Any moderately hard stone is suitable for making the concrete base; and the stone is usually crushed to pass a 1-inch ring. For a discussion of the relative merits of gravel and broken stone for use in concrete, see § 553. It is important that the sand or gravel used in the concrete base be clean, so that in tamping a film of clay may not work to the top and make a surface of separation between the concrete base and the mortar top. The following proportions are common: 1 part Portland cement, 3 parts sand, and 6 parts of unscreened broken stone (see § 555).
The concrete should be mixed rather dry in order that there may not be a film of water on the top of the concrete base which will pre vent a firm union between the top and the base. If the concrete is too moist, the mass will shake like wet clay; if it be too dry, it will rise up around the rammer like sand. In either case, the mass can not be suitably compacted by ramming, and will therefore be comparatively weak and porous after setting. The concrete should be thoroughly and uniformly tamped until moisture flushes to the surface. Particular care should be taken that the concrete base is well consolidated along the outer edges, so that frost will not break them up. This point is often neglected, because tamp ing the edges is likely to crowd the forms out of place.
It is important that the upper surface the concrete base should be exactly parallel to the top of the finished walk. To determine whether this condition is fulfilled draw a properly-made template over the tops of the side forms.
The concrete is cut into blocks by laying a straight edge on the marks previously made on the side forms (§ 942), and resting a short but broad blade, see Fig. 156, against the straight edge and driving it downward by striking it with an iron concrete-tamper. After the blade has been driven to the bottom of the concrete, it is drawn out and moved along, and the process is repeated until the concrete is cut through across the entire width. Some engineers specify that the space made by the cutting tool shall be immediately filled with fine sand; but it is better to leave it entirely open (see § 942).
The proper proportion of sand to cement depends upon the voids in the sand. There should be enough, and only enough, cement to fill the voids. If there is not enough cement to fill the voids, the sand will not be held with the maximum strength; and if there is an excess of cement, the walk is liable to crumble under travel, since neat cement will not resist abrasion as well as sand and cement. Sand is not as good as screenings, but is cheaper and is mutt more commonly employed.
Sand frequently contains a considerable proportion of soft and easily decomposed constituents which renders it unfit for use in the wearing coat of cement sidewalks, since the friable grains soon pulverize and blow away, leaving a hole or pit in the surface, which not only looks badly but also tends to hasten the destruction of the walk. Granite screenings are frequently used instead of natural sand, but some granites contain mica, hornblende, and feldspar which render them undesirable for use in cement walks. Crushed quartz is best for this purpose, but is expensive on account of the difficulty of crushing it. Pure silica sand is entirely satis factory. Screenings are considerably more expensive than sand (see § 955), and if used, should be perfectly free from fine dust.
The thickness of the wearing coat depends upon the amount of traffic, a thickness of an inch being employed where the traffic is light, and 1 inch where it is heavy. The mortar for the wearing coat should be mixed rather dry, and should be applied before the cement in the concrete base has begun to set, in order that the two layers may firmly unite. The mortar is to be brought to a uniform thickness by laying a straight edge on the side forms and drawing it longitudinally along the walk. The mortar should then be rubbed and compressed with a float (a plasterer's wooden spreading trowel) to expel the air bubbles and the surplus water. Just as the cement in the top coat begins to set, it is to be rubbed smooth and hard with a plastering trowel, sufficient pressure being employed to force the top and bottom layers into close contact so that they may firmly adhere.