ncc idot presentation.ppt - cptech center · shrinkage compensating (expansive) supplement expands...
TRANSCRIPT
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IDOT Bridge Decks:
Ongoing IDOT Research
&
An FHWA Process Review
James Krstulovich, P.E.Concrete Research Engineer
Bureau of Materials & Physical Research
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Transverse cracks at regular intervals at both positive and negative moment regions commonly observed
Longitudinal cracking occurs after extensive transverse cracking
Concrete Girder Steel Girder Crack
Bridge Deck Cracking in Illinois
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�Shrinkage compensating (expansive) supplement
�Expands in volume at early age
� If restrained: develops compressive stress that may compensate tensile stresses from shrinkage
Types K & G Expansive Cementitious Material
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Total heat released during hydration similar to ordinary portland cement paste.
Rate of Heat Evolution Total Heat Released
0 8 16 24 32 40 48 56 64 72Time (hrs)
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
Rate
of
He
at
Evo
luti
on
(W
/g)
OPC
OPC+K
OPC+G
0 8 16 24 32 40 48 56 64 72Time (hrs)
0
40
80
120
160
200
240
To
tal
He
at
(J/g
)
OPC
OPC+K
OPC+G
Characterizing Materials
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Restrained Expansion
(ASTM C 878)
Unrestrained Deformation
(ASTM C 1698)
Characterizing Materials
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Measurements starting at 6 hrs Measurements starting at Final Set
0 2 4 6 8 10 12 14Sealed Curing (days)
0
1000
2000
3000
4000
Exp
an
sio
n (
mic
rostr
ain
)
OPC+K
OPC+K+'F'FA
OPC+K+'C'FA
OPC+K+SF
0 2 4 6 8 10 12 14Sealed Curing (days)
0
1000
2000
3000
4000
Exp
an
sio
n (
mic
rostr
ain
)
OPC+K
OPC+K+'F'FA
OPC+K+'C'FA
OPC+K+SF
Unrestrained Expansion of Paste
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MF: Mortar Factor
CF: Cement Factor
~25% reduction in drying shrinkage by optimizing mix
0 20 40 60 80 100 120Age (days)
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
Le
ng
th C
han
ge
(%
)
CF:610, MF:0.86
CF:610, MF:0.82
CF:630, MF:0.82
Optimizing Mixes
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� Focus: Cracks due to temperature and shrinkage
� Cracking due to restraint provided by superstructure
� Cracking (tensile) stress depends on many other factors
such as
� Girder Stiffness� Deck Thickness
� Slab Reinforcement (including shear studs)� Support Conditions� Skew Angle
� Type of Bearings
� Form Types
� Others
Large-Scale Study
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�Simulate bridge superstructure
�Concrete surface, rebar, and girders instrumented to measure strains, temperatures and deformations
�Data obtained will be used to calibrate FEM model for a full scale 3-span bridge
Large-Scale Simulation
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� Dimensions: 7’ x 10’, 8” deck thickness, 6’ girder spacing
� Longitudinal reinforcement fixed to rigid steel C-channels to
simulate continuity of longitudinal reinforcement
� Shear studs included
Large-Scale Simulation
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0 20 40 60 80 100 120
Age (days)
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
Le
ng
th C
ha
ng
e (
%)
-800
-600
-400
-200
0
200
400
600
800
Str
ain
(µ
ε)
OPC (0.44)
OPC+K (0.44)
Expansion
Shrinkage
Start of drying (7d)
Small-Scale & Large-Scale Study
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� Correlating shrinkage reduction from small-scale testing
with…
� tensile strain reduction in reinforcing bar from large-scale
simulation:
i.e., maximum strain registered by longitudinal bar was 75 µε
compared to 500 µε expansion in small-scale testing
The external restraint (C-channels) and higher percentage
of reinforcement in large-scale deck simulation created a
much stiffer system which resulted in lower strain values in
reinforcing bar compared to that in small-scale concrete
prisms.
Linking Small-Scale Testing with Large-Scale Simulation
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�How does compressive stress developed by expansion mitigate tensile stress due to shrinkage?
�How much does external restraint impact such mitigation?
�Does it matter if creep reduces the developed compressive stress?
� Elastic modulus plays important role in
governing deformation under load (or creep).
Linking Small-Scale Testing with Large-Scale Simulation
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Type K Bridge Deck Poured 2012
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Balling in Mix
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Balling in Mix Blisters
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0 20 40 60 80 100 120 140
Age (days)
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1
Le
ng
th C
ha
ng
e (
%)
-1000
-800
-600
-400
-200
0
200
400
600
800
1000
Str
ain
(µ
ε)
No SRA
SRA(3.5 L/m3)
SRA (4.45 L/m3)
SRA (6.35 L/m3)
SRA (7.5 L/m3)
Start of drying (7 days)
SRA has been found to be effective reducing shrinkage;
effectiveness depending on dosage.
Also Evaluating SRAs
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IDOT Bridge Deck Construction:
An FHWA Process Review
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�Conducted in 2012
�Observed 27 bridge decks pours
�Consisted of BMPR, BBS, Construction, Districts, FHWA, and Industry
�Resulted in 14 Findings, Recommendations
FHWA Process Review
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1. Fogging
2. Curing
3. Exterior beam rotation
4. Too much hand finishing
5. Use of vibratory screeds
6. Ambient temperature restrictions
7. Concrete testing methods
8. Alternative curing materials
9. Bridge deck grinding
10. Cleaning leaking mortar off the beams
11. Concrete delivery rates
12. Plastic chair supports for deck reinforcement
13. Location of finishing machine rails
14. Training course for bridge deck construction
Findings, Recommendations
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�Foggers now banned from finishing machine
� Their use was ineffectual or counter-
productive
�Handheld foggers to be used until curing mats are placed
� Minimum pressure 2,500 psi - like a backyard
power washer
Item 1: Fogging
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� Not a result of the process review
� Instead of referring to the evaporation chart to determine
if fogging is needed, the following equation was added:
Evaporation Rate
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�Long delays in placing cotton mats
�Common excuse: trying to avoid marring the deck
�Revise language to de-emphasize marring
Item 2: Curing
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Item 3: Exterior Beam Rotation
� Illinois Center for Transportation study
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� 21 of 27 decks had hand finishing over all or the
majority of the deck – undesirable practice
� Goal: Let the finishing machine do the work
� Limit to problems found during straightedge
testing and for those surfaces not reached by
the finishing machine
Item 4: Hand Finishing
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� Now allow vibratory screed in lieu of finishing machine
� Allowed on deck pours up to 24’ wide
� Vibratory screeding followed by finishing with hand-
operated longitudinal floats having blades not less than
10 ft long and 6 in. wide
Item 5: Vibratory Screeds
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�Currently no ambient temperature restrictions for deck pours
�Concrete temperature of 90F at discharge� Ice or water chillers
� Nighttime pours
� Water/shade stockpiles
Item 6: Ambient Temperature
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�85F air temperature requirement may or may not force nighttime deck pours� Contractors may have trouble bidding
Low bidders maybe assumed daytime pours
� District may not want nighttime pours
� Several day wait for cooler weather
Ambient Temperature Debate
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James Krstulovich, P.E.
Concrete Research Engineer
Dan Tobias, Ph.D., S.E., P.E.
Acting Engineer of Concrete & Soils
Matt Mueller, P.E.
Engineer of Tests
Questions