National Academies Press: OpenBook

Validating the Fatigue Endurance Limit for Hot Mix Asphalt (2010)

Chapter: Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit

« Previous: Chapter 5 - Uniaxial Tension Results and Analyses
Page 59
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 59
Page 60
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 60
Page 61
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 61
Page 62
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 62
Page 63
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 63
Page 64
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 64
Page 65
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 65
Page 66
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 66
Page 67
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 67
Page 68
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 68
Page 69
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 69
Page 70
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 70
Page 71
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 71
Page 72
Suggested Citation:"Chapter 6 - Examination of LTPP Database for Indications of an Endurance Limit." National Academies of Sciences, Engineering, and Medicine. 2010. Validating the Fatigue Endurance Limit for Hot Mix Asphalt. Washington, DC: The National Academies Press. doi: 10.17226/14360.
×
Page 72

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

59 Introduction Flexible pavements have traditionally been designed to limit load-related cracking. The more traffic, the thicker the HMA layer to limit the load-related cracks to some design limit. As noted, however, industry has been proposing the use of an endurance limit as a mixture property for HMA layers. The endurance limit is defined as the tensile strain below which no fracture or fatigue damage occurs and applies to cracks that initiate at the bottom of the HMA layer. Almost all design and analysis procedures that use the endurance limit concept assume that one value applies to all HMA mix- tures and temperatures. Values that have been used vary from 65 to 120 ms. This section of the report has three objectives: discuss the incorporation of the endurance limit design premise into mechanistic-empirical based pavement design procedures, confirm the reality and values suggested for the endurance limit, and recommend field studies to support use of this con- cept in the MEPDG software. Including the Endurance Limit Design Premise into Mechanistic-Empirical-Based Pavement Design Procedures All mechanistic-empirical pavement design procedures can be grouped into three types relative to wheel-load-induced cracking. These are as follows: 1. Design procedures that use the equivalent axle load and equivalent temperature concepts—The equivalent tem- perature is determined based on an annual or monthly basis. These procedures typically use the cumulative dam- age concept to determine the amount of fracture damage over the design period for each structure. The DAMA Pro- gram would fall within this category (62). 2. Design procedures that use the equivalent temperature concept but the axle load distribution for each axle type— These procedures also use the cumulative damage con- cept to determine the amount of fracture damage for each structure. The PerRoad Program would fall within this category (63). 3. Design procedures that calculate and use pavement tem- peratures at specific depths over some time interval, gener- ally less than a month—These procedures typically use the incremental damage concept to determine the amount of fracture damage within specific time intervals and at spe- cific depths within the pavement structure. The MEPDG would fall within this category (64). The equivalent temperature concept simply defines one temperature for which the annual or seasonal damage equals the cumulative damage determined at monthly or more fre- quent intervals. The equivalent temperature is used to estimate the dynamic modulus for calculating the tensile strain at the bottom of the HMA layer on an annual or seasonal basis. All M-E based design procedures, regardless of the group, use Miner’s hypothesis to calculate fracture damage, and assume that wheel-load-related alligator cracks initiate at the bottom of the HMA layer and propagate to the surface with continued truck loadings, with the exception of the MEPDG. In addition, all M-E based design procedures use the max- imum tensile strain at the bottom of the HMA layer as the pavement response parameter for calculating fracture damage and predicting the amount of alligator cracks. Those design procedures apply the endurance limit design premise in one of three methods, which are summarized as follows: 1. The introduction of the endurance limit design premise into those design procedures that use the equivalent tem- perature and equivalent axle load concepts is straight for- ward. Stated simply, the maximum tensile strain is cal- culated at the equivalent temperature and axle load and C H A P T E R 6 Examination of LTPP Database for Indications of an Endurance Limit

compared to the endurance limit. The HMA layer thickness is simply determined for which the maximum tensile strain equals or is less than the endurance limit. Figure 6.1 illus- trates the use of the endurance limit within this method. 2. The introduction of the endurance limit into those design procedures that use the equivalent temperature concept but use the actual axle load distribution is also fairly straight forward. The maximum tensile strain is calculated at the equivalent temperature for each axle load within the axle load distribution. The axle load distribution for each axle type is used to determine the probability of the tensile strain exceeding the endurance limit. The designer then considers that probability of exceeding that critical value in designing an HMA layer for which no fatigue damage would accumulate over time. Figure 6.2 illustrates the use of the endurance limit within this method. One concern with this method is that the higher loads result in signifi- cantly higher damage indices; an increase in axle load will result in an increase in damage to a power of about four. Thus, the probability of cracking is much higher than the probability of a specific tensile strain being exceeded. 3. Those design procedures that use the incremental dam- age concept establish a threshold value for the tensile strain, below which the fracture damage is assumed to be zero. In other words, the procedure simply ignores calculated tensile strains that are equal to or less than the value set as the endurance limit for determining the incre- mental damage within a specific time period and depth. Successive runs have been made with the MEPDG to deter- mine the difference in calculated fracture damage with and without using the endurance limit as an HMA mixture property. Figures 6.3 and 6.4 illustrate the increasing 60 0 20 40 60 80 100 120 140 160 180 7 98 10 11 12 13 14 15 16 17 18 19Te ns ile S tra in , B ot to m o f H M A, m ic ro -s tra in s HMA Thickness, inches Standard Mix; Conv. Low Modulus, Conv. High Modulus, Conv. Standard Mix, Full-Depth Endurance Limit Figure 6.1. Tensile strains calculated for an 18-kip single-axle load for the equivalent annual temperature for different HMA mixtures. Standard Mix, Conv. Low Modulus, Conv. High Modulus, Conv. Standard Mix, Full-Depth 70 75 80 85 90 95 100 105 7 98 10 11 12 13 14 15 16 17 18 19 Pr ob ab ili ty o f E xc ee di ng En du ra nc e Li m it (65 m icr o- st ra in s), % HMA Thickness, inches Figure 6.2. Probability of exceeding the endurance limit for different HMA mixtures using typical axle load distributions and seasonal temperatures.

maximum tensile strains for varying single-axle loads for different dynamic modulus values and HMA thicknesses, respectively. Version 0.9 of the MEPDG did not include the endurance limit design premise in the recalibration process of the design methodology or software. In other words, Version 0.9 assumes that any tensile strain in the HMA layer induces some fac- ture damage. Two types of load-related cracking are pre- dicted for designing flexible pavements in accordance with the MEPDG—alligator cracking and longitudinal cracking in the wheel path. Alligator cracking, the more common crack- ing distress used in design, is assumed to initiate at the bottom of the HMA layer. These cracks propagate to the surface with continued fracture damage accumulation. Longitudinal crack- ing in the wheel paths is assumed to initiate at the surface and propagate downward. The MEPDG assumes that both types of cracking are caused by load-induced tensile strains. That hypothesis, however, has yet to be confirmed. As noted above, the new MEPDG uses an incremental dam- age index. Fracture damage is computed on a grid basis with depth for each month within the analysis or design period. Temperatures are computed with the Integrated Climatic Model at specific depth intervals for each hour of the day. These temperatures are then grouped into five averages at each depth interval for each month. The fatigue cracking (alligator cracking) equation is used to calculate the amount of fracture damage for each depth interval and month. The monthly damage indices are then summed over time to predict the area of fatigue cracking at each depth interval. 61 6 16 26 36 Single Axle Load, kips Summer, 250 ksi E=450 ksi E=650 ksi Winter, 900 ksi Endurance Limit 0 20 40 60 80 100 120 140 160 Te ns ile S tra in , B ot to m o f H M A La ye r, m ic ro -s tra in s Figure 6.3. Increasing tensile strains for varying single-axle loads for different seasons or dynamic modulus within those seasons (HMA thickness equals 15 in.). 0 50 100 150 200 250 300 6 8 10 12 14 16 18 20 M ax im um T en si le S tra in , m ic ro -s tra in HMA Thickness, inches 8-kip Load 14-kip Load 18-kip Load 22-kip Load 28-kip Load 34-kip Load Endurance Limit Figure 6.4. Increasing maximum tensile strains for varying single- axle loads for different HMA thicknesses (HMA dynamic modulus equals 450 ksi; equivalent annual modulus).

All design methods that accept the endurance limit design premise assume that the endurance limit is independent of the mixture and temperature. That endurance limit is the tensile strain below which no fracture damage occurs. If an endurance limit value was an input into the MEPDG or used within other M-E based design methods, the question becomes, what value should be used as the endurance limit? The purpose of this section is to use the LTPP database to try and answer three questions related to the endurance limit design premise, as follows: 1. Do field observations of alligator cracking support the exis- tence of an endurance limit as an HMA mixture property? 2. If the field observations support the endurance limit the- ory or hypothesis, what is the tensile strain below which no more alligator cracking has been exhibited? 3. Is the endurance limit independent of mixture type and dynamic modulus? Defining the Endurance Limit— A Survivability Analysis A survivability analysis was used to try to answer the above questions using the LTPP database. The survivability analysis completed within this project is an expansion of work com- pleted using the LTPP database in the mid-1990s. This section of the report describes the use of survival curves in determin- ing the thickness or level of tensile strain at which only lim- ited cracking has occurred over long periods of time. Development and Application of Survival Curves Survival or probability of failure analyses have been used for decades in actuarial sciences. They have also been used in the pavement industry to determine the expected service life of pavement structures for use in life cycle cost analysis, and to compare the mean and standard deviation of the expected service life for different design features and site factors in eval- uating the adequacy of the design procedure (65, 66). Survival curves are uniquely useful because every point on the curve represents the probability that a given pavement section will be rehabilitated or exceed a specific level of distress. Survival analysis is a statistical method for determining the distribution of lives or “Life Expectancy,” as well as the occur- rence of a specific distress for a subset of pavements. Since not all of the pavements included in the analysis have reached the end of their service life or a specific level of distress, mean values can not be used. The age or amount of alligator crack- ing and probability of occurrence are computed considering all sections in the subset using statistical techniques. Survival curves are typically based on age but can also be based on traffic loadings or the probability of exceeding a spe- cific level of distress. The age or condition at failure must be based on a clearly defined condition. Mathematical models are best fitted to the points in the survival curves to predict the probability of survival or failure as a function of age, thick- ness, cumulative traffic, or some other pavement feature. The general form of these models for use in life cycle cost analysis is as follows (67, 68): where, Failure = Existing pavement is overlaid or reconstructed, or a specified level of distress has been exceeded; Age = Number of years since construction (new pave- ment or overlay); ESAL = Cumulative equivalent single 18-kip axle loads since construction (new pavement or overlay), millions; and a, b, c, d = Regression coefficients determined from the analysis. The probability of survival is 1 minus the probability of fail- ure. Optimization is typically used to determine the regression coefficients that best fit the survival points. A survival analysis also can be completed using a specific level of distress and pavement response value. In other words, the survival curves can be used to define the probability that a specific area of alligator cracking will be less than some specified amount for different HMA thicknesses or tensile strains at the bottom of the HMA layer. It is important to note that survival curves for pavements are necessarily based on previously built designs, materials, construction, and maintenance. The data used to develop the survival rates or probability of failure represent typical con- struction, materials, mixture designs, and thicknesses that have been built by agencies within the past time period rep- resented by the data. These can be defined as “benchmark” survival curves. The reliability of a pavement depends on the length of time it has been in service and design features and site factors that are not properly accounted for in a thickness design proce- dure. Thus, the distribution of the time to failure of a pave- ment type or thickness level is of fundamental importance in reliability studies. A method used to characterize this distribu- tion is the failure rate, or rate of occurrence, for a specific level of distress. The failure rate can be defined as follows. Probability of Failure = + + −( ) a e d b ESAL c1 26 ( ) Probability of Failure = + + −( ) a e d b Age c1 25 ( ) 62

If f(t) is the probability density of the time to failure of a given pavement type and thickness, that is, the probability that the pavement will fail between times t and t + Δt is given by f(t)* Δt, then the probability that the pavement will fail on the interval from 0 to t is given by The reliability function, expressing the probability that it survives to time t, is given by Thus, the probability that the pavement will fail in the inter- val from t to t + Δt is F(t + Δt) − F(t), and the conditional probability of failure in this interval, given that the pavement survived to time t, is expressed by Dividing by Δt, one can obtain the average rate of failure in the interval from t to t + Δt, given that the pavement survived to time t by For small Δt, one can get the failure rate, which is The failure rate is expressed in terms of the distribution of failure times. A typical failure rate curve is composed of three parts or can be grouped into three areas, as shown in Figure 6.5 and defined as follows: 1. The first part is characterized by a decreasing failure rate with time and is representative of the time period during which early failure or premature failures occur. This area or time typically represents pavements that were inadequately designed or built, using inferior materials. 2. The second part is characterized by a constant failure rate. A constant failure rate represents the time period when chance failures occur, or the failure occurs at random with pavement age. In some survival methods, this area is referred to as the useful life of a pavement. 3. The third part is characterized by an increasing failure rate with time. This area or time represents the reverse of the Z t f t R t f t F t ( ) = ( )( ) = ( ) − ( )1 31( ) F t t F t t R t +( )− ( ) ( ) ⎡ ⎣⎢ ⎤ ⎦⎥ Δ Δ 1 30( ) F t t F t R t +( )− ( ) ( ) Δ ( )29 R t F t( ) = − ( )1 28( ) F t f t dx t ( ) = ( )∫ 0 27( ) first part, and when failure is a result of multi-distresses as related to a combination of parameters over time (for example, exponential growth increases in traffic, past the design period from which thickness was determined). The failure rate can be determined by organizing the per- formance data in terms of the distribution of pavement age exceeding a critical level (failure) versus the distribution of age for those pavements exhibiting a value lower than the critical value. Figure 6.6 shows a typical probability of failure relationship from actual data included in the LTPP database for roughness measured on flexible pavements in the general pavement studies (GPS-1 and GPS-2) and special pavement studies (SPS-1) experiments. GPS-1 sections consist of HMA on granular base. GPS-2 sections consist of HMA on bitumi- nous, hydraulic cement, lime, fly ash, or other pozzolan bound or stabilized base. SPS-1 sections are part of a strategic study of structural factors for flexible pavements. Given the above definition of each part of the probabil- ity of failure relationship with time, the failure rate can be defined as Assuming that the failure rate is constant within the second part, and replacing Z(t) with α, the distribution of failure times is an exponential distribution as shown below. Many survival curves or, conversely, the probability of fail- ure, are based on the above relationships and assumptions. Unfortunately, the failure rate within the second part is not usually constant, and the failure rates for the first and third parts are not inversely proportional to one another. For these f t e t( ) = [ ]−α α ( )33 f t Z t e Z t dx t ( ) = ( ) ⎡ ⎣⎢ ⎤ ⎦⎥ − ( )∫ 0 32( ) 63 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0 10 20 30 Age of Pavement, years Fa ilu re R at e Premature Failure Part Chance Failure Part Wear Out Failure Part Figure 6.5. Typical failure rate relationship for pavement structures.

cases, which are typical for pavements, the failure rate can be estimated by the following relationship: Thus, This density function is termed the Weibull distribution, and is typically used in failure analyses. LTPP Database to Establish the Initial Survival Curve A survivability analysis was completed by Von Quintus et al. for the Asphalt Pavement Alliance to determine the expected age for an amount of fatigue cracking that would f t t e t( ) = ( ) ⎡⎣ ⎤⎦− −αβ β α β1 35( ) Z t t( ) = ( ) −αβ β 1 34( ) result in rehabilitation of the roadway (69). The test sections used in the survival analysis were from the GPS-1 and GPS-2 experiments. Figure 6.7 shows the distribution of pavement age for the GPS-1 and GPS-2 test sections (LTPP database version 13.1/NT3.1 released in January 2002), and Figure 6.8 shows the number of test sections with different areas of alli- gator (fatigue) cracking. As shown in Figure 6.8, many of the LTPP test sections have no alligator cracking. Figures 6.9 and 6.10 show the survival curves from the LTPP data for different levels of alligator cracking that would cause some type of rehabilitation activities. As shown, the average life (50% probability) to crack initiation and a low cracking amount (less than 10% of wheel-path area) is 19 and 23 years, respectively. A similar survivability analysis was completed by Von Quintus in 1995 for a subset of the test sections included in the GPS-1 and GPS-2 experiments. The test sections were randomly selected from the LTPP program for the thicker 64 0 20 40 60 80 100 120 0 10 20 30 40 Age, years Pr ob ab ili ty o f E xc ee di ng R ou gh ne ss L ev el , % Low Roughness, >1.5 m/km Moderate Roughness, >2.0 m/km Excessive Roughness, >2.5 m/km Figure 6.6. Illustration of results from a survivability analysis or probability of exceeding a critical roughness magnitude. 0 10 20 30 40 50 No. of Test Sections Age, years = 0 27 38 17 29 35 46 39 23 25 27 16 16 17 7 1 1 3 9 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 Figure 6.7. Frequency histogram of pavement age at the time of the distress survey for the LTPP GPS test sections included in the Asphalt Pavement Alliance study (69).

65 0 20 40 60 80 100 120 140 160 180 No. of Test Sections Area of Fatigue Cracking, sq. m. = 177 65 45 27 16 15 14 9 1 0 0-10 13-50 50-100 100- 150 150- 200 200- 300 300- 400 400- 500 Figure 6.8. Histogram of the number of test sections with different levels of alligator cracking (69). 0 20 40 60 80 100 120 0 10 20 30 40 Age, years Pr ob ab ili ty o f C ra ck in g Le ve l, % Crack Initiations Low Cracking, >10% Moderate Cracking, >20% Excessive Cracking, >50% Figure 6.9. Graphical illustration of the probability of failure or exceeding a specified area of alligator cracking (69). 0 10 20 30 40 50 60 0 10 20 30 40 Age, years A re a of F at ig ue C ra ck in g, % 50% Probability of Occurrence 25% Probability of Occurrence 75% Probability of Occurrence Figure 6.10. Probability of occurrence for alligator cracking (69).

HMA layers. This survivability analysis was completed to try to estimate a value for the endurance limit based on alligator cracking observations within the LTPP program, rather than just using values estimated from limited laboratory testing programs. This survival analysis was a desk-top study that has yet to be formally documented. The LTPP data were used to determine the probability of occurrence of different amounts of alligator cracking for different HMA thicknesses and ten- sile strains. The EVERSTRESS Program was used to calculate the maxi- mum tensile strain at the bottom of the HMA layer for each test section using the equivalent annual temperature and equivalent (18-kip) single-axle load concepts. The HMA modulus value used in the calculation of tensile strain was determined using the Witczak equation (70) based on volumetric data and physical properties of the HMA for the equivalent annual temperature. The modulus values for the other pavement and soil layers were based on resilient modulus testing per- formed in the laboratory. Figure 6.11 shows the survival curve from that limited study. A magnitude of 2% cracking was used in this initial survival analysis because of the mea- surement error in alligator cracking with time. A small mea- surement error could result in significant changes to this definition of the endurance limit. In summary, the endurance limit was determined to be 65 ms at a 95% confidence level for an 18-kip single-axle load applied to the pavement at the equivalent annual temperature for each LTPP site included in the analysis. Preliminary Definition of the Endurance Limit as an HMA Mixture Property The AAMAS project sponsored by NCHRP recommended use of the indirect tensile strength and modulus tests to esti- mate the fatigue strength/life of specific HMA mixtures (59). Figure 6.12 illustrates that relationship between HMA mod- ulus and tensile strain at failure, where both properties are determined at 77°F (25°C). Points below the line in Figure 6.12 are assumed to have inferior fatigue properties, and those above the line exceed the fatigue strength/life of the standard mixture. Laboratory tests and field observations of alliga- tor cracking have been used to check the validity of this rela- tionship over time. More alligator cracking has been observed where the tensile strain at failure is less than that value from Figure 6.12 for a specific HMA modulus value based on the equivalent temperature concept. Von Quintus used this relationship to estimate or define the endurance limit for different HMA mixtures as 1% of the tensile strain at failure measured in accordance with the test protocol from the AAMAS study (59). That definition has yet to be confirmed and validated. Updated Survivability Analysis Using LTPP Data The survivability analysis completed for this project included the same test sections from the 1995 study, plus additional test sections within the GPS-1, GPS-2, and SPS-1 experiments. A subset of the LTPP test sections was used, which was ran- domly selected to cover all environmental regions, soil types, and HMA thicknesses. The additional test sections used in the updated survivability analysis were from the GPS-1, GPS-2, and SPS-1 experiments—LTPP database version VR 2004.06, release 18.0 (2004). Figure 6.13 shows the distribution of HMA thickness for all test sections included in the updated survivability analysis. Figure 6.14 shows the distribution of pavement age for the test sections with more than 10 in. of HMA that were used to update the survival curve (Figure 6.11). As shown, the age of 45% of the thicker test sections included in the updated study is greater than 15 years. Many of these additional test sec- tions were from the SPS-1 experiment that was excluded from 66 50.00 55.00 60.00 65.00 70.00 75.00 80.00 85.00 90.00 95.00 100.00 40 60 80 100 120 140 160 180 200 220 240 260 280 300 Su rv iv al , % Tensile Strain, micro-strains Fatigue Cracking <2% Figure 6.11. Survival curve for flexible pavements developed from data included in the LTPP GPS-1 and GPS-2 experiments (undocumented study, 1995).

67 10 100 1 10 100 1000 10000 Total Resilient Modulus, ksi Te ns ile S tra in a t F ai lu re , m ils /in . Note: ms and mils/in. are the same unit. Figure 6.12. Relationship between modulus and tensile strain at failure to estimate the fatigue strength of HMA Mixtures at 77F (25C) (59). < 2 2.1-4 4.1-6 6.1-8 8.1-10 10.1-12 12.1-15 15.1-20 > 20 Number 94 65 88 101 104 92 71 88 25 0 10 20 30 40 50 60 70 80 90 100 110 N um be r o f s ec tio ns in e ac h th ic kn es s la ye r Thickness Layer of HMA Layers Figure 6.13. Distribution of HMA thickness for the test sections used in the updated survivability analysis. 0 10 20 30 40 50 60 70 <11 11 12 13 14 15 16 17 18 19 20+ N um be r o f T es t S ec tio ns Age, years HMA Thickness > 10 inches Figure 6.14. Distribution of age for those test sections with HMA layer thicknesses in excess of 10 in.

the initial study to estimate the endurance limit. The reason that the SPS-1 test sections were excluded from the study in 1995 is that most of the projects within the SPS-1 experiment were relatively new at that time. The other important parameter in the survivability analy- sis is the truck traffic applied to each of these test sections. Without significant truck traffic, defining the endurance limit from field observations has limited meaning. Figure 6.15 shows the distribution of the cumulative number of 18-kip ESALs for the test sections included in the updated surviv- ability study that have HMA thickness in excess of 10 in. The cumulative truck traffic for these thicker test sections is con- sidered moderate traffic with most test sections having less than 15 million cumulative 18-kip ESALs. In summary, the test sections with the thicker HMA lay- ers are not new pavements (Figure 6.14), but do have truck traffic levels that are lower than what would be considered heavy truck traffic (Figure 6.15). This level of truck traffic is a concern to the definition established for the endurance limit. Much higher levels of truck traffic are needed to vali- date the endurance limit design premise with field observa- tions and data. HMA Thickness-Based Definition The asphalt industry has proposed some maximum HMA thicknesses that are believed to be resistant to alligator crack- ing. The LTPP database was used to determine the level of HMA thickness at which none or little alligator cracking has been observed on HMA pavement surfaces. Figure 6.16 com- pares the amount of fatigue cracking (percent of wheel-path area) from the most recent distress survey and HMA thick- ness. As shown and expected, the test sections with thinner HMA layers generally have more fatigue cracking. However, there are an appreciable number of test sections with thicker HMA layers (15 in. or more) that have levels of fatigue crack- ing exceeding 5%. Figure 6.17 compares the maximum tensile strain calcu- lated for each section and HMA thickness. The modulus of the HMA layer was determined using the equivalent temper- ature concept for an 18-kip ESAL, as described previously for the original survivability analysis. As shown and expected, the tensile strains decrease with increasing HMA layer thickness. Figure 6.18 compares the maximum tensile strain at the bottom of the HMA layer and the amount of fatigue cracking observed on the LTPP test sections from the most recent distress survey included in the LTPP database. As shown and expected, the test sections with the lower tensile strains have less fatigue or alligator cracking. Maximum Tensile-Strain-Based Definition An updated survival curve to the one presented in Fig- ure 6.11 was developed for the additional alligator cracking data and LTPP test sections. Figure 6.19 shows the results from the survival analysis for a range of fatigue cracking levels. The results from the updated survival analysis are signifi- cantly different from the 1995 desk-top study. In fact, the updated survival curve for the 1% and 2% alligator crack- ing levels would indicate that there is no endurance limit for these sections. The relationships shown in Figure 6.19 for the 1% and 2% cracking levels have a peak survival rate sig- nificantly less than 100% and then begin to decrease with lower tensile strain values. Some of the GPS test sections that were without alligator cracks in 1995 now have some alligator cracks recorded in the LTPP database for the test sections with the thickest HMA layers. Possible reasons for the significant difference in results 68 0 20 40 60 80 100 120 <5 5-10.0 10-15.0 15-20 20-25 25-30 >30 N um be r o f T es t S ec tio ns Range of Cumulative Axles, millions of ESALs HMA Thickness > 10 inches Figure 6.15. Distribution of cumulative equivalent single-axle loads for the test sections used in the survivability analysis with HMA layer thicknesses in excess of 10 in.

69 10 100 1000 0 5 10 15 20 25 30 Te ns ile S tra in , M ic ro -S tra in HMA Thickness, inches Series1 Power (Series1) Figure 6.17. Comparison of the maximum tensile strain at the bottom of the HMA layer and HMA thickness. 0 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 A re a Fa tig ue C ra ck in g, % HMA Thickness, inches Series1 Log. (Series1) Figure 6.16. Comparison of area fatigue cracking (area alligator cracking based on a percent of wheel-path area) and HMA layer thickness. 0 10 20 30 40 50 60 70 80 90 100 10 100 1000 10000 Fa tig ue C ra ck in g, % Tensile Strain, micro-strains Fatigue Cracking Log. (Fatigue Cracking) Figure 6.18. Comparison of the area fatigue cracking and maximum tensile strain computed at the bottom of the HMA layer.

or the survival curve from the one developed in 1995 are sum- marized as follows: • The SPS-1 projects were added to the updated analysis. It is expected that including the SPS-1 projects did not cause this difference in findings, unless the fatigue cracking ini- tiated from some other design-site feature that would have a higher probable occurrence within the SPS-1 test sec- tions, as compared to the GPS sections. In addition, the study completed for the Asphalt Pavement Alliance con- cluded that there was a possibility that the GPS test sections selected by the individual agencies for the LTPP program are biased towards the better performing pavements. The SPS-1 projects were built during the LTPP program and would not be biased toward better performing pavements. It is expected that this is not the reason for the difference in survival curves. • There was a change in the LTPP definition of longitudinal cracking in the wheel path. The change in definition defi- nitely could have affected the updated survival curve. Many of the previously measured longitudinal cracks that are as- sumed to have initiated at the surface are now recorded as alligator cracking and are assumed to have initiated at the bottom of the HMA layer. The cracking maps and video dis- tress data logs can be reviewed to segregate longitudinal cracks with crack deterioration along the edges from tradi- tional alligator cracks. This evaluation process is time con- suming. Figure 6.20 graphically presents the change in per- centage of survival sections as a function for varying alligator cracking levels for different tensile strains at the bottom of 70 50.00 55.00 60.00 65.00 70.00 75.00 80.00 85.00 90.00 95.00 100.00 0 50 100 150 200 250 300 350 400 Su rv iv al , % Tensile Strain, micro-strains Fatigue <1% Fatigue <2% Fatigue <4% Fatigue <8% Figure 6.19. Survival curves based on the maximum tensile strain at the bottom of the HMA layers of flexible pavements included in the LTPP program. 50 55 60 65 70 75 80 85 90 95 100 0 1 2 3 4 5 6 7 8 9 Su rv iv al , % Alligator Cracking, % of Wheel Path Tensile Strain=50 mils/in. Tensile Strain=75 mils/in. Tensile Strain=100 mils/in. Tensile Strain=150 mils/in. Figure 6.20. Percent survival of test sections for different levels of alligator cracking and tensile strain at the bottom of the HMA layer.

the HMA layer. As shown in Figure 6.20, the 150 ms (mils/ in.) curve deviates from the other relationship. Errors in measuring small amounts of alligator cracking as well as a change in the definition for alligator cracking could have caused this anomaly. This indicates that other types of crack- ing may be included as fatigue cracks for pavements with strain levels of 100 ms or less at the bottom of the asphalt layer calculated using an equivalent annual temperature and 18-kip axle load. To determine the cause of the anom- aly requires that forensic investigations be completed on these much thicker HMA sections to determine the cause of the recorded alligator cracking. • The location where alligator cracks recorded in the LTPP database initiated are assumed. As noted above, alligator cracks are assumed to initiate at the bottom of the HMA layer and propagate to the surface. The validity of this assumption would have an effect on the survival curve. In addition, the maximum tensile strain at the bottom of the HMA layer was based on the assumption of full-bond between all HMA lifts. If partial bond exists between two lifts near the surface, load-related cracks can initiate at that location and propagate downward as well as upward. A full forensic investigation will be needed to determine the location of where these cracks, recorded in the LTPP data, initiated and the mechanism (debonding between adjacent HMA lifts) that resulted in those cracks. • The initial and updated survivability analysis was performed assuming that stripping or moisture damage is not present within the HMA layer. Stripping and moisture damage were adequately identified during the initial sampling and coring program for the GPS test sections. For the SPS-1 projects, stripping or moisture damage may have occurred on some of the projects and resulted in premature alligator cracking for the thicker sections. This possible cause for the difference in findings can be resolved with forensic investigations. • An additional reason or explanation for the difference in results is that there is no endurance limit for HMA mixtures. In summary, it is still believed that the endurance limit is an HMA mixture property. Based on the results from the updated survival analysis, however, forensic investigations of the test sections with the thicker HMA layers are needed to confirm the location of crack initiation and other assump- tions used noted above in the survivability analysis. Affect of Polymer Modification on Field Performance Von Quintus et al. (71) conducted a study to quantify the effects of polymer modification on pavement performance. Sites were selected from the LTPP database, NCAT Test Track, FHWA Accelerated Loading Facility (ALF), and a number of Canadian provinces and U.S. states with good records of performance and material properties. For each polymer mod- ified section, a control mix or two to three unmodified sites were selected for comparison. The unmodified sections were termed companion sites. The performance of the polymer modified asphalt (PMA) test sections and their companion sections was compared using a normalization technique that is based on computing damage indices for each test section. This normalization tech- nique uses M-E models to reduce the effect from confounding factors between projects. The M-E performance prediction models were calibrated to local conditions using performance data from companion sections (without any additive or mod- ifier in the HMA mixture). This local calibration procedure was used to estimate the true effect of PMA because of the vari- ation and errors associated with the models selected for use. Figure 6.21 presents a comparison of the fatigue cracking predicted using the locally calibrated fatigue cracking equation 71 0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 Predicted Fatigue Cracking, % M ea su re d Fa tig ue C ra ck in g, % Companion Sites PMA Sites Line of Equality Figure 6.21. Comparison between the predicted and measured fatigue cracking for neat HMA and PMA mixtures (71).

and that actually measured for the sites. The data for the unmodified sections fall along the line of equality, whereas the data for the polymer modified section indicate that the actual cracking is less than the predicted cracking. Fig- ure 6.22 shows the damage index (DI) or ratio applied loads to allowable load before failure occurs versus measured fatigue cracking. Figure 6.22 also indicates that pavements constructed with PMA can withstand a larger percentage of their maximum load repetitions for a given level of cracking. Both Figure 6.21 and 6.22 support the findings in Chapter 4 that indicate the polymer modified PG 76-22 should have a higher endurance limit. This also indicates that the endurance limit is mixture specific. 72 Figure 6.22. Comparison between the fracture damage index and measured fatigue cracking for neat HMA mixtures and PMA mixtures (71). 0.01 0.1 1 10 Locally Calibrated Fracture Damage Index Trend Line for Companion Sites Companion Sites PMA Sites 0 10 20 30 40 50 60 70 80 90 100 M ea su re d Fa tig ue C ra ck in g, %

Next: Chapter 7 - Sensitivity of Pavement Thickness to the Endurance Limit »
Validating the Fatigue Endurance Limit for Hot Mix Asphalt Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Report 646: Validating the Fatigue Endurance Limit for Hot Mix Asphalt explores the existence of a fatigue endurance limit for hot mix asphalt (HMA) mixtures, the effect of HMA mixture characteristics on the endurance limit, and the potential for the limit’s incorporation in structural design methods for flexible pavements.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!