Concrete Roads
and Their Construction

MECHANICAL DEVICES FOR MAKING ROADS 145 the production of concrete of such low quality that its failure to withstand the strains put upon it was inevitable. The coarse aggregate should be passed through a series of steol cylinders, the perforations in which are arranged so as to form a series of sieves or riddles through which the material passes consecutively, a portion of it being separated by each screen. A commonly used device consists of a single cylinder, 6-14 ft. in length, the circumference of which is divided into four or more Fig. 95.—Baxter’s Ballast-washing and Grading Machine, fitted with petrol motor. The machine, which is taken from street to street as required and connected to the nearest hydrant, was used by the Borough Council of Southwark for washing and grading the material from the old macadam roads. sections, each of which consists of a series of perforations of defin- ite size, the finest being at the entrance end of the cylinder. When an ungraded coarse aggregate is passed through such a cylinder, the smallest fragments pass out through the smallest perforations, the remainder travel forward as the cylinder revolves, pieces of increasingly larger size being separated until the largest “ stones ” fall out at the exit end of the cylinder and are returned to the crusher for further ti’eatment. The chief objection to such an arrange- ment is that the separation or grading is very inefficient. The slope or inclination of the screening cylinder and the speed at