Shear Strengthening of Pretensioned Prestressed Concrete


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SHEAR STRENGTHENING OF PRETENSIONED PRESTRESSED CONCRETE COMPOSITE FLEXURAL MEMBERS

TECHNICAL. REPORT STANDARD TITL.E PAG E 3. Ree;,i."I·. Catalo" No.
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March 1991
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R. Aboutaha a~d N. Burns

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Research Report 1210-2

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Center for Transportation Research

The University of Texas at Austin

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T". 0' Austin, Texas 78712-1075

Re search Study 3- 5- 89/1-1210

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Texas State Department of Highways and Public

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Transportation; Transportation Planning Division

p. O. Box 5051

14. S""lori", A,.ncy C..d.

Austin, Texas 78763- 50S 1

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Study conducted in cooperation with the U. S. Department of Transportation, Federal Highway Adminis tra tion. Research Study Title: "Influence of Debond ing of Strands on the Behavior of Composite Prestressed Beams"
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Strengthening is required due to inadequacy which typically results from a poor design, a change in usage, or a change in design loads. Prestressed composite beams require special attention in connection with their behavior in horizontal shear at the composite interface. Beams lacking adequate shear reinforcement experience brittle shear failure unless they have low flexural stiffness. Such beams can be strengthened so that they can develop their flexural capacity and behave in a more ductile manner at failure.
External post-tensioning system is often a desirable strengthening solution when a major portion of a member must be strengthened or when the cracks which have formed must be closed. This research work studies the behavior of retrofitted prestressed composite beams that originally lacked shear reinforcement and have a smooth interface bonded with epoxy. Before retrofitting these beams experienced sudden horizontal shear failure. Ductile flexural failure occurred after being retrofitted by external prestressing bars. This research studied how the mode of failure of prestressed composite flexural member could be changed from a sudden shear failure to a ductile flexural failure by utilizing external prestressing bars. It studied the effect of these prestressing bars on the required development length of prestressing strands. The provisions of ACI Code 318-89 for the design of prestressed concrete composite beams were compared with the test results. Recommendations are made for the use of external prestressing systems as an effective strengthening procedure.

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shear, strengthening, prestressed

No restrictions. This document is

concrete, composite concrete beams,

available to the public through the

horizontal shear, external post-

National Technical Information Service,

tensioning, development length

Springfield, Virginia 22161.

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SHEAR STRENGTHENING OF PRETENSIONED PRESTRESSED CONCRETE COMPOSITE FLEXURAL MEMBERS
by
R. Aboutaha and N. Burns
Research Report 1210-2 Research Project 3-5-89-1210 "Influence of Debonding of Strands on Behavior of Composite
Prestressed Beams"
Conducted for Texas
State Department of Highways and Public Transportation In Cooperation with the
U.S. Department of Transportation Federal Highway Administration
by
CENTER FOR TRANSPORTATION RESEARCH BUREAU OF ENGINEERING RESEARCH THE UNIVERSITY OF TEXAS AT AUSTIN
March 1991

NOT INTENDED FOR CONSTRUCTION, BIDDING OR PERMIT PURPOSES
N. H. Burns, P.E. (Texas No. 20801) Study supervisor
The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration. This report does not constitute a standard, specification, or regulation.
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PREFACE
This report is the second in a series which studies the transfer and development lengths of pretensioned prestressed strands. It studies the development length of pretensioned prestressing strands confined with external active transverse prestressing bars, which to a great extent reduces the required development length.
This report reviews the previous research work on the use of prestressing stirrups and shear strengthening of concrete members. It presents the different types of strengthening techniques used in reinforced concrete structures.
This work is part of Research Project 3-5-89-1210, entitled "Influence of Debonding of Strands on Behavior of Composite Prestressed Beams". It presents detailed investigation on the effect of external transverse post-tensioning bars on the behavior of pretensioned prestressed concrete composite flexural members. This research was conducted by the Phil M. Ferguson Structural Engineering Laboratory as part of the overall Research program of the Center for Transportation Research of The University of Texas at Austin. The work was sponsored jointly by the Texas State Department of Highways and Public Transportation and the Federal Highway Administration under an agreement with The University of Texas at Austin and the State Department of Highways and Public Transportation.
This portion of the overall study was directed by Dr. Ned H. Burns who holds the Barrow Centennial Professorship in Civil Engineering, in cooperation with Dr. James O. Jirsa who holds the Janet S. Cockrell Centennial Chair in Engineering, and Dr. Michael E. Kreger, Associate Professor of Civil Engineering. The design, fabrication and installation of the external post-tensioning system was done by Riyad S. Aboutaha, Graduate Research Assistant. Testing was accomplished with the great assistance of Asit Baxi, Bruce Lutz, Bruce Russell, Ozgur Unay and Les Zumbrunnen, Graduate Research Assistants.
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SUMMARY
Recently, the repair and strengthening of existing structures has grown to occupy a significant share of the concrete construction market. Strengthening is required due to inadequacy which typically results from a poor design, a change in usage, or a change in design loads.
Prestressed composite beams require special attention in connection with their behavior in horizontal shear at the composite interface. Beams lacking adequate shear reinforcement experience brittle shear failure unless they have low flexural stiffness. Such beams can be strengthened, so that they can develop their flexural capacity and behavior in a more ductile manner at failure.
External post-tensioning systems are often a desirable strengthening solution when a major portion of a member must be strengthened or when the cracks which have formed must be closed. This research work studies the behavior of retrofitted prestressed composite beams that originally lacked shear reinforcement and have a smooth interface bonded with epoxy. Before retrofitting these beams experienced sudden horizontal shear failure. Ductile flexural failure occurred after being retrofitted by external prestressing bars. This research studied how the mode of failure of prestressed composite flexural members could be changed from a sudden shear failure to a ductile flexural failure by utilizing external prestressing bars. It studied the effect of these prestressing bars on the required development length of prestressing strands.
One unretrofitted and four retrofitted simply-supported beams were tested under two static stationary concentrated loads until failure. The provisions in the ACI Code 318-89 for the design of prestressed concrete composite beams were compared with the test results. Recommendations are made for the use of external prestressing systems as an effective strengthening system.
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IMPLEMENTATION This report summarizes the results of test programs involving five pretensioned concrete composite flexural members lacking horizontal shear reinforcement. External post-tensioning bars have been used and proved their effectiveness in strengthening of prestressed concrete flexural members. This study shows that external prestressing bars can be utilized as external transverse reinforcement to prevent sudden shear failure in beams lacking adequate shear reinforcement. Sudden shear failure, an unfavorable mode of failure, can be changed to a ductile flexural failure, which is a more favorable mode of failure, by using external post-tensioning bars. Poorly designed girders and existing girders that are incapable of carrying new extra loads, can still be used utilizing external post-tensioning bars. It can be utilized in strengthening of pretensioned prestressed beams reinforced with strands lacking adequate development length. This research work proved that due to the active confinement of prestressing strands by the external prestressing bars, development length for the strands as short as 50% of the recommended values can be used and the beam can still develop its full flexural capacity.
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TABLE OF CONTENTS

Page

CHAPTER ONE - Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1 1.2 Limit States Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2 1.3 Objective of Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2 1.4 Scope of this Research. ................................................ 3 1.5 Review of Literature .................................................. 3

CHAPTER TWO - Modes of Failure of Prestressed Concrete Composite Flexural Members .......... 7

2.1

Mechanics of Prestressed Concretel3, 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 L·un·iS t tates De·S1lg3n1' 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 Ultimate Limit States ............................................ 8

2.2.2 Serviceability Limit States ......................................... 8

2.3 Modes of Failure of Prestressed Composite Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3.1 2.3.2

Flexural Failure ................................................ 9 Shear Failure14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11

2.3.2.1 Flexure-Shear Failure ..................................... 11

2.3.2.2 Web-Shear Failure ....................................... 11

2.3.3.

2.3.2.3 Horizontal Shear Failure (Composite Sections) ................... 11 Bond Failurel4, 22 •••••••••••••••••••••••••••••••••••••....•••.• 12

CHAPTER THREE - Strengthening Techniques for Prestressed Concrete Flexural Members . . . . . . . .. 15

3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15 3.2 Retrofitting Techniques ................................................ 15
3.2.1 Epoxy Injection12 •••••••••••••••••••••••••••••••••••••••••••••• 15 3.2.2 Bonding of External Reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 16 3.2.3 Conventional Reinforcement ...................................... 17 3.2.4 Post-Tensioning ............................................... 17

CHAPTER FOUR - Test Program .................................................... 23

4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 23 4.2 Test Specimen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 23
4.2.1 Details of Beams .............................................. 23 4.2.2 Strengthening System and Procedure ................................ 23 4.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 24 4.3.1 Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 24 4.3.2 Prestressing Strands ............................................ 24 4.3.3 Ordinary Reinforcement ......................................... 24 4.3.4 Prestressing Bars .............................................. 25 4.3.5 Epoxy Adhesive ............................................... 25 4.4 Test Setup .......................................................... 26 4.4.1 Description of Test Setup ........................................ 26 4.4.2 Loading Pattern and Increment .................................... 28

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Shear Strengthening of Pretensioned Prestressed Concrete