Materials and Design.

—The physical properties of materials—their tensile, compressive, and shearing strengths, their elasticity, brittleness, etc.—while important elements in the durability of pavements, the design of the pavement, its thickness, the proportioning and mixing of parts, the laying, as well as the subgrade and its treatment are all elements that count very much also. No matter how good a material it can easily be spoiled in the handling. Some materials like vitrified brick and stone will last indefinitely on a little-used street while others like asphalt and creosoted wood block are much better for considerable wear. The use of definite and often meticulous specifications is to insure good materials and proper manipulation of the same, while the plans are carefully prepared ahead, so that durability and satisfaction may result.

GIVING A MACADAM ROAD AN APPLICATION OF TARVIA BINDER

This is Followed by a Coat of Screenings and then the Road is Rolled Again.

A ROAD OF MIXED ASPHALT AND CONCRETE BEING TESTED OUT

The effect of character, weight, and density of traffic has been frequently mentioned and will again be referred to in what follows. There is no doubt a relationship between materials and design and the character and amount of traffic. A cinder road may be perfectly acceptable for a park drive where the traffic is light, but absolutely worthless under heavy commercial trucking.

Resistance to traffic varies with different road surfaces. A smooth hard surface offers a very great deal less resistance than does a rough or soft surface. To illustrate, a horse is said to be able to pull directly on the traces one-tenth his own weight without being overworked. With a resistance of 100 pounds per ton (earth road in medium condition) a team of horses weighing 1200 pounds each could draw over a level road 2 × 120010 × 100 = 2.4 tons. On a concrete, asphalt or brick pavement having a tractive resistance of 30 pounds per ton the team could draw 2 × 120010 × 30 = 8 tons. In other words the load that can be drawn is inversely as the tractive resistance. Here speed was not considered. It was the natural walking gait of the horse about three miles per hour. If the speed is greater the load must be cut down proportionally. With a truck the direct pull is the effective power of the engine in foot-pounds per minute divided by distance in feet per minute; and the load that can be drawn is the direct pull times the tractive resistance. Thus if a truck may exert h effective horse power = 33,000h foot-pounds per minute, and the speed is v miles per hour, the load T, in tons, that may be hauled on a road having a tractive resistance of t pounds per ton, is

T = 33,000h 5280v60 · t = 375hvt.

Therefore a truck of 20 effective horse-power will haul over a road whose tractive resistance is 100 pounds per ton at a speed of 10 miles per hour a load of

T = 375 × 2010 × 100 = 7.5 tons;

and on a smooth road with a tractive resistance of 30 pounds per ton at the same speed, 25 tons, or the same load 7.5 tons may be drawn at a speed of 33¹⁄₃ miles per hour.

It must be remembered that when the speed is increased the tractive resistance is likewise increased. The air resistance is in about the ratio of the square of the velocity, so that 33 miles per hour would be too great in the last case.

Experiments to determine the tractive resistance due to the surface vary considerably, for it is impossible to secure like conditions of surface smoothness and cleanliness, to say nothing of hardness. The tractive resistance will with some materials vary with the temperature. That of sheet asphalt, for example, may be twice as much in summer as in winter. The tractive resistance may not be directly proportional to the load although it is customary to express it in pounds per ton. It is conceivable that a heavy load because it sinks into the road crust may require a greater number of pounds to move it than a light load that does not greatly sink in. This also leads to the effect of width of tire and diameter of wheel. Many experiments have shown the tractive force to be less with wide than narrow tires, due, no doubt, to the unequal sinking into the road crust. Likewise wheels ought, for the same reason, to show less resistance for large diameters; in fact some engineers give it as varying inversely as the diameter of the wheel.

The results of tests, while varying much, show in a general way, the direct pull necessary to draw a load at slow speed on the level in well-lubricated wagons to be approximately as follows:

	Lbs. per Ton	μ = coefficient of Resistance
Upon Steel rails	10	¹⁄₂₀₀
Sheet asphalt, good condition	20	¹⁄₁₀₀
Asphaltic macadam or concrete, good condition	20	¹⁄₁₀₀
Concrete, good condition	20	¹⁄₁₀₀
Brick, good condition	20	¹⁄₁₀₀
Broken stone water-bound macadam, good condition	30	³⁄₂₀₀
Gravel, good condition	30	³⁄₂₀₀
Sand clay, good condition	60	³⁄₁₀₀
Earth, best condition	67	¹⁄₃₀
Earth, medium condition	100	¹⁄₂₀
Earth, poor condition	300	³⁄₂₀