Cushion Layer - Brick

A long template requires considerable force to draw

it forward, some contractors using one or two horses for this purpose, and it is difficult to move backward. Other contractors, therefore, use a template equal to one quarter of the width of the pavement. For a pavement 30 to 40 feet wide, screeds made of by scantlings are placed at the crown. in the gutters, and also midway between the crown and the gutter, being bedded on a thin layer of sand so that their tops conform to the finished surface of the proposed sand cushion. The position of these screeds is deter mined by measuring down from a string stretched from curb to curb. The template may be made of a 1-inch by plank, with a 1-inch by 2-inch handle braced by two 1-inch by pieces. The edge should be hollowed out to fit the curved surface of the pavement, although often this is not done. The middle ordinate for the curved cutting edge of the template may be com puted which m is the middle ordinate in inches, C the crown of the pavement in inches, d half the length of the template in feet and D half the width of the pavement in feet.

After the sand for the cushion layer has been distributed with shovels, the template should be drawn slowly over it several times, any depressions that develop being filled by sprinkling sand into them with a shovel. A considerable quantity of sand should be

drawn along in front of the template, as this aids materially in packing the bed. It is necessary to draw the template several times to pack the sand well, particularly if there are wet and dry spots, as the successive jarring of the sand grains causes them to settle more closely together. When the sand cushion is properly packed it will have a uniform, smooth, velvety appearance, and will not look rough, porous, and grainy.

The surface of the cushion layer is often prepared with a short lute or scraper without any screeds; but the template and screeds secure a more uniform surface and also give a greater compression and a more even bed. With hand luting the surface of the pavement is almost certain to be covered with saucer-like depressions after it has been rolled. Hand luting should be pro hibited except where the use of the template is impossible, as around man-hole covers, at street intersections, etc.

A considerable part of the difference in tractive resistance between brick pavements No. 4 and Nos. 5 and 6 of Table 8. page 29, is due chiefly to the difference in the preparation of the sand cushion, the remainder of the difference being in the rolling of the brick (§ 771).