Within each grade of Oxidized bitumen 110/30, as measured by penetration or viscosity, the asphalt cement will also be graded according to its quality. When the asphalt cement is graded according to its penetration, an asphalt cement having a higher viscosity at the same penetration generally is of a higher quality. Similarly, when asphalt cements are graded according to their viscosity, an asphalt cement which has a higher penetration at a given viscosity is of a better quality.
By way of example, a high quality 85 – 100 pen paving asphalt cement would have viscosities at 135°C above about 280 cSt while a lower quality paving asphalt cement of similar penetration would have a viscosity in the range of from about 150 cSt to about 250 cSt. Similarly, a high quality AC-20 paving asphalt cement would have a penetration from about 50 to about 80 while a lower quality paving asphalt cement would have a penetration from about 40 to about 60.
The following table provides examples of paving asphalt cements conventionally manufactured from two crude oil sources. Crude oil source A provides a high quality asphalt cement. Crude oil source B provides a low quality paving asphalt cement that would be outside many user agency specifications. As is explained hereinafter, the high quality asphalt cement ex crude oil ‘A’ may also be referred to as a “high quality paving asphalt precursor”. Similarly, the low quality paving asphalt cement may also be referred to as a “low quality paving asphalt precursor”. The table also provides three examples of premium asphalt cements which are prepared according to the teachings of this invention and which exhibit superior pavement performance. These premium asphalt cements were prepared from a high quality paving asphalt precursor, a low quality paving asphalt precursor and a mixture thereof.The foregoing Table sets out the characteristics of several 85-100 pen asphalt cements. As mentioned above, in various countries, asphalt cements are graded by penetration. In other countries, such as the United States, asphalts are graded persuant to viscosity. In this regard, reference has been made above to ASTM standard D-3381. Asphalt precursors may be categorized, not only by penetration, but also by viscosity. Accordingly, in reference to ASTM standards, a “high quality paving asphalt precursor” is a vacuum tower bottom residue from refining of crude or crude blends suitable for production of paving asphalt cements meeting the requirements specified in ASTM D-3381, Table 2. A “low quality paving asphalt precursor” is a vacuum tower bottom residue from refining of crude or crude blends which are not suitable for production of paving asphalt cement meeting requirements specified in ASTM D-3381, Table 2, but are suitable for production of asphalts meeting the requirements specified in ASTM D-3381, Table 1. Further, a “non-paving asphalt precursor” is vacuum tower bottom residue from refining of crude or crude blends which are not suitable for production of asphalt cement meeting the requirements specified in ASTM D-3381
High quality paving asphalt cements may be obtained by careful selection of the crude oil from which the asphalt cement is to be made and the operating conditions used to obtain the vacuum tower bottoms. By making an appropriate selection, a high quality paving asphalt cement may be obtained. Such crude oils and operating conditions are known to those in the art. For ease of reference such VTB and similar materials which may be used as a high quality paving asphalt cement or from which a high quality paving asphalt cement is obtained comprise the high quality paving asphalt precursors. Conversely, those paving asphalt precursors from which a high quality paving asphalt cement may not be obtained are referred to hereinafter as “non-high quality paving asphalt precursors”. Similarly, those VTB and similar material from which a lower quality paving asphalt cement is obtained or which may be used as a lower quality paving asphalt cement comprise the low quality paving asphalt precursors. Non-high quality paving asphalt precursors include low quality paving asphalt precursors and non-paving asphalt precursors.
A disadvantage with this selection system is that only a small number of crude oils will render a high quality paving asphalt cement. Further, once these materials are transported, such as in a pipeline, they will be mixed, at least to an extent, and may no longer yield a high quality asphalt cement.
Since a high quality paving asphalt crude oil is not always locally available, other alternatives have been developed. For example, performance enhancers such as polymers and additives are known in the art and have been incorporated into asphalt cements to meet the requirements for high quality paving asphalt cements. Examples of such polymeric performance enhancers are styrene butadiene styrene (“SBS”), ethylene vinyl acetate (“EVA”), styrene butadiene rubber (“SBR”) and polyethylene. One disadvantage with this approach is that the incorporation of polymers into asphalt dramatically increases the cost of the resultant paving asphalt mixture.