18 BULLETIN 249, U. S. DEPABTMENT OF AGEICTJLTUEE. 



(2) They reduce the noise made by the impact of horses' hoofs and 

 steel-tired wheels. 



(3) They remove the principal objection to bituminous expansion 

 and contraction joints. 



(4) They overcome the somewhat objectionable glare of concrete 

 pavements in strong light, though this objection may also be overcome 

 with much less cost by sprinkling the pavement with crude water- 

 gas tar. 



The principal disadvantages may be inferred from what has 

 already been said. It is also well to note that, where traffic condi- 

 tions are such as to make a bituminous surface practicable on a con- 

 crete road, a bituminous-surfaced macadam road might also be prac- 

 ticable and would certainly be cheaper to construct, unless the diffi- 

 culties involved in securing suitable stone for the macadam were 

 very unusual. 



In constructing bituminous surfaces on concrete it is essential to 

 have the surface of the concrete entirely clean and free from laitance 

 when the bitumen is spread. Generally about one-half gallon of 

 bitumen to the square yard is put on in either one or two applications, 

 by hand or by means of pressure distributors. It is sometimes swept 

 with hand brooms in order to make it adhere better to the pavement. 

 Hot applications have hitherto been almost exclusively used, though 

 there is no apparent reason why materials which could be spread cold 

 might not be employed with equally satisfactory results. 



After the bitumen has been spread as described, it is covered with 

 coarse sand, pea gravel, or stone chips, applied at the rate of 1 cubic 

 yard to from 75 to 100 square yards of surface. The road may be 

 opened to traffic almost immediately after the sand or stone chips are 

 spread. Plate V shows the conditions of different concrete roads 

 with bituminous wearing surfaces after certain periods of service. 



It is realized that the above discussion of bituminous wearing sur- 

 faces falls very far short of furnishing a guide for undertaking work 

 of that kind. The available data upon this subject, however, are not 

 considered sufficient to form a basis for a more comprehensive discus- 

 sion. Not onty have contradictory results been reported by different 

 engineers concerning the same methods of construction, but the re- 

 sults now being obtained from carefully conducted experiments by 

 the Office of Public Roads with different materials and different 

 construction methods do not yet seem to warrant any definite state- 

 ments as to what materials are best adapted for such work nor which 

 construction method will give the best results, though they do indi- 

 cate in a general way that tars are preferable to asphalts for this 

 purpose. 



