CEMENT SIDEWALKS. This is the term usually applied to an artificial-stone walk composed of a hydraulic-cement con crete base and a cement-mortar top. Such a construction is sometimes called a concrete walk, and sometimes a granolithic walk. Numerous patents have from time to time been issued for various details of cement walk construction, but the essential features are not covered by patents. Within the past few years cement walks have become very common, not only in the cities of the Mississippi Valley, where natural stone suitable for walks is quite expensive. but also in the eastern cities, where suitable natural stone is plenty and cheap. Cement walks are smooth, pleasing in appearance, reasonably cheap, and when well constructed are very durable.
The foundation for a cement walk should be practically the same as that for a brick walk (see § 922). Since the cement walk is composed of large rigid blocks, it apparently does not require so heavy a foundation as a brick walk; but, on the other hand, a slight settlement of the foundation is more serious with a cement walk than with a brick one, which fact seems to show that the cement walk should have a heavier foundation than a brick one. Water freezing under a cement walk is liable to crack and displace the blocks; and therefore if the soil is retentive or is not already artificially underdrained, it is wise to lay a line of tile longitudinally under the proposed walk, which shall have a suffi cient outlet to carry the subsurface water entirely away. It is not uncommon to excavate the foundation 11 or 2 feet below the surface of the proposed walk, apparently that the porous founda tion may act as a drain or a reservoir to prevent water from stand ing against the lower side of the concrete and perhaps freezing and lifting the walk; but a tile subdrain is cheaper and more effective than a deep but undrained foundation. If the underdrainage is even fairly good, a depth of 6 inches of cinders or clean gravel will make a satisfactory foundation, provided it is firmly and uniformly tamped; and with poor drainage the thickness need not be more than 8 inches. On residence streets in small cities, 4 inches of
cinders or gravel with fair drainage would doubtless be sufficient, although the foundations are usually made much thicker.
The finished surface of the subgrade should be made exactly parallel to the top of the proposed walk. To secure this condition, some engineers specify that the depth of the subgrade is to be gaged by a template run on the top of the forms after they have been placed.
The edge of the walk is marked by a 2-inch by 4-inch scantling securely staked in position with its top face in the plane of the top of the finished walk. These scantlings should be blocked up so as accurately to maintain the longitudinal grade of the walk, and should also be so securely staked that they will not be crowded out by the tamping of the concrete.
The forms for short curves should be made by sawing the proper curve out of an inch plank, and then nailing .enough of them to gether to give the proper thickness. Care should be taken in joining the straight form to these curves, to prevent an unsightly change of direction. Large curves can be made by using a finch by 4-inch plank on edge for the side, and springing it into the proper curve and staking it fast; but care must be taken with the ends of adjacent pieces to secure a uniform curve..
In residence districts of small cities, the base is usually 3 inches thick; but on residence streets of large cities it is often 4 inches, and on business streets it is sometimes 5 inches. The base consists either of a rich natural-cement con crete or of a rather lean Portland-cement concrete. The latter is the more common, and the relative merits of the two classes of cements for this purpose will be considered presently (see § 945). For a discussion of the theory of proportioning the concrete, see § 549; and for the method of mixing, see § 557.