20 BULLETIN 249, U. S. DEPARTMENT OF AGRICULTURE. 



Rather than construct a concrete culvert sufficiently far in ad- 

 vance for the subgrade 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 perforin. 



The accompanying diagrams, Plate XI, figures 3 and 4, illustrate 

 mixer organizations for the two sizes of mixers in general use, which 



