Agents of Destruction Broken-Stone Road

Wind. The strong winds which prevail over a large part of this continent, in connection with long droughts, remove the dust from between the fragments of the stone. This effect is more serious on the less frequented highways, since the sparse traffic does not make enough dust to renew the cement in the crevices between the stones, particularly as some of it is also liable to be washed away by rains.

Rain. The action of violent rains in destroying broken stone roads is an important factor, and somewhat peculiar to this country. The rain often removes the binding dust between the top stones to such a degree that they become loosened. This effect is specially important on slopes of considerable declivity, as is shown by the "raveling out" which often occurs on steep grades or on roads which have an excessive crown. This indicates that on grades the crown should be only sufficient to turn the rain water speedily into the side ditches.

A moderate amount of water, if gently applied, is an advantage to a broken-stone road, since the cementing action of the binding material is greater when wet, and for this reason sprinkling is an important factor in maintenance (§ 380).

Frost. Although freezing and thawing has little or no effect upon the individual pieces of stone (§ 737), frost has a very injurious effect upon the road as a whole. It is desirable that the binding material shall at all times be wet, and if the construction is defective or the maintenance poor, there is a liability of con siderable water being present in the road-bed; and this water in freezing expands, breaks the bonds, and makes the road open and porous. A road that has been well filled and thoroughly rolled

will have only a small per cent of voids, and therefore there will be but little water retained in the body of the road and consequently be but little damage from frost.

Decomposition of

Stone. It is sometimes claimed that chemical decomposition of the stone is a cause of deterioration of a broken-stone road; but this effect is so small as to be entirely inappreciable. On the contrary, the decomposition of the stone may be an advantage to such a road. Many of the rocks employed in road building are slightly soluble in water containing carbonic. nitric, or sulphuric acid gathered from the atmosphere, or in water containing humic acid derived from the decomposition of vegetable and animal matter; and consequently any rain or surface water which penetrates a broken-stone road may dissolve the fine stone dust and afterwards deposit it in the interstices lower down, where it will act as a cementing material. This may be, at least in part, the explanation of the well known fact that a crushed-stone road, for a time at least, improves with age. The improvement is doubtless also in part due to the gradual infiltration into the smaller interstices of the binding material of very fine particles of dirt from the surface.