Concrete Roads
and Their Construction

138 CONCRETE ROADS secure the maximum pressure on the roller, whilst still retaining sufficient “ weight ” on the rear wheels to ensure satisfactory driving up steeply inclined gradients. For road-making, a steam-roller should have sturdy construc- tion and ample boiler capacity, with the boilers and cylinders mounted so as to facilitate quick repairs. The design should be as simple as possible, as such machines work continually under adverse conditions and frequently in out-of-the-way places. In order that the roller may be stable, its centre of gravity should be low, yet there should be sufficient space above the ground to prevent the engine being damaged by obstacles which the roller has failed to crush or by the machine having to work on very irregular ground. As steam-rollers are not required the whole of the time, it is convenient to attach a belt to the fly-wheel and from this to drive a crusher, screen or other machinery which can be operated at intervals. Motor Rollers in all sizes have been in successful use for a great many years. They are manufactured in both the single and double cylinder types, and some of them can be run on paraffin, the advantages of the motor-rollers being that they can Joe operated by one man, that keeping them supplied with fuel is less costly, and that they do not require a constant supply of water. Machines at present on the market are manufactured by Messrs. Barford & Perkins, Peterborough, and the Austin Manufacturing Company of Chicago. Preparing the Concrete The chief mechanical devices used in preparing the concrete are ci’ushers for the coarse aggregate, sand washers, screens, measuring devices, appliances used for transporting the raw materials as well as the concrete “ slop,” mixers and engines for the supply of power. It is convenient to consider each of these separately. Crushers should reduce the large lumps of stone or other material used for coarse aggregate so as to produce angular fragments of the required size. Machines which produce rounded pieces are useless for road-concrete. Jaw-crushers and gyratory crush- ers are the most satisfactory ; in the former the lumps are crushed between two plates, one of which moves towards and away from