The top of the binding course should be perfectly dry when the wearing coat is laid, to prevent its being separated from the course below by the formation of steam. Asphalt should not be laid in cold weather, since the paving mixture may become chilled between the mixing plant and the street, and particularly when it comes in contact with the cold foundation.
The chief points requiring skill in the working of the asphalt sur face are : (1) in avoiding inequalities of the surface, especially depres sions which prevent the rapid removal of storm water; (2) in secur ing a very thorough consolidation of the gutters, which otherwise rot rapidly; and (3) in thorough rolling. The asphalt mixture can not be fully compacted by simple pressure, but requires the kneading action of repeated passages of the roller. The lubricating quality of the warm asphalt aids in this action, so that under the roller the grains of sand are wedged together and the finer particles worked into the voids, until the mass becomes more dense than dry sand alone could be made. This is proved by the fact that if all the bitumen be extracted from a fragment of pavement of known vol ume, it is found to be quite impossible to reduce the dry sand ob tained to as small a volume as it occupied in the pavement.
On streets with flat grades the gutters do not drain well; and as moisture is likely to injure asphalt, the gutter is painted with a coat of hot asphalt or is laid with hydraulic cement concrete, stone, or brick. If the gutter is fairly well drained, and the asphalt is thor oughly compacted, the first method will give reasonably good re sults. The gutter may be painted with a swab or a broom; but the work is most easily done with a gutter painter, which consists of a cast-iron box about 14 inches square having a slot in the bottom, carried by a rigid handle on each side.
To protect the asphalt along street-car tracks and sidewalk crossing-stones, where there are vibrations and pounding of the wheels of vehicles, it is customary to lay a line of granite blocks or vitrified blocks—usually headers and stretchers alternately. The toothing of the bricks or the stone blocks into the asphalt adjacent to car tracks, gutters, etc., is to do away with the continuous joint and thus to prevent wheels from wearing a rut; but there is so much difficulty in sufficiently compacting the asphalt between the pro jecting blocks, that the toothing is of doubtful value. After sev eral years' trial it has been abandoned in Washington, D. C.