Concrete Roads
and Their Construction

144 CONCRETE ROADS suddenly and the pressure being just as suddenly relieved the engine will “ race ” badly until pulled up more or less suddenly by the next piece of stone. This jerkiness of action, is largely inevitable ; it is much less serious in a steam-engine than in a gas, petrol or oil-engine or an electric motor. If one of the last four is used, it should be much more powerful than is strictly necessary in order that it may not be suddenly stopped, and pos- sibly damaged, by a stone of unusual hardness. Where very finely powdered material is used, an entirely different type of crushing apparatus is required ; this cannot suitably be used on the site of a road, and such special material —’which is seldom required—should therefore be bought ready for use. Washers.—-As the presence of some kinds of clay is detrimental to the setting and hardening properties of concrete, and as it is always desirable to wet both the fine aggregate and sand thoroughly before putting them in the mixer, it is convenient to wash them prior to use, though this process is often omitted on account of its cost. Such washing consists essentially in stirring up the material with a sufficiently large volume of flowing water to bring the clay and “ dirt ” into suspension and to carry it away. Wash- ing machines used for this purpose therefore consist of somejtorm of container fitted with agitators and some means of carrying tho sand forward in ono direction, whilst the water, bearing away the clay, etc., flows in another. A considerable number of types of washers are used in various industries, but they require adaptation before they can suitably be used for the fine aggregate and sand used for concrete. As all types of washers necessarily require very large volumes of water, they cannot usually bé employed on the site of a new road, but are preferably erected on the site whore the fine aggregate or sand occurs or at some central site to which it is taken, treated and then delivered in a washed state to the road-makers. The Baxter Ballast Washer and Grader, illustrated in Fig. 95, is, however, being used on the site by the Southwark Borough Council, London. Screens are essential for the production of a properly graded aggre- gate, and failure to use them has, on several occasions, resulted in