4B - Creep and Shrinkage

8/14/2019 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



Bridges

5. Conclusions


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CEB-FIP 1970 Model Code



FIB 2010 Draft Model Code

ACI-209

BP3


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1. Introduction



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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Documents

4B - Creep and Shrinkage