Optimising Precast Bridge Girders for Sustainability With the use of High Performance Concrete Doug Jenkins - Interactive Design Services Joanne Portella– DMC Advisory, Melbourne. Daksh Baweja – DMC Advisory, Melbourne, The University of Technology, Sydney.

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

Alternative Concrete Mixes

Component Emissions

Embodied Energy Calculation

Typical Super T Girder Section

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.

Alternative Girder Dimensions

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.

Typical Grillage Layout

Beam / Slab Detail

Live Load (Max Moment)

Girder Bending Moments

Live Load Deflections

Live Load Deflections Maximum allowable deflection (AS 5100) = 47.5 mm. Decks Type 2-E and 2-D exceeded this limit by 3% and 11% respectively. Deflections may be reduced by: Using the next deeper girder Using a higher strength concrete Providing momemt continuity over the pier

Emissions Analysis Results

Emissions Analysis Results

Research and Development Optimise SCM content for in-situ slab Optimise design procedures for high strength concrete Shear strength Creep and shrinkage losses Deflection limits ULS design factors

R&D – Optimise ULS Design Rectangular Section; 90 MPa

Research and Development Post-tensioning at the precast yard Use of ultra high strength concrete Geopolymer concrete For precast work In-situ top slab

Conclusions SCM’s 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.

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.

Conclusions Engineering is the art of directing the great sources of power in nature for the use and convenience of man. - Thomas Tredgold, 1828 .