Page:Popular Science Monthly Volume 90.djvu/749

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Hammering Road-Beds to Make Them Even

��A one -man -operated machine imparts more than a thousand hammer shocks a minute

��MAXY concrete road foundations fail because they are made of a porous mortar or of a mortar in which the materials are not properly mixed and bound together. In the first case, water enters the porous mortar, freez- es, and cracks the entire sur- face; in the second case, the surface wears uneven- ly because there are many patches of improperly bound stone in some places and areas of pure mortar in others. To overcome these difficul- ties, concrete road foundations are now hammered into place as if b\' a thousand hammers to form a


���A concrete roadway being nammered into unifcrmity after the mechanically mixed base has been deposited

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��A motorcycle motor with an unbal- anced fly-wheel supplies the vibration

��road-bed which is of exceptional density and uniformity.

An entirely new method is employed in laying the hammered concrete roadway.

After the con- crete base has been mixed mechanically and deposited in place a lay- er of screened hard clean stone is spread. On top of this is laid a series of narrow slat- ted platforms. These plat- forms are shaken by a \" i b r a t o r , which con- sists of a motorcycle motor with an unbalanced fly'wheel. The metal frame on which the flywheel is mounted is pushed back and forth by one man. As the unbalanced fly- wheel revolves it causes the entire frame- work to vibrate violently, imparting from one thousand to twelve hundred shocks each minute to the wood platforms. These shocks, aided by the weight of the vibrator and the operator, force the hard stones down into the concrete and at the same time drive all air bubbles, cavities and surplus mortar out of the concrete base. The grout or fluid concrete is forced to the surface. It is later swept off to expose the top layer of rock, upon which the final wearing surface of a special grade of coal tar is applied. The tar serves to keep the moisture in the concrete so that it will not dr>' out too rapidly and provides a pro- tective coat so that the road may be thrown open to traffic in a few days instead of a few weeks. It also provides a wearing surface which will last several years; after which the dense concrete may be used as the surface or some other top dressing applied. The top dressing of tar is only one-quarter-inch thick and wears evenly.

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