72 BULLETIN 724, U. S. DEPARTMENT OF AGRICULTURE. 
grade from being more or less damaged. Where hauling over the 
prepared subgrade is permitted it always tends to produce a lack of 
uniformity by increasing the degree of compactness of the parts 
traversed, and this may result easily in unequal settlement with con- 
sequent cracking of the foundation and road crust. 
If the locality of the construction is one where rains are heavy and 
frequent, or in a season having such rains, judgment should be exer- 
cised in placing the materials on the subgrade very far in advance 
of the time they are needed for use, because under such conditions 
the subgrade may become saturated under the material piles and not 
dry out before the concrete is placed. 
According to present practice concrete for road foundations is 
almost invariably mixed in a machine mixer. Hand mixing can be 
afforded only where the work to be done is inconsiderable and rarely 
produces as uniform concrete as machine mixing. In this discus- 
sion, therefore, it will be assumed that a machine mixer is employed 
in all cases. To accomplish satisfactory work under the conditions 
met ordinarily in foundation construction the concrete mixer should 
possess the following features: (1) It should be of the batch type, 
so that uniform proportioning of the material may be assured; (2) 
it should be fitted with a traction attachment so that it may be 
moved along the road under its own power as the work progresses: 
(3) it should include a device for distributing the concrete. This 
device may consist of a boom with a bucket traveler, a chute, or a 
revolving tube. The boom and bucket is to be preferred generally 
because it enables a greater area of foundation to be placed from one 
position of the mixer than either of the other devices, and conse- 
quently machines so equipped require less moving than others. 
Sometimes the concrete is mixed in a stationary mixer and hauled 
to its place in the foundation, but this method is objectionable be- 
cause it permits a separation of the component materials to take 
place between the time of mixing and the time of depositing. If a 
uniformly dense concrete is to be produced it must be deposited in 
the foundation immediately after it is mixed. Failure to secure 
-uniform density impairs the strength of the concrete, and. in extreme 
cases, may cause cracks to develop in the foundation, thus greatly 
lessening its power of distributing stresses clue to concentrated 
loads. 
The principal precautions essential to observe in mixing and plac- 
ing concrete in a road foundation are to see — 
1. That the amount and time of mixing is sufficient .to secure a thorough 
distribution of the different materials through the mass and complete hydration 
of the cement. Tests have shown that increasing the time a batch of concrete 
remains in the revolving drum of a mixer, within reasonable limits, has very 
much the same effect as increasing the proportion of cement. The efficiency 
with which mixers work, as expressed in the strength of concrete, varies 
