10 BULLETIN 249, U. S. DEPARTMENT OE AGRICULTURE. 



base before the concrete is placed. Otherwise the irregularities in 

 the telford surface would prevent the pavement from contracting 

 and expanding readily and would thus cause cracks to occur at fre- 

 quent intervals. 



When old macadam or gravel roads are to be surfaced with con- 

 crete it is advisable to scarify the entire surface to a depth of several 

 inches before the subgrade is shaped to receive the concrete. If this 

 is not done, it is almost impossible to prevent a lack of uniformity 

 in the subgrade wherever it is necessary to grade or shape up any 

 part of the old road. 



It has been suggested, with an apparent show of reason, that a 

 thin cushion of sand might be advantageously used under concrete 

 pavements. The purpose of this construction is to facilitate the 

 sliding of the pavement, due to expansion and contraction, and thus 

 to increase the allowable distance between contraction joints. So far 

 as is known there are no experimental data which bear on this subject. 



The form work required for concrete pavements is very simple 

 and inexpensive. Ordinarily the forms may consist of 2^-inch boards 

 having a width equal to the edge thickness of the pavement, though 

 metal forms are in general more economical and are always to be 

 preferred. The forms should be set before the subgrade is finished, 

 in order to serve as a guide for the finish grading, and should be 

 securely held in place by means of stakes driven on the shoulder 

 side to such depth that they do not extend above the top of the forms. 

 Care should be taken to see that the forms bear uniformly on the 

 subgrade, as otherwise they are likely to sag while the concrete is 

 being struck off and tamped, and thus produce an irregular surface. 

 It is also well to have the ends of the different sections fastened 

 together in such a manner that no relative displacement is possible. 



The forms should always be set true to line and grade, and where 

 curbs or gutters are to be provided they must be modified to suit the 

 requirements for these features. 



MIXING AND PLACING THE CONCRETE. 



When a considerable area of concrete pavement is to be laid it is 

 usually economical to employ a mechanical mixer for mixing the 

 concrete (PI. II, fig. 1). Hand mixing is much more expensive 

 than machine mixing, and hand-mixed concrete is rarely as uniform 

 as machine-mixed concrete either in consistency or in the distribu- 

 tion of the component materials. Since lack of uniformity is be- 

 lieved to be one of the most potent causes for the formation of cracks, 

 machine mixing is greatly to be preferred. There are several makes 



