As a binder
At this stage, in order to avoid confusion, it is necessary to clarify the meaning of the terms bitumen and asphalt, as well as their adjectives bituminous and bituminous binder.
The reason is that, nowadays, the above terms have different meanings, certainly among European and American specifications and practicing engineers.
In Europe and according to CEN EN 12597 (2000), bitumen is virtually an in-volatile, adhesive and waterproofing material derived from crude petroleum or present in natural asphalt, which is completely or almost completely soluble in toluene and very viscous or almost solid at ambient temperatures.
According to the same specification, asphalt is a mixture of mineral aggregate and bituminous binder.
In addition, bituminous is the adjective applicable to binders and mixtures of binders and aggregates containing bitumen.
Hence, bituminous binder is the adhesive material containing bitumen.
Furthermore, natural asphalt is a relatively hard bitumen found in natural deposits, often mixed with fine or very fine mineral matter, which is virtually solid at 25°C and viscous fluid at 175°C at the same time.
In American English and according to American specifications, more often the term asphalt is used rather than the term bitumen. According to ASTM D 8 (2013), ‘asphalt’ or ‘bitumen’ is ‘a dark brown to black cement-like residuum obtained from the distillation of suitable crude oils’.
Another term also used in North America is asphalt cement, which has the same meaning as asphalt.
The term asphalt binder has been utilized in the Super-pave mix design method to classify the grade of asphalt cement used in an asphalt mix based on expected performance under specific environmental conditions and anticipated traffic loading. Asphalt binder according to ASTM D 8 (2013) is ‘an asphalt which may or may not contain an asphalt modifier’.
Finally, in American specifications, the term native asphalt is used instead of the term natural asphalt used in European standards. The term native asphalt is defined as ‘the asphalt occurring as such in nature’.
In order to avoid confusion to the readers of this book, the terms bitumen and asphalt, as defined by CEN EN 12597 (2000) will be adopted throughout the text, unless reference is made to the American standards or specifications.
NATURAL ASPHALT
Natural asphalt is derived from the natural mutation of petroleum. This mutation happened a million years ago under the influence of bacteria. This bacterial decomposition of petroleum resulted in reduced content of saturated hydrocarbons, mainly n-alkylenes, and of light oils, which became heavier and more viscous, taking the form of asphalt.
According to CEN EN 12597 (2000), natural asphalt is defined as ‘a relatively hard bitumen found in natural deposits, often mixed with fine or very fine mineral matter, which is virtually solid at 25°C but which is a viscous fluid at 175°C’.
Trinidad asphalt
The asphalt that comes from Trinidad Island is the most widely known natural asphalt, in semi-solid or very viscous form, which is currently used in highway engineering constructions.
Deposits are superficial and form ‘lakes’, which is the reason that this natural asphalt is also known as ‘Pitch lake or Lake asphalt’. The largest lake asphalt can be found in the south part of Trinidad Island, near La Brea.
The total surface of the lake is approximately 500,000 m2 (50 ha), with a maximum depth of approximately 90 m, and it is estimated
that there are approximately 15 million tonnes of asphalt. This constitutes one of the largest deposits of very good quality natural asphalt in the world (Chilingarian and Yen 1978).
The paradox is that although the asphalt is mined, the deposits remain almost the same.
Surface mining is possible since the lake asphalt surface is hard enough to withstand the loads of the mining machinery.
Trinidad natural asphalt contains a small amount of water and foreign materials, mainly topsoil and stones, which are removed once the asphalt is heated, at 160°C, and ‘sieved’.
The final product, known as Trinidad Lake Asphalt (TLA) or Epuré, has the following typical composition: 54% asphalt, 36% mineral matter and 10% organic matter. Its specific gravity varies from 1.39 to 1.44 g/cm2. The TLA’s typical chemical composition, together with the typical chemical composition of other natural asphalts, is given in Table 3.1.
The pure asphalt (Trinidad Epuré) is very hard (penetration, 0–2 dmm; softening point, 93°C–99°C) to be used alone in the production of bituminous mixtures.
Hence, it is mixed with soft crude petroleum bitumen (usually 200 pen [dmm]) in order to produce suitable paving asphalt (a proportion of approximately 50/50 results in a 50-pen asphalt/bitumen).
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