CUSHION LAYER - BRICK This is a layer of sand between the foun dation and the wearing course of brick, to secure a uniform bearing for the latter. The proper thickness of this layer will depend upon the regularity of the upper surface of the concrete foundation and also upon the uniformity of the size of the bricks. It should be thick enough to give a uniform support to the bricks, and any greater thickness does no particular harm except that it is a little more difficult to spread exactly uniformly a thick layer than a thin one. In common practice, the thickness varies from 1 to 21 inches, but usually from 1 to 2 inches. A thickness of 1-inch is enough to fill up all reasonable irregularities of the foundation, but it is not enough to permit a uniform bedding of the brick by rolling. Ap parently a 1-inch cushion is not thick enough to permit the sand to flow sufficiently to adjust itself to the inequalities of the brick. Unless the blocks are unusually uniform, the cushion layer should be 2 inches thick.
The thickness should be as uniform as possible, so that the bricks will settle evenly during the rolling; and therefore the top of the concrete foundation should be carefully finished with a surface parallel to the surface of the pavement. Not infrequently loose fragments of stone are left on the surface of the concrete, a result which is very undesirable, since they necessitate a thicker cushion and at best prevent the bricks from coming to a uniform bearing. With good workmanship in laying the concrete, there will be no loose pieces of stone on the surface ; and if they do happen to get there, they should be removed before laying the cushion coat.
In adjusting the thickness of the sand cushion adjoining a concrete gutter, care should be taken that the upper surface of the brick after being rolled is not below the upper edge of the gutter.
When the sand cushion is laid on a foundation of broken stone (§ 562), care must be taken to roll the stone so that the jar of the traffic will not cause the sand to work into the broken stone, thus permitting the pavement to settle and to become rough and uneven. If the broken stone is rolled until the surface of the layer is firm and solid and does not shake under the foot in walking over it, unless the stone is very hard and tough there is not much danger of the sand sifting into the stone.
The sand for the cushion should preferably be so fine as to be of a soft, velvety nature. and should contain no pebbles of any considerable size, or loam. or vegetable matter. The size of pebbles permissible depends upon the thickness of the sand bed.
Pebbles will prevent the brick from having a uniform bearing; the loam is likely to be washed to the bottom of the layer and cause the brick to settle; while the vegetable matter will decay or wash away, and leave the brick unsupported. The sand should be dry when it is spread. Even a small per cent of moisture in the sand adds considerably to its volume, particularly if it is fine ; and hence if the sand when laid is wet and dry in spots, the cushion will not be of uniform thickness when dry. The shrinkage of the sand cushion away from the brick, under certain conditions causes an unpleasant rumbling of the pavement when heavy vehi cles pass over it (see § 781).
The spreading of the sand should be carefully done, so as to secure a uniform thickness and to have its upper surface exactly parallel to the top of the finished pavement. After the sand has been distributed approximately to the proper thickness with a shovel, the surface should be leveled by drawing over it a tern plate conforming exactly to the curvature of the cross section of the proposed surface of the pavement. Practice differs consid erably as to the length of the template.
Some contractors make the template the full width of the pavement, if that is less than about 30 feet, and for a wider pave ment make the template half the width of the street. This form of template must be made of a 2-inch pine plank of sufficient width to permit of the cutting of its lower edge to the proper curva ture, which may be determined by the method explained in § 310 (page 200). If the template is long, it must be braced to prevent bending and sagging; and it must have a long and substantial handle at each end by which to draw it forward, and another handle at each end by which to carry it backward. It is desirable that the template shall have considerable weight to keep it from lifting up as it is drawn forward; and when being drawn forward, the face of it should lean backward a little to keep it from lifting up. At each end there should be a roller or a metal runner to carry the template along the top of the curb or along the edge of the con crete gutter. If it is to run on top of the curb, a roller also should be provided to keep it away from the curb. If the length of the template is equal to half the width of the street. one end of it may run upon a screed, or wood strip, equal in thickness to that of the cushion layer, placed in the center of the street. If there is a car track in the street, one end of the template may be made to run on the rail.