Enhancing Compression Flange Behaviour of Beams with FRP Prestress

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Enhancing Compression Flange Behaviour of Beams
with FRP Prestress
Chris Burgoyne Engineering Dept. University of
Cambridge
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1988

• ACI Fall Convention, Houston
• Tony Naaman organised a session on
• External Prestressing of Bridges
• Published as SP-120

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Aramid prestressing tendons had been
developed We had concluded that reinforcing with
FRP was not economic as strain capacity of fibres
could not be utilised Prestressing was
logical Strain capacity absorbed in prestress
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• Beam reinforced with an FRP with strain capacity
  of 0.0015
• Neutral axis very high

0.0012
0.015
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Section prestressed with FRPand enhanced
concrete strain capacity
0.007
Prestrain
0.008
0.007
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Thorpe Marsh Power Station
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Aramid rope prestressing
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The top flange explodes!
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Over-reinforced

• Beams prestressed with FRP are truly
  overreinforced
• (Beams reinforced with FRP are often tecnically
  over-reinforced but concrete reaches strain limit
  before tendon snaps failure therefore soft)

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Problem

• How to make a beam over-reinforced with FRP into
  a ductile flexural element?
• Tension element cant be ductile
• Must make concrete fail first and in a ductile
  way
• Confine concrete

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Effect of loose confinement
50 mm spacing
35 mm spacing
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Effect of close confinement
10 mm spacing
20 mm spacing
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Confinement Reinforcement

• Sections with steel are under-reinforced, \
  confinement reinforcement does little
• Sections with composite reinforcement are
  over-reinforced, so confinement reinforcement can
  increase both strength and strain capacity

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Confinement Hoops
Axial stress varies with depth
Confinement is passive
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• Analyse concrete under active confinement
• Break stresses into hydrostatic and deviatoric
  components (Kotsovos model)
• Hence determine response to passive confinement

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Analysis of overlapping spirals using local f.e.
model
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Resulting stress-strain curves
Axial stress
Axial strain
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Rectangular beam
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With hoops added
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Beam section
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Beam 7.3 m clear span
Prestressed with two external deviated aramid
cables with 600 kN breaking load. 2-point loading
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Compression hoops
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Peak Load
Cover concrete lost
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Post-failure loading
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Reinforcement for compression
Amount of fibre in compression zone is 1/6th the
amount of tension reinforcement Confining
spirals triple strain capacity in
compression Economically worthwhile use of
material Uses material properties effectively
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Improvements
Large amount of unconfined cover concrete
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Improvements
Needs automated fabrication
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3-D Textiles
Form of reinforcement

• We have used spirals
• Individually most effective
• But if they overlap, are there easier ways to
  make them?

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Raschel Knitting
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So what will beams look like?

• Partially bonded internal pre-tensioning with
  resin based rods
• External post-tensioning with resin-free ropes
• Novel forms of shear reinforcement
• Confinement reinforcement in the compression zone

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Compression Hoops
Mesh Shear Reinforcement
External Prestressing
Prestressing With Controlled
Or Intermittent Bond
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0.0012

• Concrete reinforced with steel reinforcement
• Gives good strain distribution.

0.0012
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Parafil termination
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Two reinforced concrete footbridges with GFRP
bars
Both are much thicker than they would be with
steel bars
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Tied arch bridge in Norway GFRP reinforcement
but tie is aramid rope
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Hamamatsu Bridge
Strengthened by aramid prestressing ropes
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Tring footbridge
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Box Lane Footbridge Stoke-on-Trent
Steel deck and towers with CFRP cables
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Tank Bridge

• Aramid ropes for attachment

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No-steel Bridge - Long section
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No-steel Bridge - Cross-section
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Cost/unit force - 2004

• Prestressing steel 1 7-wire strand
• Reinforcing steel 3 plus bending
• GFRP pultrusion 6 straight
• Aramid fibre 4 fibre only
• Aramid rope 12 terminals
• AFRP pultrusion 12 straight
• CFRP pultrusion 12 straight
• PBO 15 fibre only
• So it has got more expensive!