ROADS AND PAVEMENTS 401 



selecting and grading the stones. This is done with great care 

 in the bitulithic pavement, the aggregate of which consists 

 of stones of different sizes proportioned so as to reduce the voids 

 to a minimum. 



Concrete Highways. The destructive effect of modern 

 traffic on the public highways has also led to extensive experi- 

 ments in the construction of road surfaces in which Portland 

 cement is used as a binder. Although still in the experimental 

 stage, this form of construction promises a great development 

 \n the near future. 



In constructing concrete pavements, a great variety of 

 methods are employed, and many of them are patented. 

 The types of construction most in use are: the one-course 

 pavement, the two-course pavement, and the grouted pave- 

 ment. 



The one-course pavement consists of one layer of Portland- 

 cement concrete about 6 to 8 in. deep laid on a properly pre- 

 pared subfoundation. The cement used should be of the best 

 quality, the aggregate should consist of hard and tough material, 

 and the proportion of the different materials must be such as 

 to fill all the voids. 



The two-course pavement consists of a layer of Portland- 

 cement concrete about 5 in. thick upon which is laid a 1J- to 

 2-in. wearing surface consisting of cement mortar prepared 

 from the best Portland cement and a fine aggregate properly 

 graded and capable of resisting abrasion. To secure proper 

 binding between the two courses, the top course should be 

 placed before the concrete in the base course has set. The 

 advantage of this type of construction is that in many cases 

 it allows the use of a cheaper grade of material for the concrete 

 in the lower course. On the other hand, the one-course type 

 of construction eliminates the danger of a loose-top such as is 

 liable to occur in the two-course type of construction. 



The grouted pavement is a two-course pavement in which 

 the first layer is formed of broken stone instead of concrete. 

 The broken stone is firmly compacted by rolling, and a Port- 

 land-cement grout is poured upon it until it flushes the surface. 

 Upon this surface is then spread a thin layer of stone of about 

 the size of peas, after which it is again rolled and grouted. 



