1. AN OVERVIEW OF THE AGEING OF BITUMINOUS BINDERS
2017 Annual Southern African Transport Conference, 10 – 13 July 2017
Presented by:
Johan O’Connell

2. Acknowledgements
CSIR parliamentary grant funding
Prof. Steyn (UP)

3. Framework Background SARA composition
Mechanisms of ageing and the effect on chemical composition
Factors that influence the rate of ageing
How to measure the rate of ageing
Binder selection and specification
Conclusion

4. Courtesy: Virginia Asphalt Association
Introduction
A road is designed to consist of defined layers of engineered strengths with the sole purpose of transferring traffic loads to the subgrade at acceptable levels.
Courtesy: Virginia Asphalt Association

5. Courtesy: Virginia Asphalt Association
Introduction
Often an asphalt mix layer is found at the surface to ensure (amongst other functions) that the underlying pavement layers are protected from moisture ingress.
Courtesy: Virginia Asphalt Association

6. Introduction
Stiffness and viscous response varies with temperature and time (ageing)

7. Modes of failure accelerated by increasing stiffness
Thermal or Shrinkage cracking – top down form of cracking
Fatigue cracking – bottom up
Low temperature cracking – top down

8. Cracking = potholes = base failure
Vehicle maintenance and repair costs can increase by up to 120 %
Asset Value of SA roads – > R1 tn

9. Goals Understand the mechanism of ageing Improve binder selection
Retard / Prevent (Additives, design)
Predict the appearance / maintenance schedules
Improve binder selection
Select tests and specifications to prevent age-prone bitumens from being used
Select ageing simulation methods for these tests to best predict actual ageing

10. SARA composition
Separation of bitumen into fractions using chromatography
Saturates (waxes)
Aromatics
Resins
Asphaltenes (by filtration)
Ratio of components affects the stiffness of bitumen
Bitumen components must
be in chemical balance!
Maltenes

11. SARA composition During Ageing: Saturates remain constant
Aromatics transform to Resins
Resins transform to asphaltenes
Asphaltenes increase in size and polarity
In time compositional balance is destroyed

12. Mechanisms of Ageing
Ageing mechanisms may apply to the bitumen only or they may apply to the bitumen as well as any additive/modifier that may be present – focusing on bitumen only
Volatilization
Oxidation involving the incorporation of oxygen into the molecular structure of the bituminous binder.
External oxidative coupling
Internal oxidative coupling
Exudation
Steric hardening

13. Volatilization “evaporation”
As bitumen ages, the components of lower molecular mass may dissipate into the atmosphere.
High temperature process associated with a decrease in binder mass.
Extent of volatilization depends on method of manufacture of the bituminous binder. Less applicable to current South African bitumen sources.
Traditionally, South African bituminous binders have been manufactured using high temperature processes, including blowing, thereby removing lower molecular mass components during the manufacturing process.

14. Volatilization Binder Mass Loss after RTFOT Original 0,5% Petrol
Diesel
50/70 Binder 1
+ 0.11
- 0.40
- 0.29
50/70 Binder 2
+ 0.08
50/70 Binder 3
+ 0.03
35/50 Binder 1
+ 0.06
35/50 Binder 2
+ 0.05
35/50 Binder 3
+ 0.02
Traditionally, South African bituminous binders have been manufactured using high temperature processes, including blowing, thereby removing lower molecular mass components during the manufacturing process.

15. Oxidation involving the incorporation of oxygen
Oxygen taken up into the molecular structure of the bitumen – UV an important initiator and accelerator
Relates to an increase in binder mass.
The incorporation of oxygen takes the form of a carbonyl functional group, which manifests itself as a ketone, carboxylic acid or aldehyde. This allows for hydrogen bonding, facilitates oxidative coupling
Traditionally, South African bituminous binders have been manufactured using high temperature processes, including blowing, thereby removing lower molecular mass components during the manufacturing process.

16. Oxidation involving the incorporation of oxygen
Water: molecular mass = 18
Butane gas: molecular mass = 58
As the oxygen content in asphaltenes increase, hydrogen binding facilitates the agglomeration of one or more asphaltene molecules.. Large increase in apparent molecular mass
Traditionally, South African bituminous binders have been manufactured using high temperature processes, including blowing, thereby removing lower molecular mass components during the manufacturing process.

17. Oxidation involving the incorporation of oxygen
Stiffness is related to the mobility of the molecules – Introducing polarity slows motion – increases stiffness
Traditionally, South African bituminous binders have been manufactured using high temperature processes, including blowing, thereby removing lower molecular mass components during the manufacturing process.

18. External oxidative coupling
Joining of two different molecules to form a larger heavier structure – UV an important initiator and accelerator
Increase in oxidation number - oxygen is not incorporated. Oxygen is reactive – Incorporation from the previous step facilitates the process.
Oxygen may be eliminated or may be a catalyst
Relates to a decrease in binder mass.
This is the process by which bitumen is “blown” and converted to harder grades in refineries. This relates to a decrease in binder mass.

19. External oxidative coupling
This is the process by which bitumen is “blown” and converted to harder grades in refineries. This relates to a decrease in binder mass.

20. Internal oxidative coupling
Same as external oxidative coupling, but within the same molecule
This is the process by which bitumen is “blown” and converted to harder grades in refineries. This relates to a decrease in binder mass.

21. Exudation From the Latin exsūdāre, "to (ooze) out like sweat”
Over time, certain component oils in the maltene phase in the bitumen of the bituminous binder may become absorbed by the voids/pores in the aggregate, which results in a stiffer binder.
Exceptional circumstances – related to pore size and bitumen compositional balance
Really ageing?
Traditionally, South African bituminous binders have been manufactured using high temperature processes, including blowing, thereby removing lower molecular mass components during the manufacturing process.

22. Steric hardening
Process by which the molecules rearrange themselves into a more closely packed state of least energy – affects mobility as intermolecular forces are involved – like compaction?
Associated with greater thermodynamic stability.
Time requirements in the penetration test - conditioning period is 1 to 1.5 hours in air and 1 to 1.5 hours in water. During this phase the penetration may change by as much as 1 dmm per hour.
Delayed penetration test

23. Factors that influence the rate of ageing
Chemical composition of the bitumen
Voids present within the asphalt layer
The presence of antioxidants such as lime. Polymers, and zinc-based compounds may counteract some ageing effects
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

24. Factors that influence the rate of ageing
Rate of diffusion of oxygen through the binder
The film thickness of the bituminous binder around the aggregate
The viscosity of the bituminous binder which regulates the diffusion rate of oxygen through the binder.
Partial pressure of oxygen
Air Temperature
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

25. Factors that influence the rate of ageing
The thickness of the asphalt layer which regulates the diffusion rate of oxidized material throughout the layer.
Climate which determines the effects of moisture, temperature and cloud cover.
Aggregate which may promote exudation or catalyse oxidation
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

26. Factors that influence the rate of ageing
Latitude
Relates to the distance travelled through the atmosphere or angle of incidence of the sun’s radiation.
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

27. Measure the rate of ageing
Physical
Penetration
Softening Point
Viscosity
DSR (G*/Sin δ, ΔTc )
BBR (S,m)
Trends differ, depending on the test selected
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

28. Measure the rate of ageing
Chemical
SARA UV IR
AFM
Trends differ, depending on the test selected
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

29. Bitumen selection and specification
Two main phases of bitumen ageing
Short Term Ageing (STA)
That which an asphalt mix undergoes during manufacture, storage, transport and placement.
High temperature process favouring high energy oxidation mechanisms and volatilization.
High rate of ageing which requires limits on the temperature and time during this stage
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

30. Bitumen selection and specification
Long Term Ageing (LTA)
That which an asphalt mix undergoes after placement over the lifetime of the pavement.
For the purposes of defining LTA with specification purposes in mind, LTA is often defined as the in-service ageing that occurs over 5 – 10 years
Medium temperature process ranging between - 5°C and 70°C in South Africa.
Favours lower energy oxidation mechanisms
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

31. Bitumen selection and specification
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

32. Simulation of ageing
Both STA and LTA varies widely – depending on all the factors that have been discussed. Also acceleration of ageing requires high temperature and/or pressure, thereby changing the reaction mechanisms.
Require standard methods as indicators for a purchase specification
RTFOT
PAV
Forced draft oven (163°C)
Moving thin film in glass bottle
Rack rotates at a prescribed rate
Air jet (air flow 4000 ml/min)
Time: 85 minutes
Time: hours
Temperature: 90 to 110ºC
Pressure: 2.1 kPa

33. Simulation of ageing and specification
To ensure binders are not prone to premature ageing, specifications are required in order to limit fatigue properties of binders after RTFOT and PAV ageing
New SA PG specifications, currently under evaluation, monitors binder stiffness ratio of RTFOT:Original and PAV:RTFOT
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

34. Conclusion
Fatigue properties of bituminous binders and the asphalt mix properties combine to play an important role in the rate of ageing that an asphalt layer will undergo. This, in turn, has an effect on future pavement deterioration (cracking, pothole formation)
Understanding ageing enables
Engineers to slow it’s progress
Improvement in binder specifications
Development models to predict crack initiation
Scheduling of maintenance
which is, in turn, determined by the crude source and method bitumen manufacture.
, which regulates accessibility to oxygen.
which regulates the diffusion rate of oxygen through the binder.
Molecules are in motion

35. Thank you