BINDING THE ROAD FOR BROKEN--STONE The interstices between the frag ments of stone should be filled with a fine material which will act mechanically to keep out the rain water and thereby keep the subgrade dry, and also to support the fragments and pre vent them from being broken. and which will act physically and possibly also chemically to bind or cement the fragments into a single more or less solid mass. The proper binding of the stone is the most important part of the construction of a broken stone road.
The material employed to fill the interstices in a broken-stone road is usually called the binder. and sometimes the filler.
Clay and loam are frequently used. Their merit is that they are cheap, are easily applied. and have a high cementing power (see Table 20. page 187) ; but they are easily affected by water and frost, and when wet act more as a lubricant than as a binder. Clay or loam binder will give a smooth road without much rolling, but such a road is liable to be very dusty in dry weather, and muddy in wet weather. When clay or loam is employed as a binder, the utmost care should be taken that no more is used than just enough to fill the voids.
Shale and slate are only hard and compact clay, and their only merit is that they give a smooth surface with but little rolling. They are speedily reduced to dust, and then have all the disadvantages of clay. They have fair cementing power—see Table 20, page 187.
Sand is often used as a filler, and if composed of fine, clean, hard grains, gives fair results; but sand which is resistant enough for a good binding material usually consists of silica or quartz, neither of which has a high cementing power (Table 19 and 20, page 186). If the grains are coated more or less with iron oxide, or if accom panied by bits of ironstone (clay cemented with iron oxide), sand makes an excellent binding material, since the iron possesses con siderable cementing power. This form of binder is particularly valuable in making repairs over an opening when a roller is not available, or when water for washing in the binder is scarce. Low grade iron ore has been used for a binder—either alone or mixed with stone dust.
Fine screenings—the finest product of the stone crusher, say, from or I inch to dust—from the stone used in the body of the course is the most desirable material for a binder, partly because it helps to utilize the entire product of the crusher, partly be cause of its high crushing strength. and partly because the stone
is usually selected for the high cementing power of its dust. Lime stone has very high cementing power, but is soft and pliable. Trap has a fair cementing power, and is hard and durable. Limestone screenings require less rolling, but the trap dust makes a more durable road.
Sometimes the detritus removed from the surface of a stone road during maintenance or preparatory to making repairs. is employed as a binder. At best, such material is very poor for this purpose. It is worn out and has performed its duty; and, besides, it is com posed largely of manure and vegetable and earthy matter—all of which are very undesirable in a hinder. Such detritus is more valuable as a fertilizer than as a road material.