Asphaltes are combinations of bitumen and calcareous matter, sometimes found in nature, sometimes artificially formed.
Natural asphaltes are superior to artificial imitations, probably because in them the bitumen is more thoroughly incorporated with the limestone or other calcareous matter.
The natural asphalte is generally ground, mixed with sand and a further proportion of bitumen, and run into moulds. When thus mixed it is known as mastic.
Uses, Advantages and Disadvantages. - Patent asphalte (or mastic) is waterproof, fireproof, easily applied, and to some extent elastic, it can therefore be used with advantage for many purposes.
It is an admirable material for the damp-proof courses of walls (see Part II. p. 214), also as a waterproof layer over arches or flat roofs, or for lining tanks. It is useful for floors that require a very smooth surface, as in racket courts; also for those that have to resist water, as in wash-houses, and for skirtings of such floors. When spread and brought to a smooth surface it wears well in footpaths, makes substantial and almost noiseless carriageways, but is very slippery in damp weather.
Good mastic should be proof against frost and damp, tough not brittle, and uninflammable. It should withstand a temperature of from 140° to 160° Fahr. without softening to any appreciable extent, and should not become so fluid as to run down below a temperature of 260° Fahr.1
The following remarks are necessary, however, in order to understand the peculiarities of the different kinds of asphalte described below.
There are two principal methods by which asphaltes may be applied to a surface : (1) by being melted, spread, and rubbed to a smooth surface.
(2) By being ground to powder, spread, and consolidated by ramming.
Of these methods the first is the more convenient in many positions, but asphaltes laid as compressed powder appear to be the most durable under considerable wear, as in carriage-ways.1
When the surface is at a slope exceeding about 1/10, the asphalte is apt to run if exposed to the sun, unless a good key can be obtained.
For steep inclinations and for vertical work (such as the linings of tanks) the face must be roughed, the joints well raked out and filled with asphalte, the whole surface free from moisture and warmed; the asphalte is then applied in successive thin coatings. Where the moisture cannot be got rid of, it is necessary to build the face of the wall with asphalte joints, to which the covering asphalte adheres. Plates of asphalte are sometimes used.
"Minute holes are noticeable in compressed asphaltes shortly after they are laid, which seem after a time to close up or disappear,, while others open. The cause of these has not been satisfactorily explained." 2
There are several different asphaltes in the market. A few of them will now be described.
Seyssel Asphalte, known also as Claridge's Patent Asphalte, is made from a bituminous rock found at Pyrimont Seyssel, in the Jura mountains.
It is a limestone saturated with bitumen, and contains about 90 to 92 per cent carbonate of lime and 10 to 8 per cent of bitumen.
This material is ground, mixed with grit and with heated mineral tar until the mass has thoroughly amalgamated and become reduced to a mastic. It is then run into moulds to form blocks.
The asphalte is imported in this form by the Pyrimont Seyssel Asphalte Company, from whose circular most of the following information is obtained : -
There are three qualities in the market -
1. Fine, without grit, used for magazine floors and as a cement for very close joints in brickwork.
2. Fine-gritted, for covering roofs and arches, lining tanks, as a cement for brickwork, and for running the joints of stones.
3. Coarse-gritted, containing more and larger grit; used for pavements and floorings where great strength is required, as gun-shed floors, tun-room floors, margins of stall floors, etc. In gateways for heavy carriage traffic small pieces of granite chippings, etc., are introduced.
1 Report of Engineer, City of London, 1871.
2 Clark on Koads.
Coal is objectionable on account of the smoke it creates; coke injures the material and destroys the caldron.
The following directions are from the circular of the company : -
"The fire having been lighted in the caldron, put into the boiler 2 lbs. of mineral tar, to which add 56 lbs. of asphalte, broken into pieces of not more than 1 lb. each. Mix the asphalte and tar together with the stirrer, till the former becomes soft, and then place the lid on the caldron, keeping up a good fire. In a quarter of an hour repeat the stirring, and add 56 lbs. more asphalte, in similar sized pieces, distributed over the surface of that in the caldron. Again cover the caldron for ten minutes, after which keep the contents constantly stirred, adding by degrees asphalte in the proportion of 112 lbs. to 1 lb. of tar, until the caldron is full and the whole is thoroughly melted.1 When fit for use the asphalte will emit jets of light smoke and freely drop from the stirrer."
The asphalte is removed from the cauldron in ladles, poured over the concrete foundation, or other place where it is to be applied, brought to a smooth surface with wooden rubbers, and finished, either with a mixture of slate-dust and silver sand in equal parts, or roughened by grit stamped in while the asphalte is soft.
Val de Travers Asphalte is from a rock found at Neuchatel in Switzerland.
It is said to be richer in bitumen than the asphalte from Seyssel, containing from 11 to 12 per cent, and sometimes as much as 20 per cent.
The material is laid in two different ways - either in powder, compressed, by ramming, into a solid condition, or by melting and spreading, as in the case of Seyssel asphalte.