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A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers (2018)

Chapter: APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION

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Suggested Citation:"APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION ." National Academies of Sciences, Engineering, and Medicine. 2018. A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers. Washington, DC: The National Academies Press. doi: 10.17226/25304.
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Suggested Citation:"APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION ." National Academies of Sciences, Engineering, and Medicine. 2018. A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers. Washington, DC: The National Academies Press. doi: 10.17226/25304.
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Suggested Citation:"APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION ." National Academies of Sciences, Engineering, and Medicine. 2018. A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers. Washington, DC: The National Academies Press. doi: 10.17226/25304.
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Suggested Citation:"APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION ." National Academies of Sciences, Engineering, and Medicine. 2018. A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers. Washington, DC: The National Academies Press. doi: 10.17226/25304.
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Suggested Citation:"APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION ." National Academies of Sciences, Engineering, and Medicine. 2018. A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers. Washington, DC: The National Academies Press. doi: 10.17226/25304.
×
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Suggested Citation:"APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION ." National Academies of Sciences, Engineering, and Medicine. 2018. A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers. Washington, DC: The National Academies Press. doi: 10.17226/25304.
×
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Suggested Citation:"APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION ." National Academies of Sciences, Engineering, and Medicine. 2018. A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers. Washington, DC: The National Academies Press. doi: 10.17226/25304.
×
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Suggested Citation:"APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION ." National Academies of Sciences, Engineering, and Medicine. 2018. A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers. Washington, DC: The National Academies Press. doi: 10.17226/25304.
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E-1 APPENDIX E. COLLECTION OF TOP-DOWN CRACKING DATA FOR MODEL DEVELOPMENT AND CALIBRATION Accurate and meaningful data is an essential prerequisite for the development of a reliable top-down cracking model. It means that: 1) The recorded distress is genuine top-down cracking; 2) The data is complete to provide inputs for model construction, validation, and calibration; and 3) The data is representative of nationwide geographic and climate conditions. To satisfy these requirements, the authors have assembled the known descriptions of top- down cracking from the technical literature, field observations, accelerated test sections, and test tracks to determine those pavement conditions and structures that promote top-down cracking. From a synthesis of this work, a set of criteria are developed in the search for top-down cracking data:  Distress criterion;  Thickness criterion;  Climate criterion; and  Stabilization criterion. The distress criterion identifies which type of pavement distress forms top-down cracking. Based on the comprehensive literature review conducted above, Table E-1 presents the type of distress identified as top-down cracking and associated example literature. It is widely accepted that top-down cracking is the longitudinal wheel path cracking. There are also researchers believing that top-down cracking starts as longitudinal cracks, and eventually deteriorates into an extensive network of longitudinal cracks connected by short transverse cracks. The thickness criterion identifies which type of pavement structure that most likely suffers top-down cracking. The literature review conducted identifies the pavement structure that most likely suffers top-down cracking, as documented in the literature and shown in Table E-2. It can be seen that top-down cracking is observed in both thin and thick asphalt layers. The climate criterion is used to ensure that the collected top-down cracking data covers nationwide geographic and climatic conditions. The climatic conditions of each pavement section needs to be taken into account to develop pavement temperature sub-models for the final top-down cracking model. The U.S. is split into the characteristic four climatic zones: 1) wet- freeze (WF); 2) dry-freeze (DF); 3) wet no-freeze (WNF); and 4) dry no-freeze (DNF). The stabilization criterion is used to discriminate top-down cracking from other surface cracking based on the type of base course. Distinction needs to be made between lime or cement stabilized base courses and bituminous stabilized base courses. Two types of pavement distresses are generated due to over-stabilization:  The base courses with too much lime or cement stabilization often develop block cracks due to shrinkage cracks subsequent to construction and these cracks reflect upward through the overlying surface layer forming block cracking surface distress patterns. This is clearly not top-down cracking.  There is a well-known case of cracking in the wheel path of thin asphalt layers resting on a cement stabilized base layer. Repeated traffic loading generates a longitudinal bending crack in the cement stabilized layer which then reflects up through the asphalt surface layer and intersects with a transverse shrinkage crack which also occurs in the

E-2 cement stabilized base. Where the two cracks intersect, a diamond-shaped cracking pattern (frequently called Y-cracking) develops which traps water and feeds it down into the sublayers. This is clearly not top-down cracking although a longitudinal crack appears in the wheel path. Table E-3 presents examples of literature review with respect to these two types of distresses. Even though longitudinal cracks or a network of longitudinal cracks with transverse cracks are recorded in the database, the data should be discarded if the base course is over- stabilized by the lime or cement. Table E-1. Identified Pavement Distress as Top-Down Cracking Pavement Distress Sources Findings Longitudinal wheel path cracking Washington field evaluation (Uhlmeyer et al. 2000) 24 pavement sections, 33 of 143 field cores with identified top-down cracks & 56 of the rest of the cores with full depth cracks of thin surface layers Florida field observation & field cores (Garcia 2002; Roque et al. 2004; Myers 1998; 2002) 90% of asphalt pavements with surface cracking; increased appearance of longitudinal wheel path cracking Indiana field evaluation (Pellinen et al. 2004) Cracks initiate from the surface & do not penetrate deeper than thin surface layer Colorado field observation & field cores (Harmelink and Aschenbrener 2003) 25 pavement sections, 17 out of 25 sections with observed top-down cracks Minnesota MnROAD observation (MnDOT 2002; Holewinski et al. 2003; Worel 2003) Cracks located in wheel path & do not propagate through entire layer Japan field observation (Matsuno and Nishizawa 1992) Visual observations of longitudinal surface cracks within or very close to the wheel path United Kingdom field evaluation (Bensalem et al. 2000; Nunn 1997; 1998) Field cores with identified top-down cracks Accelerated loading device (De Freitas et al. 2005) Wheel-tracking test of 17 mixtures that simulated the range of mixture characteristics for top-down crack initiation Longitudinal and/or transverse cracks Michigan field observation & field cores (Baladi 2003; Svasdisant 2002) 13 flexible & 5 rubblized pavement sections; 34 of 67 field cores with observed longitudinal crack; top-down cracks are longitudinal or transverse cracks; majority of longitudinal TDCs outside the wheelpaths, with a few in the wheelpaths France field evaluation (Dauzats and Rampal 1987) Longitudinal surface cracks located on the centerline side of the slow lane

E-3 Table E-2. Identified Thickness of Asphalt Layer with Top-Down Cracking Asphalt Layer Thickness Literature Findings Thick Uhlmeyer et al. (2000) Washington HMA surfaced pavements (> 160 mm) Nunn (1998) United Kingdom HMA pavements (> 180 mm) Mun (2003) Surface damage increases with increasing thickness Merrill (2000) Greater surface tensile stress in thicker pavement (layered elastic and finite element analysis) Bensalem et al. (2000) United Kingdom field cores (>160 mm) Thin Svasdisant (2002) Michigan field observation (<120 mm) Worel (2003) Minnesota MnROAD project Thin & thick Myers et al. (1998) A wide range of thickness (50 – 200 mm) Table E-3. Pavement Distresses Excluded from Top-Down Cracking Type of Pavement distress Excluded Literature Comments Blocking Cracking on stabilized base Metcalf et al. (1999) Accelerated Loading Facility (ALF) tests George (1968); Adaska and Luhr (2004) Shrinkage characteristics of cement- treated soil Y-cracking on stabilized base Tayabji et al. (1998) Y-cracking versus shrinkage of differentbase courses Ley et al. (2011) Observed more cracking on cement stabilized bases in Texas After identifying top-down cracks in asphalt pavements, the authors have examined the data from the Long-Term Pavement Performance (LTPP) database, the literature, and other sources, paying particular attention to obtain adequate information needed to develop various top-down cracking sub-models. Two steps are taken: 1) Assemble data elements from different pavement sections that have been identified from the following resources: a. LTPP database; b. National Center for Asphalt Technology (NCAT); and c. State Departments of Transportation (DOTs). The availability of these data elements in terms of both quality and quantity is preliminarily evaluated, as summarized in Table E-4. These data elements can be used for model development and calibration of the proposed models. 2) Categorize the collected pavement sections into each climatic and thickness category. The climatic categories refer to the climatic zones (WF, DF, WNF, and DNF). The thickness categories are defined based on the thickness and stabilization criteria mentioned above, which contain: a. Thin asphalt layer (less than 4 inches or 100 mm) on unstabilized base course;

E-4 b. Thick asphalt layer (greater than 4 inches or 200 mm) on unstabilized base course; c. Thick asphalt layer (greater than 4 inches or 200 mm) on stabilized base course. Table E-4. Pavement Data Available for Development, Calibration and Validation of Top- Down Cracking Models Data Source Data Elements Data Quality Model Type Data Quantity LTPP Observed Distresses  Crack amount for low severity  Crack amount for medium severity  Crack amount for high severity Available for crack- related models 278 pavement sections Material & Structural Properties  Material properties  Layer thickness  Laboratory testing Available for material-related model Falling Weight Deflectomete r (FWD) FWD for different aging time to backcalculate change of layer moduli with aging Available for material property model & aging- related model Climate  Hourly solar radiation  Air temperature  Wind speed  Emissivity, absorption, & thermal conductivity coefficients Available for temperature-related models Traffic  Traffic volume  Vehicle class distribution  Traffic growth & axle load distribution factors Available for traffic load-related models NCAT (West et al. 2012) Observed Distresses  Crack amount Available for crack- related models 6 test sections Material & Structural Properties  Materials  Layer thickness  Laboratory testing Available for material property model & crack- related model FWD  Backcalculated moduli Available for material property model & aging- related model

E-5 Climate  In-situ temperature  Mid-depth temperature Available for temperature-related models Traffic  Axle truck weight  ESALs Available for traffic load-related models Florida DOT (Myers 1998; 2002) Observed Distresses  Crack width & crack depth  Crack severity Available for crack- related models 44 pavement sections Material & Structural Properties  Asphalt surface layer thickness  Base type Available for material property model FWD  Backcalculated moduli Available for material property model & aging- related model Climate  Mean annual air temperature  Thermal stress Available for temperature-related models Traffic  Tire-pavement contact stress  ESALs in 20 years  Load spectrum Available for traffic load-related models Michigan DOT (Svasdisant 2002) Observed Distresses  Crack length at first crack  Crack length development vs. time Available for crack- related models 18 pavement sections Material & Structural Properties  Material properties  Layer thickness  Laboratory testing Available for material property model & crack- related model Climate  Mid-depth asphalt layertemperature Available for temperature-related models FWD  Backcalculated moduli Available for material property model & aging- related model Indiana DOT (Pellinen et al. 2004) Observed Distresses  Crack amount at low severity  Crack amount at medium severity  Crack amount at high severity Available for crack- related models 3 pavement sections

E-6 Material & Structural Properties  Materials  Layer thickness  Treatment  Laboratory testing Available for material property & crack-related models FWD  Backcalculated moduli Available for material property model & aging- related model Climate  Maximum & minimum temperature  Hourly temperature variation Available for temperature-related models Traffic  Average effective structural number  ESALs  Pavement-tire contact stress Available for traffic load-related models Washington State DOT (Uhlmeyer et al. 2000) Observed Distresses  Crack depth  Age to first crack Available for crack- related models 24 pavement sections, 143 field cores Material & Structure Properties  Asphalt layer thickness Available for material property models FWD  Backcalculated moduli Available for aging-related model Traffic  ESALs to first crack  ESALs at time of coring Available for traffic load-related models Take the LTPP data for example. The LTPP online database, “LTPP InfoPave”, is explored to collect pavement sections with top-down cracking. In each category, the collected data of each pavement section includes: 1) Amount of longitudinal wheel path cracking at low severity level, medium severity level, and high severity level, respectively; 2) Base type and base treatment; 3) Layer thickness; 4) Mixture design; 5) Construction time (aging time); 6) Weather; and 7) FWD In the search of top-down cracking data from LTPP, a thorough evaluation of the GPS-1, GPS-2, GPS-6A, GPS-6B, and SPS-1 is completed. The identified pavement sections in each LTPP category are shown in Table E-5. The total number of qualifying pavement sections is 278. Figure E-2 shows the distribution of these 278 pavement sections within the different climate zones in the United States and Canada. The number of pavement sections in each climate and thickness category are given in Table E-6. The pavement sections that have been identified are well distributed in all of the four principal climatic zones and are also well distributed in the states of the United States and in the provinces of Canada.

E-7 Table E-5. Number of Pavement Sections Identified from LTPP LTPP Category Pavement Description Number of Sections GPS-1 AC on unbound base 156 GPS-2 AC on bound base 65 GPS-6A Existing AC overlay on AC pavement 4 GPS-6B AC overlay with conventional asphalt cement on AC pavement, no milling 2 SPS-1 Strategic study of structural factors for flexible pavements, new/reconstructed AC pavements 51 Table E-6. Number of Pavement Sections with Top-Down Cracking in Each Climate Zone Pavement Structure Description Number of Pavements Sections in Each Climatic Zone Total Pavement Sections WF DF WNF DNF Thin/ unstabilized Thin asphalt layer (less than 4 inches) on unstabilized base course 10 15 22 18 65 Thick/ unstabilized Thick asphalt (greater than 4 inches) layer on unstabilized base course 47 25 26 16 114 Thick/stabilized Thick asphalt layer (greater than 4 inches) on stabilized base course 32 18 29 20 99 Total Pavement Sections 89 57 77 54 278

E-8 Figure E-2. Distribution of Identified Top-Down Cracking Data from LTPP Database

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TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 257: A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers develops a calibrated mechanistic-empirical (ME) model for predicting the load-related top-down cracking in the asphalt layer of flexible pavements. Recent studies have determined that some load-related fatigue cracks in asphalt pavement layers can be initiated at the pavement surface and propagate downward through the asphalt layer. However, this form of distress cannot entirely be explained by fatigue mechanisms used to explain cracking that initiates at the bottom of the pavement. This research explores top-down cracking to develop a calibrated, validated mechanistic-empirical model for incorporation into pavement design procedures.

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