The natural rock having been ground to powder, is subjected to great heat in a revolving boiler. The boiler may be on the spot, or the powder may be brought in a hot state in closed iron carts.
A foundation of Portland cement concrete having been formed, its surface is spread over with the powder, which is then compressed by means of hot iron rammers into one homogeneous layer without joints, and impervious to moisture.
Carriage-ways are generally laid by this method.
The asphalte is melted in boilers as above described, a small quantity of bitumen being gradually added.
1 Practice, however, best regulates the quantity of tar to properly flux the asphalte. In exposed situations, particularly on the coast during cold and other unfavourable weather, a strong fire is necessary to be kept up, and at such times the asphalte work is longer in execution. On this account the tar is more quickly consumed, and a small quantity will have to be added A somewhat larger proportion of tar is also necessary in the application of asphalte to brickwork, and also in running the joints of stones. In warm climates an excess of tar must be avoided. From the first lighting of a caldron about 31/2 hours will be occupied before the entire mass with which it is to be filled will become melted. The subsequent operation will occupy about half an hour less time.
It is carried in ladles from the caldrons to the concrete foundation prepared for it, and spread in a liquid state over the surface and allowed to cool.
About 18 parts asphalte and 2 parts grit are used for roofs, linings, tanks, etc.
About 16 parts asphalte and 2 parts grit for flooring, footways, stalls, etc. etc.
Rather more bitumen is added in the roofs than the floors, but the amount depends, of course, upon circumstances.
Limner Asphalte is obtained from Limner, near Hanover.
The asphalte is broken up and mixed with clean grit, together with a small quantity of bitumen.
The mixture is melted in caldrons, and laid in two thicknesses, the lower stratum having coarser grit in it than the other.
Brunswick Hock Asphalte is obtained from mines at Vorwohle, in Brunswick, Germany.
Montrotier Asphalte is a French production, and is laid in compressed powder. Mastic Asphalte comes from Spain, and is laid in small blocks.
There are several other so-called asphaltes in which the natural substance is mixed with various ingredients.
Among these may be mentioned the following : -
Barnett's Liquid Asphalte is made from natural or artificial asphaltes, mixed with powdered oxide of iron and a small proportion of mineral tar.
The materials are melted and laid, as before described, on a concrete foundation.
Trinidad Asphalte is a mixture of Trinidad pitch, broken stone, chalk, and other ingredients, and is laid hot, in the form of powder.
Patent British Asphalte is a mixture of quicklime, pitch, sawdust, and ground iron slag, heated and laid in a semi-liquid state.
Inferior Asphaltes are also made with coal-tar pitch boiled with chalk and sand.
Pitch plays an important part in asphaltes, and it will be well to distinguish between the different varieties.
Mineral Pitch, or bitumen, is the constituent that makes asphalte so valuable.
In fact, strictly speaking, solid bitumen is asphalte; the rock asphalte, generally known by engineers as asphalte, is merely stone saturated with asphalte.
It used to be found in large quantities on the Dead Sea (Locus asphaltites), and thus obtained the name of Bitumen of Judea.
Natural bitumen is found also in the island of Trinidad.
Bitumen contains an oil which in coal tar is very volatile, and escapes, leaving the tar brittle.
Coal tar is very brittle at the freezing point, and softens at 115° (Fahr.), whereas true bitumen is tough at 20°, and will not soften at 170° (Fahr.)
Coal Tar Pitch is the residue obtained by distilling coal tar.
This material is sometimes used instead of bitumen for mixing with asphalte.
It is, however, brittle, softens more under heat, is easily crushed, and if altogether inferior.