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1 Fundamentals and Application of Stress Ratio in Concrete Pavement Design Edward H. Guo Consultant April , 2012 FAA Working Group Meeting

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2 Acknowledgements This work was sponsored by the FAA Airport Technology R&D Branch, ANG-E260, Dr. Satish Agrawal, Manager This work was sponsored by the FAA Airport Technology R&D Branch, ANG-E260, Dr. Satish Agrawal, Manager The views expressed and the conclusions drawn are those of the author alone The views expressed and the conclusions drawn are those of the author alone This presentation does not represent a standard, specification, or regulation, nor does it necessarily represent a methodology to be introduced into pavement design standards This presentation does not represent a standard, specification, or regulation, nor does it necessarily represent a methodology to be introduced into pavement design standards

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3 OUTLINE Can a “STRESS” in a concrete pavement be reliably predicted ? How? Fundamentals for understanding the “STRESS” in “Stress Ratio”, DF = σ/R Can a “STRESS” in a concrete pavement be reliably predicted ? How? Fundamentals for understanding the “STRESS” in “Stress Ratio”, DF = σ/R Advantages and disadvantages of using DF in concrete pavement design for highway and airports Advantages and disadvantages of using DF in concrete pavement design for highway and airports How to Maximize the Advantage of Using DF and Minimize its Uncertainty Effects by Full Scale Testing? How to Maximize the Advantage of Using DF and Minimize its Uncertainty Effects by Full Scale Testing?

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4 Can the “STRESS” in a concrete pavement be reliably predicted ? Fundamentals for understanding the STRESS in “Stress Ratio” - σ/R

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55 Why Stress is so Uncertain for Prediction? The classic mechanics was started by Isaac Newton. How could he have made such a great contribution? The classic mechanics was started by Isaac Newton. How could he have made such a great contribution? Classical mechanics was started from the definition change for “constant velocity”. A circular motion with constant speed in magnitude had been defined as a motion with zero acceleration. Newton changed it. A constant velocity must remain constant for both – magnitude and direction. Classical mechanics was started from the definition change for “constant velocity”. A circular motion with constant speed in magnitude had been defined as a motion with zero acceleration. Newton changed it. A constant velocity must remain constant for both – magnitude and direction. How to learn above for improving researches in pavement engineering? How to learn above for improving researches in pavement engineering?

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66 Definitions of Three Types of Variables (I) It can be defined and quantified (measured) by both Math and Science Thickness 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

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Advantages and Disadvantages of using DF in Concrete Pavement Design for Highway and Airports 7

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8 Advantage of Using “Stress” A Bridge to Evaluate and Predict “Performance” Known Thickness 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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9 Excellent Match between Strains from Measurements and Prediction Exllent Match can be reached by both 2D and 3D Programs by selecting appropriate input data. POWER OF MODELING By Univ. Ohio, Supported by FHWA By FAA’s NAPTF

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10 “STRESS” is a Variable - Objective in Engineering. Observation on Strain-Crack in CC6 Strains were different under same load but the recorded “cracking strains” were similar

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11 Uncertainties in Stress Prediction – Long-Term Response (I) 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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12 Uncertainties in Stress Prediction 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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13 Uncertainties in Stress Prediction 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). Strains - Load at South Strains - Load at North

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14 Uncertainties in Stress Prediction Short - Term Response (II) 1998

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15 Uncertainties in Stress Prediction Discussion of Built-in Curling Approach Through Equivalent Temperature 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 definition and appropriate procedure for quantification. Satisfaction of above requirements still needs more work. As a mechanistic model, “Built-in Curling” requires correct definition and appropriate procedure for quantification. Satisfaction of above requirements still needs more work. ETG approach was used by Guo ten years ago. It can’t simulate the effects of localized stress due to non-load excitation. ETG approach was used by Guo ten years ago. It can’t simulate the effects of localized stress due to non-load excitation. Localized stress Global Curling

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16 has to be quantified through other SUBJECTIVE variables; Total “STRESS” can not be objectively measured in science (experiments) since it has to be quantified through other SUBJECTIVE variables; Therefore, “STRESS” is not a variable predictable by any mechanistic model, neither 2D, nor 3D finite element procedure. How can we minimize the uncertain effects of the STRESS for pavement engineering? Therefore, “STRESS” is not a variable predictable by any mechanistic model, neither 2D, nor 3D finite element procedure. How can we minimize the uncertain effects of the STRESS for pavement engineering? Different DF ranges are used for highway and airport pavements. Different DF ranges are used for highway and airport pavements. “STRESS” is also a Variable that can not be Objectively Measured in Science (Experiments and modeling).

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How to Maximize the Advantage of Using DF and Minimize its Uncertainty Effects by Full Scale Testing? 17

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

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19 Top-down Crack, from Initiation to Full Depth, 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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20 Top-down Crack, from Initiation to Full Depth, CC6, South DF => N => Modeling =1558 was measured by gages 27-28

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

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

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23 Bottom-up Crack, from Initiation to Full Depth, CC6, South (III) The crack was detected at pass 2251 by strains and observed after passes by survey DF => N => Modeling

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

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25 SUMMARY AND RECOMMENDATION Known Thickness 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 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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26 Thank You !

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