Concrete Roads
and Their Construction

140 CONCRETE ROADS which it revolves impel the materia! much too rapidly through it, with the result that, instead of all the pieces of aggregate of any given size passing through the desired perforations, some of them are carried forward and pass through larger openings. Even when baffles are inserted in the cylinder the grading1—though better—is far from satisfactory. To avoid this serious objection, Messrs. Johnston and Chapman Co., U.S.A., supply a screen which consists of three or more con- centric cylinders (Fig. 92), arranged so that the outermost ones have the smaller perforations. By this means the largest pieces are separated first, and no pieces can pass to the next section of the screen unless they are of the proper size to do so. Such an arrangement has the further advantage of only allowing the smaller pieces to come into contact with the finer portion of the screen, so that these can be made of thinner metal and they are not so rapidly spoiled by wear and tear. Rectangulai’ screens are occasionally employed—-chiefly for small quantities and for relatively crude grading. If properly arranged, however, a series of rectangular screens can be made to work with remarkable efficiency especially for the finer aggregates, and with little or no expenditure of power. Rectangular screens may be of two kinds, horizontal or inclined, the latter being usually more satisfactory and requiring less labour. An excellent type of inclined screen is the “ Newaygo ” screen macle by Messrs. Sturtevant Engineering Co., Ltd., which consists of a sheet of perforated steel plate inclined at an angle of about 45 degrees and arranged so that it is vibrated by a number of hammers which rotate on shafts above the sieve and periodically strike raised projections on the sieve provided for the purpose, thus keeping the apparatus constantly in a state of vibration. The material is supplied to the uppermost end of the sieve by means of a screw conveyor, and as it falls down the incline the small particles pass through the perforated plate whilst the coarser ones run down either into a box below or on to a coarser screen. The vibration of the screen prevents the holes in the sieve becoming clogged, unless the material is very sticky. In some cases, several screens of varying fineness are suspended one below another, so that the material passing through one sieve passes on to a finer one below it, and so on until the whole of the material is satisfactorily graded, when it may be mixed in the required proportions,