noiselessness and low traction with certain types of bituminous materials; very good from sanitary standpoint.

DISADVANTAGES. Among the disadvantages attendant upon the use of bituminous macadam amd gravel pavements should be noted: increase in cost over bituminous surfaces on macadam and gravel roads; slipperiness when some bituminous binders are used on certain grades; dependence upon climatic conditions in

order to carry on construction properly; variability in results and lack of uniformity in composition of wearing surface secured due to uneven penetration, see Fig. 135, uneven distribution, see Figs. 136 and 137, and segregation of road metal.

CAUSES OF FAILURE. The causes of failure of bituminous macadam and bituminous gravel pavements may be considered under the following heads, bituminous material and methods of construction.

Bituminous Material. Unfortunately many are the instances where unsuitable bituminous materials have been employed. In some cases the materials were satisfactory in themselves, but were used improperly. See Fig. 138. Many engineers having charge of bituminous work do not appreciate the fact that different types of bituminous materials have entirely differ

FIG. 138.

ent physical properties and require entirely different treatment in use, although they may have been purchased under one and the same specification covering chemical and physical properties. In some cases entirely unjustifiable combinations of materials are employed. For instance, one case is in mind where an asphalt of excellent characteristics was used for the first applica

FIG. 140. Soft Seal Coat Torn up Due to Adhesion to Tires.

tion, while for the second application an asphaltic oil having decidedly solvent and fluxing properties was employed. Overheating of the material has likewise proved the cause of many failures as thus the properties of the materials are sometimes changed and in many cases the materials are ruined.

Methods of Construction. Under the heading construction, we find failures due to the uneven distribution of the bituminous material in some cases when horse drawn or power driven distributors are employed. This type of failure, however, is more frequently due to the improper use of hand pouring pots and hand drawn distributors. Many unsatisfactory bituminous macadam pavements result from the use of the wrong sizes of broken stone. One instance will be cited where a hard broken stone ranging from 2 to 32 inches was used for the

wearing surface. See Fig. 139. After rolling, 11⁄2 gallons of bituminous material were applied and the road finished with a layer of chips. Failures due to the rapid formation of fine cracks caused by the rocking movement of the individual stones under traffic, finally resulting in raveling and general disintegration, are of common occurrence. Segregation of sizes of stone preventing uniform penetration results in weak spots in some cases and "fat" spots in others. In certain cases after a rain the construction has been carried on before the broken stone immediately below the surface has dried out. Many of the causes attributed to the failures of bituminous surfaces may likewise apply to bituminous macadam and bituminous gravel pavements. See Fig. 140.

CHAPTER XIV

BITUMINOUS CONCRETE PAVEMENTS

DEVELOPMENT. The first bituminous concrete pavement was probably constructed about 1840 in Nottingham, England, while in the United States the first construction of this type of pavement occurred at Knoxville, Tenn., in 1866. From 1870 to 1875 there were about 70,000 square yards of bituminous concrete pavements laid in Washington, D. C. From 1888 to 1893 many yards of coal tar distillate pavements were laid in Washington because Congress had prohibited the use of sheet asphalt pavements in the District. From 1880 to 1891 several sections of bituminous concrete pavements using coal tar as the bituminous material were laid in Ontario, Canada. Another early bituminous concrete pavement was built in Concord, New Hampshire, and is still in use today. During the closing period of the 19th century attention was directed in England to the details of construction of bituminous concrete pavements for use on highways outside of built up districts. In the United States at the opening of the 20th century Fred J. Warren urged the use of bituminous concrete as a pavement for streets in competition with sheet asphalt, wood block, and brick pavements. Based on experimental work during 1906, 1907, and 1908, Rhode Island in 1909 was the first State to adopt the bituminous concrete pavement as a standard type of construction for use on state highways. Since 1910 there has been a rapidly growing appreciation of the inherent value of many different types of bituminous concrete pavements for use on highways outside built up districts as well as on certain streets of municipalities. Instances of development in this field of construction will be cited later in connection with each of the types of bituminous concrete pavements. Two notable contributions to the literature relative to the historical development of bituminous con