PORTLAND CEIMENT CONCEETE ROADS. 3 



2. They present a smooth, even surface, which offers very little 

 resistance to traffic. 



3. They are practically dustless and may be easily cleaned. 



4. They may be maintained at comparatively small cost. 



5. They may be made to serve as a base for some other type of 

 surface when resurfacing becomes necessary. 



The principal disadvantages are : 



1. They are somewhat noisj- under steel-tired traffic. 



2. They are subject to cracking, and wherever a crack develops it 

 must be given frequent attention in order to prevent deterioration of 

 the pavement. 



3. On account of the sharp line of separation between the pave- 

 ment and the shoulders and the marked difference in hardness, an 

 abrupt and dangerous depression is sometimes formed at the edge of 

 the pavement which reduces the effective width of the roadway. 



A finished concrete road is shown in Figure 1, Plate X. 



MATERIALS USED IN CONCRETE ROADS. 



Concrete consists of a mixture of water, cement, sand, and gravel 

 or stone or other similar materials. It is customary to refer to the 

 sand as the fine aggregate, and to the gravel or stone as the coarse 

 aggregate. Durable, clean, well-graded aggregates are absolutely 

 essential to the success of a concrete pavement. Mixed aggregates, 

 such as bank-run gravel or crusher-run stone, should not be used 

 except under rigid laboratory control. For a successful concrete 

 pavement, each of the different aggregates should be properly graded 

 and kept clean and separate until proportioned to place in the mixer. 



CEMENT. 



Portland cement of a character satisfactory for use in pavement 

 construction is at present manufactured in nearly every section of 

 the country. The product of all cement plants is not always en- 

 tirely uniform and of equal excellence, and even if it were uniform 

 immediately after manufacture this condition might easily be 

 changed by age or exposure. These facts make it imperative that 

 cement for use in concrete pavements be subjected to very rigid tests. 

 It should meet the requirements of the specification for Portland 

 cement contained in Circular 33 of the United States Bureau of 

 Standards and also issued by the American Society for Testing 

 Materials, and. accepted generally as the standard specification. 



FINE AGGREGATE. 



Sand is almost universally used as a fine aggregate for concrete 

 pavements. In exceptional cases stone screenings have been used, 

 but the use of screenings is not recommended, as the presence of dust 



