Note 10 — PART I SPECIFICATIONS FOR STREET ROADWAY PAVEMENTS

The ratios of the materials may appropriately be varied with the strength and soundness of the sub-foundation, the amount of travel on the street, and with the thickness of foundation it is proposed to use. Where good materials are used and the work is properly done, a 1:3:5 concrete six inches thick is sufficient for streets of the heaviest travel. For streets of light travel a 1:5:9 concrete will usually give entirely satisfactory results. The most economical thickness for a concrete foundation is an important consideration. The strength of concrete may be said to increase, within usual limits of practice, with the ratio of cement in it. The strength of concrete beams or slabs increases in the ratio of the square of their depth. To secure a required amount of strength in a pavement foundation, we may therefore vary the richness of the concrete and the depth of the foundation so as to secure the requisite strength at the least total cost of materials and labor. This will be influenced by the cost of materials and labor in each particular locality.

Within certain workable limits there is no reason why the same principles of proportioning the strength of a pavement foundation to the work required of it should not be applied as are employed in designing other engineering structures.

The practice, usual in many cities, of adopting general specifications requiring a standard thickness of foundation and composition of concrete, and applying these to all streets, regardless of the quantity and character of travel which the pavement is expected to carry, is illogical and often very wasteful. If such a standard foundation is sufficient for the streets of heaviest travel, it is obviously a sheer waste of money to use it on the suburban streets carrying the lightest travel. It is therefore better in preparing standard specifications for pavement in any city to leave blanks for the ratios of the concrete and for the thickness of the foundation, to be filled in, in each individual case, as the judgment of the engineer may dictate.

While it is important that the foundation of any pavement shall be adequate, it is inexcusable to waste money in providing superfluous strength. For the great majority of suburban streets, carrying but little except the local travel, a foundation four inches thick made of good Portland cement concrete in the ratios of 1:4:8 will prove entirely satisfactory. Hundreds of such streets paved over a foundation of that thickness, made of natural cement concrete in the ratios of 1:2:4 can be cited where the foundation has proved entirely satisfactory.

The character and firmness of the sub-foundation must, of course, be taken into consideration in designing the foundation.