CONSTRUCTION IN DISTRICT NO. 4

ARIZONA HIGHWAYS Construction In District No. 4

In the southern district of the Arizona Highway department, or District No. 4, there has been laid, in the past year or year and a half, approximately 145 miles of oiled roads, together with 3.10 miles of bitulithic and concrete pavement, listed as follows:

F.A.P. 97 B and C-Chandler Coolidge 18.80 miles

F.A.P. 94 B and C-ChandlerCoolidge 2.00 miles F.A.P. 94-D-(1½ oil and 1½ cut back asphalt)-CoolidgeTucson 22.00 miles

F.A.P. 94-E-Coolidge-Tucson 8.40 miles

F.A.P. 94-F-Coolidge-Tucson 15.90 miles

F.A.P. 86-E-Tucson-Nogales 9.20 miles

F.A.P. 86-C-Tucson-Nogales 7.70 miles

F.A.P. 90-A-Tucson-Nogales 22.00 miles

F.A.P. 18-A-Tucson-Nogales 2.80 miles F.A.P. 23-A and B-Florence Jct. Florence ...14.80 miles

Casa Grande-Picacho 16.50 miles

Tucson-Florence .......; 5.10 miles

Tucson-Nogales-bitulithic pavement .60 miles

F.A.P. 1 and 94-A-Concrete pavement 2.50 miles I have listed these as Federal Aid projects, as I wish to refer to them later. There are two types of oil roads represented, the 65 plus road oil and the cutback asphalt, this product being Grade "E" asphalt cut back with 15 to 20 per cent kerosene oil.

The accompanying plates, 1 to 5 inclusive, show the grading of the mineral aggregate for each of the several jobs, together with some other jobs not listed above. I am showing these plates in an endeavor to find some common factor in the grading of the mineral aggregate which will insure an excellent oil cake, but I find in all instances shown above, with the exception of one which I will refer to later, the curves for the grading seem more or less erratic. The only con-

ARIZONA HIGHWAYS

clusion I can draw from this is that the upper end of these curves from the one inch material to the 10 mesh material is not so important so long as the rock content does not fall below 25 per cent, and I have found that the best rock content to procure is around 40 per cent, but the lower end of the curves must be watched carefully for a more or less uniform gradeing from the 10 mesh through the 200 mesh. Any lack of 200 mesh below seven or eight per cent is rather dangerous when using the above 65 plus road oil, but this objection does not seem to be serious in the use of cut-back asphalt.

As we all know, the theoretical mixture for this mineral aggregate is based upon the same theory that we try to follow in the mixture of concrete; that is, the several different gradings filling the voids within the above sizes, the oil or asphalt taking the place of the cement as a binding agency, making up a matrix or "hold together" agent. As stated above, regardless of the erractic nature of these curves all of the gradings shown on the above charts have produced excellent oil cakes, which have been hard, tight and abrasive-proof, with the exception of the Casa Grande-Picacho oil cake, as shown on plate No. 5. This oil cake, 16½ miles long, was laid from two pits. The curve on plate No. 5 is one taken from pit No. 2, which showed an excessive lack of fine material from the 10 mesh through the 200 mesh, especially lacking in the 200 mesh content. This gave us an oil cake which I considered more or less porus, owing to the lack of the finer mesh material. This cake stands very well the compressive elements but is easily moved out of place by the abrasive elements. It is poor policy to build a cake of this nature, but it has an advantage which I will cite later.

We have found that in using cut-back asphalt it is more difficult to mix and lay than the road oils, as this material, mixed with mineral aggregate, gives a much stiffer and harder mixture than the above oils. Cut-back asphalt must be put on rather hot and must be mixed before laying until practically all the kerosene has been worked out of it.Our only experience on a large job with cut-back asphalt has been on F. A. P. 94-D, to an extent of practically 10 miles. This section was laid during very hot weather, and great care had to be taken in drying out the kerosene cut back and also in the final lay down. This material, as pointed out above, is very stiff, and if great care is not taken a rough surface is the result.

In oil processing today we have the oil cake practically under control; but the greatest enemy of oiled roads is the under-ground water, which occurs, not in large quantities, but in quantities sufficient for capillary action to bring it to the surface. In laying these oil mixtures the subgrade is tried out and tested for dryness by digging test holes in the subgrade to a depth of six to eight inches; but I find this capillary water may be anywhere from one to two feet below the surface of the subgrade, and in laying an oil cake upon this surface it acts as a porus plaster. Augmented by the heat of(Continued on Page Sixteen)