improved and the architect plans his building from the grade of the street upon which is the main entrance.

To overcome this a break of grade should be made at the property-line whenever the rate of grade exceeds 2 per cent. A grade of 2 per cent on an intersection insures a maximum difference of 0.6 at A when the sidewalk width is 15 feet as in the figure. When this width is more or less than 15 feet the rate of grade should be modified so that in no case shall there be a greater difference in property-line grades as calculated above than 0.6 of a foot. This difference can be reduced to a minimum by reducing the standard slope of the sidewalk on one side and increasing it on the other. On residence streets where buildings set back from the street this rule can be somewhat modified.

The following is recommended as a form for arranging curbelevations in grade ordinances, the body of the ordinance specifying that the grade shall be a straight line between given points:

The above method has been used successfully at Omaha, Neb.

CHAPTER XV.

ASPHALT PLANTS.

THE machinery used for the preparation of the asphalt mixture is technically termed the plant. Its province is to take the different materials and mix them in the proper proportions for the pavement, and the arrangement that will properly do this the most cheaply and expeditiously is the best.

When it is understood just what is required of the plant, it is simply a problem in mechanical engineering as to its exact design. Like all machinery a large first cost means a small outlay for operation and maintenance. But some contractors prefer to carry out details by manual labor rather than by machinery, giving as a reason that it is less liable to cause delay and in the end is more economical. All details must be governed by circumstances and the conditions of each case.

Before taking up methods of construction, it will be well to discuss somewhat the capacity and location of the plant, the two being associated more or less, especially in cities that cannot be supplied from one location.

Capacity.

In cities of 150,000 or even 200,000 inhabitants there is no necessity for more than one plant from an economical standpoint. Where different companies are operating in the same city, each must have a plant of its own; but it is proposed to discuss the question with a view to providing for a community as economically as possible, for the time may come when municipalities will lay asphalt pavements. New York City has agitated the question, and Toronto is now considering it.

Assuming that weather conditions will permit asphalt to be

laid from the first of May to the first of December of each year, and that there will be twenty days per month suitable for work, there will be 140 working days each season, and a plant having an output of 1000 square yards per day would lay 140,000 yards per year. This would be as much as would be often required in a city of the size mentioned above. It is perfectly practicable, however, to construct one of greater capacity, and it is a matter of record that one Brooklyn plant laid 250,000 square yards of pavement in 1897. When, however, such an amount is laid, the hauls in many instances must be excessive and then there comes up the question of

Location.

This is not decided simply by the streets to be paved. All material used must first be taken to the plant, and as sand forms about 85 per cent of the entire wearing surface, the question of its supply becomes an important factor in determining the location. If sand must be brought from outside, a plant near the railroad or water-front is almost a necessity.

The price of real estate must often be taken into consideration, as what would otherwise be an ideal location might be prohibitive on account of the fixed charges if the cost of the necessary land should be too great.

There is also a certain amount of dust in and around every asphalt plant, and if located in the vicinity of expensive machinery, enough damage may be done to cause a nuisance.

There has been one case of the removal of a plant from a desirable locality simply on account of the destruction to machinery in adjoining buildings by the dust being carried into open windows.

Territory within 24 miles from the plant can be economically supplied with material. Working ten hours per day, an ordinary team will haul four loads for that distance; but if the haul be much increased, the loads would be reduced to three and the cost of hauling correspondingly increased.

Assuming the wages of a team to be $5 per day and that each load will lay 33 square yards of surface, at four loads per day the

cost of hauling will be 3.8 cents per yard for surface and 1.9 cents for binder, or 5.7 cents complete. With three loads per day per team the cost would be 7.6 cents per yard. Where but a small amount of work is projected outside of the 24-mile limit, the extra expense would not be much, but in establishing more than one. plant in the same city it should be seriously considered.

It is probable, however, in making a final decision upon a location, that the governing principles will be cost of real estate and the facility for receiving the crude material.

The actual work to be performed by the plant is heating the sand and stone for binder, making the asphaltic cement, and sometimes the stone-dust, and mixing these different ingredients in their proper proportions.

Heating the Sand.

This is an important function. To lay 1000 square yards of pavement will require approximately 60 cubic yards of sand and 30 cubic yards of stone for binder. The plant should have a capacity somewhat in excess of that amount, so that a stoppage of a few hours would not interfere with the supply for the mixer. As sand retains its heat when in bulk for a long time, no loss ensues from the storage of quite an amount.

Sand comes to the plant as a rule damp and often wet. All moisture must be evaporated and the material brought to a temperature of 400°. The sand should be heated as uniformly as possible, for if any portion be too hot, it is liable to burn and injure the asphalt even when the average temperature would be satisfactory. Just what temperature asphalt will stand without injury. is not positively known, but there is no necessity of heating it much. above 400° F.

The sand is generally heated in hollow steel cylinders or drums set at a slight vertical angle. Upon the inside of the cylinder angleirons are bolted at intervals. The cylinder is set in a solid frame and surrounded by an iron jacket. The heat is applied to the space inside the jacket and the cylinder revolved. The sand is carried on the angle-bars up the side of the cylinder for a certain distance, when it falls to the bottom, is again carried up, the operation

being continued as long as the sand remains in the drum. The drum being set at an angle, the sand falls vertically and advances towards the lower end a distance equal to the sine of the vertical angle. The time sand is retained in the drum is regulated by the rate of its revolutions. In addition to the above the capacity of the sand drum will be governed by its diameter and length.

Some heaters are constructed on a somewhat different plan, the heat-space being in the centre of the drum, and the sand falling directly upon the heated cylinder. The other details are practically as given above. An attachment is made by which all dust and steam are carried off.

As the heated sand is delivered at the end of the drum, it is carried by elevators to the storage-bin on the mixing-platform.

Asphaltic Cement.

To lay 1000 square yards of pavement will require approxi mately 25,000 pounds of asphaltic cement for the top and 6000 pounds for the binder, when made from Trinidad asphalt. The refined asphalt is melted in kettles and the petroleum residuum or other flux is then added. As a different amount of flux is used for the binder cement, separate sets of kettles will be necessary for economical operation. Three kettles should be provided for the surface mixture and two for the binder. As it requires about eight hours to thoroughly mix the oil and asphalt, as soon as one kettle is emptied it should be immediately refilled so that there will always be some cement ready for use.

The mixing, or agitating as it is generally called, is done mechanically and also by means of air.

The mechanical method consists simply of an arrangement by which what is practically a paddle-wheel is revolved in the kettle, keeping the material in constant motion.

With the air process a series of perforated pipes are laid in and around the inside of the kettles. By means of a pump air is forced into these pipes and through the perforations to the surface, thoroughly mixing the oil and asphalt. It is extremely important that these two ingredients become thoroughly incorporated with each other, so that the flux shall act upon all the asphalt, convert