Concrete Roads
and Their Construction

MECHANICAL DEVICES FOR MAKING ROADS 153 misuse ; they do not require any power, such as is always the case with mechanical measurers. The Measurement of Water is most satisfactorily effected by means of a tank of the required capacity which is fixed at a suitable height above the mixer, and is so arranged that a definite quan- tity of water—-neither more nor less than is required—-is delivered on opening the tap. An ordinary water-saver syphon tank with a ball-valve controlling the feed is excellent for the purpose, as it delivers the water with the utmost rapidity and can be refilled automatically from a largei’ tank without any trouble. Water- measuring tanks fitted with taps are often troublesome, and the delivery of the water is usually too slow. The water tank used in connection with a concrete mixer should be of ample size, yet not excessively large. For the size of mixer most suitable for road work, a tank delivering 17 or 18 gallons is satisfactory. Elevators.—The cement, aggregate and sand must usually be lifted from the ground level to a short distance above the inlet of the mixers. If the material is transported by means of a conveyor no further arrangement for raising it is needed, but in other cases some form of elevator is desirable. Such elevators may conveni- ently form a part of the mixing plant. The use of an elevator reduces the cost of charging the mixer and, under certain condi- tions, inci’eases the output by reducing the time required to charge the mixer. The advantage is very marked when the mixer is located at a considerable elevation above the level at which the aggregate, etc., is delivered. Many elevators which have been in use in the past are primitive and crude in design and arrangement, but several more recent designs are quite satisfactory. Among the latter is the Ransome Elevator (Fig. 98), which consists of an elevating skip of rectangular form to avoid undesirable clogging when discharging its contents. Fhe skip is controlled by a single lever, by the use of which all operations of lifting, discharging, lowering and steadying during its descent are controlled. Mixers.—The machines used for mixing concrete are arranged in two classes, according as they are operated by hand or mechanical power. Hand-Mixers are chiefly useful for small repairs, as they are very