Binding the Road for Broken-Stone

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.

Nature of the Binder.

The binding material or the filler should be finely divided so as to be easily worked into the inter stices, should have a considerable resistance to crushing so as to properly support the pieces of crushed stone; and should not change its physical condition when wet. Various materials have been employed—clay. loam. shale. sand. and limestone and trap rock screenings.

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.

Applying the Binder.

There is a difference of opinion among competent engineers as to the best method of applying the binding material. Some apply it on the top of each course, and some on top of only the last course. In the first case, all the voids from the bottom to the top of the road are filled with fine material; in the second case, the binder usually fills the voids of the top course only. Those who advocate the first method claim that the whole mass should be filled to prevent the stones from moving under the traffic, and also to prevent the soil from working up from below; while the advocates of the second method claim (1) that filling the top layer is sufficient to hold the stone in place near the surface, (2) that the stones of the lower courses have no ten dency to move, (3) that the unfilled voids of the lower course pro mote drainage, and (4) that as the upper layer wears away, the dust will wash down into the lower open spaces in such a manner as always to keep the 3 or 4 inches just below the surface properly bound. If the stone is hard, or if the lower courses are not thor oughly rolled, applying the binding material only on the top of the last course practically fills the voids to the earth foundation; but of course it is cheaper to apply the filler on the top of each course than to attempt to fill all of the voids by applying it on the top course only. If the stone in the lower courses is soft, or if the top of the next to the last course is thoroughly rolled, applying the binder on the top fills the voids in the top course only. It is suffi cient to fill the voids of the.top course.