FOUNDATION.
Choice of Materials—There are several forms of foundation suitable for brick pavements, viz.: Concrete, gravel, macadam, a course of brick laid fiatwise, or a layer of plank. Concrete is by far the most common foundation ; and apparently a plank foundation was never used except in a single city, and it has, been abandoned there.
It seems to be the common belief that only a 6-inch layer of concrete is a suitable foundation for a brick pavement. The truth is that in some cases a 6-inch layer of concrete is unnecessarily thick, while in other cases a layer of gravel or of broken stone will make an equally good and more economical foundation. Less skill is required with a gravel or broken stone foundation than with a concrete foundation. Those who have made cuts into concrete pavement foundations report that in many cases the concrete is no better than layer of broken stone without cement, due apparently to carelessness, or inefficiency, or dishonesty in the construction. The process of placing gravel or broken stone is simpler, and therefore there is less danger of inferior work; and the gravel or broken stone requires less hand labor, which is an advantage to contractors in these days of inconsiderate demands of laboring men. As to whether concrete, gravel, or broken stone be used for the foundation in any particular case depends upon local prices and the local condition; and right here is where the city needs engineering advice of a high order, for a single word in the specification may add hundreds of dollars to the cost of the work without any return. It is more scientific and usually more profitable to give time to the consider ation of the specifications beforehand than to the Niggling with the contractor afterwards.
The various forms of foundations will be considered separately.
it seems to be the general belief that a 6-inch concret base is necessary for a brick pavement; at least, this foundation is used indiscriminately for business and residence streets, and is used indiscriminately also on the stiffest soil and on the softest. A 6-inch concrete foundation is ordinarily used under an asphalt pavement, which is a more or less flexible layer from 2 to inches thick; while the same thickness of concrete is ordinarily used with a brick pavement having a cement filler, which is a very rigid layer from 5 to 7 inches thick. Is there any evidence that the foundations of asphalt pavements are generally too weak ? An engineering journal recently contained an account of a test of the supporting power of an "asphalt pavement, made by hauling over it a truck weighing 22,300 pounds and giving a pressure of slightly over three tons on two wheels having tires four inches wide. The founda tion consisted of a 4-inch layer of natural-cement concrete mixed in the proportion of one part cement, two parts sand, and five parts crushed stone. The asphalt wearing surface was two inches thick. The subgrade consisted of "soft wet clay which has been much disturbed by many trenches for sewers and for water and gas pipes." The pavement had been in use twelve years when tested, and had shown no signs of failure. "The above load was hauled over this pavement from end to end and produced no effect upon the pavement except to make a slight depression in the asphalt where the wheels stood for half an hour, the day being warm." Does this prove that the concrete foundation of brick pavements are generally needlessly thick ? Surely if four inches of concrete over soft clay and under a 2-inch asphalt wearing-coat can support such loads, six inches of concrete is needless under brick pavement with cement filler.
Let us consider this question from another point of view. There are three and only three reasons for constructing a pavement, viz.: (1) To secure a-smooth surface for case of cleaning and to decrease tractive resistance; (2) to secure an impervious roof to prevent rain water from softening the subgrade; and (3) to interpose a layer thaf shall distribute the concentrated load of the wheel over so great an area of the subgrade that it can safely support the load without depres sion. For the moment, we are not concerned about the smoothness of the surface, and hence nothing will be said here about the first reason for constructing a pavement. The wearing surface of a brick pave ment is practically impervious whether sand or cement filler be used, and consequently a concrete foundation is not necessary to secure a water-tight roof to protect the subgrade. Therefore, the concrete foun dation of a brick pavement acts only to distribute the load of the wheel over the subgrade. The concrete distributes the load by virtue of its ability to act as a beam ; and this property is due to the cement which the concrete contains. If there were no cement in the concrete, the layer of gravel or crushed stone would distribute the concentrated load of the wheel over a considerable area. ' The pressure of the wheel is transmitted downward in diverging lines; and if the point of contact of the wheel is considered as the apex of a cone having its base on the subgrade, it may be assumed that the load of the wheel distributed nearly uniformly over the base of this cone. It is unwise to attempt here to go into the mathematics of the subject further, but the efficiency of a layer of broken stone in distributing a concentrated load is proved by the fact, that, under favorable circumstances as to soil and drainage, 4 inches of broken stone has successfully carried considerable travel, while 6 inch macadam roads are quite common in a number of states. If 4 or 6 inches of macadam without any other pavement will carry travel, the same thickness will certainly make a good foundation for a brick pavement under ordinary conditions—particularly if a cement filler is used, since the filler gives the course of brick a considerable transverse strength, as will be discussed later. The writer recently saw a piece of brick pavement with sand filler which is laid directly upon the black loam of the Illinois corn belt, which for six .or eight years has carried the heaviest travel of a city of three or four thousand inhabitants, and which is still in good condition. A tile drain was laid at each side of the street, the subgrade was well rolled, and the ,paving bricks (not blocks) were laid upon a layer of sand and small gravel only one or two inches thick. Probably no small part of the success of this pavement is due to the fact that a prominent intelligent and successful local business man acted as inspector. The writer does not advocate the general adoption of this form of construction; but cites this case to show what can be done by intelligence and care, and to prove that a layer of concrete is not always necessary. A needlessly expen sive form of construction is not only money wasted, but deters the con struction of other pvaements.