Sometimes the mortar is inadvertently made too wet, and an excessive amount of water appears in floating and troweling, par ticularly on a cool damp day. To take up this water, dry cement is sometimes sprinkled over the surface; but this practice is very undesirable, since it leaves the surface too rich in cement and likely to be spotted in color. A surplus of cement makes the surface of the walk more friable than though the proper proportion of sand had been used. The best method of removing this excess of water is to absorb it with a dry mixture of cement and sand of the pro portions used for the top coat, and then there will be no excess of cement and no spottedness.
In troweling, particular care must be taken to consolidate the edges of the blocks; and the general tendency to trowel the blocks low in the center must be carefully guarded against, as these de pressions retain water after a rain and keep the walk needlessly wet. The troweling should be done so that when a 4-foot straight edge is laid in any direction upon the walk a space greater than 1/8 of an inch will never be found under it, and seldom a space greater of an inch will be found.
Troweling for an excessively long time is very objectionable, since it is liable to work an excess of cement to the surface, a result which makes the walk more slippery and less durable.
While completing the troweling, the wearing coat is to be separated into blocks by laying a straight edge to the marks pre viously made upon the side forms, and with the point of the trowel cutting through the wearing coat exactly over the cut previously made in the concrete base. The joint is then finished by rubbing it with a tool similar to that shown in Fig. 157. The edge of the walk also is finished by running over it a tool similar to that shown in Fig. 158, the front face of the tool as shown being placed next to the wood frame.
Some engineers specify that after the troweling has been finished and the joints and edges have been rubbed down, the entire surface shall be brushed with a damp bristle-brush, to remove the trowel marks. The brush-finish gives a uniform dull surface that appears better than the surface left by the trowel. See § 527.
Other engineers require that the surface shall be marked with a toothed roller somewhat like that shown in Fig. 159, page 608, the object being to render the walk less slippery (see § 952).
After the wearing surface is finished, the walk must be protected from the weather and other injury until it has thoroughly set. It is well to shield the walk from the direct rays of the sun and from strong winds for at least one day, in order that the water required for the setting of the cement. may not be lost by evapori
zation. If the weather is dry, it is well to keep the walk moist by sprinkling it frequently; but it should not be sprinkled until the surface has hardened, lest it be pitted by the drops of water. In a very dry time, the necessity for frequently sprinkling the surface may be obviated by covering the walk with sand, straw, etc.
The forms should not be removed until the cement has set so hard that there is no danger of injuring the edge of the walk in re moving them.
Joints. The walks should be formed in blocks from 3 to 8 feet square, to prevent settlement of the foundation or contraction by cold from making unsightly irregular cracks. If the walk is 5 with sand to prevent the tar from being tracked over the surface of the walk. Damage by expansion is more common after the walk is two or three years old than before, owing to the fact that when first made there is more or less empty space in the joint which takes up the expansion; but this space gradually becomes filled with dirt, and no longer absorbs the expansion. The damage from expan sion is not very serious. For example, a contractor who has laid 250,000 square feet of cement walk in the past ten years has been called upon to repair only five breaks due to expansion.