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. 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. 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
6. Tests Used in PG Specifications
Construction
RV
Asphalt Binders
DSR
Superpave Binder Tests
BBR
6
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
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. 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
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. Short Term Aging
• Rolling Thin Film Oven Test (RTFOT)
– Simulates aging from hot mixing and construction
Asphalt Binders
Superpave Binder Tests
14
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. 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
18. Fatigue Cracking
Function of repeated traffic loads over time
(in wheel paths)
Asphalt Binders
Superpave Binder Tests
18
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. 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
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
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. 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
28. Low Temperature
• Two rheological tests
– Bending beam
• Stiffness
– Direct tension
• Strength
Asphalt Binders
Superpave Binder Tests
Low Temp
Cracking
[DTT]
[BBR]
28
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
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.