The Mechanisms of Healing of Asphalt Pavement

1. The Mechanisms of Healing of Asphalt Pavement By DingXin Cheng For Dr. Lytton�s Micromechanics Class

2. 2 Outline Surface Free Energy Concept Fatigue and Healing Analysis Mechanical Testing Results Healing Mechanisms

3. 3 Surface Free Energy Surface energy is the energy needed to create or heal a unit surface area of material. In a liquid, the surface energy is the same as surface tension. In an asphalt-aggregate system, surface energy is primarily composed of a nonpolar component - the Lifshitz van der Waals compoment; and a polar component � the Acid-Base component. The relationships between the surface energy and its components are identified by these two equations. Where G is total surface energy, Glw is Lifshitz-van der Waals component and GAB is the acid-base component. The acid-base component can be further divided into acid, G+ and base component, G- in the second equation. We should mention that the surface energy and its components are measurable and total surface energy is important in terms of the fracture, while the components are more important in terms of healing effect.Surface energy is the energy needed to create or heal a unit surface area of material. In a liquid, the surface energy is the same as surface tension. In an asphalt-aggregate system, surface energy is primarily composed of a nonpolar component - the Lifshitz van der Waals compoment; and a polar component � the Acid-Base component. The relationships between the surface energy and its components are identified by these two equations. Where G is total surface energy, Glw is Lifshitz-van der Waals component and GAB is the acid-base component. The acid-base component can be further divided into acid, G+ and base component, G- in the second equation. We should mention that the surface energy and its components are measurable and total surface energy is important in terms of the fracture, while the components are more important in terms of healing effect.

4. Measurement of Asphalt-Wilhelmy Plate This is the Wilhelmy plate device made of the Cahn balance. The Dynamic Contact Angle or DCA software directly acquires data from the Cahn balance and can automatically calculates the advancing and receding contact angles. This is a relatively easy method to obtain the surface energies of wetting and dewetting, and the results are quite reliable. The standard deviations of contact angle measurements are normally very small.This is the Wilhelmy plate device made of the Cahn balance. The Dynamic Contact Angle or DCA software directly acquires data from the Cahn balance and can automatically calculates the advancing and receding contact angles. This is a relatively easy method to obtain the surface energies of wetting and dewetting, and the results are quite reliable. The standard deviations of contact angle measurements are normally very small.

5. Measurement of Aggregate The surface energy of aggregate is measured by the Universal Sorption Device (USD). The aggregate sample is put into the isolated sample chamber. When different solvent gas are introduced into the chamber at different vapor pressure, the mass of gas solvent adsorbed onto the aggregate surface are measured by a highly accurate Magnetic Suspension Balance system. The surface energy of the aggregate can is calculated from these measurements. The USD experiment is more intensive and takes a longer time to complete, but it can accommodate the peculiarity of the irregular shape, size, mineralogy, and rough surface texture of the aggregate. The surface energy of aggregate is measured by the Universal Sorption Device (USD). The aggregate sample is put into the isolated sample chamber. When different solvent gas are introduced into the chamber at different vapor pressure, the mass of gas solvent adsorbed onto the aggregate surface are measured by a highly accurate Magnetic Suspension Balance system. The surface energy of the aggregate can is calculated from these measurements. The USD experiment is more intensive and takes a longer time to complete, but it can accommodate the peculiarity of the irregular shape, size, mineralogy, and rough surface texture of the aggregate.

6. 6 Fatigue shift factor It is well established that in the fracture fatigue process a considerable shift factor separates laboratory fatigue data from field fatigue results. This shift factor can vary from about 3 to over 100. The shift factor used by the Asphalt Institute in MS-1 is 18.4. Work at Texas A&M over the past 20 years has shown that a considerable portion of this shift factor is due to fracture healing where microcracks, especially in cohesive regions of the mixture, at least partially, rebond and heal resulting in extended fatigue life. Evidence of this healing process has been clearly demonstrated in laboratory and field work at Texas A&M and North Carolina State by Little, Lytton, and Kim et al.It is well established that in the fracture fatigue process a considerable shift factor separates laboratory fatigue data from field fatigue results. This shift factor can vary from about 3 to over 100. The shift factor used by the Asphalt Institute in MS-1 is 18.4. Work at Texas A&M over the past 20 years has shown that a considerable portion of this shift factor is due to fracture healing where microcracks, especially in cohesive regions of the mixture, at least partially, rebond and heal resulting in extended fatigue life. Evidence of this healing process has been clearly demonstrated in laboratory and field work at Texas A&M and North Carolina State by Little, Lytton, and Kim et al.

7. 7 Fatigue and Healing Model -(1) Fatigue model without rest period Based on the Schapery�s fundamental law of viscoelastic fracture mechanics, the familiar Paris�s law model is used to assess crack growth rate during fatigue tests. In the Paris equation, c is crack length; Capital N is the number of load cycles; Jv0 is the maximum viscoelastic J integral during loading time Dt; A and small n are constants. A and n can be determined experimentally or they can be approximated from direct measurements of material properties. The exponent n can be determined by knowing the slope of the compliance-time relationship, mf . A is more complex and can be approximated by knowing creep test parameter D1 and mf as well as the reference modulus, ER; the fracture process zone length, ?; the wavefunction, W(t); and the surface energy of fracture, ?f. From the model, one can see that the higher the value of surface energy, the longer the fatigue life of asphalt pavement will be, assuming the other parameters are same. Based on the Schapery�s fundamental law of viscoelastic fracture mechanics, the familiar Paris�s law model is used to assess crack growth rate during fatigue tests. In the Paris equation, c is crack length; Capital N is the number of load cycles; Jv0 is the maximum viscoelastic J integral during loading time Dt; A and small n are constants. A and n can be determined experimentally or they can be approximated from direct measurements of material properties. The exponent n can be determined by knowing the slope of the compliance-time relationship, mf . A is more complex and can be approximated by knowing creep test parameter D1 and mf as well as the reference modulus, ER; the fracture process zone length, ?; the wavefunction, W(t); and the surface energy of fracture, ?f. From the model, one can see that the higher the value of surface energy, the longer the fatigue life of asphalt pavement will be, assuming the other parameters are same.

8. 8 Fatigue and Healing Model -(2) Fatigue model considering the healing effect A modified form of Paris� law was developed in previous research at Texas A&M on the subject of fracture fatigue and healing. As shown in the first equation, the rate of crack growth is modified by the amount of fracture healing that occurs per rest period. This rate of healing per load cycle, dh/dN, is, in turn, governed by the second equation shown in this slide. In this equation, (?t)h is the healing period; h? is a constant, and h1 and h2 are short-term and long-term healing rates, respectively, which have been proven to be influenced by nonpolar and polar surface energies, respectively. A modified form of Paris� law was developed in previous research at Texas A&M on the subject of fracture fatigue and healing. As shown in the first equation, the rate of crack growth is modified by the amount of fracture healing that occurs per rest period. This rate of healing per load cycle, dh/dN, is, in turn, governed by the second equation shown in this slide. In this equation, (?t)h is the healing period; h? is a constant, and h1 and h2 are short-term and long-term healing rates, respectively, which have been proven to be influenced by nonpolar and polar surface energies, respectively.

9. 9 Healing Curve

10. 10 Short-Term Healing Rate vs. Non-Polar Component The effect of the Lifshitz van der Waals component of surface energy is to reduce the level and rate of short-term healing, h1. This relationship is explained mathematically in a relationship derived by Lytton during previous work on this project at Texas A&M. In that relationship short-term healing is affected primarily by Lifshitz van der Waals surface energy but also by mixture compressive compliance as well as other mixture properties.The effect of the Lifshitz van der Waals component of surface energy is to reduce the level and rate of short-term healing, h1. This relationship is explained mathematically in a relationship derived by Lytton during previous work on this project at Texas A&M. In that relationship short-term healing is affected primarily by Lifshitz van der Waals surface energy but also by mixture compressive compliance as well as other mixture properties.

11. 11 Long-Term Healing Rate vs Polar Component The effect of the Acid-Base component of surface energy is to increase the level and rate of long-term healing, h2. This relationship is explained mathematically in a relationship derived by Schapery. In that relationship long-term healing is affected primarily by the Acid-Base surface energy but also by mixture compressive compliance as well as other mixture properties. The effect of the Acid-Base component of surface energy is to increase the level and rate of long-term healing, h2. This relationship is explained mathematically in a relationship derived by Schapery. In that relationship long-term healing is affected primarily by the Acid-Base surface energy but also by mixture compressive compliance as well as other mixture properties.

12. 12 Summary of Effect of Surface Free Energy on Fracture Fatigue Healing Healing potential is promoted with Reduced ?LW Increased ?AB We can see from the previous evidence that fatigue life is extended when fracture resistance is enhanced and healing potential is maximized. Fracture resistance is enhanced by maximizing totals surface energy of cohesion and adhesion, while healing potential is maximized by minimizing ?LW and maximizing ?AB .We can see from the previous evidence that fatigue life is extended when fracture resistance is enhanced and healing potential is maximized. Fracture resistance is enhanced by maximizing totals surface energy of cohesion and adhesion, while healing potential is maximized by minimizing ?LW and maximizing ?AB .

13. GLW and Healing Potential The primary application of surface energy of asphalt in this study was to assess healing potential. For this analysis, the advancing contact angle is used. The non-polar Lifshitz-van der Waals component is inversely correlated with healing meaning that a low GLW component relates to a high early healing rate, while a high GLW component relates to a low early healing rate. MAKE COLUMNS SOLIDThe primary application of surface energy of asphalt in this study was to assess healing potential. For this analysis, the advancing contact angle is used. The non-polar Lifshitz-van der Waals component is inversely correlated with healing meaning that a low GLW component relates to a high early healing rate, while a high GLW component relates to a low early healing rate. MAKE COLUMNS SOLID

14. GAB and Healing Potential Based on the healing model, asphalts with higher Acid-Base component of surface energy are associated with greater healing potential. Accordingly, AAD should have the smallest healing potential and AAM should have the greatest healing potential among the unmodified binders tested, with AAA being intermediate. This is directly compatible with healing tests performed on mixtures and repeated many times by Little et al at Texas A&M and by Kim et al. at North Carolina State. The modified HCR demonstrated a substantially greater healing potential than any of the unmodified binders which is fully compatible with the Acid-Base results shown here. MAKE COLUMNS SOLIDBased on the healing model, asphalts with higher Acid-Base component of surface energy are associated with greater healing potential. Accordingly, AAD should have the smallest healing potential and AAM should have the greatest healing potential among the unmodified binders tested, with AAA being intermediate. This is directly compatible with healing tests performed on mixtures and repeated many times by Little et al at Texas A&M and by Kim et al. at North Carolina State. The modified HCR demonstrated a substantially greater healing potential than any of the unmodified binders which is fully compatible with the Acid-Base results shown here. MAKE COLUMNS SOLID

15. 15 Testing Results and Healing Analysis- Aging Effect Aging has a big influence on the surface energy of the asphalt binders. From the advancing angles, aging increases the Lifshitz-van der Waals component while reducing the Acid-Base component of surface energy of High Cure Rubber asphalt. Thus, according to these data, the aging reduces the healing ability of asphalt binder.Aging has a big influence on the surface energy of the asphalt binders. From the advancing angles, aging increases the Lifshitz-van der Waals component while reducing the Acid-Base component of surface energy of High Cure Rubber asphalt. Thus, according to these data, the aging reduces the healing ability of asphalt binder.

16. 16 Asphalt Binder Fatigue Testing including Healing Effects This figure shows the experiment results from the DMA fatigue experiment conducted by Kim, Little et al. HPI refers to the recovery in the dynamic modulus measured during DMA testing. We have identified this as a measure of microcrack healing in this experiment. FLI refers to fatigue life increase and actually means the percent of fatigue life increase that is realized due to the healing effect. As you can see, the HCR has higher healing potential and longer fatigue life increase than AAM, and AAM has higher healing potential and longer fatigue life increase than AAD. These are consistent with the surface energy modeling results and is also consistent with mixture fatigue testing. USE SOLID COLORS FOR COLUMNSThis figure shows the experiment results from the DMA fatigue experiment conducted by Kim, Little et al. HPI refers to the recovery in the dynamic modulus measured during DMA testing. We have identified this as a measure of microcrack healing in this experiment. FLI refers to fatigue life increase and actually means the percent of fatigue life increase that is realized due to the healing effect. As you can see, the HCR has higher healing potential and longer fatigue life increase than AAM, and AAM has higher healing potential and longer fatigue life increase than AAD. These are consistent with the surface energy modeling results and is also consistent with mixture fatigue testing. USE SOLID COLORS FOR COLUMNS

17. 17 DMA Fatigue Data for AAM-1 This is typical DMA fatigue data for asphalt AAM � a good healer. Note that fatigue damage, according to my colleague Y. R. Kim occurs at the second inflection point. Note the recovery in dynamic modulus each time a rest period occurs. More importantly, note the dramatic shift in fatigue life from *** cycles to *** cycles due the the application of ten 30-second rest periods.This is typical DMA fatigue data for asphalt AAM � a good healer. Note that fatigue damage, according to my colleague Y. R. Kim occurs at the second inflection point. Note the recovery in dynamic modulus each time a rest period occurs. More importantly, note the dramatic shift in fatigue life from *** cycles to *** cycles due the the application of ten 30-second rest periods.

18. 18 DMA Data Fatigue for AAD-1

19. 19 Asphalt Mixture Healing Testing -by Si, et al. Si, et al. conducted fatigue experiment including healing effect on different combinations of aspahlt-aggregate mixtures. Extended index indicates the healing effect in terms of extended fatigue life. From their testing results, the descending order of healing within the mixtures are HCR, AAM and AAD. Once again these mixture tests are totally consistent with the premise that low Lifshitz van der Waal and high Acid-Base surface energy components promote good healing, and healing is an important component in the fatigue process. In this figure the term BG stands for Brazos gravel aggregate, LS stands for limestone aggregate. Beyond the dash, is the type of asphalt binder. SOLID COLUMNSSi, et al. conducted fatigue experiment including healing effect on different combinations of aspahlt-aggregate mixtures. Extended index indicates the healing effect in terms of extended fatigue life. From their testing results, the descending order of healing within the mixtures are HCR, AAM and AAD. Once again these mixture tests are totally consistent with the premise that low Lifshitz van der Waal and high Acid-Base surface energy components promote good healing, and healing is an important component in the fatigue process. In this figure the term BG stands for Brazos gravel aggregate, LS stands for limestone aggregate. Beyond the dash, is the type of asphalt binder. SOLID COLUMNS

20. 20 Result Comparison Table

21. 21 Mechanisms of Healing

22. 22 Acid-Base Components of Surface Energy

23. 23 Acid-Base Attracting Energy Potential Two Particles

24. 24 Attracting Energy Potential of One Mole Material

25. 25 Repulsion Due to Electron Orbital Overlap

26. 26 Net Acid-Base Attraction Energy

27. 27 Potential Energy of Acid-Base Interaction Versus Inter-particle Distances

28. 28 Lifshitz-van der Waals Components London Dispersion Force Debye Induction Force Keesom Orientation Force

29. 29 Net Lifshitz-van der Waals Potential Energy

30. 30 Potential Energy of Lifshitz-van der Waals Interaction Versus Inter-particle Distances

31. 31 Comparison of AB and LW Potential Energy Changing with Inter-particle Distances

32. 32 Healing Mechanism

33. 33