CONSTRUCTION OF SHEET PAVEMENTS.
The construction of asphalt pavements in this country is, in the main, in the hands of two or three large corporations, and methods of construction vary but little, the differences in the various pavements being principally due to differences in the composition of the materials used.
Concrete base. As a sheet asphalt surface has no power to sustain loads, acting only as a wearing sur face, which must be held in place from below, it is essential that it be placed upon a very firm, unyielding foundation. It is consequently nearly always placed upon a concrete base, which is commonly formed of hydraulic cement mortar and broken stone, prepared as described in Art. 47. In the use of this base, it is necessary that the mortar be fully set, and the concrete thoroughly dry before the asphalt is laid upon it, as the placing of the hot surface upon a damp foundation will cause the blistering and possible dis integration of the surface by the steam generated from the base by the heat of the material.
For moderate or heavy traffic in cities, the concrete base is commonly made 6 inches thick. For lighter traffic a less depth, 4 inches or 5 inches, is sometimes employed. The depth necessary will depend upon the nature of the road-bed as well as the weight of the traffic. It should be greater as the subsoil is less firm and well drained.
Bituminous base. Sometimes a base has been used consisting of a layer of broken stone four or six inches thick rolled into place and coated with asphalt or coal tar paving cement. This is known as a bituminous base. The advantage which has been claimed for it is that the foundation and surface material become joined into a single mass, with the effect of anchoring the surface and preventing the formation of weathering cracks and wave surfaces, which are sometimes found when the hydraulic base and light surface layer are employed. The hydraulic base is commonly preferred to the bituminous base, which is practically obsolete, because it forms an unyielding structure, not likely to be forced out of place by the weight of traffic at any point where the support of the road-bed may be weakened.
Macadam base. •In surfacing streets with asphalt which have previously been macadamized, it is some times possible to use the old macadam as a base for the asphalt. This offers a good base in so far as it can be used without disturbance. It is difficult, however, to change the grade or reduce the crown without destroy ing the bond of the macadam. Old brick and stone pavement's may also be used in the same way, where they can be used without disturbing them.