Page:Popular Science Monthly Volume 91.djvu/429

 Measuring the Wear of Roads

A means by which the right material for the right place is determined

��THERE are now approximately two million, three hundred thousand miles of public roads in the United States, not including the streets of our incor- porated towns and cities. The materials which are used in them differ, varying from concrete and macadam to cob- ble stones and dirt. The determining of the right material for the right place is a problem the solution of which would save many millions of dollars.

The impor- tance of this consideratio n has led the Office of Pub- lic Roads and Rural Engi- neering to attempt

���Steel block -"'>«--- Electric cable— *C__^'

��The equipment with which the wear on roads is measured. The concrete blocks hold the fine wire which is stretched across the road

��to secure definite in- formation on the wearing qualities of dif- ferent materials under different conditions of traffic. For this purpose, the office has adopted a method which is very accurate. A brass plug is permanently buried in each side of the road, three-quarters of an inch below the surface. Between these plugs a wire is stretched by means of the

��concrete blocks shown in the photographs. One of these blocks is pivoted on the ver- tical rod which passes through it in front, the weight of the block being utilized to keep the wire taut. By placing the vertical rod of each block so as to stand on its respective brass plug, and by then adjusting the length of the wire until the bubble- level shows the rod to be "plumb," the fine wire will always stretch across the road in a certain line which will notvary,how- ever much the road may be worn down.

Having es- tablished a measuring line, the rest is easy. A steel block is placed upon certain spots across the road and a caliper measures the vertical distance from it to the wire. This can be obtained very accurately because the cal- iper screws upward and when the top of it just touches the wire, an electric circuit is closed and a buzzer rings. Repeating this measurement reveals the amount of wear.

���A fine wire is stretched across the road. By noting the differences in distance between it and the road at intervals of time the rate of wear of the road is readily determined

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