38 BULLETIN" 257, U. S. DEPARTMENT OP AGRICULTURE. 



The section which was constructed directly under the supervision 

 of the Office of Public Roads carries voluminous and heavy suburban 

 traffic, both vehicle and motor, and has received no maintenance or 

 repairs since its original construction. A new 40-foot steel truss 

 bridge was constructed across the slough last October, which resulted 

 in unusually heavy additional traffic. For 20 feet on either side of 

 the new bridge the macadam has been destroyed. The remainder 

 of the experiment now presents an unsatisfactory appearance and is 

 very wavy and much worn. There is a series of potholes of various 

 sizes and depths, but there is, however, no apparent disintegration. 

 The former shoulders have been used as a roadway until they have 

 worn through the subgrade and are largely used by lighter traffic 

 to avoid jolting. The crown is well maintained, and the penetra- 

 tion has been effective in general, with an excess of oil in places. 

 The unevenness of wear, as previously noted, has been due primarily 

 to a weak and poorly prepared foundation. Wherever the bond is 

 broken the contributory cause appears to be large water-worn gravel 

 too near the surface, and lack of uniformity in the distribution of 

 the bituminous material, which latter permits early raveling. 



SUPPLEMENTARY REPORT OF EXPERIMENT MADE AT AMES, IOWA, 



1910. 



OIL-ASPHALT GRAVEL. 



The original report of this experiment was published in Circular 

 No. 94, and reports of annual -inspections were given in Circulars 

 Nos. 98 and 99, Office of Public Roads, and in U. S. Department 

 of Agriculture Bulletin No. 105. 



When visited on December 19, 1914, there was about 1 inch of 

 snow on this road. Scraping this snow from spots revealed about 

 one-half inch of oily dust, covering a well-bonded surface. On 

 warming bits of bituminous material in the hands it still developed 

 a perceptible stickiness. 



It was learned that an attempt was made in August to remove 

 certain irregularities on the surface and to raise the center of the 

 crown by means of a light two-wheeled single-blade grader. This 

 cut away the high smooth places on the surface, leaving the gravelly 

 mixture exposed. However, the traffic compacted this surface and 

 it bonded again in very good shape. There was and is now some 

 roughness perceptible to automobile travel caused by the uneven dis- 

 tribution of the material which was brought to the center for the 

 purpose of raising the crown. 



Traffic on this road is considerable, although it consists mainly of 

 automobiles, carriages, and light wagons, since most of the heavy 

 trucking enters the college grounds by another road. 



