Optimising Precast Bridge Girders for Sustainability With the use of High Performance Concrete

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Optimising Precast Bridge Girders for
SustainabilityWith the use of High Performance
Concrete
Doug Jenkins - Interactive Design Services Leigh
McCarthy The University of Technology,
Sydney. Daksh Baweja The University of
Technology, Sydney.
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Introduction

• Focus of emissions reduction strategies in
  Australia has been on cement reduction.
• Can significant emissions reductions be made with
  the use of high strength concrete?
• Outline of study
• Effect of high strength concrete and high
  supplementary cementitious material (SCM) content
  on total CO2 emissions.
• Typical 2 Span freeway overbridge
• 5 grades of concrete
• 3 deck types

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Alternative Concrete Mixes
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Component Emissions
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Embodied Energy Calculation
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Typical Super T Girder Section
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Design Constraints

• High strength concrete allows increased prestress
  force and/or reduced bottom flange depth.
• Pretension force limited by concrete strength at
  transfer and number of available strand
  locations.
• Provision of post-tensioned cables allows higher
  total prestress force.
• Reduced girder depth will often provide
  additional savings to emissions and cost (not
  considered in this study).
• Live load deflection may control minimum girder
  depth.
• Moment connection over pier reduces deflections.

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Alternative Girder Dimensions
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Design Options

• Type 1 - Fully Pre-tensioned Design Typical
  current practice Standard Super-T girders with
  in-situ top slab and link slab.
• Type 2 - Post-tensioned Design As Type 1 but
  post-tensioned after casting top slab.
• Type 3 - Post-tensioned Continuous Design As
  Type 2, but with full structural continuity over
  the central support.

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Typical Grillage Layout
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Beam / Slab Detail
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Live Load (Max Moment)
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Girder Bending Moments
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Emissions Analysis Results
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Emissions Analysis Results
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Conclusions

• SCMs allowed significant reductions in CO2
  emissions in all cases, compared with the
  standard reference case concrete.
• High SCM concrete showed greatest reduction, but
  reduced compressive strength at transfer, and
  increased curing period.
• Emissions from the 80 MPa and 100 MPa concretes
  were about equal to the 65 MPa concrete.
• Higher strengths allowed the use of a reduced
  depth of girder, with associated savings in other
  works.

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Conclusions

• Precast post-tensioned girders allowed
  significantly higher levels of prestress, and
  reduction in concrete volumes and emissions.
• Structural continuity over the central support
  allowed an additional small saving in emissions.
• The overall reduction of CO2 emissions was not a
  simple function of the reduction of Portland
  cement in the concrete, but was also based on how
  the material properties of the concretes used
  influenced the structural efficiency of the
  design.