4B - Creep and Shrinkage
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Transcript of 4B - Creep and Shrinkage
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VIRGINIA CONCRETECONFERENCE
March 3-4, 2011
Presented by:
Teddy Theryo, P.E.
Parsons Brinckerhoff
SEGMENTAL BRIDGE GROUP
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Definitions
Creepis time dependent deformations of concreteunder permanent loads (self weight), PT forces andpermanent displacement
Shrinkageis shortening of concrete due to drying andis independent of applied loads
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Factors Affecting Creep
Concrete mix proportion Cement properties
Curing conditions
Size and shape of members
EnvironmentAge at loading
Stress level
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Factors Affecting Shrinkage
Concrete mix proportion
Cement properties
Aggregate properties
Curing conditions Size and shape of members
Environment
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In structural concrete creep and shrinkage strains arecoexist and occur together.
The rate of both creep and shrinkage decrease with time.
Theoretically the creep and shrinkage are considered
diminished at 10,000 days (27 years) after construction. For practical purposes the ending time of 4,000 days (11
years) is also commonly used in creep and shrinkagecalculations .
Mathematically the non linear shape of creep andshrinkage has been assumed as hyperbolic, exponential orlogarithmic.
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Strain
Strain
Time Time
Creep strain
Instantaneousstrain
TYPICAL CREEP TIMECURVE TYPICAL SHRINKAGE TIMECURVE
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Drying
creepBasiccreep
Totalcreep
Shrinkage
Nominalelastic strain
Time (t t )0t0
Strain
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0 50 100 150 200
Instantaneousrecovery
Creep recovery
Residualdeformation
500
1000
1500
Strain on application
of load
Time since application of load - days
Strain-10-6
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1. Introduction
2. Understanding of Creep & Shrinkage
3. Code Development of Creep & Shrinkage4. Impact of Creep & Shrinkage on Post-Tensioned
Bridges
5. Conclusions
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Relationship between creep and elastic deformations
cr = el =
E28
where: cr= creep strain
el= elastic strain
= stressE28 = elastic modules of concrete at age 28 days
= creep factor
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4.0
3.5
3.0
2.5
2.0
1.5
3.72
3.03
2.57
2.222.00
1.70
1.44
1.0
0.5
0 3 7 14 2128 4256 3 4 56 9 1 1.5 2 3 5
Days Months Years
1.2
0
1.07
1.0
0
0.9
6
0.9
1
0.9
4
0.9
0
0.8
8
t
DURATION OF LOADING
TOTALELASTICANDCREE
PSTRAIN
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Mcr(t) =(1 e- (t)) (MIIMI)
MFinal(t) = MII+ (MIMII) e- (t)
where: (t) = creep factor at time te = Base of Napierian logarithms
= 2.7182
MI = Movement due to permanent loads before
change of statical systemMII = Movement due to the same loads applied on
changed statical system (build onfalse-work)
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Free Cantilever Statical System
Changed Statical System (Midspan Continuous)
MFinal (t)
L L
MI M =I
Fixed Fixedq
qL2
8
MIIM =II
qL2
12qL2
24
MII
MIMcr (t)
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el(t )0
cr(t )
P P
Pef Pef
Cantilever Beam
Simple Beam
el( )t0cr(t )
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P
Post-Tensioned BeamP
P P
Pef Pef
el(t )0
el(t )0el(t )
PT Tendon
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1. Introduction
2. Understanding of Creep & Shrinkage
3. Code Development of Creep & Shrinkage4. Impact of Creep & Shrinkage on Post-Tensioned
Bridges
5. Conclusions
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CEB-FIP 1970 Model Code
CEB-FIP 1978 Model Code
CEB-FIP 1990 Model Code
FIB 2010 Draft Model Code
ACI-209
BP3
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1. Introduction
2. Understanding of Creep & Shrinkage
3. Code Development of Creep & Shrinkage4. Impact of Creep & Shrinkage on Post-Tensioned
Bridges
5. Conclusions
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There are two major impacts of creep and shrinkageon structural concrete
Deformations (simply supported and indeterminatestructures)
Redistribution of stresses / forces on indeterminate
structure, including support reactions
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CL
CLIn-span HingeIn-span Hinge
Mid-span HingeBearing &
Expansion Joint Bearing
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Expansion Joint
Bearing
Old Generation of Midspan Hinge(not recommended)
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Mid
-SpanHinge
In-SpanHinge
5.1%
S1.8%
2.5
5.0
7.5Deforma
tion(cm)
Span Length: 79m (260 feet)
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Deck Profile basedon As-Built Dwgs
ExistingDeck Profile
ReferenceLine
C EXP. JT. NO. 3LSTA. 67+16.50
C PIER 9LSTA. 68+16.59
BEGIN S.E. TRANSITIONSTA. 68+18
C PIER 8L
STA. 65+74
0.36
0.46
0.8
2
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Deck Profile basedon As-Built Dwgs
ExistingDeck ProfileLine
C EXP. JT. NO. 3LSTA. 67+16.50
C PIER 9LSTA. 68+16.59
C PIER 8L
STA. 65+74
0.49
0.35
0.8
4
Reference
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Active Hinge(proposed by Jean M. Muller)
Active hinge memberMidspan expansion joint
Typical internaldiaphragm
Hydraulic jack
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SlidingExpansion Joint
CL Mid-Span
Steel Strong Back
Fixed
Elastomeric Bearing
Teflon Surface (typ)
Mid-span Hinge with Strong Back
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-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 200 400 600 800
Distance Along the Bridge (ft)
VerticalDisplacement(in)
L
L
@ TFo
creep
0.079 Degree 8-6
3-6
12-0
Lcreep = 0.079 x 3.5 x 12 = 3.31
Assuming 50% of the creep had been correctedcamber during segment casting.
Lavailable gap at 60F in 2010o
Abutment 1 = 3-3/4 - 0.5 (3.31) = 2.09 vs 1.75
Abutment 29 = 3-3/8 - 0.5 (3.31) = 1.75 vs 1
Point of rotationcreepV
AbutmentBack Wall
Camber Diagram of Unit 1 at T =
End Span Girder Rotation at Abutment 1(Varina-Enon Bridge Case Study)
Elastomeric Bearing
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Expansion Joint at Abutment
Abutment
Span 1
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X CL
Top Plate
Bottom Pot
>X
CLTop Plate
X min.
CL
CLBottom
Pot
CL BottomPot
creep at T =
Top Plate
creep at T =e =
Ideal/preferredposition at T=
Incorrectposition at T=
Correct bearing &joint expansionpreset at construction
Expansion
Joint
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Over Extended of Bearing Top Plate
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Torsional Creep Deformation in Horizontally Curved Bridge
A
A
GOODBAD
Roadway Axis
Girder Axis
SupportAxis
SECTION A-A
BAD STRATEGY GOOD STRATEGY
Top AbutmentElevation
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Introduction
Understanding of Creep & Shrinkage
Code Development of Creep & Shrinkage Impact of Creep & Shrinkage on Post-Tensioned
Bridges
Conclusions
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In order to avoid the negative impacts of long-termcreep and shrinkage:
1. Good understanding of creep and shrinkage behaviors
2. Accurate estimation of creep and shrinkage on structuralconcrete design
3. Proper counter measures of long-term creep andshrinkage effects
4. Implement simple structural details
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