In building such roads two tracks of concrete, one for each wheel are provided. The sub-grade should be well ditched in the center with cross ditches every fifty feet, as is done with the pole road. It has been suggested that the ditches holding the track be six inches deep and twenty-six inches wide. They are filled to the top with concrete and built with a lip four inches high and four inches wide along the outside on top of the main surface to serve as a guard rail. No forms are necessary except for the guard lip.
A word of caution here may not be amiss. Concrete roads of this nature must be regarded as only experimental, for no specific data are available for determining the proper section of concrete to be used for carrying heavy loads on so narrow a bearing surface. It is evident that the carrying capacity of such strips of concrete would be greatly affected by the character of the sub-base. It will therefore be impossible to specify a standard that can be used under all conditions.
The use of the concrete guard rail is one of the disadvantages of this road. The edges of the rail cannot be made rounding except by special forms and the rubbing of the tires against this rough surface would greatly reduce the tire mileage. In addition, the rail is so exposed to weather and hard wear that it cannot be relied upon to serve effectively for any great length of time. The placing of forms is also a considerable item of expense in building such a road. A method which would eliminate such an expense and at the same time provide a more practical rail would be an advantage.
Figure 2. Cross section of concrete road. Scale—1 inch equals 2 feet.
It has already been said that guard rails are unnecessary with a thirty inch track except on sharp curves and otherwise dangerous places. However, where rails are necessary the wooden rail fastened by bolts embedded in the concrete as illustrated above, is quite effective and readily installed. This consists of a four by six inch plank placed on edge and drift-bolted to the concrete every three to five feet by a three-quarter inch bolt. These bolts are placed in the concrete when it is poured and should be embedded six inches. This will provide a rail less expensive to build than a concrete rail and one which will last longer and save on tires. Replacements are easily made by removing the nuts and placing a new plank in place of the old. With a guard rail of this type, there is left a twenty-six inch track for the wheels to run in.
Experiments by W. D. Pence (Journ. West. Soc. Eng. Vol. VI, 1901, Page 549) on 1 : 2 : 4 concrete give an average value of 0.0000055 inches per degree Fahrenheit for the coefficient of expansion. The richer the concrete, the greater the change in dimension. Due to the expansion, in laying the concrete the track must be broken every twenty-five or thirty feet by placing a half-inch board in the ditch when the concrete is being filled in. Later this board is removed and the joint filled with asphalt so that the concrete may expand without danger of cracking the road.
Cost. The best mix to use in building this road is what is known as the 1 : 21⁄2 : 5. For one cubic yard of concrete, the following amounts of materials will be used for the above mix: 1.21 barrels of cement, 0.46 cubic yards of sand, and 0.92 cubic yards of stone. At the present prices, the cost for the materials for this road is about twenty cents a cubic foot or about $4,400 a mile. The total cost of the road including the necessary grading, ditching and labor, will be from $7,000 to $9,000 per mile.
One of the big advantages of the concrete road is the large gain in traction secured when operating on steep grades. A motor truck will haul up a twelve per cent and down a fifteen per cent grade in wet weather on concrete due to the roughened surface on which the tires do not easily slip. This, of course, would be dangerous to attempt on the other types of roads. Another advantage is the small item of upkeep necessary. A road well laid in the first place should need no repair except to replace worn guard rails as they show signs of weakening. The concrete road, however, will not be generally used except on the mainline by the larger concerns, or for short distances on steep grades where greater traction is desired.