PORTLAND CEMENT CONCRETE ROADS. 25 
CURBS AND GUTTERS. 
To prevent erosion of the side ditches and the danger of washouts 
on relatively steep grades, some form of paved gutter, or combined 
curb and gutter, must be used. The amount of erosion depends upon 
the velocity of the water and the kind of soil. On soils of loose tex- 
ture a small accumulation of water on grades as low as 3 per cent is 
sufficient to cause considerable erosion ; while some soils of dense tex- 
ture are not materially eroded on grades as high as 6 per cent. The 
grade, therefore, on which it will be necessary to use a paved gutter 
will depend upon the kind of soil. In general, it will be found desir- 
able to provide paved gutters on all grades greater than 5 per cent. 
A paved gutter, or the combined curb and gutter, may often be 
used to advantage in reducing the amount of grading in through and 
hillside cuts. For example, in deep cuts the amount of grading can 
often be reduced as much as 35 per cent by omitting the shoulders 
and side ditches and providing curbs along the edges of the pavement, 
so that the sides of the pavement serve as gutters. Similarly on 
heavy hillside work, by omitting the shoulder and ditch next to the 
hill and using a curb on one side, a considerable saving in grading 
can be effected. Inasmuch as the use of curbs confines traffic to the 
pavement, the width of the pavement should be slightly increased 
where curbs are employed. If curbs are used in connection with a 
standard 18-foot pavement with earth shoulders, the width between 
curbs should be at least 20 feet. 
The paved gutter, or the combined curb and gutter, can be con- 
structed as an integral part of the pavement, but this operation is 
a slow, tedious one which slows up the laying of the main body of 
the pavement and prevents the use of a mechanical finishing machine 
to the best advantage. Better results will be obtained if the regular 
width of pavement is constructed first and the gutter, or curb and 
gutter, constructed later. If this procedure is adopted, the gutter, or 
curb and gutter, should be tied to the main pavement by short pieces 
of reinforcing steel. This can be accomplished by drilling holes in 
the pavement forms midway between the top and bottom and insert- 
ing bars 3 feet long, spaced 2J to 3 feet apart, so they will project into 
the pavement about one-half their length. The bars should not be 
bent to conform to the gutter section until the forms have been re- 
moved. Joints should be placed in the gutter, or the curb and gutter, 
at points where joints exist in the pavement. Typical details of 
circular and V-shaped gutters and a combined curb and gutter are 
shown in Figure 6. 
BITUMINOUS SURFACE TREATMENT. 
A coating of bituminous material and sand, gravel, or stone chips 
applied to the surface of a concrete road is known as a bituminous 
