Not infrequently pavements having a concrete foundation are found which have settled over trenches. Does not this prove that the ordinary concrete foundation is not strong enough ? No, it simply proves that the foundation over the trench was not strong enough. There are two remedies for this condition of affairs. Either consolidate the filing in the trench better by rolling or tamping as described above, or make the concrete thicker over the trench. The first is cheaper and more scien tific. In no case is it justifiable to thicken or strengthen the founda tion over the entire street simply because trenches occupying from 5 to 10 per cent of its surface may not have been properly back-filled.
Before considering substitutes for concrete foundations, let us exam ine the concrete a little further. Formerly it was the custom to use a rich natural-cement concrete, because it was cheaper than a Portland cement concrete of equal strength. A few years ago nearly all the Portland cement used in the country was imported, while now nearly all of it is of domestic manufacture; and further, it is not only home made, but is both better and cheaper. Although natural cement is marvelously cheap, a of a given strength can be made cheaper of Portland than of natural cement. Experiments made at the Uni versity of Illinois show that a concrete composed of one part cement, eight parts of coarse sand or fine gravel, and eight parts of screened or broken stone was considerably stronger in compression and also in bending than a concrete composed of one part natural cement, three parts of the same sand, and three parts of the same broken stone. These proportions have been practically tested in the construction of half a mile of. pavement with the greatest satisfaction to all parties concered. Prices vary greatly with the locality, but in most, if not all, cases Portland-cement concrete is cheaper in proportion to strength than that made with natural cement.
Before dropping the subject of concrete foundations, a few words should be said in condemnation of the quite general practice of leaving the upper face of the concrete needlessly rough and irregular, with loose stones strewn over the surface. To secure a uniform surface for the pavement, the cushion coat should be of uniform thickness, and hence the top face of the concrete should be practically parallel with the sur face of the finished pavement. Also any loose stones on top of the concrete causes the brick to be broken during the rolling and produces inequalities in the surface of the finished pavement. Both of these effects can be eliminated without appreciable expense by a little care.
Gravel—Where gravel is cheap, it is better to use a thicker layer of gravel without cement than a 6 inch layer of gravel with cement, i. e., a 6 inch layer of concrete. To secure a good foundation, the gravel must be properly used, which apparently is seldom or never done.
The usual method seems to be to dump the gravel upon the subgrade directly from wagons, and then to level off between the piles with shovels. By this process the lower part of the original piles is much more compact than the space between the piles; and rolling does not materially lessen the inequalities, since the roller, being a cylinder of considerable length, rides upon the tops of the piles and does not com press the gravel between them. The result is that soon after the pave ment is completed, the natural settlement of the gravel foundation causes the surface to be full of depressions.
The better and cheaper method is to level off the piles with a scrap ing road-grader, (the ordinary "road machine") and then thoroughly harrow the gravel with a long-toothed harrow, after which the founda tion should be rolled. For the best results, the gravel should be spread in layers not more than three or four inches thick. Brick pavementh upon gravel foundations laid by this method have shown no depressions after many years, while those constructed with the utmost care by the preceding method with the same gravel on the same soil have been full of holes. This is another example showing that cheaper materials and proper methods intelligently give better results than expensive materials improperly used.
Broken-stone—Where broken stone is cheap, it is better economy to use more stone and omit the cement from the foundation, i. e., use broken stone alone instead of concrete. If the rock is soft or contains much fine material as it comes from the crusher, it should be screened to take out all dust and most of the pieces up to say 1/4 inch in greatest dimensions. . The broken stone may be hauled to the street in wagons, and dumped upon the subgrade. It may be spread by hand with forks or rakes, or it may be spread with a scraping road-grader, the latter method being the cheaper. In spreading the stone care should be taken that the several sizes are not separated too much, and that the piles on which the stone was dumped from the wagons are not left too high. The layer of stone should be rolled until the individual stones do not move as one walks over the surface, or until the surface stones are not easily kicked out with the foot. After the completion of the rolling, the surface of the broken stone should be impervious to the sand to be used in the cushion coat. With most stone this condition will be secured by the crushing of the top layer of the broken stone during the rolling; but if there are spots that are porous, throw on a few shovelfuls of fine stone and roll again. If the stone is hard, it may be necessary after the rolling is nearly completed to apply a thin coat of finer or softer stone. Of course the top of the foundation should finally be left smooth and of proper grade and crown.