EVALUATION OF THE ORIENTATION OF 90 AND 180 REINFORCING .

EVALUATION OF THE ORIENTATIONOF 90 AND 180 REINFORCING BARHOOKSbyNichole PodhorskyLesley SneedNUTCR257

DisclaimerThe contents of this report reflect the views of the author(s), who are responsible for the facts and theaccuracy of information presented herein. This document is disseminated under the sponsorship ofthe Department of Transportation, University Transportation Centers Program and the Center forTransportation Infrastructure and Safety NUTC program at the Missouri University of Science andTechnology, in the interest of information exchange. The U.S. Government and Center forTransportation Infrastructure and Safety assumes no liability for the contents or use thereof.NUTC###

Technical Report Documentation Page2. Government Accession No.1. Report No.3. Recipient's Catalog No.NUTC R2574. Title and Subtitle5. Report DateEvaluation Of The Orientation Of 90 And 180 Reinforcing Bar HooksJanuary 20126. Performing Organization Code7. Author/s8. Performing Organization Report No.Lesley SneedProject # 0002810210. Work Unit No. (TRAIS)9. Performing Organization Name and AddressCenter for Transportation Infrastructure and Safety/NUTC programMissouri University of Science and Technology220 Engineering Research LabRolla, MO 6540911. Contract or Grant No.DTRT06-G-001412. Sponsoring Organization Name and Address13. Type of Report and Period CoveredU.S. Department of TransportationResearch and Innovative Technology Administration1200 New Jersey Avenue, SEWashington, DC 20590Final14. Sponsoring Agency Code15. Supplementary Notes16. AbstractThis report describes test results of a study initiated to evaluate the potential influence of hook tilt angle of standard reinforcing hookson the bond strength of concrete. The topic of the evaluation of the orientation of 90 and 180 degree reinforcing bar hooks in concretemembers was identified by the Concrete Reinforcing Steel Institute (CRSI) as high-priority for the reinforcing steel industry. In thetest program conducted, a series of single bar and multiple bar specimens was designed and tested to examine bar behavior andpotential group effects that may exist in wide flexural members with multiple bars, such as a slab or footing. Using beamendspecimens, 90 and 180 degree standard reinforcing hooks were placed at varying angles to compare the angle of tilt and to comparethe two hook types. Twelve single bar specimens and twelve multiple bar specimens, each containing either No. 5 or No. 8 standardreinforcing bars,were tested by axially loading the reinforcing bar(s) in tension. Measuring the bar displacement and strain at varyinglocations along the bar, load-displacement curves obtained were utilized in the analysis of hook tilt. Based on the results, designrecommendations for tilted hooked bar anchorages were made. For No. 5 bars and smaller with concrete compressive strength, f’c,greater than 4500 psi, spacing between 0.5 and 2 times the hook length, A, and concrete cover equal to or exceeding the values used inthis study, tilting reinforcing hooked bars from vertical at any angle did not compromise the structural integrity. For No. 5 bars andsmaller with concrete compressive strength less than 4500 psi, spacing less than 0.5 times the hook length,A, or concrete cover lessthan the values used in this study, further study is needed.Additionally, further study is needed for bars larger than No. 5.17. Key Words18. Distribution StatementBridge superstructures, reinforcing bars, developmentNo restrictions. This document is available to the public through theNational Technical Information Service, Springfield, Virginia 22161.19. Security Classification (of this report)20. Security Classification (of this page)unclassifiedunclassifiedForm DOT F 1700.7 (8-72)21. No. Of Pages17222. Price

ABSTRACTThis report describes test results of a study initiated to evaluate the potential influence of hooktilt angle of standard reinforcing hooks on the bond strength of concrete. The topic of theevaluation of the orientation of 90 and 180 degree reinforcing bar hooks in concrete memberswas identified by the Concrete Reinforcing Steel Institute (CRSI) as high-priority for thereinforcing steel industry. In the test program conducted, a series of single bar and multiple barspecimens was designed and tested to examine bar behavior and potential group effects thatmay exist in wide flexural members with multiple bars, such as a slab or footing. Using beamend specimens, 90 and 180 degree standard reinforcing hooks were placed at varying angles tocompare the angle of tilt and to compare the two hook types. Twelve single bar specimens andtwelve multiple bar specimens, each containing either No. 5 or No. 8 standard reinforcing bars,were tested by axially loading the reinforcing bar(s) in tension. Measuring the bar displacementand strain at varying locations along the bar, load-displacement curves obtained were utilized inthe analysis of hook tilt. Based on the results, design recommendations for tilted hooked baranchorages were made. For No. 5 bars and smaller with concrete compressive strength, f’c,greater than 4500 psi, spacing between 0.5 and 2 times the hook length, A, and concrete coverequal to or exceeding the values used in this study, tilting reinforcing hooked bars from verticalat any angle did not compromise the structural integrity. For No. 5 bars and smaller withconcrete compressive strength less than 4500 psi, spacing less than 0.5 times the hook length,A, or concrete cover less than the values used in this study, further study is needed.Additionally, further study is needed for bars larger than No. 5.ii

TABLE OF CONTENTSPageABSTRACT . iiLIST OF ILLUSTRATIONS . vLIST OF TABLES . xNOMENCLATURE . xiSECTION1. INTRODUCTION . 11.1. PROBLEM STATEMENT . 11.2. SCOPE AND OBJECTIVES . 21.3. SUMMARY OF CONTENT . 22. LITERATURE REVIEW . 42.1. INTRODUCTION . 42.2. BOND STRESS DISTRIBUTION. 42.2.1. Mechanism of Bond Transfer . 42.2.2. Bond Stresses on Straight Deformed Bars . 52.2.3. Bond Stresses on Hooked Deformed Bars . 62.3. BOND TEST TYPES . 82.3.1. Pull-out Test . 82.3.2. Beam-end Test . 92.3.3. Beam Anchorage Test . 92.4. PREVIOUS STUDIES . 92.4.1. Minor, 1971 . 92.4.2. Jirsa and Marques, 1972 . 102.4.3. Minor and Jirsa, 1975 . 102.4.4. Marques and Jirsa, 1975 . 112.4.5. Pinc, Watkins, and Jirsa, 1977. 122.4.6. Johnson and Jirsa, 1981 . 122.4.7. Hamad, Jirsa, and D’Abreu, 1993 . 122.4.8. Ehsani, Saadatmenash, and Tao, 1995 . 13iii

2.5. ACI 318 CODE (2008) PROVISIONS FOR DEVELOPMENT OF STANDARDHOOKS IN TENSION. 133. EXPERIMENTS. 163.1. INTRODUCTION . 163.2. MATERIAL PROPERTIES . 163.3. SPECIMEN DESIGN AND DIMENSIONS . 163.4. TEST RESULTS . 204. ANALYSIS . 274.1. INTRODUCTION . 274.2. DISCUSSION OF RESULTS . 274.2.1. Effect of Hook Tilt Angle. 284.2.2. Effect of Bar Size . 284.2.3. Effect of Hook Type . 314.2.4. Effect of Multiple Bars . 314.2.4.1 Effect of bar spacing . 324.2.4.2 Effect of bar position. 324.2.4.3 Multiple bar and single bar comparison . 324.3. COMPARISON TO LITERATURE . 335. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS . 425.1. SUMMARY . 425.2. CONCLUSIONS . 425.3. RECOMMENDATIONS . 43APPENDICESA. TEST PROGRAM . 43B. SPECIMEN DESIGN PROCEDURE . 65C. TEST RESULTS. 74D. ANALYSIS OF TEST VARIABLES . 125REFERENCES . 157ACKNOWLEDGEMENTS . 159iv

LIST OF ILLUSTRATIONSFigurePage1.1. Construction photos1 of tilted reinforcing hooked bars . 11.2. Recommended bar details for solid slabs (CRSI Design Handbook 2008). 21.3. Schematic of a hooked bar in concrete slab . 22.1. Bond force transfer mechanisms (ACI 408R-03) . 42.2. Relative rib area (ACI 408.3R-01) . 52.3. Splitting failure and pullout failure shearing cracks (ACI 408R-03) . 62.4. Bond stresses on a straight bar (Minor and Jirsa 1975). 62.5. Bond stresses on a hooked bar (Minor and Jirsa 1975) . 72.6. Behavior of hooked anchorage (Minor 1971) . 72.7. Loss of bond of bar and crushing at bend (Jirsa and Marques 1972) . 82.8. Schematic of bond tests (ACI 408R-03) . 92.9. Slip wire instrumentation (Minor and Jirsa 1975) . 112.10. Minor and Jirsa beam-end specimen (Minor and Jirsa 1975) . 112.11. Ehsani et al. beam-end specimen (Ehsani et al. 1995) . 142.12. Development length for standard hooked reinforcing bars (ACI 318-08) . 153.1. CRSI Design Manual hook detail (CRSI Design Manual 2008) . 173.2. 180 degree single bar specimens . 193.3. 90 degree single bar specimens . 213.4. Multiple bar specimens – end view . 213.5. Single bar specimen test setup . 253.6. Multiple bar specimen test setup . 264.1. Influence of tilt angle on maximum normalized bar stress for Groups 1-3. 294.2. Influence of tilt angle on bar displacement for Groups 1-3. 294.3. Influence of tilt angle on maximum normalized bar stress for Groups 16-21. 304.4. Influence of tilt angle on bar displacement for Groups 16-21. 304.5. Influence of tilt angle on bar displacement for Groups 32-37. 314.6. Influence of bar spacing on bar displacement for Groups 12-15 . 344.7. Influence of bar spacing on bar displacement for Groups 28-31 . 344.8. Influence of group effect on normalized displacement for Groups 56-59 . 35v

4.9. Influence of group effect on normalized displacement for Groups 60-63 . 36A.1. Compression and splitting tensile strength tests . 48A.2. Compressive strength history for all concrete mixtures . 48A.3. Reinforcing steel tensile coupons . 50A.4. Typical stress-strain curve for steel reinforcing bars. 51A.5. Summary of reinforcing steel yield and ultimate strength . 51A.6. Formwork for specimens . 53A.7. Reinforcing bar(s) inside formwork before concrete placement . 54A.8. Concrete placement . 55A.9. Moist curing of concrete specimens . 55A.10. Single bar specimen test frame (side view) . 57A.11. Single bar specimen test frame (top view) . 57A.12. Single bar specimen test frame (isometric view). 58A.13. Multiple bar test frame (side view). 58A.14. Multiple bar test frame (top view) . 59A.15. Multiple bar specimen test frame (isometric view) . 59A.16. Hydraulic jack and anchorage systems for test frames. 60A.17. Instrumentation placement . 61A.18. As-built instrumentation photos: strain gages . 61A.19. As-built instrumentation photos: displacement wires . 62A.20. DCVT and string transducer photos . 62A.21. Hydraulic jacks . 63B.1. Minor and Jirsa beam-end specimen (Minor and Jirsa 1975) . 66B.2. Ehsani et al. beam-end specimen (Ehsani et al. 1995). 66B.3. Strut and tie model from ACI 318-08 . 67B.4. Modified beam-end specimen . 67B.5. 180 degree modified beam-end specimen . 69B.6. 90 degree modified beam-end specimen . 69B.7. 90 degree beam-end specimen width increase . 70B.8. 180 degree beam-end specimen width increase . 70B.9. CRSI Design Manual hook detail (CRSI Design Manual 2008) . 71vi

B.10. 90 degree hook, 0 degree (nominal) tilt, 0.5 A spacing, multiple bar specimen . 71B.11. 90 degree hook, 22.5 degree tilt, 0.5 A spacing, multiple bar specimen . 71C.1. Dissection process (specimen BE-5-180-90 shown) . 75C.2. Dissected specimen BE-5-180-0 . 76C.3. Dissected specimen BE-5-180-90 .

members was identified by the Concrete Reinforcing Steel Institute (CRSI) as high-priority for the reinforcing steel industry. In the test program conducted, a series of single bar and multiple bar specimens was designed and tested to examine bar behavior and . ACI 318 CODE (2008) PROVISIONS FOR DEVELOPMENT OF STANDARD .