AN OVERVIEW OF THE AGEING OF BITUMINOUS BINDERS

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. 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. 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 • 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.

14. Volatilization

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

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

17. Oxidation involving the incorporation of oxygen Stiffness is related to the mobility of the molecules – Introducing polarity slows motion – increases stiffness

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.

19. External oxidative coupling

20. Internal oxidative coupling Same as external oxidative coupling, but within the same molecule

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?

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 dmmper hour.

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

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

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

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.

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

28. Measure the rate of ageing Chemical SARA UV IR AFM Trends differ, depending on the test selected

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

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

31. Bitumen selection and specification

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

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

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

35. Thank you