Discussion of Oil Roads

FEBRUARY, 1933 ARIZONA HIGHWAYS 5 DISCUSSION of the OIL ROADS By JULIAN W. POWERS, MATERIAL ENGINEER

So much has been written of the oil road, that what may be here presented will, no doubt, be a repetition of at least some of its characteristics: To most people the oil road is considered a pavement, and as such receives an unjust criticism when it shows certain types of failure such as rolling, breaking along the edges and general disintegration. Regardless of the general impression that the oil road is a pavement, it is actually an oiled treatment which conserves the surfacing material, and for all practical purposes serves the same purpose as a pavement. It, however, is subject to many idiosyncrasies. The binder, ie, oil, is not always stable in the presence of some of the material it is mixed with, and water. First we'll take up some of the reasons for failure, and then after reviewing the failures, explain what may be done to overcome them, and to build better and more stable roads, regardless of the kind of traffic.

In the early stages of the building of oil roads in this State, it was thought that the oil road was a pavement, and as such could be placed upon practically any kind of a base. Experience has shown that not only was this a false premise, but that the oil road really needs a better base than any other type of improved riding surface. It is for this reason that many of the earlier roads built by the department have had more or less mediocre success.

In addition to the poor bases on which some of these oil roads were put, the use of any kind of material was thought to be satisfactory to make them. This supposition also was proven to be false, and we now know that for best results the materials have to be comparable to those used in asphaltic concrete. The reasons for failure from the above causes are all tied up in the one word -“water”-While water is one of man's best friends in an arid country, it is his worst when it comes to the oil road. Inferior subgrades usually are high capillary, and feed water into an oil cake. Inferior materials usually are more receptive to having water films surrounding them, than oil. As a consequence, the water displaces the oil and will do one of several thingsEither the oil cake will become so depleted of oil by being pushed to the surface and dissipated that it ravels because of insufficient binder, or it will roll because of an excess-i. e., the oil not being able to be pushed out either because of a seal coat over the cake, or because of its low void content acts with the water to give an excess of oil and water. This excess of binder crowds the grains (sand and gravel) apart, and then traffic action forms the waves frequently encountered on the oil road. With some materials placed on adequate subgrade the water works from the top. Certain alkalies and clays will cause an oil to emulsify. Oil a material of this character, and rain with resulting traffic agitation will give a perfect emulsion. When this occurs, the oil is splashed, along with a certain amount of surfacing off onto the roadside. Enough rain, and the road is down to unoiled material and the oiled surface is in the drain ditch.

Enough of the history of oil road trouble-let's see what has been done to correct these troubles Early in 1929 and 1930, tests were devised by this department to select materials that were not subject to any vagaries of nature except the threat of water entering from the subgrade. We know that some materials have more affinity for water than for oil, but cannot, for economic reason prohibit their use. In order to overcome this, we adopted the method of the U. S. Bureau of Public Roads for testing subgrades. We firmly believe that if we only place oil mix work