moved to a depth of not less than 1/4 inch and the new surface must, when compressed, be not less than 1⁄2 inch in thickness. The heat shall be applied in such a manner as not to injure the remaining pavement. All burnt and loose material shall at once be completely removed and, while the remaining portion of the old pavement is still warm, shall be replaced by new and freshly prepared wearing surface made and laid in strict accordance with these specifications."

"Conditions at Expiration of Guarantee. In addition to the proper maintenance of the pavement during the period of guarantee, the contractor shall, at his own expense, just before the expiration of the guarantee period, make such repairs as may be necessary to produce a pavement which shall:

(a) Have a contour free from depressions of any kind exceeding 3% of an inch in depth as measured between any two points 4 feet apart on a line conforming substantially to the original contour of the street.

(b) Be free from cracks showing disintegration of the surface mixture.

(c) Contain no disintegrated surface mixture.

(d) Not have been reduced in thickness more than 3% of an inch in any part.

(e) Have a foundation free from such cracks or defects as will cause disintegration or settling of the pavement or impair its usefulness as a roadway."

Cost Data. Based upon experience in the construction and maintenance of sheet asphalt pavements in the District of Columbia for many years, the following conclusions * have been reached by the Engineering Department of the District.

"It is actually cheaper to maintain pavements at an average age of eleven years, resurfacing when the cost of patching becomes excessive, than it is to maintain them on a lower standard and of a greater average age. In order to bring the average age down to eleven years Congress has lately increased the appropriation, and in another year or two we shall have arrived at the point

See Municipal Journal, April 4, 1912.

where the pavements can be maintained with the maximum of economy. Very soon, when the average age of our pavements has been reduced to eleven years, we can compute by a simple formula the amount that will be required for asphalt pavement maintenance. A most careful study shows that we shall then need 101⁄2 cents each year for each square yard of pavement in existence if we continue to operate under the contract system, or approximately 92 cents per square yard per annum if we procure a municipal plant, following the example of the more carefully managed American municipalities.

"At the present time the prices paid for resurfacing and repairing asphalt pavements in Washington are as follows:

"Laying standard asphalt pavement (21⁄2 inches asphalt surface, 2 inches binder before compression, with 6-inch concrete base), $1.68.

"Laying standard asphalt surface, 21⁄2 inches before compression, 64 cents.

"Laying standard asphalt surface, resurfacing by the heater method, per cubic foot, 66 cents.

"Laying asphalt binder in connection with resurfacing work, per cubic foot, 28 cents.

"Laying standard asphalt surface for repairs, cuts, and miscellaneous work, 57 cents per cubic foot.

"Laying asphalt binder for repairs, cuts, and miscellaneous work, 43 cents per cubic foot.

"Laying standard asphalt surface for repairs, etc., within the space required by law to be kept in repair by street railway companies, 63 cents per cubic foot.

"Laying asphalt binder for repairs, etc., within the space maintained by the street railway companies, 48 cents per cubic foot."

Information relative to the yearly cost of maintenance of sheet asphalt pavements extending over a number of years has been recorded by the Bureau of Highways of the Borough of Brooklyn, N. Y. Typical examples are cited in the following table, No. 22, of pavements on which the guarantee has expired.

CHAPTER XVI

WOOD BLOCK PAVEMENTS

DEVELOPMENT. Russia is credited as being the first country to use a pavement constructed with wood blocks. Although wood has been used in this country in the construction of both corduroy and plank roads, neither type could hardly be called a wood pavement. The first wood block pavements in this country were laid in New York and Philadelphia about 1835, in England about 1838, and in Paris about 1880. The blocks first used were round or hexagonal in shape. Round cedar block was extensively used in the Middle West some years ago. The patent office contains many records of different types of wood block pavements that were patented at different times between 1840 and the present. One of the first methods in which rectangular blocks were used was the Nicholson pavement. This pavement was laid in many cities of this country, between 1860 and 1870, and was perhaps the most successful wood pavement up to that time. The blocks, which were 3 inches thick and 6 inches long, were laid in parallel courses with 1-inch joints on a plank sub-base and the joints were filled with hot gravel and coal tar. Little care was taken in selecting the wood for the blocks, and it was not until 1872 that a concrete foundation was used. In the development of this type of pavement, it was found that the rectangular block was the best shape to use, while other details of construction, such as proper foundation and joint fillers, were found to have a direct bearing on the success of the pavement. It has also been satisfactorily proved that unless the blocks are treated by some preservative process, the life of the pavement is considerably shortened. The success of the present type, which has been developed by the gradual improvement of the early methods of construction, is evidenced by the large amounts that have been laid in London, Paris, and

the United States. It has been extensively used in many of the largest cities of this country, among which might be cited the following: Greater New York, Boston, Chicago, Minneapolis, Detroit, Cincinnati, Toledo, and Indianapolis. It has also proven to be an excellent pavement for surfacing the roadways of bridges, many instances of which can be found.

THE WOOD

WOODS COMMONLY USED. Very little thought was given to the kind of wood used in the earlier types of wood pavements, and without doubt this fact hastened the failure of many of the pavements constructed. The "Nicholson" pavement was sometimes constructed of blocks made of soft pine. The round blocks with which the streets of Chicago were largely paved in the early days of the city were cedar. Among the other woods which have been used in various cities of the United States may be mentioned oak, cypress, hemlock, Washington red cedar, cottonwood, mesquite, Osage orange, redwood, Douglas fir, tamarack, long-leaf yellow pine, short-leaf pine, Norway pine, and black gum. A great many of the wood pavements in England are constructed with Swedish deal. Experiments have also been tried there with camphor wood from Borneo, oak, beech, and both Australian and American red gum. The Australian woods, jarrah and karri, which are extremely hard and dense, were first used in London about 1891. They are only used to a slight extent now in England, the softer woods being preferred. The wood pavements of Australia, however, are largely constructed of jarrah, which is one of the principal woods of that country. In France, pine of Landes and of Gascogne are used to a great extent. Red pine of Nord, or Sylvestre pine, commonly but improperly called red fir of Nord, is also used. Little attention was given to the proper seasoning of the wood used in the earlier pavements, and only in rare instances were the blocks treated with any kind of materials which serve to preserve the wood. The importance of these things has been realized in the development of the modern wood pavement so that