SHEET ASPHALT PAVEMENT
Note.—A number of distinct varieties of asphalt are now used for asphalt pavements, either alone or mixed. These different varieties differ from each other quite widely in their physical and chemical properties. Thus, in the form called “refined asphalt” some of their properties are shown by the following table, the data for which is taken from the second edition of Richardson’s “The Modern Asphalt Pavement.”
| COMPARATIVE PROPERTIES OF DIFFERENT REFINED ASPHALTS | |||||
|---|---|---|---|---|---|
| Trinidad, average | Bermudez, average of two samples | Maracaibo, average of six samples | Calif. “D” grade, average of two samples | Gilsonite, average of two samples | |
| Softens, degrees F. | 180 | 165 | 225 | 132 | 280 |
| Flows, degrees F. | 190 | 175 | 236 | 151 | 300 |
| Penetration at 78° F. | 7 | 24 | 21 | 48 | 0 |
| Loss, heated to 325° for 7 hours, %. | 1.1 | 3.7 | 3.2 | 1.7 | 1.6 |
| Loss, heated to 400° for 7 hours, %. | 4.0 | 8.8 | 5.5 | 7.1 | 2.6 |
| Bitumen soluble in CS2. | 56.5 | 95.5 | 93.9 | 99.3 | 99.4 |
| Inorganic, other than bitumen. | 36.5 | 2.2 | 2.9 | 0.3 | 0.5 |
| Bitumen soluble in naphtha. | 35.6 | 65.6 | 51.1 | 69.6 | 47.2 |
| Bitumen sol. in carbon tetra-chloride. | 98.7 | 99.0 | 93.2 | 95.7 | 99.8 |
| Fixed carbon. | 10.8 | 13.7 | 17.2 | 17.6 | 13.2 |
The practice has been heretofore, and is at the present time, to attempt to make specifications for asphalt pavements broad enough to include all the various varieties of asphalts, under general requirements which shall admit these, and any new varieties that may appear on the market suitable for the purpose, the object being to permit a wide range of competition. This makes it exceedingly difficult if not impossible to frame specifications that shall be sufficiently explicit and at the same time sufficiently broad to admit these several differing materials. This practice has been adhered to in these specifications, though in this respect they are far from satisfactory to the author. So long as it continues to be the policy of cities to admit these various varieties of bitumen under the same general requirements for crude and refined material, such objectionable specifications cannot be avoided. Even with the great latitude now provided they exclude some materials with which good pavements have been made.
Two remedies for this unsatisfactory condition seem practicable.
1. A city might purchase a sufficient supply of refined asphalt for its use after asking for proposals under suitable specifications with alternative requirements for the different varieties on the market, and after bids are received and the samples accompanying them have been properly examined in the laboratory, award contracts for a supply of one or more kinds, as might seem best for the interests of the city. Stocks of these would be delivered, tested and stored accordingly, in good time for the season’s work. Specifications for construction with special reference to the kind of asphalt it is proposed to use could then be prepared, the contractors to be supplied with asphalt at the city storage yard at a stipulated price per ton. This plan would possess a number of advantages. A similar plan is quite commonly in use with reference to hydraulic cement.
2. Specifications might be framed with special reference to the properties and qualities to be possessed by the asphaltic cement, permitting a liberal range as to the crude and refined bitumens to be used in manufacturing this cement. This would be considered, at the present time, a radical departure from well established custom, but the author sees no reason why it should not be satisfactorily employed.
A sheet asphalt pavement is composed of two essential elements; a mineral aggregate made up of sand of assorted sizes and mineral “dust” and a bituminous cement. When properly compounded, manipulated and compressed these elements make up a bituminous concrete suitable for use as a wearing surface for streets and roads.
The character of the sand is important and we have now sufficient knowledge from experience to specify a sand that will give, approximately, the best results.
The bituminous cement is, however, the element of most importance, and upon its suitability for the purpose depends very largely the utility and durability of the pavements made with it.
It is important that this asphaltic cement shall possess certain properties and qualities, most of which we are now able to define satisfactorily, but others require further practical and experimental study, and some tests not now in use would doubtless be desirable.
It is not a matter of importance what particular crude or refined materials enter into the composition of this cement if the resulting product is satisfactory in use. The prime requisite is a paving cement that shall possess in a high degree the chemical and physical qualities required for making an asphalt pavement of the best quality. If we can devise standards and tests that will enable us to secure such a cement we need not be concerned about its antecedents.
It would be well worth while for paving engineers and those who have laboratory facilities to give attention to this matter. If it shall be found practicable to define satisfactorily the qualities the cement should possess without reference to the materials from which it is compounded, a great advance will have been made, and our asphalt paving specifications could not only be greatly simplified, but much greater precision and definiteness secured.
While great advances have been made in the art of building sheet asphalt pavements and in the framing of specifications for its construction, too many of the specifications still in use are antiquated, indefinite and unsatisfactory. Some of these contain requirements that, if literally enforced, would prevent the attainment of the best results. They are largely survivals of the time when little was known either practically or technically of the science and art of constructing the pavements, outside of the promoters and contractors in the business, who consequently dictated, in a large measure, the specifications used. City engineers were compelled to rely largely on the presumption that the guarantee clauses of the contracts would insure good results, and allowed the contractor wide latitude in the conduct of the work.
While there is undoubtedly much yet to learn, even by the experts, in the matter of the materials to be used, a quite satisfactory working basis has been arrived at, particularly as to the practical side of the work, and a large mass of data accumulated by study and experience is available to the municipal engineer, and the services of independent experts is readily obtainable. There is no longer any good reason, therefore, why the character of the materials to be used, the methods followed, and the quality of the work secured should not be quite definitely and fully specified in the same manner and to the same extent as in the case of other kinds of pavement and with equally satisfactory results.