.••...••..••••... 12

2.4 Material Characteristics of Original Test Specimen... 12 2.4.1 Concrete...................................... 12 2.4.2 Reinforcement .•••.•••••••••••••••••••••••.•••• 16

2.5 Strengthened and Repaired Specimens ••••.•••••••••••.. 16 2.5.1 Strengthening Technique (Specimens 1-2, 1-3) •• 16 2.5.2 Repair Technique (Specimen 1-1R) ••••••.•••••.• 16

2.6 Strengthened Specimen 1-2 .••••.•••..••.•.•.•.•••••••• 16 2. 6 • 1 D et ai 1 s ....... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 2.6.2 Calculated Strengths ••....••.••••••.•••••••••• 16

2.7 Strengthened Specimen 1-3 .•.•....•.•..••.•.....•..•.• 19 2.7.1 Details .....••............••.•...••........•.• 19 2.7.2 Calculated Strengths .............•............ 19

2.8 Repair of Specimen 1-1 •••••••••.••••••••••••••••••••• 19 2.9 Fabrication of Strengthened Specimens .•.••........... 19 2.10 Material Characteristics of Strengthened Specimens .•• 25

2.10.1 Shotcrete ...•..•.....................•..•..... 25 2 • 1 0 • 2 Rei nf 0 r c em en t ................................. 2 9 2. 1 0 . 3 Epo xy ..•..•.•..•...•.•..••.•..•..••..•..•..•.• 29

2.11 Loading History .•••.•••••.•••..••••••••.••••••••..••• 29

3 LOADING SYSTEM AND INSTRUMENTATION ......•.......•..•....•• 33

3. 1 3.2

Loading System •.•...•....•..•........•...•...•.•..•.. Ins t rumen tat ion ..•...•••.•...•..•..•.•...•....•..••.. 3.2. 1 3.2.2

Loa ds •.••.•••.•••.••..•....••.•.•••.••.••.••.. Def 1 ect ions

v

33 36 36 36

CONTENTS (continued)

Chapter Page

3 LOADING SYSTEM AND INSTRUMENTATION (continuet)

3 . 2 . 3 S t r ai ns ........•..•.•......•..........••.••..• 36 3 • 2 • 4 Sl i P .......................................... 3 6

4 BEHAVIOR OF SPECIMENS..................................... 39

4.1 Introduction......................................... 39 4.2 Description of Test Results ••..•.••.••.••••••.•••..•• 39

4.2.1 Load-Deflection Curves .•••••••••••..•••••••••• 39 4.2.2 Crack Patterns ••.•.•••...•••..•.•••••.•••••.•• 43 4.2.3 Strain Distributions •••••••••••.•••••••••••••• 48 4 • 2 • 4 Sl i P .••.••.•.•.••..•••..•..••••.••.•.••••••••• 52

5 DISCUSSION OF TEST RESULTS ••.••••••••.•••••.•••••••••••••• 59

5. 1 Introducti on .......................... I • • • • • • • • • • • • • • 59 5 • 2 T es t Res ul ts ••.•••••••••••••••••.••••• ,.............. 59

5.2.1 Specimen Stiffness and Capaci ty .••.••••.•••••• 59 5.2.2 Strains in Reinforcement ...................... 61

5.3 Canparison of Observed and Predtcted Bellavior •••••••• 64

6 SUMMARY AND CONCLUSIONS •••••••••••••••••••• , •••••••••••••• 67

NOTATION

6.1 Summary of Investigation ••••......•..•.••••••..•••.•• E7 6.2 Summary of Test Results ••••••••••.•••• , •••••••••••••• 68

6.2.1 Original Column Specimen ...................... 68 6.2.2 strengthened and Repaired SpecimE~ns •••.••••••• 68

6.3 Conclusions.......................................... 69 6.4 Additional Research ••••••••••••.••••••.•••••••••••••• 70

71

REFERENCES •.•••••••••••••...•••••••••••••••••••••••.••••••••••••••• /'3

vi

Table

2. 1

2.2

2.3

2.4

2.5

2.6

2.7

5. 1

5.2

5.3

TAB L E S

T es t Pro gr am S umm ar y •••••••••••••••••••••••••••••••••••••

Column Concrete Strength ..••.•.••••.••..•••••••••••••••••

Steel Properti es (Ori ginal Specimen) •••..•••.••.••••.••••

Shotcrete ................................................ Shot cr et e S trengt hs .•.••.•..•..•..••........•.•...••..•••

Steel properties (Jacket #3 Long.) •••••••••••••••••••••••

Concresive 1411 Mechanical Properties .......•.••.••..•...

Secant Stiffness in First Cycle to Various Drift Levels ••

Reduction in Secant Stiffness Between 1st and 3rd Cycles •

Lateral Capacity •...•.......•....•.........•..•........•.

vii

Page

8

"15

'15

30

:30

30

60

50

64

FIG U RES

Figure Page

1 • 1 Captive column concept 3

1 .2 Relation between end shears and moments in a column subjected to sidesway ..•.•...........•..•............•..•. 3

1.3 Failed columns at Misawa Canmercial High Scrool, Tokachi-Oki earthquake, Japan, 1968 ..•.....••.•...•.••.••• 4

1.4 Techni ques to increase shear capacity ....•................ 5

2.1 Details of ori gi nal specimen 11

2.2 Shear capacity-interaction diagram (original specimen) •••• 13

Details of end block reinforcement 1 4

2.4 Construction of formwork ••••••••••.•••••••.••••••••••••••• 14

2.5 Stress-strain curve for ori ginal specimen steel reinforcement....................................... 15

2.6 Details of Specimen 1-2 •..••...•.....•...•.•..•..••..•.••• 17

2.7 Shear capacity-interaction diagram (Specimens 1-2, 1-3) ... 18

2.8 Details of Specimen 1-3 20

2.9 Post-test, Specimen 1-1 21

2.10 Loose cover rEmoved, Specimen 1-1 ............•.•.••..•...• 21

2.11 Jacket reinforcement, Specimen 1-1 ......................... 22

2.12 Crossties secured by epoxy, Specimen 1-1 .....•.••....••.•. 22

2.13 Screed guides ............................................. 23

2.14 Shotcrete test panels ..•..•........•...••....•..•.•.....•• 24

2 • 1 5 S hot cr e tin g .................................•............. 2 6

2.16 Finishing shotcrete 27

viii

Fi gure Page

2. 17 Panel (top) core samples 28

2. 1 8 Pan el ( bot tan) cor e s am pI es .•...•••.••••..• 0 • • • • • • • • • • • • • • 28

2.19 Stress-strain curve for jacket steel (113) ••••••••••••••••• ~:1

2.20 Lateral displacement history •••.•••.•.••••. o.............. ~1

3.1 Loading rams ..•••.•.......•••.•.••....••..•.•......••.•.•. ~:4

3 . 2 T es t set up. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4

3.3 Restraining rams.......................................... 35

3.4 Vertical and horizontal posi tioning system................ 35

3.5 Strain gage locations (original specimen) ...•.........•..• 37

3.6 Strain gage locations (jacket) ••.••••.•••••.••.•.••••••••• 37

3.7 Slip wire instrumentation ••••.•...••••...•..••••••••••.••. 38

4. 1

4.2

4.3

4.4

Load-deflection

Load-defl ecti on

Load-defl e ction

Load-defl ecti on

curve,

curve,

curve,

curve,

Specimen 1-1 · ...................... Specimen 1-2 · ...................... Specimen 1-3 · ...................... Specimen 1-1R ......................

4.5 Load-deflection envelopes, Specimens 1-1,

40

41

l.j 1

1-2, 1-3 and 1-1R .......••.•....••...........•.••...•.•..• 42

4.6 Crack patterns, NW column faces, Specimen 1-1 •••.•••...••• 44

4.7 Crack patterns, Specimen 1-2

4.8 Crack patterns, Specimen 1-3

4.9 Crack patterns, Specimen 1-1R ....•...........•.......••... 47

4.10 Envelopes of strain distribution, NW longitudinal #6, Specimen 1-1 •....••.•....•.••.••.......••.•••..•...•.••..• 49

4.11 Envelopes of strain distribution, NW longitLdinal #6, Specimen 1-3 ..••••....•..•••.•..•.•.....•..••.••.•..•...•. 49

ix

Figure Page

4.12 Envelopes of top gage strain, #6/#3, NW loneitudinal bars, Specimen 1-3 ....•.........•...•.......•.•........••....... 50

4. 13 Strai n envelopes, N-S di recti on, ori gi nal cc,l umn ties, Specimen 1-1 ..•.••••••••••••••.••.•..•.•••.•••••.••• 51

4.14 Strai n envelopes, E-W di recti on, ori gi nal c(ll umn ties, Specimen 1-1 .••..•.•.•...•...•.••••...•.••.••..•••.• 51

4.15 Strain envelopes, mid-height ties, Specimen l-1R .••....•.. 53

4.16 Strain envelopes, mid-height ties, ~;pecimen 1-3 ••••••••••. 54

4.17

4. 18

4. 19

4.20

4.21

4.22

Strain envelopes, N-S crossties, Specimen 1-3

Strain envelopes, E-W crossti es, Specimen 1·-3

Strain envelopes, N-S crossties

Strain envelopes, E-W crossti es

Envelope of mi d- hei ght jacket sl i p, Spe ci merl

Envelope of bottom jacket slip, Specimen 1-]

1-3 .....•....

55

55

56

56

57

57

5.1 Predicted envelopes of longi tudina1 bar strains ••••..•...• 62

x

C HAP T E R

INTRODUCTION

1 • 1 Background

Reinforced concrete buildings in seismic zones are repaired or strengthened for three reasons: 1) to repair earthquake damage and obtain improved performance during future events; 2) to comply with local building codes and regulations when the building's use is changed; and 3) to satisfy the building owner's con(~ern for the safety of the occupants and protection of his financial investment.

The existing building must be thoroJghly analyzed to determine the strengths and weaknesses of the orig:.nal lateral force­resisting system, considering the building's functi~ns and aesthetics. Strengthening schemes may involve the use of materjals different from tho s e 0 f the 0 rig ina 1 s t r u c t u r e , and the in t era c t ion 0 f tho ::;, e materials must be understood. The scheme selected nust not create new areas of weakness, and must be economically feasible.

The need for information on the repair arid strengthening of reinforced concrete structures is apparent. Several National Science Foundation- sponsored workshops on this topic have been held in the United States, and a number of U.S. research inst:~tutions (Portland Cement Association, University of Michigan, Univer~;ity of California­Berkeley) have studied repair techniques [1,2]. Repair and strengthening problems have received more attentie>n in Japan [3,4J. Because experimental work in the area of t4 epair and strengthening is very complex and expensive, most studies have involved small scale specimens. In addition, there has been little dialogue between researchers and designers who must incorporate re~,earch results into practice. These two concerns are addressed in the overall research program discussed in Chapter 2.

1.2 Objectives and Scope

The objective of this study was to evaluate several repatr and strengthening techniques for reinforced concrete short columns. Short columns under constant axial compression were subjected to reversed cyclic deformations. Two columns were strengthened before testing, and one column was repaired after testing. Individual column test results were compared. Repair and strengthen:.ng techniques Wel"e evaluated in terms of strength, stiffness, and damc,ge repair.

1

2

1.3 Short Columns in Structures

Field reports following various damaging e3rthquakes indicate that columns are vulnerable structural elements, particularly if they fa i lin she a r • She a r -d 0 min ate d be h a vi 0 r ism 0 s t com m 0 n inc 0 I u m n s having shear-span depth (a/d) ratios less than 2.~; [5,6,7,8], Short columns exist in structural systems either as part of the original design, or as the result of structural or architectural changes made during the life of the structure. Members origjnally designed as short columns can behave satisfactorily under lateral loads if designed for sufficient shear resistance. Hcwever, short (cr "captive") columns are sometimes produced unintentionally [8,9] whE~n clear column height is reduced by stiff elements that restrict the lateral deformation of the colu mn over a portion Jf its length (Fi g. 1.1). This change in length is important because the applied shear and moment on a column are related by its length, as shown in Fig. 1.2. The original column may have been properly designed to develop its flexural capaci ty before failing in shear. Due to the reduction in length, the captive column will often fail in shear before developing its flexural capacity. Post-earth iuake structurE.l investigations report many failures of captive columns restrained by structural or non-structural elements (Fig. 1.3).

1.4 Short-Column Repair/Strengthening Techniques

Severe seismic loading of columns with small shear-span/depth (a/d) ratios and widely-spaced transverse reinfclrcement generally results in shear-dominated fai lure, leading to structural collapse by the formation of a single- story sidesway mechani~,m. Hhile this can be prevented by increasing column shear capaci ti, it must be done economically, and without large increases in flexu:~al capacity, which would i ncrea se appl i ed shear s. Fi gur e 1.J~ illus t rates four methods now available for increasing the shear capacity of a vulnerable column: 1) encase it with rectangular or circular steel sections; 2) encase it with steel straps; 3) confine it by using welded wire fabric; and 4) confine it by adding spliced. ties. A jacket of shotcrete or cement grout is then applied to protect the added steel and make it act integrally with the original column.

Jacketing can increase the shear resistance of the column, but may adversely affect the building's seismic re~istance: decreased (a/d) ratio and increased moment capacity make she3r-dominated column failure more likely; increased column stiffness decreases the building's fundamental period and increases seismic-induced lateral forces. However, jacketing of the original column is still beneficial; 1) due to increased confinement, column shear performanl:!e is adequate even at lower (a/d) ratios; 2) judicious selection and placement of jacket longitudinal steel minimizes increases in column

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flexural capacity; and 3) increased column Sheal" capacity offsets increases in se is mi c-i nduced lateral forces.

Retrofitting techniques can be evaluated experimentally in terms of strength and sti ffness. The behavior of a colu mn, ini tially tested, repaired and strengthened, and then retested, can be compared to the behavior of initially strengthened columns, and a correlation developpd between retrofitting technique and performance.

C HAP T E R 2

EXPERIMENTAL PROGRAM

2.1 Introduction

The purpose of this study was to evaluate ::everal repair and strengthening techniques for reinforced concrete short columns. Three test specimens were constructed using normal weight concrete and Gr. 60 reinforcement. One specimen was testE,d in it:) original form; repaired, strengthened, and retested. The remaining two specimens were strengthened prior to testing .. The specimens were numbered sequentially (1-1, 1-2, 1-3) and the repaire(j specimen was designated by 1-1R. In each test, constant axial compression and numerous cycles of reversed lateral deformations were applied to t:1e specimen. The primary objective of this test program was to study the effect of different strengthening or repair techniques on the strength and response characteristics of reinforced concrete short columns. In this chapter, the experimental program will be discw3sed. Much of the information is summarized in Table 2.1.

2.2 Overall Research Program

This investigation was part of a lar'ger study of the behavior of reinforced concrete frame systems subjected to cyclic lateral deformations. The overall research program was devoted to evaluation of various repair and strengthening techniques for R/C frame elements. The study reported herein deals only with short columns.

In the overall research program the capabiljties of a univer­si ty research laboratory and a structural engineerin[ desi gn firm were combined. Repair and strengthening is a speciali:~ed area in 'Which professional experience is extremely important. DESign requirements for strengthening techniques and details are not a3 codified as are requirements for original construction. Second, experiments were conducted using nearly full-scale specimens fabricated especially for studying repair and strengthening procedures, and not as an adjunct tc a study wi th other primary obj ecti ves. The SUCCe!3S of most repair techniques lies in the details utilized, and scaJe effects may be important.

2.3 Original Test Specimen ~

2.3.1 Design Requirements. The obj ecti ve of the proj ect was to study the behavior of a reinforced concrete column that would fail in shear if it were not strengthened. To avoid the cost of deSigning

7

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ars

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@ 2

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17

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Test

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.

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10

and constructing a new test frame, it was decided to use short column specimens of the same size as those studied in previous test programs [5,6,7,8,9J. The test specimen selected was a short column framing into enlarged end blocks, which provided both for c.ttachment of the specimen to the test frame and for anchorage of che longitudinal column reinforcement.

The prototype short column was designed c.s an l8-in. (45.7 cm) square section meeting the column design provisions of ACI '318-63 [16J, particularly Section 806 and Chapter 19. It was 4.5 ft (1.37 cm) high, and reinforced with eight 119 longitudinal bars (Pg = 0.025) and two sets of #3 ti es at 12 in. Cover was 1-1 /~' in. Transverse reinforcement spacing and 'details were aelectec. as typical for structures designed for seismic regions of the U.S. during the late 1950's and early 1960's. Col umns of such struct lres usually had transverse reinforcement which would be insufficient by current standards, resulting in shear-dominated behavior uncer severe lateral loads.

Column loads in the structures mentioned c.bove vary widely. Based on actual load data [14J, typical compressiv~ stresses ranged from about 350 psi to 550 psi, wi th an aver·age of about 450 psi. The axial load required to develop an average compressive stress of 450 psi on the prototype 18-in. square column is about 146 kips.

To reduce fabrication and testing costs, y~t permit the use of commercially available deformed reinforcement, the test specimens were constructed to two-thirds scale. Analyses indicated that the previously used connection details between the frame and a strengthened specimen would be inadequate. Those details were subsequently modified as described in Chapter 3.

2.3.2 Details of Specimen. Details of the original test specimen are shown in Fig. 2~-The two-thirds scal e model was a 12-in. (30.5 cm) square section, 3.0 ft (0.92 m) in hEight, containing eight #6 longitudinal bars, sets of special 6 mm deformed ties at E; in., and 1 in. cover.

2.3.3 Calculated Strengths. The test specimen's theoretical moment and shear capaci ties were calculated using the computer prograrr, R C COL A , de vel 0 p e d f or i n e 1 as tic f 1 ex u r a 1 an a 1 y sis of rei nf 0 r c e cl concrete sections [15J. Although moment capacity can be estimatecl fairly accurately, shear capacity is more difficu:_t to predict. The program used an empirical relationship Eq. 2.1 based on University of Texas short column test results [23J.

;-;- 0.2 N Ac "';f~ + a

J*

11

I.. 36" (92cm) .1 •••••••••••••••••••••••••• 4

· . · . · . · . · . · . · . · . · . · . · . : :

o o

o o

t t

Elevation

........ -: · · · · · : ....... :

End View

· . · . · . · . · . · . · . · . · . · . · . · . · .

o o

18" (46cm)

36" (92cm)

: ~:

· · · t ••••••• "-

.~ · · · · " ....... ; 2

· · · · · ·

........

: c:::: • . •..•...•.......••....... ~

Reinforcement

--(Scm)

4 Spaces at 8" (21cm)

#2 (+3)* Closecl Ties at 8" (21 cm)

2" PVC 12" (18")* (30em) ~

1" (1.5" *) (2.5cm)

10" (25cm)

8-#6 (#9)* Vertical ---+-+-H--.

_~l::: .... t::=====~===~ 1" (1.5" *) I

-W-~I-... =~== .. ---'I ~ * 1.625" B.·,r5" 1.625" Prototype Dimensions

(4.8cm) (22 cm) (4.8cm)

Column Sl~ction A-A Fig. 2. 1 Detai Is of original specil1en

12

0.2 N a ci*

1 <

<

a ci*

2.5

Using the above shear resistance, and the statical relations between shear end moments in a column subjected to sidesway (Fig. 1.2), a diagram of moment capacity as govel~ned by :3hear was produce<j (Fig. 2.2). Capacities are shown for both the initial (entire) and confined cross sections. The shear capacity plot indicates the level of end moment required to generate the column's she:lr capacity at any axial load. Inspection of Fig. 2.2 reveals that the shear capacity curve becomes vertical for large axial loads. This conservati ve limi': was due to the absence of test results for large axial loads. Neglecting the beneficial effects of axial compl'essive load on a column's shear capacity may be appropriate in the analysis of column:3 subjected to earthquakes. It is entirely possible that during a severe seismic event, the effect of overturning or vertical accelerations may reduce the level of axial compres:3ion significantly. Considering a 12 in. square column subjected to 450 psi compression (64.8 kips), the predicted end moment correspording to flexural failure of the initial section is about 1150 i1.-kips, and that corresponding to shear failure about 720 in.-kips. Based on the analytical model of Fig. 1.2, the corresponcling latEral capaci ties are 64 kips (flexure) and 40 kips (shear). The origj,nal column could therefore be expected to fail in shear.

2.3.4 Specimen Fabrication. DimensionE of the end block were based on t6erequirements for attaching the te3t specimen to the test frame and for anchoring the longitudinal column reinforcement. Details of the end block reinforcement are shown in Fig. 2.3.

To simplify formwork, specimens were C;:lst in two stages. First, the bottom end block was cast with the column and top block form work already in place. Four days later the (!olumn and top end block were cast. This casting sequence produced a cold j oint at the bottom of the column, and is similar to that u3ed in reinforced concrete buildings. Figure 2.4 illustrates the specimen formwork.

2.4 Material Characteristics of Original Test Specimen

2.4.1 Concrete. Ready-mixed concrete was obtained from a local supplier, with the mix proportions, as shown in Table 2.2.

1000.

800.

-(/) 600 . . 9-~ -1J as 0 ..J

:! 400. )( c(

200.

-200.

-400.

Max. Concrete Strain

Initial Sectlon=0.0030 Inlln Confined Section = 0.0 1 00 in/in

Shear Capa city

,..---- Initial Section

Confined Se(:tion

Flexur,:ll Capacity

--Initial Section

Confined Section

1600. 2000. 2400.

Fig. 2.2 Shear capacity-interaction diagram (original specimen)

13

: :

.~

+3

Tie

s

Ir~:

.....

.. f'

V

if

.. ::

:.

::

.'

.'

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w\ ~

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2"

¢ P

VC

Pip

e

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....

. N

Tie

Sp

lice

#3

Tie

s L

f9

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lH

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Fig

. 2

.3

Deta

ils o

f en

d b

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re

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en

t

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.... ___

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Fig

. 2

.4

Co

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on

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rmw

ork

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)-

TABLE 2.2 Col umn Concrete St.rength 15

Concrete Mix Desi gn (4000 psi) Concrete Pt'operti es Canpressi ve

Proportions of 1 yd 3 Age Strength, f' c (days) (psi)

Water 210 1 b 28 3831 Cement (5 sacks) 470 lb Fine aggregate 1530 1 b Test 1- 1 57 4333 Coarse aggregate ( 5/8 in. ) 1830 lb Trisene L (retarding 15 02 Test 1-') 147 4399

admi xture) (w/c = 0.45 by weight) Test 1-1 204 4627

TABLE 2.3 Steel Properties (Original Colu~n)

Bar f E Esth fu Size (k§i) (ksi) (ksi) (ksi)

#6 67 25463 0.0079 1344 112 0.1445

6mm 60* 26625 0.00265 870 88 0.0975

* 0.2% offset

75 +6 ~---,~

60

30

15

0.008 0.010 0.012 0.014

Strain (ln/ln)

Fig. 2.5 Stress-strain curve for original specimen steel reinforcement

16

The aggregate was Colorado Ri ver sand and e;ra vel. Because of congestion of reinforcement, a relatively high slump was necessary to ensure proper placement of concrete. The concrete Has ordered with a slu mp less than the desired 7 in., and water was added on site to achieve the required slump. Twelve control cylinders were cast and cured with the specimens. All three specimens were cast in the same operation, and moist-cured under polyethylene sheets for seven day,s prior to stripping the forms.

Three control cylinders were capped and tested at 28 days, and at the conclusion of the first, second, and fo'..:rth tests. Table 2.2 summarizes the results of the cylinder tests.

2.4.2 Reinforcement. Number 6 deformed r~inforcement (ASTM A - 6 1 5 Gr. 60) was use d for the Ion g i t u din a 1 s tee 1, an d 6 m m d e for me d reinforcement for the transverse steel. The 6 mm deformed bars wer'e fabricated in Sweden and obtained through the Portland Cement Association Laboratories. Mechanical characteristics of reinforcement are shown in Table 2.3, and typical stress-strain Clrves in Fig. 2.5.

2.5 Strengthened and Repaired Specimens

2.5 • 1 ~~!:~!l~~~~!l!.~~I~~~!l!.~ u e_ ~e..~ c ::.~~!l~ l=~L l=ll. Strengthening invol ved encasing the ori ginal colu mn wi th a shotcrete jacket reinforced with closely-spaced transverse steel. Addi tional longitudinal steel was placed at each corner of the jacket to support the transverse steel. Details of the strengthening technique for each specimen will be described in subsequent sections.

2.5.2 ~~e..~!.!:. !~~~~~~~~ i~e..~~!_~~~ .l=.!~l. The repair technique consisted of two operations. First, all loose cover was removed with a chipping hammer, exposing the lcngitudinal steel. Holes were then drilled through the columns, and crossties used to

anchor additional longitudinal steel were inserted and cemented with epoxy. Second, closely-spaced ties were placed around the column core, and it was encased with shotcrete. Details of the repair technique will be shown later.

2.6 Strengthened Specimen 1-2

2.6.1 ~~~~!..!.~. The strengthening tl~chnique used for Specimen 1-2 consisted of a shotcrete jacket reinforced as shown in Fig. 2.6.

2.6.2 Calculated Strengths. Theoretica:. moment and shear capacities, calculated as described previously [15J, are shown in Fig. 2.7. Inspection of Fig. 2.7 reveals that for a 64.8 kip (224 psi) axial load, the end moment corresponding to flexura:. capaci ty is about

" E 0

~ .... 0) ....... It CO tv)

I?

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~t ~ ~ · ......

-.! · · · -=-....:. · It · -.r

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~ · ii' · --=-

#3~ · .. • • lO

: : · : C\I · V' .... · ~

--=

~6(mm)

to · · · · a. · • f/) t- · :

~ · · --t -.: : .... · · · · A · · A · : ,

~ · · ·1 · · ~ ..

lO 0

- a)Elevation E o J 17"(43 em)

~ __ ----I_ .. I- 2.5·(6.4 em) I - +3

" E (,)

tv) ~ ....... .. ,... ....

T --(6 mm) at 2.5-

- Shotcrete Jacket

" E (,)

~

~ L=:::======~ b)Section A-A

Fig. 2.6 Details of Speeimen 1-;~.

18

2000.

-tn .9-.)t. -"C as 800. 0 ..J

"ii )( oc(

400.

-400.

-800.

Max. Concrete Strain Initial Section = 0.0030 in/In Confined Section = 0.0100 in/in

Flexural Capacity

r----- Confined Section ,.---- Initial Section

------6 ....... ...... ......... -.... ........ I .... ?-.

8 .8

:~

....".-

~-"'- Moment (inch kips)

Shear Capacity

rconfined Section Ilniti81 Section

Fig. 2.7 Shear capacity-interaction diagram (Sp,?cimens 1-2, 1-3).

19

1800 in.-kips, and that corresponding to shear capclcity is about 1650 in.-kipg. The corresponding lateral capacities are 104 kips .(flexurE~) and 100 kips (shear). The strengthened specimens cou ld therefore be expected to fail by combined shear and flexure.

2.7 Strengthened Specimen 1-3

2.7.1 .!2~!:.~~~~. The strengthening technique used fer Specimen 1-3, shown in Fig. 2.8, consisted of the same basic reinforced shotcrete jacket, plus 116 longitudj.nal bars at each midface. Holes were drilled through the column, and 113 crossties, secured with epoxy, anchored opposite face longi tudinal bars. HolE~s

were 1/4 in. oversize and were cleaned usl.ng a tight-fitting bottle brush. One end of each crosstie was field-bent around the midface bar before the epoxy had set.

2.7.2 Calculated Strengths. Neglecting the contribution of the crossties, theoretical moment and shear capac]. ti es are identical to those of Speci men 1-2 (Fig. 2.7) [15].

2.8 Repair of Specimen 1-1

Following the first test, Specimen 1-1 wcs removed from the test frame for repair and strengthening. After l~emoving all loose cover (Figs. 2.9, 2.10), jacket reinforcement (Fig. 2.8) was constructed identical to that used for Specimen 1-3, as shown in Fig. 2.11. r.rossties through the cracked column core were secured by epoxy (Fig. 2.12), and a shotcrete jacket was added to increase the column size to 17 by 17 in.

2.9 Fabrication 01 Strengthened Specimens

Fabrication of strengthened specimens consisted of tieing the jacket reinforcement cages and then shotcretin2.. Specimens wer'e roughened by light sandblasting using a No.6 venturi nozzle and a fine sand (No.6). As shown in Fig. 2.13, wooden 3creed guides wer'e attached to each specimen's end blocks using ramsec nails. Shotcre~e

quality was monitored using two vertical test panels (36 x 18 x 3 in.), shown in Fig. 2.14. One of the panels had a wood back while the other had a concrete back. The two types of matE~rials were used to determine if shotcrete test panel rebound charactl~ristics different from the actual column applications might altE~r quality control information. Half of each panel was reinforced identically to the jacket of Specimen 1-3, without crossties. Two sizes of core samples were to be taken from each panel. Large cores (4 x 3 in.) from the reinforced side of each panel were used to moni~~or void formation behind individual bars. Small cores (1-3/4 ) 3 in.) from the

/.0

~

E 0

~ ,.. 0) ....., " <0 en

'I'

,

~

E o C'? "lit ....., • r--,..

~

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~! I"

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0-en "lit ,..

~

• ~ 0

~

E o

A r

· · : · · ; : I~ · I- -· ~ · +3

· : ............ · I · ~ ---· +3 Crossties

· [ -r : : -'-· : · · · · · 0 · l.------· · · I ~ ~ 6(mm)

0 · 0 · 0 0 - +6 · r-- · · · · · · 0 I ? · · : · ~ A ·

a)Elevation

17"( 43 em) I ~ 2.5-(6.4 em) +3

(6mm) at 2.5"

#6

~~t-----ACI 318-77 Min. Extension

: I-r----...::::::::::..~ #3 Crossties

Shoterete Jacket

b)Seetion A-A

Fig. 2.8 Details of Specimen 1-3.

Fig

. 2

.9

Post-test~

Sp

ecim

en

1-1

Fig

. 2

.10

L

oo

se

co

ver

rem

ov

ed

, S

pec

imen

1

-1

N ~

22

~> >: (l 1=1.. <11

~> ,.L!

reI 0) ~I ::1 C,I (1) ~

(ll I ~

(ll Q) s::

• ... 1 <l)

"-I S Cll 'r-! Cll () () <l) 1-1 p..

U C/)

C'J .... ~ C'J . t{) .... ~ ~i

23

Fig~ 2.13 Screed guides

24

Fig. 2.14 Shotcrete test panels

25

unreinforced side were used for compressive strength tests. Ar. experienced contractor shotcreted and float-finished (Figs. 2.15, 2.16) the specimens and panels, both of which ,.Jere cured under polyethylene sheets for seven days. When the specimens were shotcreted, concrete temperature, slump, uni t weight., and air content were measured. Two sets of control cylinders were cast; one set wac taken using concrete directly from the ready-mix truck, the other set was taken using shotcrete placed in a wheelbarrow by the nozzle.

2.10 Material Characteristics of Strengthened Specimens

2.10.1 Shotcrete. Ready-mixed conerete was obtained from a local supplier for the shotcrete. Mix proportions for the shot crete [18J were as shown in Table 2.4.

TABLE 2.4 Shotcrete

Shotcrete Properties

Sl ump (i n.) (from truck) Uni t Weight (lbs/ft 3) (fran truck) Air Content (%) (from truck) Air Content (%) (fran nozzle)

Shotcrete Mix Design (4000 psi) Proportions for 1 yd 3

Water Cement Fine Aggregate Coarse Aggregate (3/8 in.) Sol Air (3% air entrainment) CCC 494 (water reducing agent)

(w/c = 0.38 by weight)

5-112 130

4-1/2 4

250 Ib 658 1 b

2100 1 b

750 1 b 10 oz 21 oz

The aggregate was Colorado River sand and gravel. To facilitate pumping, a small quantity of pumping agl~nt [20J was added on site. Table 2.4 summarizes the shotcrete proper·~ies.

Four-inch cores (Figs. 2.17, 2.18) were tcken from both thj~ concrete-backed panel eCB) and the wood-backed panel (WB) to determin,:; if voids were present behind the reinforcing. A single small voi j

(114 x 114 in.) was found in one (CB) of the six sa.nples. There wa.3 virtually no difference between the wood or concnete-backed panel

26

Fig. 2.15 Shotcreting

27

Fig. 2.16 Finishing shotcrete

28

Fig. 2.17 Panel (top) core sample:;

Fig. 2. 18 Panel (bottom) core samples

samples Smaller cores (1-3/4 x 3 in.) were taken from both panels, capped and tested in compression, with the results S10wn in Table 2.5.

2.10.2 Reinforcement. Jacket rei nt'orcem en'~ consi sted of 116 longitudinal bars at column midfaces, #3 crossties and longitudinal bars at corners, and 6 mm deformed ties. All U.S. sizes conformed to ASTM A-615, Gr. 60. Data on the 116 and 6 mm bars rlere provided in Section 2.4.2. Samples of the 113 bars were test:!d to obtain the averaged steel properties shown in Table 2.6, and the typical stress­s t r a inc ur ve 0 f Fig. 2. 1 9.

2. 1 o. 3 E pox y . C r 0 sst i e s (II 3 ' s) w ere 3. n c h 0 red tot he ori ginal col umn uSing-Concresi ve 14-,1, a two-com pC1nent paste epoxy bon din gage n t pro d u c e d by A d he s i veE n gin e e r i n g [1 9 J . Min i m urn mechani cal properti es are summ ari zed in Table 2.7.

2.11 Loading History

Axial compression was maintained at 64.8 ki ps for all tests, and was based on typical column load data [14J. The corresponding compressive stress was 450 psi for the 12- x 12-in. column (Test 1), and 224 psi for the 17- x 17 in. columns"(Tests 2, 3, and 4).

The lateral loading history (Fig. 2.20) Has displacement controlled. Specimen 1-1 was limited to 2% drift, while Specimens 1-2, 1-3, and 1-1R had maximum drifts of 2.5%. Each 1~est consisted of two cycles at low load levels for system checkout, fClllowed by sets of three cycles at increasing displacement lev(~ls. Drift was increased in increments of 0.5%.

30

TABLE 2.5 Shotcrete Strengths

Aver age

Age f' f' (dais) (gsi) Zpi)

From ready-mix truck 28 4:: 62 4c92 28 4/04 28 4[',10

From nozzle 28 5CJ93 5'52 28 5~'70

28 5093

Concrete-backed panel 107 3cJ51 3()37 107 3 i 43 107 2?95 107 3:;43 107 2'/53

Wood-backed panel 107 4058 2'105 107 2~;28

107 3193 107 21\44 107 1 ~104

TABLE 2.6 Steel Properties (Jacket 113 Long)

Bar Size

fJ3 75

E (ksi)

26923 0.010

Esth (ksi)

1182

fu (ksi)

0.117

TABLE 2.7 Concresive 1411 Mechanical Properties

Tensile Strength (psi)

Elongation at Break (~)

(ASTM 0638)

Compressive Yield Strength (psi)

Compressive Modulus (psi) (ASTM D695)

Heat Deflection Temperature (OF) (ASTM D648)

Slant Shear Strength (psi)

1500

• 4

8000

4.0 x 105

105

75000

Damp to Damp Concrete (AASHTO T-237)

100$ concrete failure

75

60

'in ~

'" 45 III e w

30

15

0,014

Strain (inlin)

Fig. 2.19 Stress-strain curve for jacket steel (#3)

o

~CD wci :r: u ztO -ci U

t--20 W

~C'4 uci 0:

0: 0

end 0-0

ON

.J d 0: 1

a: ..... w . ~9 .JID

d I

CD

d I

o -I

DISPLACEMENT HISTORY

-~ ~ ~

~I\AAA, 'i~VVV ! ttl t~ 1~ 13 1~ c

~ ~ ~

.

Fig. 2.20 Lateral displacement history

1 ~ 1 P I L )

I

lP

N (}I

(JI

r ,-0 0;; .,

o o (}I~

::

~ 0-1-•

-

~

r-

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b

r­(]I

N b

N tn

31

C HAP T E R 3

LOADING SYSTEM AND INSTRUMENTATION

3.1 Loading System

In this research program, the specimens wer3 loaded laterally in one direction while a constant axial load was maintained. The loading system utilized the reinforced concrete fJ oor-wall reactior. syst ern [5J in Ferguson Laboratory.

The loading system consists of three indep~mdent components: 1) a mechanically-controlled hydraulic system for' controlling the axial load; 2) a closed-loop, servo-controlled hydl'aulic system for controlling the lateral load; and 3) cross-coupled rydraulic rams for restraining the end blocks from rotating during loading.

The axial loading system was made up of a 300-kip static capacity ram connecting the specimen's upper loadLng head with thE vertical reaction frame. Axial loads were adjusted manually using ar.. Edison load maintainer [21] while moni toring load c'311 readings. The lateral loading system was composed of two rams, an accumul'ator, servo-controller, and a central pump. Each ram hact a tensile static capacity of 113 kips with a piston stroke of 12 in. The lateral rams were under displacement control. Figure 3.1 illustrates the arrangement of the loading rams with respect to the 3pecimen, and Fig. 3.2 ill ustrates the act ual test setup.

The test specimen is bounded at each end by a loading head which is a welded assembly of wide flange members. The end blocks of the specimen were attached to the loading heads by eight high-strength threaded rods, ai'ter placing a coat of gypsum plc:ster between thE>

loading head and the end block to ensure a sm'ooth bearing surface. The lower loading head is bolted to the testing floOl', while the upper' crosshead is free to translate in the north-south direction.

The test specimen represents a column bourded by very stiff' framing elements. To better model the condition of end fixity, rot at ions of t h eu p per loa din g head are res t r a i n ~ ~ d by a s y s t em of cross-coupled hydraulic rams (Figs. 3.3, 3.4). Two pairs of rams act. vertically to restrain rotations of the upper head in two orthogonal vertical planes. The remaining pair of rams is used to resist rotation of the upper head about a vertical axis.

Each pair of cross-coupled rams may extend or retract equally, as in the case of vertical translation of the upper loading head. However, because of cross-coupling, one rar1 in a pair cannot

32.-33

Axi

al

Lo

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race

d A

ga

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ll

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ral

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oa

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. 3

.1

Lo

adin

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s

Fig

. 3

.2

Test

se

tup

LV

.l>-

35 Ruction a .. me

Axial Load Frame

Horizontal Aeaembt', With Dummy Load Cell

Loading H.ad

Vertical Aaumb ... With Dummy Load Ce.

Positioning 8y.te~Schem.tlc

Bracing

Swivel Connectlona (Both Ends All Assemblies)

Fig. 3.3 Restraining rams

Moment On Loading Head

Horizontal ~ •• ernbl".

Loading Head

~ Obs.rver

lOClltton-Pean View

Double Rodded Hydraulic Actuator

Bottom Loading Head J ----L-____ .-L

Vertical and Horizontal Positioning System

Fig. 3.4 Vertical and horizontal positioning system

36

retract while the other ram extends. This resistalce to differential displacements restrains rotation of the upper loading head.

3.2 Instrumentation

Thr ee types of measur ing de vi ce:5 were used to m on i tor the performance of the specimen during testing: 1) load cells; 2) linear potentiometers; and 3) strain gages.

3.2.1 Loads. Load cells were mounted on each loading ram and on one ram in each pair of restraining rams. All load cells were monitored by the data acquisition system, and the force applied by one of the lateral loading rams was plotted on an X-Y recorder as the test progressed.

3.2.2 Deflections. Twelve linear potentiometers were used to moni tor the deflections and rotations of the speci men end blocks. The potentiometers were supported independently of the loading frame. Deflections measured by the potentiometer s were r!~cord ed by the data acquisition system. The signal from one of the lateral potentiometers, when used in conjunction with the output from a lateral ram load cell, provided a load-deflection plot along the north-south displacement axis used to monitor the response during testing.

3.2.3 Strains. As shown in Figs. 3.5 and 3.6, paper backed strain gages were attached to the tie and longitudinal reinforcement in both the column core and shotcrete jacket. Gages were located on each leg of a jacket reinforcement tie at three levels (top, mid­height, and bottom) of the column. One end of every crosstie was gaged.

3.2.4 Slip. Relative Slip between the c·riginal column and

the jacket was measured using slip wires as shown in Fig. 3.7 located at the column mid-height and near the base.

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Col

umn

Se

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Fig

. 3

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Str

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gag

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cati

on

s (o

rig

inal

spec

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)

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Ga

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oca

tio

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-1

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16

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lum

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on

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-2)

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Co

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Fig

. 3

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gag

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et)

w

38

! w

c: E :J

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3 CL

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C HAP T E R 4

BEHAVIOR OF SPECIMENS

4.1 Introduction

Portions of the experimental test resultg for all specimens (original, strengthened and repaired) are report~d in this chapter. Load-deflection curves and envelopes for all specimens are shown. Strain information is reported for Specimen 1-1 anj Specimen 1-3 only because strain data from Specimen 1-3 were found t.o be representative of both the other strengthened specimen (1-2) and the repaired specimen (1-1R). Significant variations among individual tests will be discussed in Chapter 5. Basic data for each test were obtained from load cells, displacement transducers and strain gages. Photograp1s were used to record crack patterns at the end of each load phase.

The hysteretic behavior of the test specimens under the imposed cyclic deformations and constant axial load' is presented in terms of lateral load-deflection curves. Of particular interest are the stiffness of the specimen, its peak lateral eapacity at a given deflect.ton level, its loss of lateral load capaci ty due to cycling, and the overall shape of the hysteretic loops. The slope of the load deflection curve at any point represents the tangent stiffness of the specimen. Envelopes of peak lateral load-deflect:~on values are used for direct comparison of test results. The main objective in analyzing the test results is to study the differences in behavior between the original column, and the same tYl=e of column after strengthening or repair.

4.2 Description of Test Results

4.2.1 Load-Deflection Curves. Load-deflection curves for Specimens 1-1, 1-2, 1-3, and 1-fR-are shown in Figs. 4.1 through 4.4. Inspection of the figures reveals recognizable chaJ'acteristics such as symmetry about the load axis, peak load-displacement envelope outlines and the effect of successive cycles at a constant drift level. Hysteretic behavior is referred to as "stable" when only small changes in lateral capacity are observed under cycling to constant drift levels. Large losses in specimen stiffness are characterized by "pinching" of the hysteretic loops. Poor ~nergy dissipating characteristics of a member are generally typLfied by nonstable hysteretic behavior with pinching. As expected, pinching is more pronounced for both the original column (Specimen 1-1) and the repaired column (Specimen 1-1R), than for either of the strengthened specimens (Specimens 1-2, 1-3). Peak load-deflecti.on envelopes, shown in Fig. ll.5, connect peak load-deflection values ill the first cycle to each drift level, and are used to compare hysteretic characteristics

39

4J

STaRr DRIFT [7.J -2. a -1. 5 -1. a 1.5 2. a

I " , . '

-0.8 -0.6 -0.4 O. 4 0.6 0.8 OISPLACt:~ENT CINCHESJ

SOUTH NORTH

Fig. 4.1 Load-deflection curve, Specimen 1-1

STfJRr DRIFT [Y.J -2. 5 -2. a -1. 5 -1. a 1. a 1. ~i 2. a 2. 5

i I i~--~~--~~~-r~~~~~~~~~~~-L~i iii

-1.0 -0.8 -0.6 -0. 4 O. 4 O. 6 0.8 1.0 OISPLACEMENT CINCHES]

S~UTH N~RTH

Fig. 4.2 Load-deflection curve, Specimen 1-2

STClRr DRIfT ex.J -2. 5 -2. a -1. 5

-1. 0 -0.8 -0.6

SOUTH

-1. a -0.5

-0. "

n (f)

o o

0...0 .... ex:> :J:: u 00 a:c.o o ..J

o (0 I

o o .... I

1. a 1. ,; 2. a 2. 5 , I' , I i

o. 4 I). 6 O. 8 1. 0 DISPLACEMENT [INCHES]

N(jRTH

Fig. 4.3 Load-deflection curve, Specimen 1-3

STORr DRIFT CXJ -2. 5 -2. 0 -1. 5

-1. a -0. 8 -0. 6

SClUTH

n (f)

o o

0...0 ..... (0

~ u 00 CI:W o -J

o .. I

o W I

o ro I

-I

1.5 2.0 2.5 ~~::....&.~--'---'-~-':'-4-~'~" o.'a I 1.10

DISPLAC~MENT [INCHES] NuRTH

Fig. 4.4 Load-deflection curve, Specimen I-·IR

41

n (/')

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Fig

. 4

.5

1.

0 1

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o. 4

o.

6

2.0

S

TD

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DR

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CY.

::J

2 .. 5

o. 8

1.

0 D

ISPL

ACE

MEN

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1-1

, 1

-2,

1-3

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NORT

H

.+:-­

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43

of different tests. North and south displacements generated similar peak load envelopes, and for clarity of pre~lentaticn only the north displacement curves are shown. For later'al deflections in excess of 1 percent drift, the 1 ateral load capaci ty of Specimer 1-1 is observed to decay much faster than that of either of the strengthened specimens. The repaired specimen (1-1R) was less stiff than either of the two strengthened specimens, and exhibited significantly reduced lateral capacity at deflections in excess of 2% drift.

4.2.2 Crack Patterns. Crack patterns for ~~ach of the tests were photographed at the end of each load phase, Figs. 4.6 through 4.9. Figure 4.6 illustrates typical crack patterns on the northwest faces of Specimen 1-1 after reaching lateral displacements corresponding to 0.5, 1; 1.5, and 2% drift. At 0.5% drift, flexural cracks developed near the top and bottom face of the column. Inclined shear cracks formed at 1% drift, and were approximately 1/64 in. wide at that displacement l"evel. Cycling at 1.5% drift extended those inclined cracks across the column face to a \.Jidth of 1/32 in. When 2% drift was reached, the inclined cracks widened, and concrete spalling was Observed at the column corners, and to a lescer degree on the col umn face at mi d-hei ght.

Typical crack patterns for the west face of Specimen 1-3 are shown in Fig. 4.8, for lateral drifts for 1.0%, 1.5, 2, and 2.5%. Significant flexural cracks developed at the 1% dri:~t level and were I e sst han 1 / 6 4 in. wid e • Inc lin e d c r a c k s de vel 0 p e d fro m e xis tin g flexural cracks at 1.5% drift, and a widE' crack (3/32 in.) on the opposite side of the displacement direction openEd up between the shotcrete jacket and the top and bottom end blocks. Continued cycling at 2% drift level extended and widened both inclined and end cracks, culminating in crushing and spa1ling near both tCIP and bottom end blocks at the 2.5% drift level. At peak displacements at that drift level, the end cracks were approximately 1/4 in. wide.

Similar crack patterns were observed in ~,pecimens 1-1 and 1-1R for all levels of deformation. Flexural cracks turned into inclined cracks at about 1% drift. Repeated cycling at 1% drift widened and extended those cracks, which were evenly distributed over the height of the column. The cracking patterns of Specimens 1-2 and 1-3 resemble each other, but were Significantly different from those of Specimens 1-1 and 1-1R. The strengthened specjmens showed only flexural cracking at 1~ drift. Cracks became incljned at 1.5% drift. At corresponding drift levelS, Specimens 1-2 and 1<: had fewer crack~)

and a smaller relative crack width than Specim,:ms 1-1 and 1-1R. Specimen 1-1 exhibited some spalling at column mid-height, while Specimens 1-2, 1-3 and 1-1R all exhibited nearly equal amounts of crushing near the end blocks. Summarizing, Specimens 1-1 and 1-1H exhibited shear-dominated crack patterns, and Specimens 1-2 and 1-] exhibited flexure or flexure-shear dominated crack patterns'.

"ES'

,.a

(a)

0.5

% d

rif

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) 1.

0%

d

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) 1

.5%

drif

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Fig

. 4

.6

Cra

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att

ern

s,

NlJ

colu

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fa

ces,

Sp

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en

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-1

(d)

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% d

rif

t

+'

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(a)

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rift

(b

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rift

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.,

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s, Speci~en

1-2

(d)

2.5%

dri

ft

~

VI

(a)

1. 0

% d

rif

t (b

) 1.

5%

d

rif

t (c

) 2

. 0%

d

rif

t

Fig

. 4

.8

Cra

ck p

att

ern

s,

Sp

ecim

en

1

-3

(d)

2.5%

drif

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+'

0'1

(a)

1% d

rif

t (b

) 1.

5%

drif

t (c

) 2

.0%

d

rif

t

Fig

. 4.

9 C

rack

p

att

ern

s,

Sp

ecim

en

l-1

R

Cd)

2

. 5%

d

rif

t

.f>

--

...j

48

A crude indicator of the amount of delamination between the shotcrete jacket and the original column face is the comparative hollowness of the sound produced by tapping with a hammer on the column face. All jacketed columns were investigated in this manner after testing. The east and west faces of each spec:~men sounded more solid than the north and south faces. Though the eas'~ and west column faces exhibited more damage as evidenced by wider clnd more numerous cracks, the north-south faces would be expected 1.0 indicate more delamination because of alternating extreme fiber compression and tension under north-south lateral displacements. Ti".:; north and south faces of Specimen 1-3 sounded most solid, follcwed by those of Specimen 1-2. Specimen 1-1R had the hollowe!3t sound.

4.2.3 Strain Distributions. :;train ~;ages mounted on longi tudinal and trans-versereinforcement wer-e moni tQred at each load stage for all tests. Attention was paid to the variations of strain at each drift level, and also to the history of ;3trains at given locations under increasing drift levels.

Longitudinal Reinforcement. Figure 4.10 :.llustrates, as a function of peak drift levels in the north direction, the distribution of strain along the northwest #6 reinforcing bar of Specimen 1-1. Data correspond only to load stages used to produce the load­displacement envelopes of Fig. 4.5. As expected, the plot indicates the development of tension at the top of the nortr. face, while the bottom north face of the column remains in com pres!3i on. Figure 4.11 illustrates the comparable situation in Specimen 1-3. Especially noteworthy is the development of tension at both the top and bottom of the north faces as the drift level increases. This will be discussed in Chapter 5.

Longitudinal bar strains can be used to characterize the behavior of the original column section. Because the jacket longitudinal reinforcement did not extend into the E:nd blocks, it die not develop large tension forces. Strain information from jacket bars in compression, however, can be helpful for insight into the behavior of the jacket. Figure 4.12 illustrates the history, wi th increasinE; drift levels, of strain at the top of both the #6 original column reinforcing bar and the adjacent #3 jacket reinforcing bar. For northerly displacements, the jacket bar is strained much less than the original column bar. While the jacket bar alternatE~s between tension and compression, as would be expected from conventj onal beam theory,. the original column bar experiences tension at the top under cycling in ei ther direction. This was also observed in Sp(~cimens 1-2 and 1-' 1R, and will be discussed in Chapter 5.

Transverse Rei nforcement. Fi gure 4.13 ill ustrates, for increasing drift levels, the averaged strains frol1 the rectangular ties running in the north-south direction (east-1oJest faces of the column) in Specimen 1-1. The mid-height tie experil;nced the greatest

DISPLACEMENT N~ATH .

0.0% DRIFT

o. 25% OAIFT~,: I

-800 -600 -400 -200 o 200 400

1. 5r. DRIFT

iii'

600 800 1000 1200 1400 HICROSTRAIN [IN/INJ

Fig. 4.10 Envelopes of strain distribution, NW longitudinal 116, Specimen 1-1

I I

+

IJ)

. ..

, \ \

sao

2. Or. DRIFT

2.5% DRIFT

I i

1000 1500 2000 2500 MICRO:HAAIN [IN/IN]

F ' 4 11 Envelopes of strain distribution, NW longttudinal 116, 19. . Specimen 1-3

49

-6

BA

R

STtJ

RY

D

RIF

T eX

J ~

-2.5

-2

.0

-1.5

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8 -0

. 6

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..

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....

. ••• ..

. ..

o o If)

nN

Z ~

........

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o

.....

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o

ZN

.....

a:

ex

: ....­ (f)O

cO

ex

:lf)

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~

:E o o o - o o U

i 0.10

1

,. -3

BAR~, •

,

1.

0 ...

. "1'.

5 2~O

2.5

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0

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0.8

1

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ENT

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Fig

. 4

. 1

2

En v

el 0

pes

0 f

top

gag

est r

a in

, n 6

/ 11

3 N

W I

on

g i

t u

din

a 1

b

ars

, S

pec

imen

1

-3

L,;

o

n z

o o o C'

.....

...... 0 zo ..... 0 Ut.D

z

o o o en

o o o N

o

~I STaHr OflIFT C7.J

-e e e. • 0.5 .,..#c .-

,.

M I 0 -H E I GH T TI [

BCHTCIM TIE

(. TOP TIE ~ / /

~ .... /

~/ > ~-------­

~i

5]

1.5 ,

O. 1 0.2 0.3 0.4 0.5 0.6 OISPU,CEMENT [INCHES]

NCJATH Fig. 4.13 Strain envelopes, N-S direction, original column

ties, Specimen 1-1

o o o

o o ex>

o o CD

o o • a o N

MID-HEIGHT TIE

BCJTTCJH TIE

TCJP TIE

~====!~?P~~~~~~~----'- , 1.5 i

0.3 O. 4 O. 5 O. 6 o I SPUICEMENT CINCHES]

NrlfHH

Fig. 4.14 Strain envelopes, E-W direction, original column

ties, Specimen 1-1

52

increase in strain between the 1 and 1.5% drift lEvels, which also corresponds to the formation of significant inclined cracks at column mid-height (Fig. 4.6). At 1.5% drift, onl y the top tie remained elastic. Simila:rly, Fig. 4.14 illustrates the averaged strains for ties running in the east-west direction (north-s(uth faces of the column). As before, the strain increased significan'~ly between 1 and 1.5% drift level. Figures 4.15 and 4.16 illustrate averaged strains at mi d-hei ght of Specim ens 1-1 Rand 1-3 for both j a ~ket and ori ginal column rectangular ties. Gages located on east ard west face ties exhi bi ted the greatest increase in strain following the formation of inclined cracks between the 1 and 1.5% drift displacement levels. At peak displacement, only the east-west face jacket ties approached yield. Because the core was damaged before jacketing, the jacket tie strains in Specimen 1-1R were much higher at compar3.ble drift levels than similar ties in the strengthened specimens (1-2,1-3).

Crossties. Specimens 1-3 and 1-1R had 113 (~rossties in both the north-south and the east-west directions. The purpose of these crossties was to provide confinement in the east-w ~st direction and provide both confinement and shear resist.ance in the north-south direction. Histories of strain versus lateral displ.3.cement are shown in Figs. 4.17 through 4.20. At comparable drift levels the crossties running in the east-west direction consistently experienced less stress than those in the north-south direction becau~ e the north-south crossties had to resist shear in addition to providing confinement. A comparison of north-south crosstie strain readings :~or Specimens 1-3 and 1-1R indicates that crossties in the strengthened specimen (1-3) were'strained about half as much as those in the repaired specimen-'(1-1R) at comparable drifts. The north-south crossties in both specimens developed significant strains at drifts in excess of 1 and 0.5% respectively, corresponding to the formation of inclined cracks and increasing tranverse tie strain.

4.2.4 Slip. The jacketing technique results in an interface between new and existing concrete in the strengthE~ned and repaired specimens which affects the performance of the colunns. Movement of the shotcrete Jacket with respect to the origiral column would indicate that the cross-section was not re~iisting flexurE monolithically, and could imply that the jacket was not fully effective in confining the original column nor in resisting shear.

Representative plots of jacket movement with respect to thE original column were shown in Figs. 4.21 and 4.22 :or Specimen 1-3. Posi ti ve slip corresponds to upward movement of the ~ acket wi th regard to the original column. Similar behavior was observed for all strengthened and repaired specimens. In each cas'=, slip wire data appeared to indicate that the mid-height portion of the jacket moveCl upward relative to the original column at about 1% drift. The bottom porti on of the jacket appeared to move downward wi th res pect to the original column at about the same drift. This would suggest the

o o o ex:>

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Fig

. 4

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train

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vel

op

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mid

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gh

t ti

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Spe

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V

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Fig

. 4

.16

S

train

en

vel

op

es,

mid

-hei

gh

t ti

es,

Spe

cim

en

1-3.

lr. ~

n Z

'" Z

u

o 0, O. N'

o ZO t-flf) a: .... ex: I­(f') o ex: u

/ j? TOP

I I

55

'.

:S::o o o / t +HIO-LOWEA

.,.,)C....,. /

'-'X''- ~ / / ,''-x.,.. / ./ /" t HID-UPPEA

" '. f- ... ....... ' • -'x.... ./ .y' ._---~-::...:-j BOTTOM -' '" 7----)l---x STORr DRIFT CY.J ~1-":\~"+f1 "x."'1t +

-2.5 -2; a . -1: 5 .... l.~ -~ .51 ./1.0 1.-5 ~'IO 2.5 -1.0 I -0:8 I -0:6 I -0:--4 -·~o:~·- '<3. (jA. .... - :2 6 '!"'.a-.\ ;*'0.'6 0.'8 I 1.'0

SOUTH OISPLAC~HENT CINCHES]

NOATH

Fig. 4.17 Strain envelopes, N-S crossties, Specinen 1-3

n z --

o o o -

....... 0 !So u lfl

STCJRr DRIFT CZJ

~~~~~~~~~~~~~~~~~~O~.~5~~.~~~-~~2.5 t- I I f I I

O.~···O:4-···0.6 0.8 1.0 -1. 0

SOUTH r"\ _ ..

MID-UPPER

(f') O. cr o ex: u ....... 0 X:o

lJ) I'

o o c:t ...J I

D I SPLACENEIH .. [I NCHESJ . NORTH

Fig. 4.18 Strain envelopes, E-W crossties, Specimen 1-3

56

•

\ \

)1._\ ---x.... \ .......... -\ '.~

\ \ \

\ \ \ \

\ \ + '\.

\ \ \ \ \ ~. ..... "

\ . .. \ " \ '. '" \ STl'1Rr DRIFT cY.:t··. _ .•

-2. 5 -2. 0 -1. 5 -1. V

-1. 0 -0. 8 -0.6 -0. 4

SOUTH

o o a

B~TTOM -'C ___ ~./,//

j

~' /

; ;

/ ;

; /

;

/

)I

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,// /' i HID-UPPER

;(/ .-' ..... /' .' r .'

I •

J •. l ...

/ • ,.' .' HID -LCJWER

. ·~#1.~ [.L..:i 2. 0 2. 5 -+ I I...., I I

Fig. 4.19 Strain envelopes, N-S crcssties

, '- " .t MIO-LDHER

'-

x..... • ..... ,l-.'"

BOTTOM...7' ..........

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5:7

58

presence of significant cracks between the location3 of slip wires in the shotcrete jacket. However, such cracks were not. observed.

Care must be exercised in drawing conclus:.ons based on thi!3 slip data. First, the magnitude of slip measured In all tests is a': the lower limi t of the sensi ti vi ty of the linear ~otentiometer used. Second, the linear potentiometer, mounted as shown :~n Fig. 3.12, could not distinguish between movement of the slip wire and outward movement of its support rod due to lateral expansion of tre column. Third, slip in Specimen 1-3, whose jacket was reinforced with crossties, wa:3 about twice that of Specimens 1-2 and 1-1R. Intuitively, th3 crossties would be expected to inhibit relative mo'rement between the shotcrete jacket and the ori ginal col umn.

C HAP T E R 5

DISCUSSION OF TEST RESULTS

5.1 Introduction

In this chapter, the resul ts of the four tl;sts on three will be compared. The behavior of the columns will be conpared in terms of load-deflection curves and strain distributions. fhe experimentally observed lateral capacity will be compared with com}:,uted values.

5.2 Test Results

5.2.1 Specimen Stiffness and Capacity. An indication of the relative stiffness of each specimen can be seen in Figs. 4.1 through 4.4. Unstable hysteretic behavior (pinching) is predominant in Specimen 1-1, and much less evident in the repaired or strengthened columns (Specimens 1-1R, 1-2 and 1-3). Specimen 1-1 exhibited stable hysteretic behavior for deformations up to 1% drift, but showe(j considerable loss of stiffness at 1.5% drift. Un;,table, degrading hysteretic behavior was observed for drift levels in excess of 1.5%. The strengthened specimens (1-2 and 1-3) exhibited stable hystereti(~ behavior for deformations up to 1.5% drift, after Hhich pinching and loss of stiffness became apparent. In the first cycle to each drift level, the repaired specimen (1-1 R) exhi bi ted load-deflection behavior similar to that of the stren-gthened spec.Lmens, tlut degraded much faster than the strengthened specimens under ccnstant amplitude cycling beyond 1.5% drift. Nei ther the str~enthened nor the repaired specimens exhibited the dramatic loss of stiffness observed in Specimen 1-1 beyond 1% drift.

The stiffness of each specimen can be compared graphically using the load-displacement envelopes of Fig. 4.5, which illustrate::> the improved performance of both the strengthEned and repaire,j specimens compared to the original column (Specimen 1-1). Table 5.1 summarizes the first cycle secant stiffness (applied lateral forc(~ divided by lateral displacement) of each specimen ~or various drift levels. Specimens 1-2 and 1-3 had nearly equal first-cycle stiffnesses, and both were about 10% stiffer than the repaired Specimen 1-1R in this respect.

The effect of cycling on specimen stiffn:;ss can be seen in Figs. 4.1 through 4.4. The load-displacement envelopes for the first and third cycles to equal drift levels are shown for each specimen. Table 5.2 summarizes the percentage losses in secant stiffness between

59

60

TABLE 5.1 Secant Stiffness in First Cycle to Va~ious Drift Levels

Secant Stiffness (kip/inch)

Drift Specimen Specimen Specimen Specimen Level 1-1 1-2 1- 3 1-1 R

0.25 204 259 270 235

0.5 192 242 250 218

0.75 162 220 221 193

1 .0 143 199 195 178

1.5 95 156 158 146

2.0 62 127 130 119

2.5 109 110 93

TABLE 5.2 Reduction in Secant Stiffness Between 1st and 3rd Cycles

Secant Stiffness Reduction ( %)

Drift Specimen Specimen Specimen Specimen Level 1-1 1-2 1-3 1-1 R

0.5 7 7 2 5

1 .0 13 7 3 7

1 .5 23 8 8 9

2.0 7 7 11

2.5 11 7 12

61

the first and third cycles at a constant drjft level. Specimens 1-1 and l-1R experienced the greatest degradation, and Specimen 1-3 the leas t.

The lateral resistance of Specimen 1-1 decreased as drifts were increased beyond 1%. Other specimens did not dE'grade as much in this respect. The jacketed specimens exhib1ted incJ'eased resistance wi th increased drifts up to 2.5 percent.

5.2.2 Strains in Reinforcement

Longi tudinal Reinforcement. Comparison of longi tudinal bar strains in Specimen 1-1 (Fig. 4.10) versus those of ej ther Specimen 1-2,1-3 (Fig. 4.11), or l-1R indicates that the original column had a different strain distribution from that of e:i. ther th-~ strengthened or repaired specimens. Conventional flexural theory indicates that a beam-column, subjected to equal end moments, and having a point of inflection at mid-height, will have a strain gradient ranging from tension to compression along longitudinal reinforcem~nt. Specimen 1-1, with as aspect ratio of about 1.7. confor'ms to t:1is conventional expectation. However, as mentioned in Chapter 4, plots indicate that tension develops along the entire length of t.he longitudinal reinforcement in both the strengthened and r'epaired specimens. This phenomenon can be explained in terms of two impor::,ant differences between the ori ginal and the jacketed col umns: 1) the location of the neutral axis; and 2) the effects of diagonal tension on the internal resisting moment within the column.

Prior to the formation of inclined cracks, and assuming Ii ttle bond deterioration, longi tudinal steEI strains are consistent with the predictions of simple bending theory. Analysis using the com puter program RCCOLA [15J indi cated that regar dl ess of mom ent direction, the position of the jacketed column's neutral axis placed all of the original column longitudinal reinforcenent in tension. Figure 5.1 (a) illustrates the effect of this neutral axis location on strains within a single longitudinal bar. The bottc·m portion of the bar is in tension even though it is located on the "compression" face of the col umn.

Paulay, in his study of coupling beams [2~J has shown that due to the effects of diagonal tension, flexural members with shear span/depth ratios less than about two have tensile stresses along the entire length of their longitudinal reinforcement, =ven at locations where conventional flexural theory would predi ct com )ressi ve stresses. Such a distribution is shown in Fig. 5.1 (b) and becomes more dominant with decreasing shear span/depth ratios. The addition of the shotcrete jacket to the original column deepens the ~Iection, resulting in a coupling beam effect.

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In all strengthened and repaired specimens, both effects were observed. The position of the neutral axis influenced bar stresses from the start of the tests, and the effects of diagonal tension increased as the tests proceeded. Figure 4.9 shows the shift of the strain envelope from one resembling Fig. 5.1(a) to 01e resembling Fig. 5.1 (b) at 1.5% drift. Subsequent larger drift~ lengthened the inclined cracks, shifting the strain envelope even more.

The jacket longitudinal reinforcement was not continuous into the end blocks of the specimens. It ser'ves to hold the jacket transverse steel during construction but makes no c(,ntri bution to thE section capacity. Both the strengthened and reJaired specimens exhi bi ted si milar beha vi or in both the ori ginal col umn and jacket rei nf or cern ent. Strai ns in t he or i gi nal 1 ongi tudj nal rei nf orcem ent typically started to increase at about 0.5% drift, with relatively large increases occurring wi th increasing drift. Cn the other hand, strains in longitudinal jacket reinforcement inc"'eased much more slowly with increasing drift for all specimens. For large drift values in the northerly direction, when the reinfor~ement at the top north face of both the original column and the jack(~t was in tension, the jacket reinforcement had Ii ttle strain. At sirr ilar displacement levels, the jacket reinforcement inSpecimen 1-1R exhibited higher strains. This appears reasonable, because the core of Specimen 1-1F had been badly damaged by the first test, the shotcl'ete jacket had to carry a larger share of the imposed forces. However, the jacket longitudinal reinforcement in all specimens did not develop significant tension.

Tranverse Reinforcement. DistributionJf shear between concrete and steel, and relati ve confining effects o~" transverse steel were investigated using strain gages mounted on the transverse reinforcement in both the jacket and the original c,)lumn. TransverSE~

reinforcement on the east and west faces of the column resisted norttl and south loads, and confined the concrete core. All strengthened specimens exhibited similar behavior in that drif·:.s in excess of 1~;

caused large increases in strain in transverse reirforcement on the east and west faces (north-south direction). In all specimens, this drift level also corresponded to the formation of inclined cracks at col umn mi d-hei ght.

Up to drifts of 2'%, little if any yieldirg occurred in thE~ transverse steel of the strengthened specimens. However, this was not the case for the repaired specimen. Inspection t)f transverse tie strain data for Specimens 1-3 and 1-1R indicate:: that the jacket transverse reinforcement in the north-south directi.on yielded beforf= 2% drift, and reached strai ns far in excess of thost~ recorded for the jacket ties in the strengthened specimens. This is reasonable considering that the original column core of this specimen was badly damaged in the first test, and the original column ties were therefore not fully effecti vee

64

Transverse reinforcement on the north and south faces (east­west direction) confined the column core. ~)trains in the north and south face transverse reinforcement in both the jacket and the original column were less than yield for all specimens.

5.3 Comparison of Observed and Computed Capaeity

The experimentally observed lateral eapaci ty of each specimen was determined using: 1) load-displacement curves; 2) longitudinal steel strains in the original column; 3) curve:, of strains in transverse reinforcement; and 4) observations o:~ the extent of inclined cracking.

The original column (Specimen 1-1) vJas anal:rzed as described in Section 2.3.3 using the computer program RCCOLA [1~)J. The program, assuming plane sections remain plane, analyzed a slice of column cross section and computed the col umn's flexural and shear ,~apaci ty as shown in Fig. 2.2. As mentioned previously, the mornent-axi 3.1 force curve as governed by shear capaci ty is based on Unt versi tr of Texas short column tests [23J rather than conventional ACI [17J equations. Analyses indicated that a shear failure was likely because the predicted capacity in shear was less than that in f:_exure. Specimen 1 - 1 did h a ve ash ear - do min ate d b r itt 1 e f ail ur est art i n gat abo u t 1 % drift. Longitudinal steel strains at failure were less than yield, and degradation of stiffness under cycling occ..lrred after the transverse reinforcement yielded. ThE' computed and observed capaci ties are shown in Table 5.3.

TABLE 5.3 Lateral Capaci ty

Computed Lateral Capacity Exper. (ki ps ) Observed Drift Canputed/

Specimen Capacity (% ) Observed Flexure Shear Shear

(UT) (ACI) (ki ps ) (UT) (ACI :i

1-1 64 40 31 47 0.85 0.66

1r.-2 104 100 72 90 2 1 • 11 0.80

1-3 104 100 72 88 2 1 . 1 4 o. 8;~

1-1R 104 100 72 86 2 1. 16 0.84

65

Inspection of Table 5.3 reveals that the ACI shear equations (11-6, 11-7 and 11-17) consistently underestimated the section shear capacity when compared to the results of the empirical relationship discussed in Section 2.3.3.

The strengthened specimens (1-2, (1-3) wer,= analyzed using the same program (Section 2.6.2) used for the ori ginal col umn. A number of addi tional assumptions were made in modelling the original column-shotcrete jacket combination:

1) No slip was assumed between the original column and the shotcrete jacket. The effeot of relative slip would be a smaller predicted capacity based on less than monolithic action;

2) Jacket longitudinal reinforcement was assumed not to carry any tensile stress. As shown in Fig. 4.10, a small amount of tension due to bond did develop in the jacket longitudinal reinforcement, and this could be expected to increase the predicted capacity; and

3) The shotcrete jacket was assumed to be fully effective in compression. The presence of 1/64 in. shri nkage cracks at each end block would indicate less than full jacket effectiveness at low displacement levels.

The results of the analysis, shown in Fig. 2.7 and Table 5.3, indicated that the computed capacity in shear was nearly equal to that in flexure. A combination flexure-shear failurE mode would be expected, considering the location of the neutral a:cis developed in the anaysis. Specimens 1-2 and 1-3 had flexurally-dominated failures, as evidenced by longitudinal reinforcement strains €~xceeding yield. Some degradation of stiffness occurred under cycling 3.t larger drifts.

The com puted flexural and shear capaci ti es of the repaired specimen (1-1R) were nearly equal and a combined shear and flexure failure could be expected. Specimen l-1R did exhib:.t a combination she a r - fIe x u ref a i I ur e, as e v ide n c e d by Ion g i t u din ell s tee 1 s t r a i ns slightly less than yield at peak displacements, Significant degradation of stiffness under cycling, and by strains greater than yield in transverse reinforcement. Inclined shear cracks were far more numerous and wider in Specimens 1-1 and 1-1F: than in either Specimens 1-2 or 1-3.

In summary, for the strengthened and repaired specimens, experimentall y observed 1 ateral capaci ty was about 10 to 15% 1 eSB than that computed using ACI equations and about 10% morE than that using empirical equations developed expressly for short col'lmns.

C HAP T E R 6

SUMMARY AND CONCLUSIONS

6.1 Summary of Investigation

The behavior of strengthened and/or repaired reinforced concrete short columns under cyclic deformations was studied. The primary objective of the study was to evaluate the E!ffectiveness of various techni ques for strengthening or repairing shoj"t col umns.

Based on an 18-in. square prototype col umn, three col umn test specimens were constructed to two-thirds scale, using identical geometry and reinforcement. The original specimens had a 12-in. square cross section reinforced with eight 116 longitudinal bers, sets of 6 mm ties spaced at 8 in., and 1 in. cover. Spacing of the transverse reinforcement, though greater than what would curren':ly be specified, was intended to represent the practice of column d~?sign in seismic regions of the U.S. in the 1950's and 1960's.

One of the original specimens was tested (Specimen 1-1), repaired, then retested (Specimen l-lR). Hepair cf that specimen consisted of removing loose cover, adding 113 10ngitud:.nal bars at each corner, epoxying 113 crossties in each direction at 9 in., hooked around a 116 mid-face bar, and adding 6 mm deformed tr'ansverse ties at 2.5-in. spacing. The damaged column was then encas9d with a 2.5 in. shotcrete jacket which provided a l-in. cover over the added reinforcement, resulting in a 17 in. square column. The remaining two specimens (Specimens 1-2 and 1-3) were strengthened prior to testing. Specimen 1-2 was strengthened by adding #3 longitudinal bars at each corner, 6 mm ties at 2.5 in., and a 2.5 in. sl10tcrete jacket, resulting in 1 in. clear cover and a 17 in. square column. Specimen 1-3 was strengthened similarly to the repair'ed specjmen (1-1R) using 113 longitudinal bars at each corner, 6 mm ties ,3.t 2.5 in., plus crossties hooked around #6 longitudinal bars at mid-face.

The specimens were loaded laterally using an apparatus designed to permit movement of one column end while preventing rotations of both ends. A single displacement history was used for all tests. Typically, each specimen was subjected t.o three reversed cycles of lateral displacement to drifts of 0.5,1.0,1.5,2.0, and 2.5%. A constant axial load of 64.8 kips was applied in all tests. During each test, measurements were taken at each load stage to determine the applied forces, lateral deflection, ard strains in the longitudinal and transverse reinforcement. Fixity of the column ends was also moni tored. Crdcks were marked at each peak deflection.

tb-67

68

6.2 Summary of Test Results

6.2.1 Original Column Specimen. Specim,=n 1-1 exhibited stable hysteretic behavior for-deformatiOns up to 1% drift after which considerable loss of specimen stiffness occurrej under cycling. Failure appeared to be dominated by shear, as evidenced by pinching of the hysteretiC loops, and the development cf longitudinal rei nforcem ent strai ns si gnifi cantly 1 ess than y:. eld. Extensi ve inclined cracks developed at 1% drift and steadily lengthened with cycling. Analyses indi cated that the ori ginal col umn shear capaci ty was Significantly less than flexural capacity and a brittle shear failure was predicted. The original column specimen behaved satisfactorily at drift levels less than 0.5% whl~re response was essentially elastic. However, its loss of strength and stiffness at larger drifts indicated that the original specimen ::!ould not provide adequate cyclic lateral resistance in the inelastic l'ange.

6.2.2 Strengthened and Repaired Specimens. The strengthened and repaired specimens were designed to fail in a mo:"e ductile manner than the original column. Analysis of the streng'~hened specimens indi cated a com bination flexure-shear fail ure (shear capaci ty was about equal to its fl exural capaci ty). The shotcrete jacket was assumed to behave integrally with the original co:_umn and improve inelasti c strength and stiffness due to the confini 19 interaction of the transverse and longitudinal steel.

Both the strengthened and the repaired columns exhibited greater ductility than the original column. BClth strengthened specimens (Specimen 1-2 and 1-3) exhibited similar load-deflection behavior, having stable hystereti c loops for deform 3.tions up to 1.5% drift, after which loss of stiffness became apparent. Failure appeared to be flexurally dominated, as evidenced by the development of strains in excess of yi el din the ori gi nal col urn n 1 ongi t udi Lal rei nf or cern ent. Jacketing both with and without supplementary cros:,ties resulted in much greater stiffness and strength than that of the original, unstrengthened specimen. Inclined cracks developed in both strengthened specimens at drifts in excess of 1%, coinciding with increases in measured strains in the tranSVE:rs'= reinforcement. Supplementary crossties did not significantly increase specimen strength nor stiffness in the first cycle to a gi ven drift level, but were beneficial in delaying strength and stiffness dE~terioration under repeated cycles to drift levels exceeding 2%.

The repaired specimen (l-lR) exhibited stable hysteretic behavior for deformations up to 1.5%, after which it began to lose stiffness under cycling. Failure appeared to be a combination of shear and flexure, as evidenced by yield of the original column longitudinal reinforcement, coinCiding with pinching of the hystereti c loops.

69

The repaired specimen had much greater latEral stiffness and strength than the original specimen, and about 10% less than the strengthened speci mens at the first cycle to a g:. ven dri ft level. Inclined cracks developed in the repaired specimen at 1% drift, coinciding with increases in measured strains in tne transverse ties and crossties which was in excess of yield at large drifts. Inclined cracks were longer, wider, and more numerous in the repaired specimen than in ei ther of the strengthened speci mens. The repaired speci men degraded much faster than ei ther of the strengthened speci mens under repeated cycles to drift levels exceeding 2%.

6.3 Conclusions

1) A two-thirds scale model of a typical column designed for seismic areas in the 1950's and 1960's performed poorly under reversed cyclic lateral deformations excE·eding 0.5% drift. As indicated by analysis, the column shear span/depth ratio and reinforcing details resulted in a brittle, shear­dominated failure. This situation can be remedied either by strengthening the colu mns to produce a more ducti Ie me mber, as described here, or by providing additional elastic capacity in the form of shear walls or bracing.

2) A number of techniques can be used to strengthen existing columns in order to improve their performance under reversed lateral deformations. In this study, encasement of the original square column with a shotcrete ~acket reinforced with corner longitudinal bars and clo!,ely-spaced ties significantly increased its stiffness and lateral capacity. Care was taken to develop a column whose failure would be ductile at the reduced shear span/depth ratio caused by jacketing. A 2-1/2 in. spacing of ja~ket transverse reinforcement provided increased confinement and shear resistance, and was not hard to fabricate. An upper bound to the lateral capacity of the strengthened column was calculated assuming integral behavior of thl~ shotcrete jacket and the original column. Shear capacity was predicted using equations developed from tests of short columns. ACI equations consistently underestimated the shear capacity.

3) The strengthening technique described above was varied in one specimen to increase the confinement provijed by the jacket~

Additional midface longitudinal bars were placed in the jacket, and connected by crossties grouted l.,rith epoxy through the original column. This modification did not significantly effect the monotonic stiffness or strength but did decreaSE the rate of strength and stiffness degradation under repeated cycles of reversed lateral displacements exceeding 2% drift.

70

4) When a badly damaged column was repaired by (mcasing the core with a shotcrete jacket reinforced with clm,ely-spaced ties, and with crossties connected to midface l)ngitudinal bars, the strength and stiffness were nearly equal to those of an undamaged column strengthened with the se.me jacket. The crossties and midface bars contributed sigrificantly to the confining effect of the jacket transverse re:.nforcement.

6.4 Additional Research

Based on the results of the current inv?stigation, the following future research is suggested:

1) Study is needed regarding the effects of varying the proportions of both the column and the jacket. The performance of a strengthened section may differ considerably depending on the relative proportions of the added jacket to the original column.

2) The eff ect of epoxy i nj ecti on of cr acks ina dam aged col umn needs study. Repair of the original column core may reduce the deterioration of strength and stiffness when the is cycled repeatedly to large drift levels.

a

d*

E

f' c

NOTATION

Shear span, in.

Area of concrete core, out-to-out of ties, in. 2

Gross area of cross section, in. 2

Distance from extreme compression fiber to the centroid of the tension reinforcement, in.

Distance from extreme compression fiber to the centroid of the extreme tension reinforcement, in.

Modulus of elasticity of reinforcement

Modulus of elasticity of reinforcement at the onset of strain hardeni ng

Concrete compressive strength (6 x 12 in. cllinder), psi (28 day strength for design. Strength at age of test specimen for analysis of data)

Ultimate tensile strength of reinforcement

Yield strength of reinforcement

Applied axial compression, Ibs.

Nominal shear strength of short columns, Ibs

Tensile reinforcement strain at onset of strain hardening

Ultimate tensile reinforcement strain

71

R ~ FER ENe E S

1. Hanson, R. D., "Repair, Strengthenjng and Rehabilitation of Buildings--RecoiUtilend3tions for Research," University of Michigan, ~~ort ~~~ ~~EE 17R4, October 1977.

2. Ha:.;son, R. D., "Repair and Strengthening of Reinforced Concrete M e lil b e r san d B u i 1 din g s," Pro c e e din g s , '~1 0 r k !: hop 0 n Ear t h qua k e Resistant Reinforced Concret;;-Buiiding Construction, University of California, Berkeley, July 1977.

3. Sugano, S., "Aseismic Strengthening of Existing Reinforced Concrete Bu i ld in gs," ~~~ i n ~, Sei s:n 010 gy and Earthq u ake Eng ineer in g, International Institute of Seismology and Ear:hquake Engineerir,g, Building Research Institute, Japan, March 1980.

4. "Guideline for Seismic Retrofitting (Strengthening, Toughening and/or Stiffening) Design of Existing Rejnforced Concrete Buildings," Ministry of Construction, Japan, t1a:"ch 19T7 (translated into English, 1980).

5. .Jirsa, J. 0., Maruyama, K., and Ramirez, H" IlDevelopment of Loading System and Initial Tests--Short Columns under Bidirectional Loading," CESRL Report No. 78-2, Civil Engi:leering Structures Research Laboratory~-Ttie 'UniversIty of Texas at Austin, September 1978.

6 •• Jirsa, J. 0., Maruyama, K., and Ramirez, H., IlShort RC Columns under Bidirectional Load Histories,1l Journal of Structural ~Q.~~neer:.!..Q.~, ASCE, Vol. 110, No.1, Paper -18jl~i~-January1984~~;P-=-120-137 •

7. Ramirez, H., and Jirsa, J. 0., "Ef'f'ect of' A:<ial Load on Shear

Be~3viot~ of Short RC Columns under Cyclic LatEral Deformations," f~f~~~ ~~l?.~~!i~ §Q.=,~, The University of Te):as at Austin, June 1980.

8. Woodward, K. A., and Jil~sa, J. \)., "Influence cf Reinforcement on RC Short Column Lnteral Resistance," Journal of Structural Eng i nee r i n g, AS C E, Vol. 1 1 0, No.1, Pap e r i85 f 3-:J a n u a r y -i:f84:~;P:-9~-1(j4~-----

9. r'1oot~e, M. f., "Shear Strength and Deter:.oration of Short Lightweight Reinforced Concrete Columns under Cyclic Deformation:3," unpublished ~1aster's thesis, The University of r~exa:3 at Austin, Hay 1982.

7..2-- 73