Behavior of Asphalt Binder and Asphalt Concrete

1. 1 Behavior of Asphalt Binder and Asphalt Concrete

2. 2 Mixture Classification type of binder asphalt cement liquid asphalt aggregate gradation dense-graded (well-graded) open-graded production method hot-mix (hot-laid)** cold-mix (cold-laid)

3. 3 AC Mix Design Asphalt Concrete = binder + aggregate select & proportion components that provide adequate performance over design life @ reasonable cost VOLUMETRIC process Vair > 3% to preclude bleeding, instability Vair < 8% for durability Vasp to coat, bind, & satisfy (absorption) agg WEIGH components in production

4. 4 AC Mix Design adequate performance assessed based on MIXTURE PROPERTIES stiffness stability durability flexibility fatigue resistance fracture (tensile) strength thermal characteristics skid resistance permeability workability

5. 5 ASPHALT CONCRETE MIXTURES Asphalt Concrete = binder + aggregate 3 stages of Life mixing (fluid asphalt cement) curing (viscoelastic solid) aging (environmental effects & loading)

6. 6 Behavior depends on: Temperature Time of loading (Traffic Speed) Aging (properties change with time) Factors Influencing the Behavior This specification uses tests which evaluate the fundamental material properties (stress, strain, and strain rate). Changes in asphalt properties due to temperature, rate of loading and the effect of aging are considered. Asphalt is a viscoelastic material. That is, it can both exhibit elastic and viscous properties at the same time. To demonstrate, take a penetration tin of asphalt cement at room temperature. Press your thumb into the asphalt; this will leave a large depression in the surface. Then use a hammer to hit the a remaining flat section of the asphalt; this will not leave much of an impression. Under light but long term loads, the asphalt has a noticeable viscous behavior (it flows out from under the load). Higher, but much shorter duration loads result in a primarily elastic response. This specification uses tests which evaluate the fundamental material properties (stress, strain, and strain rate). Changes in asphalt properties due to temperature, rate of loading and the effect of aging are considered. Asphalt is a viscoelastic material. That is, it can both exhibit elastic and viscous properties at the same time. To demonstrate, take a penetration tin of asphalt cement at room temperature. Press your thumb into the asphalt; this will leave a large depression in the surface. Then use a hammer to hit the a remaining flat section of the asphalt; this will not leave much of an impression. Under light but long term loads, the asphalt has a noticeable viscous behavior (it flows out from under the load). Higher, but much shorter duration loads result in a primarily elastic response.

7. 7 Permanent Deformation Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.

8. 8 Stability resistance to permanent deformation under repetitive loading rutting, shoving Marshall Stability

9. 9 Stability mechanical / frictional interlock between aggregate particles same factors that influence creep rough, angular, dense-graded aggregate ? binder (w/ voids filled) ? Sac ? degree of compaction (> 3% air)

10. 10 Stability

11. 11 Flexibility ability to conform to long-term variations in underlying layer elevations settlement (clay), heave (frost, moisture) open-graded aggregate ? binder

12. 12 Fatigue Resistance resistance to fracture caused by repetitive loading (bending) fatigue (alligator) cracking dense-graded aggregate ? binder ? degree of compaction

13. 13

14. 14 Tensile (Fracture) Strength resistance to thermal cracking important @ low temps large induced stresses (restrained contraction) weak subgrade transverse cracking primarily controlled by binder limiting tensile strength (4-10 MPa) ~ limiting stiffness dense graded aggregate ? degree of compaction ? binder

15. 15 Low Temperature Behavior Low Temperature Cold Climates Winter Rapid Loads Fast moving trucks At cold temperatures, or under very quick loads, the binder response is predominately elastic.At cold temperatures, or under very quick loads, the binder response is predominately elastic.

16. 16 Thermal Cracking Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures.Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures.

17. 17 Aging Asphalt reacts with oxygen “oxidative” or “age hardening” Short term Volatilization of specific components During construction process Long term Over life of pavement (in-service) Aging also needs to be considered in the specification as oxidation and heat hardening during tank storage, mixing and placement (short term aging) of the asphalt concrete change the properties of the original binder. Long term aging refers to the changes in binder property after 7 to 10 years of exposure to environmental factors.Aging also needs to be considered in the specification as oxidation and heat hardening during tank storage, mixing and placement (short term aging) of the asphalt concrete change the properties of the original binder. Long term aging refers to the changes in binder property after 7 to 10 years of exposure to environmental factors.

18. 18 Permeability ease w/ which air & water can pass through or into AC moisture damage, accelerated aging inversely proportional to durability dense graded aggregate ? degree of compaction ? binder

19. 19 Durability resistance to weathering & abrasive action of traffic exposure to air (aging), water, & traffic moisture damage (stripping, loss of stiffness), accelerated aging ? Sac ? binder strong, hard, clean, dry aggregate resistant to polishing, crushing, freeze-thaw effects; not water sensitive dense graded aggregate ? degree of compaction

20. 20 Mix Design select & proportion component materials to obtain desired properties @ reasonable cost properties of component materials properties of composite material economic factors & availability of materials construction methods

21. 21 Mix Design select aggregate blend determine optimum binder content balance desired properties

22. 22 Mix Design

23. 23 Mix Design selection of aggregate blend aggregate properties (primarily gradation) compactibility selection of binder content surface area of aggregates volumetrics of mixture (air voids, voids between aggregates) mechanical properties of mixture from laboratory testing

24. 24 Thermal Cracking Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures.Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures.

25. 25 Binder-Aggregate Bonding wettability viscosity (temp) composition (oxygen) durability surface chemistry (mineral composition) surface texture porosity surface condition (cleanliness, moisture)

26. 26 Binder-Aggregate Bonding ac wetting the aggregate surface low surface energy need dry aggregates polar nature of ac / electrostatic interaction mechanical bonding failure flaws @ interface stripping

27. 27 Binder-Aggregate Bonding

28. 28 Composite Material 2 components physically combined w/ some AIR VOIDS 1 continuous phase binder - viscous, viscoelastic aggregate** - solid dense aggregate skeleton w/ sufficient binder to bind and provide durability > 90% by weight aggregate

29. 29 Composite Material

30. 30 Permanent Deformation Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.

31. 31 Description of Asphalt Concrete Particulate composite material that consists of: Aggregates. Asphalt. Air voids.

32. 32 Review of the Properties of Particulate Composites The properties of the composite can be calculated from the properties of the constituents. For simplicity, assume asphalt concrete to be represented by particulate (aggregates), and matrix (asphalt and air). Also, assume elastic behavior.

33. 33 Parallel Model

34. 34 Series Model

35. 35 Hirsch’s Model gggg

36. 36 Viscoelastic Behavior of Asphalt Concrete

37. 37 Viscoelastic Models Viscoelastic Model: Mathematical expression for the relationship between stress, strain, and strain rate. Combinations of basic rheological models. The combinations mean that there are different mechanisms due to different chemical and physical interactions that govern the response.

38. 38 Basic responses

39. 39 Maxwell Model

40. 40 Kelvin Model

41. 41 Burger Model

42. 42 Asphalt Binder Behavior

43. 43 Viscous Behavior of Fluids

44. 44

45. 45 Why do we need to model the response? Conduct a creep or a relaxation test. Fit a model to the data. Determine the material parameters. Describe the material parameters based on design conditions Use the model to predict performance under different loads and applications.

46. 46 Permanent Deformation Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress.