[11]. The routine here described produces better concrete with less expenditure of labor, than the one often followed of putting all the dry materials on the concrete board before any mixing is begun. The writer has proved this from actual records covering a large quantity of work.
[12]. The objections to using mortar for plastering over the concrete are: that it is more costly than concrete; that the two materials may, under certain conditions, separate and the thin mortar surface break up under travel; that, if permitted, the mortar may be used to cover up defective concrete, and that in the case of asphalt pavements the pavement surface is more likely to “shift” on the smooth surface of the mortar than on the rough surface of the concrete. The practice of going over the fresh concrete with street brooms should not be permitted. The only argument in favor of it is that it may be used to conceal defective patches in the concrete.
[13]. See foot-note[[6]], p. [23]. The cost of filling the joints of old block pavement with mortar or grout is considerable. It will hardly ever be less than 20 cents and may exceed 35 cents per square yard, depending on the volume of joints and the local cost of material and labor. The cost of resetting and ramming the blocks with proper care will usually be from 10 cents to 12 cents per square yard, so that the cost of the foundation, exclusive of the value of the blocks, may vary from 30 cents to 47 cents per square yard.
As a good concrete foundation 6 inches in depth can be laid for from 70 to 90 cents per square yard, it is obvious that if the old blocks can be sold for as much as the difference between the cost of the old block and the concrete foundation, nothing will be saved by using the old block foundation. In at least one city, asphalt pavement has been extensively laid over old stone block foundation relaid in a very careless manner, the joints being filled usually with the old sand or loam found in the street. This practice cannot be too strongly condemned. Asphalt pavement surfaces resting on such a foundation are necessarily short-lived and unsatisfactory. The practice of opening the street to travel for a period after the blocks are relaid and before the asphalt surface is applied, helps, under favorable conditions, to consolidate the foundation, but does not remove the objections to it. If heavy rains intervene, the sub-foundation becomes saturated with water, and its resistance so reduced that the stone blocks settle out of shape, particularly in soft spots, and they are usually hastily raised and reset just before the asphalt surface is applied. The result is an insecure foundation fatal to the durability and usefulness of the pavement.
[14]. Where there is a possibility that gravel may be used, the contractor should be asked to name prices for the gravel foundation as well as the stone foundation, since, unless this be done, the change from the one material to the other might be held to be illegal.
[15]. There has been much discussion as to the suitableness of these oil asphalts, called “residual pitches,” for use in making asphalt pavements. When properly prepared from suitable asphaltic oils, so as to comply with the specifications here given, there can be no doubt that good pavements can be made with them. But as they appear on the market, being usually produced at different localities and refineries from crude oils of differing qualities, distilled by somewhat differing methods, and usually at temperatures of from 900 degrees to 1200 degrees, they are likely to vary so greatly in quality as to make their use inadvisable without careful technical inspection. Unless, therefore, the engineer is prepared to make, or to have such inspection made, it is hardly wise or safe to permit their use. They stand, in this respect, upon a footing different from the better-known natural asphalts obtained from large deposits of practically uniform character and quality, where the simpler process of refining is less likely to effect injuriously the chemical quality of the material.
It may be confidently predicted that any of these “residual pitches” which comply with these specifications will, if properly handled, make a good pavement.
[16]. The possibility that some of these compounds or artificial asphalts, may be suitable for use in pavements is not denied. But in the absence of a fuller knowledge of them than we now have, and in the light of present experience, the only safe course is to reject them.
[17]. It is not intended here to enter into an extended discussion of the fact that some asphalts are injuriously affected by water, and the bearing which this fact should have upon the selection of an asphalt for pavement purposes. There can be no doubt that modern treatment and methods of construction have tended to diminish but not wholly to prevent the disintegrating effect of water upon pavements made with such asphalt, and if the engineer could be certain that his pavements would be constructed by contractors guided by long experience and the best expert advice, he might perhaps safely disregard this provision. Since in practice he can have no such assurance, the provision is a wise one and it does not involve any serious or material hardship to the contractor.
[18]. It is now well recognized that the character and quality of the sand used is one of the most important elements in determining the utility and durability of an asphalt pavement. A satisfactory sand should be insisted on, even if it involves a very considerable increase in the first cost of the work.