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Bitulithic Pavement

surface, bituminous, sizes, stone, pavements, cement and concrete

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BITULITHIC PAVEMENT.

The name "bitulithic" is commonly applied to a pavement, the, surface of which is composed of a bituminous concrete, the aggregate being a mixture of several sizes of broken stone, so proportioned as to give a dense material with a small percentage of voids. Pavements of bituminous concrete have been occasion ally constructed for a number of years, but the intro duction of this type of pavement upon a considerable scale began about 1901, when exploited under a patent of the Warren Brothers, and most of those since con structed have been under this patent.

In the construction of pavements of this class the crushed rock is screened into several sizes, which are then mixed together in such proportions as to produce an aggregate with very small percentage of voids. Four to six screens are used, varying from about z inches to A inch openings. Sufficient quantities of the smaller sizes are employed to fill the interstices in the larger sizes; the relative proportions being determined in each instance by experiment upon the particular material in use. " After the proportions have been determined, the mineral material is passed through a rotary screen which separates it into several different groups of sizes. The proper proportion by weight of each of these sizes is secured by the use of a scale having seven beams, the exact required amount being weighed out and run into a double shaft rotary mixer. There it is combined with a bituminous cement which is also accurately weighed in the proper proportion. The whole is then thoroughly mixed together and dumped, while still hot, into carts, hauled to the street, spread, and thoroughly rolled with heavy steam road rollers.

"After the surface is thoroughly rolled, a flush coat of quick drying bituminous cement is applied to the surface. There is then applied a thin layer of hot finely crushed stone, varying from j. to inches in size, according to the roughness of the surface desired. The pavement is again heavily rolled, leaving the street in a finished condition." These pavements are commonly constructed upon bituminous foundations (see Art. 48). When the sub

foundation is not firm, and concrete foundations are required, the surface of the concrete is roughened by scattering stone of about II inch diameter lightly over it, and ramming the stones into the concrete to about half their depths. This forms a bond between the base and surface of the pavement, and prevents the creeping of the surface.

These pavements have been used with good success in many places throughout the United States. They require care and skill in construction, both in securing proper grading of the mineral aggregate and in the character and proportions of the bituminous cement. It is claimed by the advocates of this kind of construc tion that, on account of the density and firmness of the mass of stone of which they are composed, a softer bitumen may be employed, thus eliminating the danger of cracking in cold weather. In some instances, where hard stone has been used in forming the surfaces, there are indications that this construction will give better resistance to wear than the ordinary asphalt surface, but longer experience is necessary to fully test its durability. It has been successfully used upon much steeper grades than sheet asphalt, being reported as affording a good foothold to horses, and satisfactory in one instance upon a 12 per cent grade.

In constructing these pavements, as with sheet asphalt, it has been customary to rely upon the con tractor's guaranty for securing good work and no attempt is usually made to determine the character of the bituminous cement by direct tests. This is an undesirable feature of most work with these materials, and it is to be hoped that, as better information con cerning the bitumens becomes available, more satis factory specifications may become feasible. The fol lowing is an extract from the specifications used in St. Louis in 1908: "Upon the foundation shall be laid the wearing surface, which shall be composed of carefully selected sound, hard crushed stone, mixed with bituminous cement and laid, as hereinafter specified.

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