20 BULLETIN" 249, U. S. DEPARTMENT OF AGRICULTURE. 
Bather than construct a concrete culvert sufficiently far in ad- 
vance for the subgracle to be prepared before the mixer arrives, it 
may- sometimes be economical to leave out a section of the pavement 
over the culvert. • But the extra expense involved in going back and 
putting in a section of this kind after the work of laying the pave- 
ment has progressed a considerable distance ahead is usually con- 
siderable and is often underestimated by contractors. This method 
of doing the work also involves a delay in opening the road, and as 
a rule is very objectionable on that account. 
The work of preparing the subgrade and setting the forms should 
preferably proceed sufficiently far in advance of the mixer to allow 
for two or three days' run. The prepared subgrade, if properly 
drained, dries out much more rapidly after ■ rains than the rough 
grade, and thus it is possible to resume the placing of concrete much 
earlier than when the roadbed has not been shaped and rolled. A 
soaking rain will usually cause the prepared subgrade to heave 
slightly and make rerolling necessary, but ordinarily this is a very 
small item. 
OPERATING THE CONCRETE MIXER. 
In general it is economical to employ a mixer of the street-paving 
type for mixing and placing the concrete, though in some cases it 
has proved satisfactory to do the mixing in stationary mixers and 
haul the concrete out to its place in the road. This latter method is 
applicable to relatively only a very few sets of conditions, however, 
and will therefore not be discussed in detail. 
There are two sizes of street-paving mixers commonly used in 
concrete road construction. The smaller is capable of mixing a 
batch, of the proportions usually required, containing two bags of 
cement, and the larger will mix a batch containing three bags of 
cement. The larger size is economical where materials can be 
rapidly obtained and where the amount of work to be done is 
sufficient to warrant providing equipment for handling the materials 
necessary to keep the larger mixer running up to its capacity. Where 
the materials can be economically obtained only at a slow rate, or 
where the expense of providing facilities for handling large quan- 
tities of materials would be excessive, the smaller size of mixer is 
more economical to use. When efficiently operated, either size of 
mixer should ordinarily mix from 400 to 450 batches of concrete in 
a working day. 
Organizing a force of laborers to operate a paving mixer effi- 
ciently requires considerable skill in handling men. The best results 
are generally obtained when a mixer is fully manned and each 
laborer is assigned definite work to perform. 
The accompanying diagrams, Plate XI, figures 3 and 4, illustrate 
mixer organizations for the two sizes of mixers in general use, which 
