design and installation of uhpc shear keys

Design and Installation of UHPC Shear Keys

Daniel Whittemore, PE

Fuss & O’Neill

2019 Transportation Innovation Conference 2

Pre-stressed Adjacent Boxes/Slabs -Ideal for ABC Construction

Typical ConstructionAdjacent Boxes/Slabs

32019 Transportation Innovation Conference

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!


The Fight Against Reflective Cracking


Concerns of Reflective Cracking

AASHTO 4.6.2.2.1 Distribution factors assume that the bridge is acting as a monolithic unit…

Physical:

Design:


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


UHPC Shear Keys1. Design Philosophy / Research

/ Guides

2. Standard Details/Specs

3. Installation

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Why Do Shear Keys Fail?


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



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


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?


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.


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



/ Guides


3. Installation


Iowa DOT UHPC Shear Key


Iowa DOT UHPC Key Plan


Blockout Spacing, UHPC Forms



/ Guides


3. Installation

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UHPC Shear Key Installation1) Prep Beam, Then Insert Dowels


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UHPC Shear Key Installation2) Place Beams, Place UHPC


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UHPC Shear Key Installation3) Add Waterproofing



“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

design and installation of uhpc shear keys

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