ASPHALT PAVEMENTS are divided into sheet and block. Sheet asphalt consists of a binder and a wearing surface, the hinder serving to unite the foundation and the wearing surface. The binder is composed of small pieces of broken stone, united with asphaltic cement. The mix ture is spread on the foundation in a layer suffi ciently deep to give the requisite thickness after having been rolled. A final thickness of one inch is considered ample by some, but one and one half and even two inches is not uncommon. The wearing surface is composed of sand, carbonate of lime (powdered limestone). and asphaltic cement, mixed hot in varying proportions. The object is to use enough powdered limestone to fill the voids in the sand as completely as possible, and enough asphaltic cement to bind the whole together. It is also desired to make the pave ment impervious to water. The consensus of opinion is that the wearing surface should con tain 9 to 10 per cent. of bitumen and have a final thickness of about two inches. It should be spread on the binder while the mixture is hot, say at a temperature of 250° F. The spreading is effected by men with rakes, who are followed by other men with hand rollers. Next hydraulic
cement is scattered over the surface. Atter this comes first a five-ton and then a ten-ton steam roller. Thus, three rollers are used in succes sion, the lighter ones first, in order to prevent distortion of the asphalt while soft. Generally, the asphalt is laid clear up to each curb, special care being taken with the gutters, hut sometimes brick or stone is used in place of asphalt in the gutters. The need of special precautions here is the danger from the water that gravitates to the gutters. It is claimed for the Alcatraz and Bermudez products that they may be laid in glitters without harm, but some conservative engineers did not regard this as proved up to the close of 1900. Cracks are liable to occur in sheet asphalt subjected to wide variations in tem perature. Most of the asphalt pavements laid in America have been composed of Trinidad sheet asphalt, but in recent years both the Bermudez and Alcatraz products have been employed. See A