PORTLAND CEMENT CONCRETE ROADS. 29 
both sides. For a more detailed description of the use of tile drains 
see Bulletin 724, United States Department of Agriculture. 
PREPARATION OF THE SUBGRADE. 
The essential qualities of the subgrade are uniformity in grade, 
in cross-section, and in firmness. 
The purpose of the rolling to which it is customary to subject the 
subgrade is to secure uniform firmness. Whether it accomplishes 
this result is a point upon which opinions differ considerably. Cer- 
tainly no amount of rolling will result in uniform firmness if trucks 
or teams are driven over the subgrade to supply the mixer. Under 
certain conditions it is believed that no rolling is required. In par- 
ticular it is not believed necessary to roll a newly graded road which 
has been closed to traffic and which has thoroughly settled before the 
pavement is placed, providing the concrete materials are hauled to the 
mixer by means of an industrial railway. 
It is difficult to obtain uniform firmness by the use of the customary 
three-wheel type of macadam roller, because a small strip of the sub- 
grade, wheel-gauge distance from the sides of the road is subjected 
to twice as much rolling as the edges. The tandem roller is not 
open to this objection, and it is believed that a condition of uniform 
firmness can be more nearly secured with a roller of this type than 
with any other kind. 
Any soil with a clay content that is unduly compressed by rolling 
will swell considerably upon addition of moisture. Unless uniform 
firmness has been secured by the rolling, the subsequent absorption 
of moisture will result in uneven swelling which will outweigh any 
advantage which might have been obtained by rolling. For these 
reasons it is believed that, in general, light rolling is to be preferred 
to heavy rolling. 
When an old macadam or gravel road is to be surfaced with con- 
crete, the entire surface of the road should be scarified and plowed 
to the full depth of the existing surface before the subgrade is shaped 
to receive the concrete. If this is not done it will be almost impossi- 
ble to secure a uniformly firm subgrade. In case the concrete sur- 
facing is to be wider than the old road surface, the failure to loosen 
the old surfacing to its full depth will leave a hard, compact core 
in the subgrade. The uneven support afforded by subgrades with 
such hard cores is the cause of frequent longitudinal cracks in con- 
crete pavements constructed over old macadam or gravel roads. 
The uniform firmness of the subgrade should extend for a dis- 
tance of at least 1 foot beyond the edges of the paA-ement, in order 
to provide a solid support for the side forms. 
After the rolling the forms are set true to line and grade and they 
are then used as a guide for the finishing or trimming operations. 
