1.
In laying the asphalt the greatest attention and care must be paid to the preparation of a solid and dry foundation.
This is usually accomplished by removing or ramming the loose earth, and placing upon the bed a layer of coarse sand mixed with powdered limestone, in the proportion of seven parts of the former to one of the latter, and the whole is pressed or beaten solid; upon this a second layer of finer materials is laid compacted and levelled; the bed thus prepared is allowed to dry before coating it with mastic.
Fig. 2 shows the manner in which ordinary asphalting is laid down. In this figure C is the bed of coarse concrete, B the second and finer layer of the same material, and A the superior layer of asphalt.
2.
The base or concrete must be perfectly dry when the mastic is poured on, or the work will be a failure, for the moisture will be converted into steam, which, issuing through the fluid mastic, will cause the formation of holes in the latter or blister it, and ultimately the surface will crack. To counteract in some measure the evil arising from the formation of steam, fine cinder dust is sifted over the bed of concrete previous to the application of the mastic.
When asphalting suspension bridges, a sheet of canvas is usually spread over the concrete.
In asphalting damp places, such as cellars and foundations, a brick invert is always laid in asphalt beneath the concrete. This is done by placing the bricks in rows, at the proper depth and slope, and pouring a coating of asphalt about a quarter of an inch thick upon them. Before the mastic solidifies, the bricks are separated a little by passing a knife between them, thus affording the mastic an opening by which to seal up more thoroughly the connection. The concrete is afterwards laid upon this bed, and the layer of mastic upon this in the usual way. The thickness of the layer of mastic varies according to the attrition to which it is to be subjected; but the usual depth is from a quarter to one and a quarter inch.