30 



BULLETIX 1077, U. S. DEPAETMEXT OF AGEICULTUEE. 



The finishing may be accomplislied either by picks and shovels or 

 by the use of a subgrade planer which rides upon the side forms. 

 ^Tien materials are delivered to the mixer by hauling over the sub- 

 grade it is generally necessary to finish -with picks and shovels. If 

 materials are delivered by industrial railway, so that the subgrade 

 is not used for hauling, it will usually be economical to use a sub- 

 grade planer. The planer, which is generally drawn by the roller, 

 has its cutting edges so arranged that the slight excess of material 

 trimmed from the subgrade is deposited in windrows at the quarter 

 points, from which it is shoveled to the shoulders. For efficient use 

 of the planer the rough grade should be slightly higher than the 

 finished surface, a condition which is desirable in any case for the 

 reason that it leads to the construction of subgrades of more nearly 

 uniform firmness. Plate I, Figures 1 and 2, illustrate the construc- 

 tion and use of the subgrade planer and the finished subgrade. 



The cross section may be either flat or shaped to conform to the 

 finished surface of the pavement. In either case the allowable varia- 



z/fon 



i p/onk 

 '20 m:/ no/Zs 6 cenfe/"^ 



I 1 1 1 I i~ 



/ /es3 ffian fio/fcj/df/f ofpoye/ne/?/- 



Fig. 8. — Details of nail template used to test the subgrade. 



tion from the true grade and cross section is usually limited to one- 

 eighth inch. This small variation is intended primarih' to insure 

 that the full thickness of pavement will be secured at all points. The 

 subgrade is tested by means of a nail template (Fig. 8), which is 

 moved back and forth over the forms. Should the test show that 

 any portion is too low. the low area is generally filled with concrete 

 as an integral part of the pavement, though sometimes the contractor 

 is permitted to fill it with hand-tamped earth. While the engineer is 

 charged with securing the full thickness of pavement required by 

 the plans, the contractor aims to furnish that thickness and no more, 

 because any additional concrete represents loss which rapidly runs 

 into a large sum. The natural result is a subgrade as true to grade 

 and cross-section as it is practicable to obtain. 



FORMS. 



The side forms for concrete pavements may be of steel or wood. 

 Steel forms are preferable and should be used whenever the pave- 

 ment to be laid exceeds one-half mile in length, and when machine fin- 

 ishing is to be used. A number of makes of steel forms can be pur- 



