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. 
FORMS. 
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 (PL 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 
