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  • Bearing capacity, relaxation and finite element simulation for prestressed concrete beams

    reinforced with BFRP tendons

    by

    Andri Gunnarsson

    Thesis Master of Science in Civil Engineering with

    specialization in Structural Design.

    June 2013

  • Bearing capacity, relaxation and finite element simulation for prestressed concrete beams

    reinforced with BFRP tendons

    Andri Gunnarsson

    Thesis of 30 ECTS credits submitted to the School of Science and Engineering at Reykjavík University in partial fulfillment of

    the requirements for the degree of

    Master of Science in Civil Engineering with Specialization in Structural Design

    June 2013

    Supervisor:

    Eyþór Rafn Þórhallsson Associate Professor, Reykjavík University, Iceland

    Examiner:

    Baldvin Einarsson Associate Professor, University of Iceland, Iceland

  • I

    Abstract

    Fiber reinforced polymer (FRP) have over the past years become an interesting choice as a

    reinforcement for concrete structures and this is mainly so because of their good resistance to

    chemicals and weathering. Basalt fiber reinforced polymer (BFRP) is the latest FRP material

    considered as reinforcement for concrete and understanding of concrete behavior, reinforced

    with BFRP, is therefore partly unknown and still developing today. This dissertation focuses

    on increasing the understanding of BFRP as a reinforcing material for concrete. The main

    emphasis is on bearing capacity, failure modes and relaxation. Experimental results from

    three studies, considering prestressed BFRP reinforced concrete (RC) beams, were reviewed

    and compared with formulations, regarding shear capacity, from codes and guidelines. The

    experiments were also simulated in the finite element (FE) program ANSYS. Experimental

    work was then carried out to estimate the long term relaxation of BFRP tendons. This

    research produced several key findings: FE models created in ANSYS to simulate

    experimental tests gave an acceptable approach with experimental results regarding failure

    mode, failure force and stress distribution. Equations from JSCE-1997, BSI 8110, Model code

    2010, ACI 440.1R-06, CAN/CSA-S806-02, Nehdi et al, Fib and CNR-DT 203, regarding

    shear capacity, were compared with experimental results for nine beams. Many of these

    equations provided a rather conservative approach while equations from Nehdi et al and

    CAN/CSA-S806-02 gave the best safe approach when compared with experimental results.

    Finally the long term relaxation of BFRP tendons was estimated, based on over 2000 hours of

    measurements on stretched tendons. The 50 years relaxation, for tendons loaded with 50% of

    ultimate tensile strength, was estimated to be 11%. The main conclusions drawn from this

    research were that: Further use of ANSYS to explore and provide understanding of the

    behavior of BFRP RC beams is recommended. More than 4-5‰ strain should not be allowed

    in BFRP longitudinal tension reinforcement to maintain bond behavior between concrete and

    reinforcement. When developing shear capacity equations for FRP RC, use of cubic root

    when considering rigidity of FRP longitudinal bars, concrete compression strength and the

    modification factors which account for different stiffness of FRP and steel is recommended.

    Most of the shear capacity equations reviewed from codes and guidelines for FRP RC are too

    conservative which can lead to inefficient design.

    Keywords: BFRP tendons, steel tendons, shear capacity, ANSYS simulation, relaxation,

    prestressed concrete beams, FRP.

  • II

    Ágrip

    Titill á íslensku: Þol á forspenntum steinsteyptum bitum bentum með basalttrefjastyrktum

    stöngum skoðað með smástykkjaaðferð og útreikningum auk athugunar á slöknun.

    Trefjastyrkt plastefni (FRP) hafa þróast yfir í það að vera raunhæfur valkostur sem bending í

    steinsteypt mannvirki á síðustu árum en aðalástæðan fyrir þessu er gott efna og veðrunarþol

    FRP efnanna. Basalttrefjastyrkt plastefni (BFRP) eru nýjasta tegunding af FRP efnum sem

    farið er að nota í þessum tilgangi og þekking á hegðun steinsteypu bentri með þeim er því að

    hluta óþekkt og í stöðugri þróun. Þessi ritgerð stefnir á að auka skilning á BFRP sem

    bendingarefni fyrir steypu. Aðaláherslan er lögð á burðarþol, brotmyndir og slöknun.

    Rannsóknarniðurstöður úr þremur rannsóknum sem voru að skoða forspennta steinsteypubita

    benta með BFRP stöngum eru skoðaðar og bornar saman við skerþolsformúlur úr stöðlum og

    viðmiðunarreglum. Tilraunirnar voru einnig hermdar í burðarþolsforritinu ANSYS sem

    byggir á smástykkjaaðferðinni (FE). Langtímaslöknun fyrir BFRP stangir er síðan metin með

    tilraunavinnu. Þessi rannsókn gefur nokkrar lykil niðurstöður: FE líkön sem búinn voru til í

    ANSYS til að herma tilraunir gáfu ásættanlega nálgun við tilraunaniðurstöður m.t.t.

    brotmynda, brotkrafta og spennudreifinga. Jöfnur fyrir skerþol úr JSCE-1997, BSI 8110

    Model code 2010, ACI 440.1R-06, CAN/CSA-S806-02, Nehdi et al, Fib og CNR-DT 203

    voru bornar saman við tilraunaniðurstöður fyrir níu bita. Margar af þessum jöfnum gáfu

    nokkuð íhaldssama nálgun en jöfnur frá Nehdi et al og CAN/CSA-S806-02 gáfu hins vegar

    bestu öruggu nálgunina. Að lokum var langtímaslöknun BFRP stanga metin út frá yfir 2000

    klst. mælingu á strekktum stögnum. Fyrir stangir spenntar upp með 50% af mesta togkrafti, þá

    var 50 ára slöknun áætluð sem 11%. Helstu ályktanir sem dregnar voru af þessari rannsókn

    eru: Mælt er með frekari notkun á ANSYS til að skoða og auka skilning á hegðun steinsteypu

    sem bent er með BFRP. Ekki ætti að leyfa meira en 4-5‰ streitu í BFRP togbendingu til að

    viðhalda bindingu milli steypu og bendingar. Við þróun á skerþolsjöfnum fyrir FRP benta

    steinsteypu þá er mælt með notkun á þriðju rótinni þegar verið er að skoða stífni FRP

    togbendingar, þrýstistyrk steinsteypu og stuðla sem taka tillit til mismunandi stífni FRP og

    stáls. Flestar skerþolsjöfnur sem skoðaðar voru í stöðlum og viðmiðunarreglum fyrir FRP

    benta steinsteypu eru of íhaldsamar sem getur leitt til óhagkvæmnar hönnunar.

    Lykilorð: Basaltrefjastangir, stál strandar, skerþol, ANSYS hermun, slöknun, forspenntir

    steinsteypubitar, trefjastyrkt plastefni.

  • III

    Bearing capacity, relaxation and finite element simulation for prestressed concrete beams

    reinforced with BFRP tendons

    Andri Gunnarsson

    Thesis of 30 ECTS credits submitted to the School of Science and Engineering at Reykjavík University in partial fulfillment of

    the requirements for the degree of

    Master of Science in Civil Engineering with Specialization in Structural Design

    June 2013

    Student: ___________________________________________

    Andri Gunnarsson

    Supervisor: ___________________________________________

    Eyþór Rafn Þórhallsson

    Examiner: ___________________________________________

    Baldvin Einarsson

  • IV

    Acknowledgements

    Special gratitude to my supervisor, Eyþór Rafn Þórhallsson, civil engineer M.Sc. and

    associate professor at Reykjavik University, for his valuable guidance, inspiration and

    generous sheering of ideas and knowledge in order to improve this work.

    Special thanks to Indriði Sævar Ríkharðsson, mechanical engineer M.Sc. and assistant

    professor at Reykjavik University, for his appreciated and unselfish assistant regarding

    technical problems in this study.

    Thanks to all the people which helped me with the experimental work and the thesis: Jónas

    Þór Snæbjörnsson civil engineer Dr. Ing. and professor at Reykjavik University, Gísli Freyr

    Þorsteinsson technician at Reykjavik University and Hrannar Traustason electronics engineer

    at Reykjavik University.

    Also thanks to the former students of Reykjavik University that provided my access to their

    research data, literature work and good advices: Sindri Hlífar Guðmundsson civil engineer

    M.Sc., Björgvin Smári Jónsson civil engineer M.Sc. and Jónas Ásbjörnsson civil engineer

    B.Sc.

    Finally thanks to Björn Finsen for reading trough the manuscript and providing comments for

    improvements.

  • V

    Contents

    Abstract ...................................................................................................................................... I Ágrip ......................................................................................................................................... II Acknowledgements ................................................................................................................. IV List of Figures ...................................................................................................................... VIII List of Tables ............................................................