The compressed has been thoroughly described in the preceding chapter; it is sometimes used for footpaths where there is a very heavy traffic, and answers admirably.
I will, however, now deal solely with mastic asphalte, which means the rock ground to powder, mixed with a certain proportion of bitumen to act as a flux, and then subjected to heat; this is sometimes used in conjunction with fine sharp clean river sand, but more often with finely crushed stone about the size of peppercorns, and is styled “gritted asphalte.”
As long ago as the year 1838, Mr. F. W. Simms speaks of asphalte mastic from Pyrimont, near Seyssel, and says “it may be considered a species of mineral leather”[102]—a very good description of its surprisingly tough, hard, durable and pliant properties.
The usual method of the preparation of the mastic is as follows:—
According to the amount of bitumen contained in the natural stone, from 5 to 8 per cent. of refined Trinidad bitumen[103] is placed in a large caldron which is usually provided with agitators driven by steam power; when this is thoroughly melted, the powdered asphalte is added little by little, the heat being raised to between 390° and 480° F., the mixture kept well stirred and “cooked” for about five hours. It is then turned out into iron moulds, most companies having a special pattern with a trade mark for this purpose.
The caldrons generally used contain from 1¹⁄₂ to 2 tons of mastic.
Some companies in large towns are provided with caldrons on wheels, commonly called “Locomobiles,” in which case the grit is mixed with the mastic in the fixed caldrons, and the whole mass run out into the locomobiles (which are also provided with agitators worked by an endless chain attached to the axle of the wheels), and transported direct to where the work has to be done. This system, though undoubtedly the best, is not practicable except in large towns; the more usual method of laying mastic footpaths is to send the asphalte cakes to the works, where they are remelted in small round street caldrons, containing from 8 to 12 cakes each, weighing from 40 to 50 lbs., the grit being sometimes added in the fixed caldrons, sometimes in the street caldrons, this amount of grit varying from 20 to 60 per cent. according to the nature of the work.
The grit makes the asphalte more difficult to spread, but it lessens the cost and makes a very durable path. The affinity between the asphalte and grit is so great that, in breaking a sample, the actual pieces of grit will be found broken in half.
The asphalte should be spread from ¹⁄₂ to ⁷⁄₈ of an inch in thickness (if compressed 1 inch is the minimum), and should be brought hot on to the works in covered caldrons on wheels, the test of its being ready and fit to lay being made by plunging a wooden spatula into it, which should come out without any of the asphalte adhering to it, and also by jets of light smoke darting out of the mixture.
The mastic should be taken from the caldron with a warmed ladle, and put into buckets previously heated, then thrown out on the concrete (which should be perfectly dry) near the spreader, who spreads it skilfully with a wooden stave, spreader, or spatula. The surface should then be floated and dusted over with fine sand, portland cement, or stone dust.