design and installation of uhpc shear keys
TRANSCRIPT
2019 Transportation Innovation Conference 2
Pre-stressed Adjacent Boxes/Slabs -Ideal for ABC Construction
Typical ConstructionAdjacent Boxes/Slabs
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Post-tensioning Issues:1. Additional Detailing
(transverse ducts, internal diaphragms, jacking points)
2. Need for Partial-length Post-tensioning for Stage Construction
3. Easier When Girders Are Aligned in a Straight Line (i.e., not aligned with cross slopes)
4. Just more to do to finish the install!
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Concerns of Reflective Cracking
AASHTO 4.6.2.2.1 Distribution factors assume that the bridge is acting as a monolithic unit…
Physical:
Design:
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A Better Mousetrap Shearkey:UHPC Shearkeys
DESIGN GOALS:1. Ensure That the Bridge Acts Monolithically, While:
a. Eliminating Reflective Crackingb. Eliminating Transverse Post-tensioningc. Eliminating the Need for a Structural Slab
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UHPC Shear Keys1. Design Philosophy / Research
/ Guides
2. Standard Details/Specs
3. Installation
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Why Do Shear Keys Fail?
2019 Transportation Innovation Conference
Grout to Concrete Bond Strength• 400 psi
Thermal Stress• Roughly 350 psi
Live Load• Roughly 450 psi
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UHPC BasicsUltra High Performance Concrete• f’c > 22,000 psi @ 28 days• Tensile Strength above 700 psi• Eliminates Coarse Aggregates – Only Fine
Powders (2 to 5 mm)• Optimizing the Grain Size Distribution to Densify
the Mixture• Addition of Steel or Synthetic Fibers
(2% by volume)• Use of Superplasticizers to Decrease Water-
Cement Ratio (less than 0.2)• Negligible Permeability to Water• High Resistance to Chlorides• High f’c Results in Dramatically Shorter
Reinforcing Development Lengths
• …Costs $3000/CY
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Development Length in UHPCClosure Joints
AASHTO ABC Guide3.6.2.4.1
Development Length:
𝑙 8𝑑
#4 Bar in Tension Develops in 4”
AASHTO ABC Guide3.6.2.4.2
Lapped Splice
𝑙 0.75𝑙
#4 Lap Splices over 3” with Bars Spaced up to 3” between Them
…when spacing between bars does
not exceed 𝑙
…3.6.2.4 – NO PENALTY for Epoxy-coated Bars
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Surface Preparation
UHPC Shear Keys work best when the existing concrete surfaces are roughened.
AASHTO ABC Guide 3.6.2.4.2:
“Precast component interfaces onto which the field cast UHPC will bond shall be detailed to include shear keys that project into the prefabricated element with exposed aggregate or a roughened finish.”
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Why Do UHPC Shear Keys Work?
2019 Transportation Innovation Conference
Ultra High Performance Concrete to Concrete Bond Strength• 2000 psiUHPC Bond strength resists shear stress due to live load.
Thermal Stress• Roughly 350 psi
Live Load• Roughly 450 psi
Fully developed lap splices resist the thermal action.
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UHPC Shear Key Performance
“The estimated allowable tensile strength in the shear key was about 5x greater than the stress in the shear key… and therefore no cracks were anticipated.” - Ohio University
“UHPC… proved to be much better joint filler than non-shrink grout…. As it develops much higher bond strengths with the precast concrete.” - VDOT
“… field-cast UHPC connections that bond to the precast interfaces and use a lap-splice reinforcement detail can create an improved structural connection that is capable of transferring shear, moment, and axial tensile / compressive forces across the connection. The UHPC detail eliminates the need for transverse post-tensioning or a structural concrete overlay.” - FHWA-HRT-14-084
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UHPC Shear Keys1. Design Philosophy / Research
/ Guides
2. Standard Details/Specs
3. Installation
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UHPC Shear Keys1. Design Philosophy / Research
/ Guides
2. Standard Details/Specs
3. Installation
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UHPC Shear Key Installation1) Prep Beam, Then Insert Dowels
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“KEY” Take-Aways1. Evidence Shows That UHPC Work as
Grouted Keys Originally Intended
2. Allow Designer to Utilize the Shallow Profiles of Adjacent Beams/Slabs without the Historic performance issues
3. UHPC Shear Keys Are a Great Use Given Cost of UHPC Today
4. UHPC Shear Keys Are Relatively Easy to Design and Install
5. Design Guides/Standard Exist from AASHTO, Other DOTs