The first compression is given by hand rollers and tamping irons. Two sizes of hand rollers are in use : the lighter is 30 inches in diam eter, has a 24-inch face, weighs 1,000 pounds, and gives a pressure of 42 pounds per linear inch of face; the heavier is 28 inches in diameter, has an 18-inch face, weighs 1,400 pounds, and gives a pressure of 77 pounds per linear inch. Fig. 115, page 417, shows a hand roller with a fire pot inside for heating it. Tamping irons are used around man-hole covers, near the curb, etc., where the roller can not be conveniently used. Fig. 116 shows two forms of asphalt tampers. The larger has a face about 8 inches in diameter and weighs about 30 pounds; and the smaller has a face about 2i" X 5i", and weighs about 18 pounds. The former is used for general work; and the latter next to curbs, street-car rails, etc. The tamping irons are heated in a fire in an iron basket which is moved from place to place on wheels. The original method of com pression was first to run a hand roller (whose surface was prevented from taking up any of the sticky mixture by being oiled with kero sene) rapidly over the surface, four men being employed for this work. This method is still employed to a certain extent, but it has been improved upon and superseded in part by a form of roller which is attached to the front of the steam roller, and which is heated by steam. It is guided by a parallel motion from the steer ing gear of the steam roller and does away with the necessity of any one's walking on the newly laid surface.
After the first compression with hand rollers and tampers, men with hot tamping and smoothing irons, Fig. 116 and 117, proceed
to finish the gutters, joints, and all angles and edges which can not be reached with the heavier rollers. The gutters are tested with straight edges to detect depressions; and any inequalities on the surface are removed. The use of hot smoothing irons and hot rollers should be discouraged, since it is impossible always to have them of such a temperature as not to injure the pavement, and since, if the mixture is delivered at the proper temperature, and the raking and spreading is done expeditiously, they are unneces sary. Experience shows that the surface of pavements upon which hot smoothing irons were used scales and flakes off, while the face of pavements laid without such hot tools does not chip and scale off.
The first compression having been given, some natural hydraulic cement or any impalpable mineral matter is dusted over the surface to give it a more pleasing color and to prevent adhesion of the roller; and then the surface is next rolled with a steam roller of the asphalt pattern (Fig. 65, page 225). In the most approved method the pavement is first rolled with a roller weighing about 5 or 6 tons, and next with a roller weighing 10 or 12 tons. Often only the light roller is used. If the street is wide enough, the pavement should be rolled transversely as well as longitudinally; and if this is not pos sible, the roller should run as obliquely as possible, so that any little inequality which might be caused by the roller's moving lengthwise may be taken out by the cross action. The rolling should be kept up until the roller leaves no mark, a result which usually requires at least 5 hours for each one thousand square yards of surface.
The rolling should closely follow the spreading of the material, so that it shall not have time to cool before the final compression is obtained. The state of the weather also is an element to be con sidered; for if a strong wind be blowing, the material, spread over a broad surface only 2 or 3 inches thick, will cool much more rapidly than on a calm day when the temperature is considerably lower.
When the rolling is completed, the pavement may be thrown open. Traffic, if not of too heavy vehicles, is an advantage to a newly laid asphalt pavement, since the pressure of the wheels aids in consolidating the wearing coat and in clo'sing the surface, a result which helps to retain the volatile oils and prevents the entrance of water. Asphalt pavements in unfrequented streets do not wear so well as under a moderately heavy traffic.