Concrete Roads
and Their Construction
År: 1920
Serie: Concrete Series
Forlag: Concrete Publications Limited
Sted: London
Sider: 197
UDK: 625.8 Con-gl.
Being a Description of the concrete Roads in the United Kingdom, together with a Summary of the Experience in this Form of Construction gained in Australia, Canada, New Zealand and the United States of America.
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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,