2appendices_KK

Critical Loadings and Stress Locations

in Germany (9, 10, 11, 45, 50, 51), and the U.S. (4, 8, 37, 46, 47, 48, 49). They provided the technology that allowed the research team to include both a permanent built-in temperature gradient and permanent moisture gradient and a transitory temperature and moisture gradients in the design procedure in a practical manner.

The procedure developed under NCHRP 1-37A made full use of the knowledge and experience gained from these previous studies and others. The complete fatigue cracking algorithm developed as part of the design guide for JPCP considers truck axle loadings (four types of axles, lateral distribution, full axle load spectra), thermal gradients (non-linear, hourly, built-in), and moisture gradients (permanent shrinkage and transitory shrinkage) in more detail and extent than has been accomplished in the past to produce a comprehensive yet practical mechanistic-empirical cracking model (both bottom up initiated and top down initiated cracking are included). The remainder of this appendix describes in detail the mechanistic based algorithm developed to predict a combination of top down and bottom up transverse cracking of JPCP.

Transverse cracking of PCC slabs can initiate either at the top surface of the PCC slab and propagate downward (top-down cracking) or vice versa (bottom-up cracking) depending on the loading and climatic conditions at the project site, as well as material properties and the conditions during construction. Both top-down and bottom-up cracking are considered in this Guide. Damage accumulates differently for each of these different distresses and hence needs to be computed separately. The following sections explain the damage accumulation process of these distresses.

Bottom-Up Transverse Cracking (JPCP)

When the truck axles are near the longitudinal edge of the slab, midway between the transverse joints, a critical tensile bending stress occurs at the bottom of the slab, as shown in figure 1. This stress increases greatly when there is a high positive temperature gradient through the slab (the top of the slab is warmer than the bottom of the slab). Repeated loadings of heavy axles under those conditions result in fatigue damage along the bottom edge of the slab, which eventually result in a transverse crack that is visible on the surface of the pavement and work their way across the slab. Over time, some transverse cracks in JPCP deteriorate and cause roughness. Major factors that affect bottom-up cracking are PCC thermal coefficient of expansion, slab thickness, joint spacing, slab widening, tied PCC shoulder, strength of PCC, and a stabilized base.

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Acknowledgment of Sponsorship

This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO) in cooperation with the Federal Highway Administration and was conducted in the National Cooperative Highway Research Program which is administered by the Transportation Research Board of the National Research Council.

Disclaimer

This is the final draft as submitted by the research agency. The opinions and conclusions expressed or implied in this report are those of the research agency. They are not necessarily those of the Transportation Research Board, the National Research Council, the Federal Highway Administration, AASHTO, or the individual States participating in the National Cooperative Highway Research program.

Acknowledgements

The research team for NCHRP Project 1-37A: Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures consisted of Applied Research Associates, Inc., ERES Consultants Division (ARA-ERES) as the prime contractor with Arizona State University (ASU) as the primary subcontractor. Fugro-BRE, Inc., the University of Maryland, and Advanced Asphalt Technologies, LLC served as subcontractors to either ARA-ERES or ASU along with several independent consultants.

Research into the subject area covered in this Appendix was conducted at ARA-ERES. The authors of this appendix are Mr. Tom Yu and Dr. Michael Darter. Dr. Khazanovich provided the neural networks to compute stresses at the critical locations in JPCP. Data used in model calibration and verification was assembled by Mr. Leslie Titus-Glover.

Foreword

This appendix describes the mechanistic-empirical concepts involved and the development and calibration of the transverse crack prediction model for jointed plain concrete pavements (JPCP). These pavements are commonly constructed on new alignments, or for reconstruction projects, or as overlays of existing pavements. Transverse cracking initiates at either longitudinal edge of a slab (at the top or bottom of the slab) and progresses either transversely or diagonally across the slab until it reaches either the other longitudinal joint or a transverse joint (this would essentially be a diagonal crack). The key point is that it initiates at one or the other longitudinal joint. This type of cracking is considered the primary structural fatigue distress type of JPCP. All levels of transverse crack severity are included in the calibration data base.

The information contained in this appendix serves as a supporting reference to discussions presented in PART 3, Chapters 4 and 7 of the Design Guide.

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APPENDIX KK TRANSVERSE CRACKING OF JPCP

1.0 INTRODUCTION

This appendix describes the mechanistic-empirical concepts involved and the development and calibration of the transverse crack prediction model for jointed plain concrete pavements (JPCP). These pavements are commonly constructed on new alignments, or for reconstruction projects, or as overlays of existing pavements. Transverse cracking initiates at either longitudinal edge of a slab (at the top or bottom of the slab) and progresses either transversely or diagonally across the slab until it reaches either the other longitudinal joint or a transverse joint (this would essentially be a diagonal crack). The key point is that it initiates at one or the other longitudinal joint. This type of cracking is considered the primary structural fatigue distress type of JPCP. All levels of transverse crack severity are included in the calibration data base.

This procedure does not include the prediction of longitudinal cracking which initiates at a transverse joint at either the top or bottom of the slab, and progresses along the slab to the next transverse joint, but may also propagate to a longitudinal joint. Longitudinal cracking is often caused by inadequate longitudinal joint forming procedures and wanders around the longitudinal lane to lane joint. These cracks occur within the first few years of pavement life and are not the result of repeated load fatigue damage. Sometimes, however, longitudinal cracks appear to initiate either in the wheel paths or between the wheel paths and progress along the wheel paths. This often occurs when the slab is fairly thin (e.g., < 7 in as was well documented at the AASHO Road Test) this is believed to be a form of fatigue cracking in the wheelpaths. For thicker slabs it is a rare occurrence and is difficult to explain mechanistically as a fatigue based fracture since the damage calculated at various points is normally much less than damage for transverse cracking. However, longitudinal cracking does occur more often in widened slab JPCP designs and for that type of design may represent a form of fatigue initiated cracking. While this cracking model does not consider longitudinal cracking, future additions to this design procedure should fully consider this form of fracture.

This mechanistic-empirical prediction for fatigue cracking includes a comprehensive iterative damage accumulation algorithm. Damage is accumulated on a monthly basis considering both day time and nighttime hourly thermal gradients. The accumulated damage at the top and bottom of the slab is then correlated with field cracking of JPCP of projects located throughout North America. This correlation produces a calibrated model for slab transverse cracking of JPCP. Field data from hundreds of projects from LTPP and other sources are included in this calibration.

The calibration data is then used in the development of the design reliability procedure for JPCP cracking. This procedure is described in Appendix BB and is not reported herein in detail.

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2.0 BASIC APPROACH AND CONCEPTS

Summary of Previous Mechanistic-Empirical Slab Cracking Models

There have been several previous studies to develop prediction models for transverse cracking of JPCP. These studies were both empirical and semi-empirical (statistical regression using limited structural responses) and mechanistic-empirical based (using structural responses to compute fatigue damage which is calibrated with field cracking). These key previous mechanistic-empirical based studies are briefly summarized below.

• The Portland Cement Association utilized beam fatigue data since the 1930's to directly consider fatigue fracture in design. The 1966 PCA design procedure (39) was a key development in that it utilized the full axle load spectra for single and tandem axles, a structural response model to calculate critical stresses at the transverse joint, and Minor's damage model (22) to compute fatigue damage to the slab. A revised version was published in 1984 (42) that more adequately utilized edge stress as the critical loading. The total fatigue damage value was limited to less than 100 percent (or a Minor's damage value of 1.00) to limit slab fracture. No attempt was made to correlate computed damage to actual field slab cracking other than general comparison with field performance. Factors considered in this procedure are as follows.
  • Axle load spectra distribution (single and tandem).
  • Three load positions (edge, joint and near edge).
  • Critical stress from loading with the axle at the transverse joint.
  • PCC strength was third-point beam 28-day strength.
  • Thermal curling or moisture warping were not considered.
  • Fatigue damage was computed considering only the 28-day strength.
• Darter and Barenberg (41) developed a fairly comprehensive fatigue damage procedure for JPCP in 1978 at the University of Illinois under contract to the FHWA. This procedure expanded the PCA procedure to include the following factors.
  • Axle load spectra distribution (single and tandem).
  • The edge loading position was identified as critical (loads were varied transversely across the slab using a normal distribution).
  • Critical stress was computed at the bottom of the slab using a 2D FEM based on axle loading, positive and negative thermal gradient, and an equivalent negative temperature gradient to represent moisture gradient of dry on top and wet on bottom).
  • PCC strength was third-point beam 28-day strength but it varied throughout the entire design life, month by month.
  • Both positive and negative thermal gradients were considered.
  • Moisture gradient through the slab was considered to be a negative 0.5 degree F per inch of slab.
  • Fatigue damage was computed both night and day throughout the entire design life (but only at the slab bottom edge longitudinal joint).
  • A good correlation was made between fatigue cracking from limited field projects and the computed accumulated fatigue damage (note that this was the first known

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correlation between the theoretical damage calculated by Miner's damage model and field slab cracking).

• Poblete et al (38) developed a comprehensive fatigue damage procedure in the 1980's at the University of Chile that was similar to the University of Illinois approach to correlation of damage to field slab cracking. However, stresses at the top of the slab were identified as being the most critical in the dryer climate of Chile as cracks were found to initiate at the top of the slab. This procedure considered the following factors:
  • Axle load spectra distribution (single, tandem axles).
  • The wheel path loading position was used generally near the transverse joint for the non doweled joints.
  • Critical stress was computed at the top of the slab using a 3D FEM based on axle loading, negative thermal gradients, and an equivalent negative temperature gradient to represent the moisture gradient (dry on top and wet on bottom).
  • PCC strength was third-point beam 28-day strength.
  • Fatigue damage was computed for only nighttime temperature gradients only at the top of the slab.
  • A very good correlation was found between fatigue cracking from Chilean field projects and the computed accumulated fatigue damage at the top of the slab.
• A research team including Yu, Smith, Darter, Jiang and Khazanovich at ERES Consultants, Inc. under contract to the FHWA in the 1990's (2, 40). This procedure considered the following factors:
  • The edge loading position was identified as critical (however, loads were varied transversely across the slab following a normal distribution).
  • Critical stress was computed at the bottom of the slab using a 2D FEM based on axle loading, thermal gradients through the slab, and an equivalent negative temperature gradient to represent moisture gradient).
  • Axle load was limited to 18-kip ESAL.
  • PCC strength was third-point beam 28-day strength only.
  • Both positive and negative thermal gradients from solar radiation were considered.
  • Moisture gradient through the slab was approximately considered through an equivalent (negative) temperature gradient adjustment.
  • Fatigue damage was computed at the slab bottom edge both night and day throughout the entire design life (but only at the bottom of the slab but it was recognized that the top of the slab could be more critical under certain conditions such as dryer climates).
  • A good correlation was found between bottom up fatigue cracking from field projects (percent slabs cracked) and the computed accumulated fatigue damage and a calibrated cracking model developed (sections suspected of top down cracking were not included in the analysis).
• Several studies conducted on built-in temperature gradients at construction and moisture gradients through the concrete slab in the 1980's and 1990's lead to new knowledge on the initiation of slab cracking. These key studies were conducted in Chile (3, 38, 43, 44),

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Critical Loadings and Stress Locations

in Germany (9, 10, 11, 45, 50, 51), and the U.S. (4, 8, 37, 46, 47, 48, 49). They provided the technology that allowed the research team to include both a permanent built-in temperature gradient and permanent moisture gradient and a transitory temperature and moisture gradients in the design procedure in a practical manner.

The procedure developed under NCHRP 1-37A made full use of the knowledge and experience gained from these previous studies and others. The complete fatigue cracking algorithm developed as part of the design guide for JPCP considers truck axle loadings (four types of axles, lateral distribution, full axle load spectra), thermal gradients (non-linear, hourly, built-in), and moisture gradients (permanent shrinkage and transitory shrinkage) in more detail and extent than has been accomplished in the past to produce a comprehensive yet practical mechanistic-empirical cracking model (both bottom up initiated and top down initiated cracking are included). The remainder of this appendix describes in detail the mechanistic based algorithm developed to predict a combination of top down and bottom up transverse cracking of JPCP.

Transverse cracking of PCC slabs can initiate either at the top surface of the PCC slab and propagate downward (top-down cracking) or vice versa (bottom-up cracking) depending on the loading and climatic conditions at the project site, as well as material properties and the conditions during construction. Both top-down and bottom-up cracking are considered in this Guide. Damage accumulates differently for each of these different distresses and hence needs to be computed separately. The following sections explain the damage accumulation process of these distresses.

Bottom-Up Transverse Cracking (JPCP)

When the truck axles are near the longitudinal edge of the slab, midway between the transverse joints, a critical tensile bending stress occurs at the bottom of the slab, as shown in figure 1. This stress increases greatly when there is a high positive temperature gradient through the slab (the top of the slab is warmer than the bottom of the slab). Repeated loadings of heavy axles under those conditions result in fatigue damage along the bottom edge of the slab, which eventually result in a transverse crack that is visible on the surface of the pavement and work their way across the slab. Over time, some transverse cracks in JPCP deteriorate and cause roughness. Major factors that affect bottom-up cracking are PCC thermal coefficient of expansion, slab thickness, joint spacing, slab widening, tied PCC shoulder, strength of PCC, and a stabilized base.

6