investigation of early-age bridge deck cracking · pdf fileshrinkage cracking •thermal...
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Investigation of Early-Age Bridge Deck Cracking for Caltrans
Paul D. Krauss, PE Wiss, Janney Elstner Associates Northbrook, IL 847-753-6517 [email protected] Kent Sasaki, SE Wiss, Janney Elstner Associates San Francisco, CA Project Manager [email protected]
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CALTRANS’ concerns
Annually spend nearly $50 million on
deck rehabs – Early-age deck cracking
is the usual culprit!
Structure work
Traffic management
Construction
Approx. 90% of rehab jobs -
Methacrylate (HMWM)
Approx. 10% - Polymer concrete overlay
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Summary of
number of bridges
with deck cracking
2000-2010
Year # Years Post Construction
4 2 2000 96 87
2001 72 64
2002 55 41
2003 73 31
2004 43 27
2005 39 30
2006 31 24
2007 53 41
2008 44 36
2009 26*
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What is early age-cracking?
• Occurs in first several months
• Transverse
• Through-deck
• 10 to 20 mils
• 1 to 3 m apart
• Plastic cracking – also a problem
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Types of Cracking
• Plastic shrinkage cracking – Craze cracking
• Settlement cracking
• Autogenous shrinkage
• Thermal cracking
• Drying shrinkage
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Plastic Shrinkage Cracking
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Plastic Shrinkage Cracking
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Plastic Shrinkage Cracking Causes
• Inadequate curing
– Delayed wet curing
• Susceptible concretes
– Low water content
– Low w/c
– High paste
– HRWRs
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Settlement Cracking
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Autogenous Shrinkage
• First 12 to 24 hours
• Cement hydration process
• Usually simultaneous with thermal changes
• Concretes with higher autogenous shrinkage – High strength or “High performance” concrete
• High cement content
• Fine cements
• Low w/c ratios
• Fine mineral additives - silica fume
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Thermal-induced Cracking
• First 12 to 24 hours, concrete temperatures change rapidly
• Heat of hydration causes concrete to expand, cooling causes shrinkage and cracking
• Diurnal and seasonal temperature changes
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Drying Shrinkage
• Two phases
– Loss of free water = moderate shrinkage
– Loss of adsorbed water in capillary and pores = large shrinkage
• Humidity, wetting, mix affect drying shrinkage
• Differential, more shrinkage at surface (curling)
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0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Shrinkage
Autogenous
Settlement
Plastic
Thermal
Drying
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Restraint Conditions
• External
• End connections
• Beams and webs
• Friction
• Composite connections
• Internal
• Reinforcing steel
• Section shape and profile
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Research Approach
• Literature review (NCHRP, U of Kansas, others)
• Review of other DOTs
• Review of Caltrans practices
• Field and laboratory work
• Analytical studies (Equations/FEA/Lattice)
• Validation of potential solutions
• Recommendations
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WJE Expertise and Team • WJE • Twining
– Concrete materials testing
• UC Davis – Professor John Bolander, lattice modeling
• Review Panel – Gary Janco of C.C. Myers – Prof. John Bolander, UC Davis – Boris Stein, Twining, Inc. – Mohammed Fatemi, Alta Vista Solutions – Prof. David Darwin, Kansas University – Prof. David Lange, University of Illinois
Special thanks to Madhwesh Raghavendrachar and Anthony Gugino (Caltrans)
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• Bridge in Lincoln, CA
– Sacramento Area
• Bridge in Santee, CA
– San Diego Area
Case Studies
Field Work
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• Lincoln, CA
• Santee, CA
Case Studies
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Instrumentation of Bridge Decks
• Data Acquisition
– Instruments
• Strain
• Temperature
• Relative humidity
• Wind
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Temperature, Lincoln Bridge
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Temperature, Santee Bridge
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Effect of Curing Blankets
• Burlap
• Burlene
• Burlene+ insulation blankets
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Curing Techniques
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Curing Techniques
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Curing Practices • CC 2 hrs. after
placing • WC 20 hrs.
after placing
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Cracks 3-4 hrs. after placing
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Crack Map 16 weeks after placing
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Findings from the Field
• Current curing practices do not prevent plastic shrinkage cracking
• Thermal blankets applied after peak temperature reduced strains in the deck and slowed down cooling
• Application of a second coat of curing compound after wet curing reduced peak diurnal temperatures
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Analytical Studies
• Parametric studies
– Linear elastic equations
– 4CTemp&Stress
• Lattice modeling
– Nonlinear
– Explicit modeling of concrete prop.
εz =
1Ε [σ
z - μ (σ
x + σ
y)] + α T
εy =
1Ε [σ
y - μ (σ
x + σ
z)] + α T
εx =
1ε [σ
x - μ (σ
y + σ
z)] + α T
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Parametric Studies 4CTemp&Stress
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Parametric Studies 4CTemp&Stress
Two mixes: • 675# cement • 550# cement
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Parametric Studies 4CTemp&Stress
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Lattice Modeling
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Lattice Modeling
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Lattice Modeling
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Parameter & Lattice Findings
• Most benefit: reduce cement content
• Cast cooler concrete beneficial
– Limit plastic concrete temperature to 75°F (24°C) at the time of placement
• Afternoon pours better than morning pours
• Larger webs/girders cause higher stresses
• Box girder decks cool slowly
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Recommendations
Cementitious and Paste Content
• Eliminate minimum cementitious content requirement
• Maximum cementitious content of 600 pcy
• Maximum paste content of 27 percent
• Specify air entrainment of 6.0 to 8.0 percent regardless of exposure conditions
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Recommendations
Strength and Mix Design
• Min. comp. strength: 3,600 psi (25 MPa) at 56 days
• Max. comp. strength: 4,500 psi (31 MPa) at 7 or 14 days
• Optimize aggregate gradation (KU Mix)
• Keep max. slump less than 4 in. (Kelly 2.5)
• Reduce maximum shrinkage requirement
– (0.045% to 0.035% @ 28 d)
• Consider SRA’s
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Recommendations
Fly Ash and SCM’s
• Avoid silica fume
• 21-day wet curing for blended cement concrete or fly ash containing concrete
• Allow ultra-fine fly ash or other SCM’s only after testing shows no significant increase in shrinkage or cracking
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Recommendations
Curing Methods
• Immediate misting and wet curing
– Adequate equipment for wet curing
• Wet cure for 14 days
• Insulate deck
• Apply white curing compound after wet curing
• Cast in afternoon/evenings
• Hold a pre-job conference
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Closure
• Early-age cracking results from complex interactions
• Recommendations will reduce early-age cracking
• Caltrans is developing a pilot program
• Thanks to Caltrans and industry participants
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Investigation of Early-Age Bridge Deck Cracking for Caltrans
Paul D. Krauss, PE Wiss, Janney Elstner Associates Northbrook, IL 847-753-6517 [email protected] Kent Sasaki, SE Wiss, Janney Elstner Associates San Francisco, CA Project Manager [email protected]