Note 6 — PART I SPECIFICATIONS FOR STREET ROADWAY PAVEMENTS

In the great majority of cases the most satisfactory and, in the end, the most economical foundation for a pavement is hydraulic cement concrete. Old paving-block foundation, if constructed as specified in Sect. 36, will give results equally as satisfactory, but if a fair market exists for the blocks taken up from the street, it will usually be found more economical to sell them and construct a concrete foundation for the new pavement. Broken stone or gravel foundations may serve the purpose fairly well upon a street of light travel, but it should never be used on streets of considerable or heavy travel. Its lower first cost is the only thing in its favor, but this will, in nearly every case, be more than offset by the better service and greater durability of the pavement, even on streets of light travel, if laid upon an adequate hydraulic concrete foundation. Proper repairs to pavement surfaces cut into for pipe work, etc., are difficult to make and hardly ever satisfactory over broken stone foundation for the reason that the lack of cohesion in the material allows it to loosen or crumble away from under the edge of the pavement surface, and it is difficult to restore it to its original solidity and strength. The first cost saved by its use is usually not great; for whenever its use would be permissible at all, a comparatively thin and lean concrete would give better results, at a very slight increase in cost. To illustrate: On a suburban street with light travel a concrete foundation four inches in thickness, the concrete made with Portland cement in the ratio of 1 cement, 4 sand, and 8 stone, would be stronger and in every way better than a foundation eight inches in depth of broken stone. At the usual prices of materials and labor, the former may cost about $0.46 per sq. yd., and the latter about $0.40 per sq. yd.; but for the latter there would be required ⅑ cu. yd. more sub-foundation excavation, worth about four cents, so that the equivalent cost would be $0.44 per sq. yd. The difference, two cents per sq. yd., is insignificant when compared with the greater value, better service, and greater durability of a pavement on the concrete foundation. It is sometimes held that the broken stone foundation provides necessary sub-drainage. But all the standard pavements are, or soon become, impermeable to water from the surface, and seepage from the sub-foundation can be better taken care of by the sub-drainage specified in Sect. 26, which should usually cost not more than five cents per square yard of the pavement; and if drainage be required, these sub-drains should be used even with the broken stone foundation.

The practice of laying pavement surfaces, particularly those of asphalt, upon a foundation of old stone blocks, carelessly reset, with the joints unfilled with mortar, is all wrong and should never be resorted to. The integrity and durability of an asphalt pavement depends largely upon the strength and rigidity of its foundation; to lay an asphalt surface, however good, over such an old block foundation, is an inexcusable waste of money.

Old stone block and cobble-stone pavements, that have become solidified in place by long travel over them, make a good foundation for asphalt or other pavements, provided they can be utilized without taking up or disturbing the old pavement; but such cases occur so rarely that they have not been considered in these specifications.

A thoroughly consolidated old McAdam pavement, if not worn too thin, also makes a very satisfactory pavement foundation if it can be used undisturbed, or by simply trimming off the high points.

Low places in old pavements, that are otherwise satisfactory for a foundation, may be brought to the proper elevation with hydraulic concrete. “Binder” material is usually specified for this purpose in foundations for asphalt pavements, but hydraulic concrete is both better and cheaper.