Block 4 SP 14

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  • This block of instruction will cover the tests, concepts and use of the new Superpave asphalt binder specifications. At the end of this block the student will be familiar with the :
    * Concepts behind the PG asphalt binder grading system.
    * Tests used for determining performance-related asphalt binder properties.
    * Selection of an appropriate PG asphalt binder grade.
  • The asphalt binder designation is based on expected extremes of hot and cold pavement temperatures.
  • Software programs and national databases of information needed to determine the high and low temperatures for selecting the appropriate PG grade are available from the FHWA website @ www.tfhrc.gov/pavement/ltpp/ltppbind.htm and is public domain.
  • The PG asphalt binder grading system was developed to address the short comings seen in the traditional asphalt binder specifications.
  • A number of the rheological tests discussed in the General Rheology module are used for characterizing asphalt binders in the Superpave asphalt binder specification. How these tests are used in the specification will be discussed in the following slides.
  • A series of four test methods are used to assess key performance-related properties of asphalt binders. The first, the rotational viscometer, evaluates the viscosity of the asphalt binder at temperatures similar to those commonly used during production.
  • When the DSR is run at warm test temperatures, the results are used to indicate the ability of the asphalt binder to resist rutting. For this testing, a 25 mm diameter parallel plate configuration is used.
  • The viscous component of the asphalt binder response dominates its warm temperature behavior and is seen as permanent deformation. The magnitude of this deformation is increased with the time that the load is applied.
  • Permanent deformation or rutting of the HMA pavement is the result of non-recoverable or plastic deformation due to traffic loads. At the warmer temperatures, the aggregate structure carries a major portion of the loads. Stiffer asphalt binders help to keep the aggregate structure intact as well as help resist deformation in the asphalt binder matrix.
  • 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 HMA change the properties of the original asphalt binder.
    Long term aging refers to the changes in asphalt binder property after 7 to 10 years of exposure to environmental factors.
  • Short term aging is accomplished using the same RTFO oven as has been traditionally used in the AR viscosity graded specification.
    This photograph shows a close-up of the inside of the oven. Glass jars (shown in the next photograph) fit snuggly in the circular openings in the rotating carriage. As the opening of each jar passes the air line nozzle, air is blown over the continually moving thin film of asphalt binder inside. A fan in the oven keeps the temperature even throughout the chamber.
    Once the jars are loaded into the carriage, the door is shut, the temperature stabilized at 163oC, and the aging process continues for 80 minutes. An example of a clean jar before adding 35 g of asphalt binder and how it looks at the end of the test is also shown.
  • In addition to preparing simulated short term aging of asphalt binders for further tests, the mass loss due to volatilization of the light fractions can be measured. This is done by determining the mass of two jars with asphalt binder at the beginning of the test, cooling these jars at the end of the aging, then determining the mass once again.
  • The first DSR parameter used in the PG asphalt binder grading system is G*/sin , based on testing the original asphalt binder at the average 7 day high pavement temperature. At high temperatures, the potential for permanent deformation (rutting) is the major pavement distress being evaluated.
    G* in Pascals at 10 rad/sec is numerically equal to viscosity in Poise:
    G* (Pascal)/in s-1)] * (10 Poise/1 Pascal.s)
    Since increasing the viscosity of asphalt binder has been historically used as one means of helping to reduce rutting, using G* in this specification parameter makes sense. Including the sin  parameter allows a softer but more elastic (less permanent deformation per load cycle) to be used rather than simply increasing the viscosity.
  • Fatigue is addressed by assessing G* sin  after RTFO aging (simulating aging during mixing and construction) and PAV aging (simulating aging after 7 to 10 years of service). This is because fatigue cracking takes time, exposure to environmental factors, and traffic before it occurs. An intermediate test temperature is used to simulate the appropriate average field conditions.
    The configuration is changed from a 25 mm to a 8 mm diameter plate. This is necessary to keep within the torque limits of the DSR equipment.
  • Fatigue cracking shows up in the wheel paths and is the result of the tensile strength at the bottom of the HMA layer being exceeded by the flexing of the HMA pavement due to traffic loads. Since the crack starts at the bottom of the layer and works its way up, the pavement is thoroughly cracked once the they are visible.
    Some limited examples of fatigue cracks starting at the pavement surface have been noticed lately. This type of fatigue cracking has been linked to an increase in tire pressures in the newer truck tires.
  • This summarizes the testing required for this specification requirement.
  • A pressure aging vessel (PAV) treatment of the RTFO binder is used to further age the asphalt binder. This simulates long term aging changes.
  • This photograph provides an example of an older type of pressure aging vessel equipment. This old version is shown because it clearly shows all of the key elements in all PAV units (old or new). There are currently several makes and models of PAV ovens available.
  • A pressure aging vessel (PAV) treatment of the RTFO asphalt binder is used to further age the asphalt binder. This simulates long term aging changes.
  • Low temperature asphalt binder properties are determined using the bending beam rheometer (BBR).
  • At cold temperatures, or under very quick loads, the asphalt binder response is predominately elastic.
  • A length of HMA pavement can be considered to be a semi-infinite constrained beam. As the temperature drops the HMA wants to contract but is restrained. This results in internal stresses building up as the temperature drops. Thermal cracks occur when the contraction-induced stresses exceed the tensile strength of the mixture.
    A number of researchers have shown that the low temperature behavior of the HMA pavement is highly dependent upon the properties of the asphalt binder.
  • Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing asphalt binder stiffness the colder the temperatures.
  • At the current time, there are requirements for both a maximum BBR stiffness at 60 seconds into the testing time and a minimum on the slope of the stiffness-time relationship of 0.300. Work is currently being completed that will use a combination of the tensile strength and the bending beam stiffness values to estimate a critical low temperature value for cracking. This concept will be similar to the one used for evaluating the low temperature cracking potential of the mix (see the Thermal Cracking module).
  • This figure summarizes the testing required for the PG asphalt binder specification.
  • Block 4 SP 14

    1. 1. Senior/Graduate HMA Course Asphalt Binders Superpave Performance Graded Specification Asphalt Binders Superpave Binder Tests 1
    2. 2. Superpave Asphalt Binder Specification The grading system is based on Climate PG 64 - 22 Min pavement temperature Performance Grade Average 7-day max pavement temperature Asphalt Binders Superpave Binder Tests 2
    3. 3. Pavement Temperatures are Calculated • Calculated by Superpave software • High temperature – 20 mm (.8”) below the surface of mixture • Low temperature – at surface of mixture Pave temp = f (air temp, depth, latitude) www.tfhrc.gov/pavement/ltpp/ltppbind.htm Asphalt Binders Superpave Binder Tests 3
    4. 4. PG Specification Tests • Rheology – Fundamental properties related to HMA performance • Test parameters – Selected to represent in-service & construction temperatures • Asphalt binder conditioning – Environmental factors • Short and long term aging Asphalt Binders Superpave Binder Tests 4
    5. 5. Rheological Tests Selected • • • • Concentric Cylinders Dynamic shear rheometer Bending beam Direct tension Asphalt Binders Superpave Binder Tests 5
    6. 6. Tests Used in PG Specifications Construction RV Asphalt Binders DSR Superpave Binder Tests BBR 6
    7. 7. Construction Asphalt Binder Properties • Concentric cylinder rheometer – Used to insure a minimum workability (pumpability) at typical mixing temperature – Maximum viscosity 3kPa at 135oC (275 F) Asphalt Binders Superpave Binder Tests Construction [RV] 7
    8. 8. Rutting BBR RV DSR Asphalt Binders Superpave Binder Tests 8
    9. 9. Rutting • High in-service temperature – Desert climates – Summer temperatures • Sustained loads – Slow moving trucks – Intersections Asphalt Binders Superpave Binder Tests Viscous Liquid 9
    10. 10. Rutting • DSR testing – Minimum stiffness requirement • Original asphalt binder • Immediately after construction – Short term aging Rutting [DSR] Asphalt Binders Superpave Binder Tests 10
    11. 11. High In-Service Temperatures • Permanent deformation (rutting) • Mixture is plastic • Depends on asphalt binder source, additives, and aggregate properties Function of warm weather and traffic Asphalt Binders Superpave Binder Tests 11
    12. 12. Asphalt Binders Superpave Binder Tests 12
    13. 13. Short Term Aging • Simulates stiffening of asphalt binder during storage, mixing, and hauling • Function of: – Oxidation hardening • Asphalt binder reacts with oxygen • Volatilization of specific components • Simulated using rolling thin film oven Asphalt Binders Superpave Binder Tests 13
    14. 14. Short Term Aging • Rolling Thin Film Oven Test (RTFOT) – Simulates aging from hot mixing and construction Asphalt Binders Superpave Binder Tests 14
    15. 15. Mass Loss is Monitored • Calculate mass loss after RTFO Original mass - Aged mass Mass loss, % = Asphalt Binders x 100 Original mass Superpave Binder Tests 15
    16. 16. Rutting Superpave DSR test requirements: G*/sin δ on unaged (original) asphalt binder > 1.00 kPa G*/sin δ on RTFO aged asphalt binder > 2.20 kPa For the early part of the service life Asphalt Binders Superpave Binder Tests 16
    17. 17. Fatigue BBR RV DSR Asphalt Binders Superpave Binder Tests 17
    18. 18. Fatigue Cracking Function of repeated traffic loads over time (in wheel paths) Asphalt Binders Superpave Binder Tests 18
    19. 19. DSR Fatigue Cracking • Aged PG asphalt binder – Since long term performance problem, include: • Short term aging [DSR] • Long term aging • Determine DSR parameters using 8 mm plate and intermediate test temperature Asphalt Binders Superpave Binder Tests 19
    20. 20. Long Term Aging • Simulates aging of an PG asphalt binder for 7 to 10 years • Starts with RTFO aged PG asphalt binder – Additional aging with Pressure Aging Vessel (PAV) • 50 gram (1.75 oz) Asphalt Binders Superpave Binder Tests 20
    21. 21. PAV Components Bottom of pressure aging vessel Rack of individual pans (50g of asphalt / pan) Vessel Lid Components Asphalt Binders Superpave Binder Tests 21
    22. 22. PAV Testing • After RTFO aging – 50 gram sample/pan aged for 20 hours in PAV • Pressure of 2,070 kPa (300 psi) • Test temp 90, 100 or 110o C (194, 212,or 230 F) • The parameter addresses the later part of the fatigue life Asphalt Binders Superpave Binder Tests 22
    23. 23. Equipment Examples Asphalt Binders Superpave Binder Tests 23
    24. 24. Thermal Cracking RV Asphalt Binders DSR Superpave Binder Tests BBR 24
    25. 25. Low Temperature Behavior • Low Temperature – Cold climates – Winter • Rapid Loads – Fast moving trucks Elastic Solid Hooke’s Law σ=τE Asphalt Binders Superpave Binder Tests 25
    26. 26. Low Temperature • Thermal cracks – Stress generated by contraction due to drop in temperature – Crack forms when thermal stresses exceed ability of material to relieve stress through deformation • Material is brittle • Depends on source of asphalt and aggregate properties Asphalt Binders Superpave Binder Tests 26
    27. 27. Thermal Cracking Asphalt Binders Superpave Binder Tests 27
    28. 28. Low Temperature • Two rheological tests – Bending beam • Stiffness – Direct tension • Strength Asphalt Binders Superpave Binder Tests Low Temp Cracking [DTT] [BBR] 28
    29. 29. Superpave Requirements • BBR Stiffness Properties – Max stiffness, S, at 60 seconds of 300 MPa – Min rate of change in stiffness • Min slope, m, of 0.300 MPa/sec • Strength – Currently being finalized Asphalt Binders Superpave Binder Tests 29
    30. 30. Summary Construction Rutting Fatigue Cracking Low Temp Cracking [DTT] [RV] No aging [DSR] [BBR] RTFO Short Term Aging PAV Long Term Aging Asphalt Binders Superpave Binder Tests 30
    31. 31. QUESTIONS ? Asphalt Binders Superpave Binder Tests 31

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