Bitumen (or asphalt) Terminology today Part-2

Venezuela and other natural asphalts (pitch lake asphalts)

The natural asphalt of Venezuela, from the Guanoco area, is also superficially deposited and forms a lake. The covered area is larger than Trinidad Lake, approximately 4,000,000 m2, but it has lesser depth (maximum depth, 3 m). 

The quantities that exist are almost the same as those in Trinidad Island and the asphalt content is approximately 64%. This natural asphalt is not as widespread as the Trinidad asphalt.
Other asphalt lakes can be found in California (La Brea tar pits in urban Los Angeles, McKittrick tar pits in Kern County and Carpinteria tar pits in Santa Barbara County) and are mostly used as tourist attraction areas.

Rock asphalt and gilsonite

Rock asphalt is natural asphalt found in solid form. It was formed millions of years ago when layers of petroleum harden after heavier components settled while lighter components evaporated. The hardening took place, in most of the cases, within the pores of rocks (mainly limestone or sandstones); thus, a number of mineral materials are normally found within rock asphalt.

 

Rock asphalt, as natural asphalt in liquid-viscous form, was used historically in engineering works but today its use in road works is rare. Rock asphalt nowadays is used only as an additive to petroleum bitumen. Since it is a very hard material, its use is to harden petroleum bitumen.

The largest natural surface deposits of rock asphalt that are commercially exploited are found in Utah in the United States (28 km2) and Kermanshah in Iran.
The rock asphalt in the Uintah Basin in Utah has the trade name gilsonite, from S.H.

Gilson, the founder of a mining company in 1888.
Gilsonite today is a well-known additive for hardening petroleum bitumen.

Other natural rock asphalts

All the other natural asphalts in rock formation can be found at a depth ranging from 3 to 1000 m. The typical asphalt content of these deposits range from 5% to 20%. Because of this low asphalt content and the costly and time-consuming purifying procedure, the exploitation of these natural asphalts turns out to be unprofitable. As a result, their extensive usage in both highway engineering and other industrial applications is limited.

The only exception seems to be the rock asphalt of Buton Island (south of Sulawesi Island) in Indonesia. In this island, the rock asphalt forms hills and mountains. This porous rock contains hard natural asphalt (penetration of 5–10 dmm) at 15%–30% and soft limestone with fossilised shell impurities. The extraction is very simple and cheap, because of low local labour cost; thus, it is used locally for the production of bituminous mixtures.

The rock asphalt is grinded into fine grains 0–2 mm in diameter, and this constitutes the binder under the trade name Asbuton. Asbuton is mixed with aggregates and solvents and thus cold bituminous mixtures are produced for pavements with medium to low traffic volume.
Because of the binder’s specificity, to ensure the quality of the mixture and of the construction, a specific mix design methodology has been developed (MoPW 1989). In the early part of the 1990s, several pavements had been constructed or maintained successfully using the mixture designed with the specially developed mix design methodology (Nikolaides 1990, 1991a,b). Lasbutag and Latasbushir, trade names for coarse and fine bituminous mixtures, respectively, are considered to be alternative low-cost bituminous mixtures for maintenance and construction of pavements in the remote areas of Polynesia.

Natural rock asphalt can also be found in small deposits in Greece, particularly in Marathos, Paxos and Antipaxos islands; Divri; Suli; Prouso; Fteri; Kalarrytes; Dremissa village in Giona; Zakynthos; and Epirus.
Natural rock asphalt in larger deposits can be found in the Jordan Valley, Dead Sea banks, France, Switzerland, Antilles, Venezuela and Cuba.