EVALUATION OF LOW-TEMPERATURE FRACTURE PROPERTIES OF ASPHALT BINDER

ABSTRACT

 

 

To rationally predict low temperature cracking in flexible pavements, the ability of asphalt binder to resist crack initiation and propagation must be measured. Different theoretical approaches to this problem exist. The current Superpave low-temperature binder grading system is based on the measurement of stiffness and strength. This method was developed during the SHRP program and allowed significant progress in the prediction of low-temperature binder performance. However, further improvements are required, especially to better describe the low-temperature behavior of modified asphalt binders.

The use of fracture mechanics principles is a potential alternative to the current Superpave approach. Attempts have been made to apply this method to characterize the fracture properties of asphalt binders at low temperature. Researchers always assumed binders to be linear elastic. The results obtained from these studies were encouraging and concluded that fracture mechanics was a useful tool to describe binders at low temperature and leads to a different ranking of binder performance as compared to the Superpave grading system.

This study was to verify and determine the limits of the applicability of linear elastic fracture mechanics (LEFM) to asphalt binder. Alternative, fracture mechanics-based approaches were considered to account for non-linear behavior. These methods imply the laboratory determination of critical fracture parameters: linear stress intensity or fracture toughness K_IC, plastic energy rate J_IC, and time-dependent energy rate J_VC. The measurement of K_IC was found to be straightforward using existing laboratory tools. To obtain J_IC and J_VC values, more tests and data analysis are required.

Loading curves obtained from fracture testing of nine binders were analyzed using the different fracture mechanics approaches. Superpave critical temperatures were also determined for all binders. Binder characterization was completed with the dynamic thermomechanical determination of their glass transition temperature, T_g.

 

Parameters such as temperature, loading rates and binder nature were found to have a crucial influence fracture response. The validity of LEFM was proved to be limited to very low temperatures, typically around and below T_g. K_IC seems to reach a “glassy” plateau as temperature decreases. At higher temperatures, or when loading rate is lowered, more elaborate tools are required to account for binder plastic and viscoelastic behavior.

The comparison between rankings of binder performance from Superpave and fracture-based criteria confirmed the conclusions of previous studies. The application of fracture mechanics principle leads to very different binder rankings. The Superpave critical cracking temperature was found to be within the temperature range of validity of LEFM. Consequently, K_IC and another parameter based on linear fracture mechanics should be included in the low-temperature binder gradation system as a complement to the existing criteria. Plastic and viscoelastic fracture mechanics give valuable information on the behavior of binders at intermediate temperatures, typically between –20 and 5°C. Therefore, these approaches are potentially interesting to study crack initiation and propagation associated with mechanical and thermal fatigue.

TABLE OF CONTENTS

List of Figures………………………………………………………………………………………………….. vii

List of Tables……………………………………………………………………………………………………. ix

Acknowledgements…………………………………………………………………………………………….. x

Chapter 1 Introduction…………………………………………………………………………………………. 1

Background…………………………………………………………………………………………………. 1

Problem statement………………………………………………………………………………………… 3

Objective……………………………………………………………………………………………………. 4

Scope…………………………………………………………………………………………………………. 4

Methodology……………………………………………………………………………………………….. 5

Chapter 2 Literature Review………………………………………………………………………………….. 6

Introduction…………………………………………………………………………………………………. 6

Low temperature PG specifications…………………………………………………………………… 7

Glass transition temperature: Definitions and significance……………………………………… 9

Linear Elastic Fracture Mechanics (LEFM)………………………………………………………… 9

Griffith energy balance………………………………………………………………………….. 10

Stress analysis approach………………………………………………………………………… 13

Elastic-Plastic Fracture Mechanics………………………………………………………………….. 19

Crack Tip Plasticity………………………………………………………………………………. 19

Crack Tip Opening Displacement…………………………………………………………….. 20

J-Contour Integral………………………………………………………………………………… 21

Time-dependent Fracture Mechanics……………………………………………………………….. 23

Pseudo-strain Analysis…………………………………………………………………………… 23

Viscoelastic J-integral formulation…………………………………………………………… 24

Application of fracture mechanics to asphalt binder and mixes……………………………… 26

Characterization of asphalt mixes…………………………………………………………….. 27

Characterization of asphalt binders…………………………………………………………… 27

Chapter 3 Laboratory Testing………………………………………………………………………………. 29

Materials…………………………………………………………………………………………………… 29

Binder laboratory aging and storage………………………………………………………………… 29

Specimen preparation…………………………………………………………………………………… 30

Instrumentation………………………………………………………………………………………….. 31

Test procedures………………………………………………………………………………………….. 32

PG grading of asphalt binders………………………………………………………………….. 32

T_g measurements………………………………………………………………………………….. 32

K_IC and J_IC measurements……………………………………………………………………….. 33

Measurement of relaxation moduli……………………………………………………………. 34

Test results………………………………………………………………………………………………… 35

PG Gradation………………………………………………………………………………………. 35

Fracture testing……………………………………………………………………………………. 36

T_g measurement…………………………………………………………………………………… 41

Chapter 4 Data analysis………………………………………………………………………………………. 42

Introduction……………………………………………………………………………………………….. 42

Analysis using Superpave PG Grading…………………………………………………………….. 42

Derivation of low-temperature criteria………………………………………………………. 42

Comparison with values from Ergon…………………………………………………………. 44

First correlations………………………………………………………………………………….. 44

Discriminating power……………………………………………………………………………. 47

Investigation of Linear Fracture parameters………………………………………………………. 48

Statistical investigation of the influence of notch size and temperature……………… 49

Description of the influence of temperature and T_g………………………………………. 52

Validity of LEFM approach – Influence of loading rate…………………………………. 54

Alternative analyses…………………………………………………………………………………….. 58

EPFM………………………………………………………………………………………………… 58

Relation between K_IC and J_IC values………………………………………………………….. 63

TDFM……………………………………………………………………………………………….. 65

Influence of temperature………………………………………………………………………… 68

Fracture-based specification criteria………………………………………………………………… 71

Definition…………………………………………………………………………………………… 71

Comparison of binder rankings according to the different criteria……………………. 73

Conclusions regarding relative rankings…………………………………………………….. 73

Chapter 5 Findings and Conclusions……………………………………………………………………… 75

Findings……………………………………………………………………………………………………. 75

Chapter 6 Recommendations for Further Work………………………………………………………… 80

References………………………………………………………………………………………………………. 81

Appendix A Dimensionless Function for Different Test Configurations………………………… 85

Appendix B: Torsion Bar Test Specimen Preparation and Testing………………………………… 86

Appendix C: Pseudo-strain Calculations…………………………………………………………………. 89

Appendix D: Notched Specimen Mold Photographs and Schematics…………………………….. 93

Appendix E: Pennsylvania Transportation Institute and ERGON Test Results………………… 98

Appendix F: Critical temperatures derived from analyses…………………………………………. 100

Appendix G: Measured K_IC Parameters………………………………………………………………… 101

Appendix H: Measured Parameters K_IC, J_IC, and J_VC compared………………………………….. 102

Appendix I: Fracture-based low-temperature grading temperatures…………………………….. 103

Chapter 1  Introduction

Background

Many types of distresses can occur in a hot mix asphalt pavement, depending on various parameters, such as weather, traffic conditions and construction quality. Temperature plays a major role with respect to the pavement behavior. Rutting typically occurs at high service temperature. Fatigue appears in the form of alligator or map cracking around ambient, intermediate temperature. Fatigue can be defined as the repetition of cycles of loading causing a progressive failure of the pavement below its static mechanical strength. Thermal cracking is characteristic of low temperatures, around or below the materials glass transition temperature (T_g). Temperature excursions cause stresses to build up and exceed the strength, resulting in the appearance of regular, transverse cracks along the pavement.

From 1988 to 1995, the $150 million Strategic Highway Research Program (SHRP) resulted in the definition of the new, performance-based SUPERPAVE specifications [1] . They provide a range of temperature in which each binder is considered perform satisfactorily. New test methods and tools were designed, such as the Dynamic Shear Rheometer (DSR), the Bending Beam Rheometer (BBR) and the Direct Tension Test (DTT) for asphalt binders. The new Performance Grade (PG) system reportedly gives satisfactory results for neat binders. But with the increase in use of specialty and polymer-modified binders the limits of this new set of tools are reached. For these applications, insufficient correlation is observed between laboratory test results and field behavior.

Both fatigue and low temperature cracking are accounted in SUPERPAVE specifications. The fatigue criterion is defined as G*sin(δ) obtained from a dynamic test in the DSR. The validity of this parameter is currently under review and has been found to be insufficient to predict fatigue behavior of modified binders [2] . For low temperatures, the newer version of the specifications includes a critical cracking temperature computed from BBR and DT tests results. Therefore, this criterion accounts for the rheological and strength behavior of the binder. It has been proved to give reasonable predictions, but needs to be improved in order to better differentiate between modified binders.

Another approach can be used to describe failure of asphalt binders at intermediate and low temperature. Both fatigue and thermal cracking involve the initiation and the propagation of cracks in the material leading to its mechanical failure. Therefore we can use the principles of fracture mechanics to describe this phenomenon. It provides intrinsic material properties from which specification criteria could be derived. For linear elastic materials, the Linear Elastic Fracture Mechanics (LEFM) can be used [4] . The stress intensity K_I can be measured and describes the state of stress in the material around a crack. Its limiting value at failure, K_IC or fracture toughness is a material property. It provides valuable information on the failure behavior and therefore is a good candidate as a low temperature binder-grading criterion.

It is well recognized that asphalt binders are very temperature dependent materials.

Their behavior can vary from fluid (modulus < 1kPa) above 80°C to glassy solid (modulus > 1GPa) below –30°C. A change of few degrees in temperature induces dramatic changes in behavior. As a consequence, even at relatively low temperature it is possible that asphalt binders exhibit significant viscoelasticity or even non-linearity. In that case, LEFM analysis would be irrelevant and alternative fracture mechanics models would have to be considered. These approaches account for the presence of a significant amount of plastic deformation around the crack tip. They form a branch of fracture mechanics know as Elastic Plastic

Fracture Mechanics (EPFM) [4] . The crack tip opening theory was first introduced by Wells in 1961 [5] . It assumes the presence of a plastic zone or yield strip at the crack tip. The crack tip opening displacement (CTOD) is the transversal displacement at the edge of the plastic zone. It can be proposed as a fracture criterion. The J integral approach was presented by Rice in 1968 [6] . The J integral is defined as a path independent contour integral representing a non-linear elastic energy release rate. Under certain restrictions, it was shown to be equivalent to an elastic-plastic energy release rate. Crack extension occurs when the J integral crosses a critical value J_IC, which was shown to be a material property. Similar to K_IC for linear materials, J_IC could also be used as a low temperature failure criterion.

Fracture analysis can also take into consideration the time-dependency of bituminous materials. This development in fracture mechanics is relatively new. Shapery was the first to introduce the viscoelastic concept in fracture in 1975 [7] and introduced a complete theoretical framework leading to the derivation of a new set of failure parameters, such as the generalized J integral: Jv.

Problem statement

Applicability of fracture mechanics principles to testing and gradation of asphalt binders at low temperatures needs to be verified. Although studies show that fracture toughness analysis of asphalt binders gives promising results, available documentation remains limited. Furthermore it is still to be determined whether LEFM is appropriate for describing fracture behavior of asphalt binders, or if other, more elaborate models are needed (elastic-plastic models, viscoelastic model).

There is currently a need for a better fatigue criterion, and the low temperature grading systems needs to be improved for modified binders. Fracture mechanics-based analyses provide a set of material intrinsic parameters that could efficiently serve as specification criterion. This possibility needs to be investigated. The performance of the fracture mechanics approach is then to be compared to the existing SUPERPAVE specification system.

 

Objective

The objective of this study was to determine whether various fracture mechanics approaches could be applied to characterize failure of asphalt binders at low temperature. If applicable, these analyses were compared to the current Superpave recommendations for specifications to determine whether fracture mechanics-based criteria could be incorporated in the binder PG grading system.

Scope

An experimental determination of various fracture parameters was carried out for a panel of eleven different asphalt cements. These binders include two core asphalts and nine polymer-modified binders supplied by Ergon Asphalt and Emulsions. All binders were aged prior testing to simulate aging occurring during the plant mixing and the pavement service life. The binder-specific glass transition temperature, T_g is believed to have a major influence on the rheological and failure behavior of binders. Therefore T_g was determined for all cements using a dynamic analysis method.

K_IC was determined at three temperatures relatively to each binder-specific T_g value. Two fracture test methods were used and compared: double-notched traction and notched 3point bending. The influence of the crack length on K_IC was investigated, as 3 different notch sizes were used. The validity of the LEFM approach was checked using various fracture mechanics criteria. Three different strain rates were used to evaluate the time dependency of each binder. Elastic plastic and viscoelastic analysis of the fracture test data was also considered.

The BBR and DT standard tests were also run at the same temperature as the K_IC measurements and the SUPERPAVE critical cracking temperature was derived for each binder using the TSAR software. Thus, the results of the fracture mechanics analysis could then be compared directly to the current low temperature PG gradation.

 

Methodology

The following sequence of work was used to conduct the research reported in this thesis:

1. Literature review.
2. Design and construction of laboratory equipment.
3. Preparation and testing of laboratory specimen.
4. Data acquisition and analysis.

Each of these steps is reported in detail in the following chapters.

EVALUATION OF LOW-TEMPERATURE FRACTURE PROPERTIES OF ASPHALT BINDER

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