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Requisites for Road Stone

rock, toughness, resistance, hardness, traffic and power

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REQUISITES FOR ROAD STONE. The principal requi sites of a material for a broken-stone road are hardness, tough ness, cementing or binding power, and resistance to the weather. Usually any stone that is hard and tough will resist the weather reasonably well; but shales and slates, though hard and tough when first quarried, often disintegrate when exposed to the weather. The material for a road surface should also be uniform in quality or the surface will wear unevenly, and the depressions which occur where the materially is comparatively soft will hold water, thus. softening the road-bed and occasioning damage difficult to repair.

Hardness and Toughness.

These two qualities are closely related. Hardness is that property of a solid which renders it difficult to displace its parts among themselves; while toughness enables the parts to yield somewhat without being separated or broken. For road purposes, hardness is the power possessed by a rock to resist the rubbing or the abrasive action of wheels and horses' feet; while toughness is the adhesion between particles of a rock which gives it power to resist fracture when subjected to the blows of traffic. A stone may be hard and brittle, and be quickly pounded to pieces in the road, as quartz; or it may have a high crushing strength and yet be deficient in toughness, and grind away speedily under the abrasion of traffic, as some varieties of sandstones. A road metal should have enough resistance to crushing to support the load brought upon it by the wheels, and enough toughness to prevent its being readily ground into powder. A large part of the fine material is inevitably swept away by the rains and winds, or is removed by scrapers to keep the road in good condition (luring wet weather; and therefore it is important that the fragments should be tough enough not to be unduly pulverized by the traffic. Toughness is incompatible with a high degree of hardness, and in a measure makes up for a deficiency in resistance to crushing. Hardness could be measured by the resistance offered

by a rock to the grinding of an emery wheel; and toughness would be measured by the resistance to fracture when struck with a hammer.

Cementing or Binding Power.

Binding power is the prop erty possessed by rock dust to act as a cement between the coarser fragments composing a stone road. This property is of the highest value, for the strength of the binder determines the resistance of the road to the wear and tear of traffic more than does the strength of the fragments themselves. It is possessed in a very much higher degree by some varieties of rocks than by others, and its absence is so pronounced in some varieties that they can not be made to compact under the roller or under traffic without the addition of some cementing agent. This subject has been studied but little, and only by the Massachusetts Highway Commission, which offers the following tentative conclusions: * " It is difficult to say what brings about this cementation or binding of rock dust. It is clear, however, that with many va rieties of rock it is due to several causes. Experiments made on a number of different kinds of rock dust showed that the more finely they were pulverized the higher would be the cementing value when subjected to pressure, both with and without water; and an increase in pressure seems to produce a corresponding increase in cemen tation. Further than this, in a number of cases similarly made briquettes of the same rock dust gave distinct indication that destruction to the bond of cementation by impact bore a definite relation to the amount of energy expended; i. e., about the same amount of energy was required to destroy the bond in each briquette, even when applied in different loads. The inference drawn from such results would be that cementation in such materials is to a considerable extent mechanical,—that is, the interlocking of the fine particles of dust caused by pressure.

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