Fatigue crack propagation simulation of orthotropic bridge ...
10/21/2016 Bridge Lessons Expanded to Example Work ... - Bridge Lessons.pdfX‐DOT’s “Crack...
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10/21/2016 1 Bridge Lessons Expanded to Concrete Structures Brian D. Merrill, PE Associate Principal Example Work at TxDOT: Design Example Work at TxDOT: Construction & Maintenance Example Work at TxDOT: Construction & Maintenance
Transcript of 10/21/2016 Bridge Lessons Expanded to Example Work ... - Bridge Lessons.pdfX‐DOT’s “Crack...
10/21/2016
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Bridge Lessons Expanded to Concrete Structures
Brian D. Merrill, PEAssociate Principal
Example Work at TxDOT: Design
Example Work at TxDOT: Construction & Maintenance
Example Work at TxDOT: Construction & Maintenance
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Despite what you hear in the media… it’s not all that bad. Texas is:
1st in total number of bridges – by a lot
2nd in lowest in % of deficient bridges (2.6%)
3rd in overall condition
51st in Federal Bridge Funding – since 1956
“Report Card”: Biennial bridge inspections
Learn something from TxDOT?Topics
Alkali‐Silica Reaction (ASR)
Cracking
Specifications
Joints
Post‐Tensioning
TxDOT ASR “discovered” in 1994 ( fab in 1987) ASR vs DEF
Mainly in post 1986 PS Concrete Beams
1999 specs gave 5 options
Current specs: 8 Options
>120 bridges documented
Up to 1500 Bridges?
>40000 girders possibly impacted (10 yrs)
ASRWhy is ASR mainly in PS Beams?
High/early release strengths 7500 psi in 14 hours
Longer Spans Higher strength
more cement
more alkalis
more heat
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Why 1994?
Possible changes in Type III cement chemistry
due to EPA restrictions?
Switch from wet to dry process for cement
Blaine fineness changes: faster strength gain = possible change in rate of reactions, higher temps
New admixtures – effect on cement hydration/release of alkalis?
Parking garages
MSE Wall panels
Concrete façade panels
Nuclear structures
ASR: Non‐TxDOT
Alkali‐Silica Reaction
ASR – what is it?
1) Alkalis in cement (OH‐) + Reactive Aggregate + water = ASR Gel product (aggregate “dissolves”)
2) ASR Gel Product + Water = Expansion
Where are ASR Affected Bridges?
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ASR: Symptoms
Map cracking
Longitudinal Cracks in prestressed beams
Vertical cracking in columns
Oriented cracking in beam ends and bent/abutment caps
What Does ASR Look Like? PS Beams
What Does ASR Look Like? Other
Cracks form perpendicular to tensile stress
Tensile stresses are the result of material expansion
Confinement (rebar or applied loading) can provide restraint
Crack Orientation:Why are ASR cracks oriented this way?
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ASR Cracking in Prestressed Beams ASR Cracking in Columns
ASR Cracking in Caps
?
?
?
ASR: How bad can it get?
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ASR: How bad can it get?
4” PVC
34” Box Beam
ASR: Fundamental Questions
1. Has structural capacity been affected?
a) How can you monitor it?
2. Can we keep it from getting worse?
a) How can you tell if it’ll get worse?
3. How do we prevent ASR in new concrete?
1857 – Structural Assessment
4069 – Mitigation of In‐service ASR
5218 – Service life of Large ASR affected structures
1521 – Lithium Field Trials
4085 – Preventing ASR in New Concrete
4183 – Improved test Procedures for ASR
5722 – Affect of ASR on splices and development
5997 – “D” Region Assessment
IAC: Shear Strength of caps
IAC: Trap Girders
6491 – NDE for ASR/DEF
6436 – Effect of ASR on Rebar Stress
6656 – ASR Testing Modifications
ASR Research by TxDOT ≈ $8‐10M ASR – What have we learned?
1. Structurally not as bad as it looks – for now
a. Cracking in most cases not in core
2. You can slow it down in some cases
3. You can largely prevent it in new concrete – for now
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10 ‐ 15% Reduction in Compressive Strength
Reduction in Tensile Strength
Reduced Bond with Rebar/Prestressing Strands
Reduced Stiffness ‐ Increased Cracking
Increased Potential for Damage due to Corrosion
Service Life Implications?
Structural Effects of ASR ASR: What Can Be Done About it?
1. Do nothing and monitor
2. Reduce moisture availability
3. Chemical treatments (ASR only)
4. Physical treatments: confinement
5. Replacement
ASR: Treatment to reduce moisture availability
Silane
Caulk large cracks
Silicone‐resin coating
TxDOT Std. Spec 789
Caulk – not epoxy
Waterproofing (Silane)
Coating
Preventing ASR: TxDOT Std. Spec 421
10+ years at exposure site – 1st of kind in US
Companion samples in Canada
FHWA ‐ TWG Review
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Preventing ASR – Prescriptive Specs
1. Replace 20‐35% of cement with F flyash
2. Replace 35‐50% of cement with GGBFS or MFFA
3. Replace 35‐50% of cement with F flyash, GGBFS, MFFA, UFFA, metakaolin, or silica fume. Flyash must be < 35% and SF must be < 10%
4. Use Type IP or IS cements (up to 10% repl. with F ash, GGBFS or SF)
Preventing ASR
5. Replace 35‐50% cement with C ash plus >6% SF, UFFA or metakaolin, but C ash < 35% and SF < 10%
6. LiNO3 at 0.55 gal (30% sol’n) per pound of alkalis
7. Straight cement: alkali content < 3.5 lbs per cy
8. Performance option. Test mix using C1567.
TxDOT has a MAXIMUM cement content
All aggregates considered reactive
ASR: Texas’ Aggregate reactivity>60% of coarse aggregate sources
Varibility of Reactivity in Coarse Aggregate
0.00
0.05
0.10
0.15
0.20
0.25
ReadyMix Plant
ReadyMix Plant
ReadyMix Plant
Rail Car ReadyMix Plant
PrestessPlant
PrestessPlant
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Da
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xp
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ASR: Texas’ Aggregate reactivity>80% of fine aggregate sources
Fine Aggregate C 1260 Testing
0
0.05
0.1
0.15
0.2
0.25
Quarry AsIs
Quarry Quarry PrestressPlant 1
PrestressPlant 2
14 D
ay E
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n
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Topics
Alkali‐Silica Reaction (ASR)
Cracking
Specifications
Joints
Post‐Tensioning
1. “All Concrete Cracks”
2. Structural Engineers cannot change Rule #1
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Cracking: Two “Rules”
How many clientswant to hear this?
Who Fails to Reduce Cracking and How?1. Engineer
a) Failure to understand concrete behavior
b) Design deficienciesc) Specificationsd) Contractual requirements
2. Contractora) Handling/placingb) Failure to follow specificationsc) Failure to understand concrete
behavior3. Concrete Supplier – “harsh” mix
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Causes of Cracks per ACI 224.1R‐2
1. Cracking in Plastic Concrete: a) Shrinkage
b) Settlement
2. Cracking in Hardened Concrete: a) Drying shrinkage/restraint,
b) Thermal stresses,
c) Chemical reaction,
d) Corrosion,
e) Construction overloads,
f) Errors in design/detailing,
g) External loads: flexure/shear
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Drying Shrinkage: “All Concrete Shrinks”Specifications1. Shrinkage Affecting Admixtures: “better concrete thru chemistry”
a) Shrinkage Reducing Admixtures – reduce surface tension of water
– Lower strength (12‐15%)
– Negative reaction with air‐entraining agents
b) Shrinkage Compensating Admixtures – cause expansion designed to off‐set shrinkage
2. Optimized Graded Aggregates
a) Reduce paste volume
Use caution! !
Drying Shrinkage: “All Concrete Shrinks”
Detailing1. Shape
2. Restraint
Drying Shrinkage ‐ Restraint
15’
35’
Partial Contraction Joints
Joints in slab not aligned with joint in walls – lots of cracks
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Restrained Shrinkage
PCA: 36” min. dimension ACI: no specific limits** any volume that can have problems with internal stresses due to heat of hydration
TxDOT: >5’ min. dimension FDOT: >36” and V/A >12
Thermal Stresses – when do you need Mass Concrete Provisions?
Image from lusas.com
Cracking in new concrete structures – 1 year
Reduce potential for DEF
Thermal Stresses – Why should we care?
Lower cement content Use SCM’s Control placement temperature
Insulate formwork Max T<160F Max ∆T<35F
ConcreteWorks©
Thermal Stresses – TxDOT Controls
Mass Concrete + DEF
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Design/Detailing Issues: Rebar Development
Designed using sectional analysis
Cracks noticed < 1 year in service
Emergency shoring required
Design/Detailing Issues: Rebar Development
P-T Bars
Vert Passive Reinf.
P-T AnchorBlockout
Design/Detailing Issues: Rebar Development
Same Project – Diff Bents
Full P-T
Design/Detailing Issues:
Creep
Rebar Development+
Material Behavior
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Design/Detailing Issues: Addition/Accumulation of Stresses
Max M & V
PTBursting stresses
Can’t “Double-count”reinforcing
Design/Detailing Issues: Addition/Accumulation of Stresses
Contractual IssuesSPEED OF CONSTRUCTION – PERFECT STORM
Engr: 4000 psi
Contr. Orders 7500 psiNeeds 4000 in 2 days(Time/strength penalties)
$
Supplies 8500 psi Owner unhappy with cracks!
Topics
Alkali‐Silica Reaction (ASR)
Cracking
Specifications
Joints
Post‐Tensioning
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Define Quality Properties: strength, permeability, volume change, ASR resistance,…
Test methods (timing: 28 days, 56 days,…)
Acceptance criteria – sampling and testing variability
What if criteria are not met?
Reject?
Reduce payment?
Specifying Concrete
Performance Specs for Concrete: ACI 329 R‐14Cracking: Crack resistance of concrete mix – ASTM C1581 (Ring Test)
No mention of parameters or tests for in‐situ cracks X‐DOT’s “Crack Free” Spec: Up to 30% pay reduction based on crack density in bridge decks30% increase in bid prices ‐ paid more for the same “quality”
TxDOT: HPC Performance Option since 2004 – no takers
Specifying Concrete
Performance Specs for Concrete: ACI 329 R‐14
Performance Specs (per SHRP2 R07)
Pros
Contractor “Innovation”
Contractor assumes risk
Flexibility in materials and processes
Pay adjustment for performance
Cons
Less Agency control
Reduced opportunities for smaller contractors
Hard to determine perf. criteria/tests
Blurred roles between Contr/Agency
Contractor assumes risk
The “Truth”:
Contractors/suppliers don’t innovate to improve performance – they innovate to save money/increase profits
I’ve never met a contractor who can’t mess up a good material
Performance Specs = “Innovation”?
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Performance Specs – Rocks in the Road
Better for negotiated price than low bid
Risk factors: Project duration
Material changes, seasons,…
Reliability/repeatability of tests
Service life
How do you measure performance?
How long does a test take? Frequency?
What properties define durability?
Strength? Higher strength = more cracks?
Permeability? What if it cracks?
Quality Properties/Tests?
SB: f’c = 8000 psi884 lf cracks
NB: f’c = 4000 psi554 lf cracks
Performance f’c SCC Removed overdesign Slump ranges Mix design options
Prescriptive Mix Design Options (ASR) Aggregate gradations Optimized gradations Max cement content Limits for sulfates Min/max w/c ratio
TxDOT Concrete Specifications: Performance or Prescriptive?Really a Hybrid Spec
P/S beams: 6000 psi max release; 8500 psi max f’c
Lot’s of standard details
LRFD Design/Detailing Manuals
Std Bridge Deck: 8.5” with2.5” clear cover
Columns,…: 2” clear cover
All are exposed elements
TxDOT Design/Spec Practices for Bridges
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TxDOT District‐Specific Design/Spec Items
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Use the strength you NEED
High strength does not mean “crack free”
High Strength can result in MORE cracks
High cement factor: more creep and shrinkage
Admixtures usually needed (water reducer)
more difficult to place (ice, nitrogen,…)
HS concrete is more brittle
Specifications: Concrete Strength Specifications:Concrete Strength
8’x10’ Column
Calc Avg Stress: <500 psi
f’c = 5000 psi
Wanted it to be “strong”
Thermal cracking + DEF
Louetta Rd Deck Photos
Southboundf´c = 8000884 lf cracks
Northboundf´c = 4000554 lf cracks
Specifications: Concrete StrengthHigh Performance Concrete Between the Cracks
Cracking due to drying shrinkage
TxDOT Prescriptive Mix Design Options ‐HPC
1. Replace 20‐35% of cement with F flyash
2. Replace 35‐50% of cement with GGBFS or MFFA
3. Replace 35‐50% of cement with F flyash, GGBFS, MFFA, UFFA, metakaolin, or silica fume. Flyashmust be < 35% and SF must be < 10%
4. Use Type IP or IS cements (up to 10% repl. with F ash, GGBFS or SF)
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5. Replace 35‐50% cement with C ash plus >6% SF, UFFA or metakaolin, but C ash < 35% and SF < 10%
6. LiNO3 at 0.55 gal (30% sol’n) per pound of alkalis
7. Straight cement: alkali content < 3.5 lbs per cy
8. Performance option. Test mix for permeability. Max cement content enforced
TxDOT Prescriptive Mix Design Options ‐HPC Topics
Alkali‐Silica Reaction (ASR)
Cracking
Specifications
Joints
Post‐Tensioning
Why Do We Need Bridge Joints?
Allow thermal expansion and contraction
Allow translation & rotation of the structure
Other Joint Functions: seals
Keep water,… off of substructure
Protect exposed concrete edges
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Minimizing Joints – the TxDOT Way
Joints and the 90% “Rule”
90% of new bridges use PS beams
“Poor‐Boy” Continuous slabs vs Continuous spans
Don’t eliminate joints completely
Minimizing Joints – the TxDOT Way
Don’t eliminate joints completely
Change the way we build foundations
Moves the maintenance problem
Don’t make prestressed beams continuous over the bents
Not worth the trouble
Fully Continuous Units “Poor‐Boy” Continuous Spans
400’ maximum unit length
Continuous slab on simple span beams
Controlled Joint
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“Zip-strip”3/4” Chamfer orSteel Angle
Controlled Joint
4”
Undesirable
Construction, Control or Expansion Joint
Preferred Method for Control Joints
1” Fiber Board
Can You saw‐cut the controlled joint? NO!
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Preferred Method for Expansion Joints
Constr. Jt
Armor Joint (AJ)
Sealed Expansion Joints (SEJ)
Fabric Joint Underseal
Header Type Joint
Asphalt Plug
Finger Joint
Modular Joint
TxDOT Joint Types
Topics
Alkali‐Silica Reaction (ASR)
Cracking
Specifications
Joints
Post‐Tensioning
PT/Grouting Issues
Grout Issues
Going forward – Design/Specification
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Chlorides in Grout
5 bents, 5 tendons per bent , 14‐19 strands per tendon
A
B
Petrography
Significant segregation of “A” material
Many large air voids
excess moisture present
“A” material weak and brittle
“B” material appeared normal in strength
Scanning Election Energy Dispersive Spectroscopy/
X‐ray Fluorescence/ Ion Chromatography
“A” material
High levels of Chlorides > 21000 ppm
High Na and K levels
Relatively low Ca levels
“B” Material
Normal hydration
Cl levels exceed specs (>800 ppm)
Unused grout (dry): approx 1300 ppm of Cl2nd lot: very low Cl
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Contributing factors
Grout storage
Too long (up to 9 months, past shelf life)
Too hot (>120F)
Mixing/pumping issues
Several blow‐outs reported
Inconsistent procedures
“burping” tendons
Pumping pressure/speed
National Issue
“What starts in Texas changes the world”
FHWA memo dated 11/23/2011
ASTM C150: no limits on Cl content in cement
Most suppliers weren’t testing their cement sources (they are now!)
Suspect grout produced from 2002 to 2010 –16M lbs of grout that went to 38 states
Grout Segregation w/o Chlorides
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PT Design/Specification
PTI/ASBI M50.3 Grouted PT Guide Spec
New Industry standard
Updated material requirements
Inspection/Testing requirements
Protection Levels
Adoption by State DOT’s
PTI Protection Levels – Spec in PlansProtection from Structure
High Medium Low
Aggressiveness of Environmen
tHigh
Med
ium
Low
PL‐1
PL‐2(Bridges)
PL‐3
Protection Level 1A – Std grout Protection Level 1B – Engineered grout
Perm. grout cap
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Protection Level 2 Protection Level 3
PL‐2 • Elec. Isolation,• Monitorable,• Inspectable
PTI M55.1‐12
Material testing
4 grout classes
Pre‐grouting
Injection req’ts
Field testing
Post‐grout Insp
PTI Grouting Spec
Contractor: “A gambler who never gets to shuffle, cut or deal”
Low Bidder: “a contractor who wonders what he left out”
Engineer’s estimate: “the cost of construction in heaven”
Bid: “a wild guess carried out to 2 decimal places”
Critical path method: “mgmnt technique for losing your shirt under perfect control”
Definitions:
