BINDER COURSE - This is a layer about 11 inches thick of broken stone cemented together with asphaltic paving cement (§ 603), and rolled in place while hot. The purpose of this course is to bind the wearing coat and the foundation or base together, and to prevent the wearing coat from lifting from the foundation or from being pushed along in a wave.
The broken stone is screened to pass a 1-inch or a 1i-inch mesh, and after being heated not more than 5 or 10 per cent should pass a No. 10 screen. An excess of fine stone is undesirable, since more asphalt is required to coat it. and also since the coarser stone gives a rougher upper surface and therefore affords a better anchorage for the top course of the pavement.
The apparatus for heating the stone consists of a revolving steel cylinder about 30 inches in diameter and 12 to 14 feet long, set at a slight inclination. On the interior are deflectors to distribute the stone in its course through the drum. The cylinder is heated by means of either wood or fuel-oil, preferably the latter, as it can be more closely regulated. The broken stone to be heated is carried by an endless chain and bucket elevator to a hopper just above the cylinder in which it is to be heated, from which it is fed into the heat ing cylinder. The temperature of the stone as delivered from the heater is controlled, not only by the fire, but by the rate at which it is fed into the heating drum. The stone should leave the heat ing drum at a temperature of about 300° F.
The hot stone is fed into a mixer which consists of an inclined steel cylinder 2i to 3 feet in diameter and 10 to 14 feet long. On the axle of this cylinder are blades which push the material through the drum and also aid in mixing the stone and the asphaltic cement. When the mixing is well regulated, the hot stone is fed into the upper end of the mixer, the asphaltic cement at a temperature of 300° to 325° F. is poured over the stone in proper proportions from a drip ping tank, and the binder drops continuously out of the lower end of the mixer into a cart. Considerable skill is required in regulating the temperature of both the stone and the cement, and in adding the proper amount of the latter.
The asphaltic cement is the same as that used for the wearing coat (see § 603), except that it is mixed much softer. The asphalt for the wearing coat is usually mixed to a consistency represented by a penetration of 70° to 90° with the Dow needle (§ 613), while the cement for the binder course has a penetration of only 45° to 55°.
This cement is added to the hot stone in the proportion of 6 to 7 pints of cement to 1 cubic foot of stone; or in other words, the binder is mixed so as to contain about 5 per cent of bitumen soluble in carbon dioxide. Each fragment of stone should be thoroughly coated with cement, but there should be no excess. If too much cement is used, it either drains off on its way to the street, or is drawn by capillary attraction into the wearing coat and causes it to disintegrate (§ 642). if too little cement is used, the fragments of the stone will not adhere firmly together and the whole course is liable to break up under the roller. The surface of the coated stone should be bright and glossy, and should not appear dull and dead, a condition which is due either to an overheating of the stone or to a lack of cement.
While hot, the mixture is hauled to the street, distributed uni formly over the foundation, and rolled until it is cold, the founda tion having previously been made clean and dry. After being com pacted, the binder course should have a thickness of about 1 inches, and should firmly adhere to the foundation.
The above method of making the binder course is the one employed at Washington, D. C., but sometimes a practice somewhat different prevails. The stone is made very fine, one quarter to one half of an inch in greatest dimension, and the cement. is made of the same penetration as that in use in the wearing surface. The finer stone requires more asphalt to coat it, but the resulting con crete is stronger, and is less likely to pull to pieces in hauling the wearing coat to place; but with the finer stone the upper surface is smoother, and therefore offers less resistance to the pushing of the top coat into waves. On the whole the finer stone and the softer cement are preferable, since the binder then is more elastic and less liable to damage in hauling the wearing coat over it.
Formerly coal tar was used as the cement in the binder course, but it has nearly been abandoned, owing (1) to its variability and (2) to the fact that it must be used at a lower temperature (about 220° instead of 300° to 325°), and consequently chills more easily, (3) to the fact that it is a weak cement, and therefore is more liable to damage in placing the top course, and (4) because it some times contains an oil which being absorbed by the wearing coat causes the asphalt to disintegrate.