=Destructive Agencies.=--It is well to have clearly in mind at the outset that the concrete in a road surface is subjected to certain destructive agencies not usually significant in connection with the use of concrete, and these are so often disregarded that the average serviceability of the concrete road surface is sometimes much lower than it would be if built with due regard for the effect of traffic on concrete surfaces. In most structural uses of concrete, its strength in compression only is utilized, and the factor of safety is such as to eliminate to some extent failures due to inferior materials or workmanship. The concrete road surface is subjected to compression under wheel loads, to bending, causing tension in the concrete, to abrasion from wheels, and to tension and compression due to effect of temperature. The weight of the wheel loads may cause sufficient distortion of the road slab to produce rupture. The aggregates may be crushed under wheel loads if the material is too soft. Abrasion from steel tired vehicles wears away the concrete unless it is hard and durable. Changes in dimension due to the effect of change in temperature introduce tension or compression into the road slab and may result in cracks. Freezing and thawing in the subgrade subjects the slab to vertical movement and discontinuous support with the result that longitudinal and transverse cracks occur. The foregoing indicates the importance of securing good concrete for road surfacing, and that is accomplished by using suitable aggregates, by proper design of the road surface and by following established construction methods. =Design.=--The widths usually adopted for concrete roads are: for single track roads, 9 or 10 feet, and for double track roads, 18 or 20 feet. The thickness is 6 to 8 inches at the middle, varying with climatic conditions and with the kind of soil upon which the concrete is laid. The thickness at the edge is 1 inch less than at the middle except that 6-inch surfaces are usually of uniform thickness, the total crown being 2 inches. The thickness of the two course pavement is the same as would be used for a single course pavement in the same location. The surface of either width has a total crown of one or two inches to insure water running off the surface. The earth foundation is often flat, the crown being obtained by making the slab thicker at the middle than at the edge. Fig. 18 shows cross section for concrete roads. [Illustration: Fig. 18.--Cross Section for Concrete Highway] In the state of California, concrete roads four or five inches thick and surfaced with a bituminous carpet mat have been successfully constructed. Similar designs have been used in a few other places, but for general practice it is unsafe to depend upon such a thin slab. Climatic and soil conditions probably account for the success of the thin roads in California.