Bitumens have unique ‘fingerprints’ based on their origin and age state. Naturally occurring bitumens can be traced back to in situ geological phenomena of tectonic fracturing and faulting. Majority of the bitumens used in the road construction industry are by-products of crude oil fractional distillation in the refineries. In volume terms, bitumen represents approximately 3% of the barrel. The article will highlight Australian bitumens, import bitumens and the implications of these sources on binder performance in the different applications. Understanding Australian bitumens would be key in measuring field related performance given the dominant use of import bitumens so as to achieve a well-balanced and relevant comparison. 

The Source of “Australian” Bitumen

Chemical constituents and their associated changes related to refining processes of Australian bitumens contribute functional properties in the field. Australian bitumens are refined in Australia but with the parent crude oils sourced outside of Australia. Dating back to the 1940s Australian refineries produced bitumens, with recent significant investment in the bitumen manufacturing infrastructure over the last ten years. The biggest market for bitumen in Australia is in Queensland, followed by New South Wales and Victoria. The emergence of mega-refineries in Asia reduced the competitive advantage of old and relatively small Australian refineries in their current configurations. 

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In the last twenty years, the industry has begun to acknowledge that crude oil used in Australia was being sourced from the Middle East. South Korea, Singapore and Thailand are the primary sources of bitumen used in Australia whilst some bitumen suppliers import bitumen from the US manufactured crude. The distinction between sources of crude oil and bitumen is that crude oil has to be refined in Australia whereas bitumen is imported as a final product for different applications. To date, Viva Energy’s Geelong refinery is the sole manufacturer of bitumen in Australia which opens up more market for import bitumens. It is known that the crude oil reserves available in Australia are not conducive for the production of paving-grade bitumen to meet Australian specifications. The drive for import bitumens and the diversity of its supply in Australia has also been fueled by the transition of ownership from different companies which has led to a change in business priorities from crude oil refiners to energy traders in the last decade. 

Measuring and Managing Bitumen Quality

The ratio of saturates, asphaltenes, resins and aromatics determine the colloidal behaviour of bitumen. In solubility terms, the resins solubilize the asphaltenes in a continuum of saturates and aromatics based on the proportion of the SARA (saturate, aromatic, resin, asphaltene) fractions. Asphaltene content is considered a key parameter in the distinction between tender and hard bitumens. A further source of variability is the blending of various crude oils in the refining process depending on availability. A library of almost five decades of Australian manufactured bitumens exists with information on the source of each bitumen and processing conditions. Propane precipitation and blowing of bitumen yielded different binders with the latter considered the better process for rheological superiority. Australia uses a viscosity-based specification for bitumen that accommodates bitumen emulsions, polymer-modified bitumens and cutback bitumens whilst limiting the range of crudes that can be used to make bitumens. 

Global crude oil quality has plummeted, and this is evident in reduced average density and increased sulphur content. Importation of bitumen in Australia has been favoured by significant downscaling in local bitumen production. Statistics have shown a 74% reduction in the period 2010 to 2014 in addition to the closure of many production units in Australia. Re-prioritization to synthetic production of diesel, petrol and lubrication oil in the refining process has driven changes in the composition of residue available for the manufacture of paving-grade bitumens which also negatively impacted on the quality of the SARA fractions present in the import bitumens. Developing a chemical specification for bitumen to monitor import bitumen seems unrealistic as that is the responsibility of the supplier. The difficulty lies in the failure of bitumen manufacturers to determine the chemical composition before production which is a key component of crude selection and manufacturing operations. Although a universal chemical specification is difficult to achieve, the independent monitoring of import bitumens used in Australia must be conducted to ensure compliance and correlation to field related performance. 


The Impact on Bitumen Performance

In spray seal applications, bitumen durability significantly affects the longevity of sealed roads. The deterioration of bitumen is associated with aging on exposure to environmental and traffic distresses. Blowing temperatures in the refining process impact on bitumen durability, with elevated temperatures resulting in loss of the, saturates which reduces durability. Higher durability of propane precipitated asphalts is achieved as a result of pressure blowing to precipitate out larger molecules but at the expense of rheological properties. Asphalt tenderness in airport runways is attributed high susceptibility to cyclic shear creep under aircraft-induced shear stresses. Forensic investigations have shown that simultaneous change in crude oil used in the production of nominally identical batches of bitumen was responsible for the observed asphalt tenderness behaviour. Field performance deficiencies have been associated with low asphaltene content with some instances high saturate content contributory as well. 

A New Road Ahead

Informed decisions based on bitumen chemistry can be achieved through assessment of past bitumens in order to meet the demands of future bitumen provisions without compromising on quality requirements. Inconsistencies in different consignments of the same bitumen grade pose a challenge to the roads industry. Collaborative supplier to client value chains with expected product deliverables is of paramount importance in the interest of maintaining measurable bitumen properties and achieving performance goals in the field. Asset management practices will require stringent quality assurance measures with the scientific rationale to ensure an increase in bitumen surface life and improved early-life performance. Regardless of the proposed suggestions, one cannot rule out the influences of intellectual property and commercial advantage to the guarded approach. Transparent scrutiny of source, supply and processing of bitumen should enable binder ‘fingerprinting’ for the benefit of all industry stakeholders.

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