POETLAND 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. 



PREPAKATIOIsr OF THE SUBGRADE. 



The essential qualities of the subgrade are uniformity m 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 pavement, 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. 



