PORTLAND CEMENT CONCRETE ROADS. 3 
2. They present a smooth, even surface, which offers very little 
resistance to traffic. 
3. They are practically clustless 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 noisy 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 a,nd 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 
