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Fundamentals and Application Fundamentals and Application of Stress Ratio in Concrete of Stress Ratio in Concrete

Pavement DesignPavement Design

Edward H. Guo ConsultantEdward H. Guo Consultant

April 24 - 26, 2012April 24 - 26, 2012

FAA Working Group MeetingFAA Working Group Meeting

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AcknowledgementsAcknowledgements

This work was sponsored by the FAA This work was sponsored by the FAA Airport Technology R&D Branch, ANG-Airport Technology R&D Branch, ANG-E260, Dr. Satish Agrawal, ManagerE260, Dr. Satish Agrawal, Manager

The views expressed and the The views expressed and the conclusions drawn are those of the conclusions drawn are those of the author aloneauthor alone

This presentation does not represent a This presentation does not represent a standard, specification, or regulation, standard, specification, or regulation, nor does it necessarily represent a nor does it necessarily represent a methodology to be introduced into methodology to be introduced into pavement design standardspavement design standards

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OUTLINEOUTLINE Can a “STRESS” in a concrete pavement be reliably Can a “STRESS” in a concrete pavement be reliably

predicted ? How? Fundamentals for understanding predicted ? How? Fundamentals for understanding the “STRESS” in “Stress Ratio”, DF = the “STRESS” in “Stress Ratio”, DF = σσ/R /R

Advantages and disadvantages of using DF in Advantages and disadvantages of using DF in concrete pavement design for highway and airportsconcrete pavement design for highway and airports

How to Maximize the Advantage of Using DF and How to Maximize the Advantage of Using DF and Minimize its Uncertainty Effects by Full Scale Minimize its Uncertainty Effects by Full Scale Testing?Testing?

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Can the “STRESS” in a Can the “STRESS” in a concrete pavement be concrete pavement be reliably predicted ? reliably predicted ?

Fundamentals for Fundamentals for understanding the understanding the

STRESS in STRESS in “Stress Ratio” - “Stress Ratio” - σσ/R /R

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Why Stress is so Why Stress is so Uncertain for Uncertain for

Prediction?Prediction? The classic mechanics was started by The classic mechanics was started by Isaac Newton. How could he have made Isaac Newton. How could he have made such a great contribution? such a great contribution?

Classical mechanics was started from the Classical mechanics was started from the definition change for “constant velocity”. definition change for “constant velocity”. A circular motion with constant speed in A circular motion with constant speed in magnitude had been defined as a motion magnitude had been defined as a motion with zero acceleration. Newton changed with zero acceleration. Newton changed it. A constant velocity must remain it. A constant velocity must remain constant for both – magnitude and constant for both – magnitude and direction. direction.

How to learn above for improving How to learn above for improving researches in pavement engineering?researches in pavement engineering?

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Definitions of Three Types of Definitions of Three Types of Variables Variables

(I) It can be defined and quantified

(measured) by both

Math and ScienceThickness

Slab length & width

Temperature

Load

Strain

(II) It can be defined and quantified

by Math but not by Science

Foundation modulus

Joint stiffness

Elastic modulus

Stress

(III) It can be defined and quantified (measured) by

Neither Math nor Science

Distresses

Performance

Failure

……

Objective in Engineering and

Science

Objective in Engineering but can

not be objectively measured

Subjective in Engineering

Advantages and Disadvantages Advantages and Disadvantages of using DF in Concrete of using DF in Concrete

Pavement Design for Highway Pavement Design for Highway and Airportsand Airports

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Advantage of Using Advantage of Using “Stress”“Stress”

A Bridge to Evaluate and Predict A Bridge to Evaluate and Predict “Performance”“Performance”

KnownThickness

Slab length

Slab width

Material Strength

Concrete Mix

Joint type

Traffic

Environment

To Be Known

Performance

Pavement Life

Cracks …

Longitudinal

Transverse

Corner

Faulting

Roughness

STRESS

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Excellent Match between Excellent Match between Strains from Measurements Strains from Measurements

and Predictionand Prediction

Exllent Match can be Exllent Match can be reached by both 2D and 3D reached by both 2D and 3D

Programs by selecting Programs by selecting appropriate input data.appropriate input data.

POWER OF POWER OF MODELING MODELING

Comparison of the Strain Time History at Point E, OHIO Tests, Ec=4,930,000psi, k=200 pci, mu=0.15

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

10.00

15.00

-200 -100 0 100 200 300 400 500 600 700 800

Mic

ro S

train

MeasuredCalculated

-50

-40

-30

-20

-10

0

10

20

30

40

50

-10 0 10 20 30 40 50

Time

Mic

ro S

trai

n

CSG13-Upper, Measured

CSG13-Lower, Measured

CSG13-Upper, Calculated

CSG13-Lower, Calculated

The nine-slab finite element mesh, gear and sensor locations

By Univ. Ohio, Supported by FHWA

By FAA’s NAPTF

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““STRESS” is a Variable - Objective in STRESS” is a Variable - Objective in Engineering. Observation on Strain-Engineering. Observation on Strain-

Crack in CC6Crack in CC6Strains were

different under same load but the

recorded “cracking strains” were

similar

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Uncertainties in Stress Uncertainties in Stress Prediction – Long-Term Prediction – Long-Term

Response (I) Response (I) S11 at A, Top-Down Crack Indicated by Top Strain Gages

4/27/2005, 9:23 am

-20

0

20

40

60

80

100

0 100 200 300 400 500 600 700 800 900

1 = 0.25 Seconds

Mic

ro S

train

s

Gage 3, Top

Gage 5, Top

Gage 9, Top

25k 30k 35k 40k 45k 50k 55k 60k

Crack was observed after 60,000 lbs load applied

Top-Down Crack was initialized between gage 3 and 5, 5 inch from gage 5

5 1.5 ft

A1 3 5 7 9

2 4 6 8 10 Observed creep behaviors are complex. Strain increased while load

remained constant. The creeps are different in

tensile and compressive zones in Lab tests. Can it be reliably predictable in

concrete pavement?

Strain Record

Load Record

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Uncertainties Uncertainties in Stress Prediction in Stress Prediction Long- Term Response (II) Long- Term Response (II)

Micro cracks in tensile zone of concrete pavement lead to

significant decrease of the measured modulus E. Then the measured strains were over-estimated. The stress predicted by mechanistic

model is lack of reliability.

Strains released by coring

Strains released by saw-cut

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UncertaintiesUncertainties in Stress Prediction in Stress Prediction Short-Term Response (I) Short-Term Response (I)

Change of interface led to sign change of surface strain.

Case-for-case match by modeling has more

significance for academic work (science) than for

applications (engineering).

-10

-8

-6

-4

-2

0

2

0 2000 4000 6000 8000 10000 12000 14000 16000

Mic

ro-S

train

s

CSG-1A CSG-1B

CSG1

LoadLocation

VD2

-1

0

1

2

3

4

5

6

0 2000 4000 6000 8000 10000 12000

Loads, Kgs

Mic

ro-S

train

s

CSG-1A

CSG-1B

CSG1 LoadLocation

VD6

Strains - Load at South

Strains - Load at North

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UncertaintiesUncertainties in Stress Prediction in Stress Prediction Short - Term Response Short - Term Response

(II) (II)

1998

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Uncertainties in Stress PredictionUncertainties in Stress PredictionDiscussion of Built-in Curling Approach Discussion of Built-in Curling Approach

Through Equivalent Temperature Through Equivalent Temperature Gradient (ETG)Gradient (ETG) The “one-to-one” relationship between “Deflection” and

“Stress” employed in modeling but could not be verified by experiments;

As a mechanistic model, “Built-in Curling” requires correct As a mechanistic model, “Built-in Curling” requires correct definition and appropriate procedure for quantification. definition and appropriate procedure for quantification. Satisfaction of above requirements still needs more work.Satisfaction of above requirements still needs more work.

ETG approach was used by Guo ten years ago. ETG approach was used by Guo ten years ago. It can’t simulate the effects of localized stress It can’t simulate the effects of localized stress due to non-load excitation. due to non-load excitation.

Localized stress

Global Curling

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Total “STRESS” can not be objectively measured in science (experiments) since it has to be quantified through has to be quantified through otherother SUBJECTIVESUBJECTIVE variables variables;;

Therefore, “STRESS” is not a variable Therefore, “STRESS” is not a variable predictable by any mechanistic model, neither predictable by any mechanistic model, neither 2D, nor 3D finite element procedure. 2D, nor 3D finite element procedure. How can How can we minimize the uncertain effects of the we minimize the uncertain effects of the STRESS for pavement engineering? STRESS for pavement engineering?

Different DF ranges are used for highway and Different DF ranges are used for highway and airport pavements.airport pavements.

““STRESS” is also a Variable that STRESS” is also a Variable that can not be Objectively Measured in can not be Objectively Measured in

Science (Experiments and Science (Experiments and modeling).modeling).

How to Maximize the How to Maximize the Advantage of Using DF Advantage of Using DF

and Minimize its and Minimize its Uncertainty Effects by Uncertainty Effects by

Full Scale Testing?Full Scale Testing?

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Full Scale Test Provides Full Scale Test Provides an an

Excellent Opportunity Excellent Opportunity

If the values of E in a pavement are the If the values of E in a pavement are the same, the same, the uncertainties of DF can be reduced.uncertainties of DF can be reduced.(1)(1)(F) and (F) and (P) are measured from the (P) are measured from the same structure;same structure;(2)(2)The fatigue behaviors (DF vs. N) for The fatigue behaviors (DF vs. N) for pavement and beam are comparable. pavement and beam are comparable.

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Top-down Crack, from Top-down Crack, from Initiation to Full Initiation to Full

Depth, CC6, NorthDepth, CC6, North

DF => N => Modeling

The full-depth crack was

detected at pass 1100 by strain

gage and observed after

2112 pass

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Top-down Crack, from Top-down Crack, from Initiation to Full Initiation to Full

Depth, CC6, SouthDepth, CC6, SouthDF => N => Modeling

2205-647=1558 was measured by gages 27-28

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Bottom-up Crack, from Bottom-up Crack, from Initiation to Full Initiation to Full Depth, CC6, South (I)Depth, CC6, South (I)

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Bottom-up Crack, from Bottom-up Crack, from Initiation to Full Initiation to Full Depth, CC6, South (II)Depth, CC6, South (II)

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Bottom-up Crack, from Bottom-up Crack, from Initiation to Full Initiation to Full Depth, CC6, South (III)Depth, CC6, South (III)

The crack was detected at pass 2251

by strains and observed after 11616 passes

by survey

DF => N => Modeling

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Next Step: Find Different “Fatigue Next Step: Find Different “Fatigue Behaviors between Beam and Behaviors between Beam and

PavementPavement

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SUMMARY AND RECOMMENDATION

KnownThickness

Slab length

Slab width

Material Strength

Concrete Mix

Joint type

Traffic

Environment

To Be KnownPerformance

Pavement Life

Cracks …

Longitudinal

Transverse

Corner

Faulting

Roughness

STRESS

The “stress” is objective in engineering (advantage) but can not be measured objectively (disadvantage). Therefore, it can not be accurately predicted by

mechanistic models. DF has been used and will be continuously used as bridge for both highway and airport concrete pavement design. How to improve?

(1) Full scale tests (2) Analysis of more

field data

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Thank You !